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Terms in this set (154)

a specialized cell transmitting nerve impulses; a nerve cell. A neuron also known as a neurone or nerve cell) is an electrically excitable cell that processes and transmits information through electrical and chemical signals. These signals between neurons occur via synapses, specialized connections with other cells. Neurons can connect to each other to form neural networks. Neurons are the core components of the brain and spinal cord of the central nervous system (CNS), and of the ganglia of the peripheral nervous system (PNS). Specialized types of neurons include: sensory neurons which respond to touch, sound, light and all other stimuli affecting the cells of the sensory organs that then send signals to the spinal cord and brain, motor neurons that receive signals from the brain and spinal cord to cause muscle contractions and affect glandular outputs, and interneurons which connect neurons to other neurons within the same region of the brain, or spinal cord in neural networks.

A typical neuron consists of a cell body (soma), dendrites, and an axon. The term neurite is used to describe either a dendrite or an axon, particularly in its undifferentiated stage. Dendrites are thin structures that arise from the cell body, often extending for hundreds of micrometres and branching multiple times, giving rise to a complex "dendritic tree". An axon (also called a nerve fiber when myelinated) is a special cellular extension (process) that arises from the cell body at a site called the axon hillock and travels for a distance, as far as 1 meter in humans or even more in other species. Nerve fibers are often bundled into fascicles, and in the peripheral nervous system, bundles of fascicles make up nerves (like strands of wire make up cables). The cell body of a neuron frequently gives rise to multiple dendrites, but never to more than one axon, although the axon may branch hundreds of times before it terminates. At the majority of synapses, signals are sent from the axon of one neuron to a dendrite of another. There are, however, many exceptions to these rules: neurons that lack dendrites, neurons that have no axon, synapses that connect an axon to another axon or a dendrite to another dendrite, etc.
If we compared the human body to a computer, then the nervous system would be the motherboard. It is the main control unit for the body, and through the nervous system, other functions in the body are regulated. Therefore, the nervous system is one of the most important systems in the human body as its effects can be seen in all other systems.

The nervous system communicates through the use of cells, called neurons. These cells participate in cell-to-cell communication for the purposes of regulating bodily processes. This is done through the generation of electrochemical stimulation that relays from neurons to other neurons and effector (target) cell. The delivery of this stimulation is going to be mediated by a portion of the neuron known as the axon.

Structures of Axons
Axons are extended regions of the neuron cell membrane. It starts from a portion of the cell body, known as the axon hillock. From there, the axon extends towards the target cell to what is known as the terminal. Along the cell membrane of the axon will be ion channels and ATP-driven pumps that will regulate ion concentrations within the axon. These ion concentrations will establish the resting membrane potential, which is the electrochemical charge of the membrane when the neuron is at rest.

Structures Found in a Neuron
Neuron
Some axons will also have additional structures to assist with communication. In areas of the nervous system that require faster communication, the axons will contain insulation, known as myelin sheaths. This insulation speeds up the transmission of cell-to-cell communication and stimulation. Not all axons will have these sheaths, but the ones that do function quicker.

Communication via Axons
At rest, the membrane potential of an axon is typically -70 millivolts. This charge is established by ATP-driven pumps in the membrane known as sodium/potassium pump. This pump ensures that more positive ions are outside of the membrane compared to inside of the cell. When the neuron depolarizes (becomes positively charged), it will transmit this communication down the axon by way of voltage-gated (electrically controlled) ion channels that open up to allow for the charge to relay along the axon. This takes place until the charge reaches the axon terminal.

The axon terminal is the site of neurotransmitter release. Neurotransmitters are chemical messengers that are released from the axon and received by effector cells. This process is critical for delivery of the message to the cells and tissues that are being controlled. The terminal, then, is the final point of stimulation in the axon before the charge is delivered.

Axon Terminal: Site of Neurotransmitter Release
Axon terminal
Conclusion
The axon is the portion of the cell responsible for delivering cell-to-cell communication. Through transmission of electrical charges and the release of neurotransmitters, axons are able to control target cells in order to regulate bodily processes. Therefore, axons are the key components of neuronal function for the nervous system and other systems under nervous control.
Your brain is like a machine with many different parts that all work together. The synaptic cleft may not be the most well known part of the brain, but it is vital for brain function. Read on to find out more about what it does and why it matters.
Your Brain
Your brain is an amazing machine with lots of work to do. The neurons, or nerve cells, in the brain are responsible for communications that make all processes possible. Communication can happen two ways: electrically or chemically. When communication is chemical, the synaptic cleft comes into play.

Function of the Synaptic Cleft
The synaptic cleft, by definition, is a tiny opening between neurons. When scientists study the synaptic cleft, they are looking at how information is relayed from one neuron to another, but we will dive deeper into this later on in the lesson.

The synaptic cleft is seemingly just an empty space, so you may think that it isn't important, but don't be fooled. Think of neuron communication like traveling to a different country—neurons don't all speak the same language. So you may be wondering, how does the information get translated? That's right...the synaptic cleft helps to decode the message. When the electrical signal reaches the presynaptic ending, it is translated into a chemical message that then diffuses across the synaptic cleft to the postsynaptic cell. The receiving neuron takes this information and translates the chemical message back into electrical signals, which then heads into the next neuron where the process is repeated. Let's take a look at how other parts of the brain come into play and how they work together.

The Neuron
Neurons are the most basic unit of the brain. Your brain has billions of neurons that use electrical signals to communicate with other neurons about all types of things, such as sending hunger pains or picking up a pencil. Neurons have projections called axons and dendrites. Axons bring information away from the cell, and dendrites carry information to the cell. The spot where neurons come together to communicate is called a synapse.

The Synapse
The synapse is like a wire that connects two cells together. Neurons pass information to each other through the synapse.

The synapse contains four main parts:

An ending with neurotransmitters
The presynaptic ending
The postsynaptic cell
The synaptic cleft
The neurotransmitters are nerves that carry information, and they are located just before the synaptic tip. The presynaptic ending is located in the synapse and is responsible for sending information out. The postsynaptic cell is a cell which has places for the neurotransmitters to land, or receive information. The synaptic cleft, as we know, is the space located between the presynaptic and postsynaptic endings.

Neurons communicate by sending an electrical signal. Let's break down how this works.

Neurotransmission in Action
Neurons communicate by sending out an electrical signal, and they start a chain reaction when they are stimulated by signals. Every neuron on the path takes up the signal and passes it to the next neuron. The dendrites pick up the impulse and send the message to the axon, which then delivers it to the next neuron. Then, the process begins again with another neuron. Finally, once the message hits its target, like a muscle or gland cell, the neurotransmitter is stimulated and causes action. All of this happens in about seven milliseconds.



neurotransmitter

Neurons Communicate
When one neuron communicates with another, it sends an electrical impulse through the presynaptic ending. This releases neurotransmitters into the synaptic cleft, or the space between presynaptic ending and postsynaptic cell. Now, the neurotransmitters can move across the synaptic cleft and bind together with the postsynaptic cell. Take a look at this transmission:



synapse

Lesson Summary
Every single thing you do as a human depends on your neurons. They send electrical impulses that bring messages to other parts of the brain, as well as your nervous system. When the signal gets to the end of the dendrite, it sends a message to the next neuron waiting to receive it through a messenger called a neurotransmitter. While the axons are waiting to receive the message, it first crosses the synapse, or wire between neurons.

The synapse has three parts:

The presynaptic ending, which releases the message
The postsynaptic cell, which receives the message
The synaptic cleft, the microscopic space between the pre- and post-synaptic endings
The synaptic cleft acts as a translator, taking the information from the sending neuron and converting it to an understandable piece of information for the receiving neuron. Without the synaptic cleft, neurons wouldn't be able to properly communicate, and messages like, 'I'm thirsty,' wouldn't get sent or received.
When Kendra wants to move, she moves, and when she wants to stop and rest, she does. This is thanks to the neurons that she has in her nervous system. But, how exactly do they work?

Like Kendra, neurons aren't active all the time. When a cell is firing, it is in action, but when it is not firing, it is at rest. The resting potential of a neuron is the condition of the neuron when it is resting. There is still potential for it to fire, but it is not firing at the moment, which is why it is called the resting potential. Think about the resting potential like when Kendra is at the starting line; she's not moving yet, but she's ready to move at a moment's notice.

So, what is a neuron like during a resting potential? To understand that, you need to know that both inside and outside of the neuron is a liquid that's filled with many different ions and anions. Especially important when talking about the resting potential are the sodium and potassium ions.

Both sodium and potassium ions are positive, but the number of each type of ion inside and outside of the cell determines what the charge of the liquid is. For example, at rest there are more potassium ions inside the cell and more sodium ions outside of the cell. This makes the inside of the cell more negative than the outside of the cell during a resting potential.

Kendra gets that during resting potential, a neuron is more negative inside than outside. But what happens when a cell fires?

Like Kendra taking off for a race, when a cell fires it becomes active. At that point, sodium ions flood the inside of the cell, and potassium ions flow out of the cell. That makes the charge inside the cell more positive than the outside of the cell. This is the opposite of a resting potential, and it causes the neuron to send a message to the next neuron in line--like when Kendra runs the relay and hands the baton off to her teammate.
Ok, so Angela and Jodie want to pass messages, but first, Angela needs to get to the roof of her apartment building.

An action potential is a chain reaction down the length of an axon, which causes the neurotransmitter to fire at the neighboring neuron. It's kind of like Angela walking up the stairs in her building, from floor to floor, until she reaches the roof and is able to send Jodie her message. How does an action potential work? To understand, let's think about Angela. Before she can get to the roof, before she can even begin to climb the stairs, she has to start somewhere. If she's in the lobby of her building, she's in the normal starting place.

A resting potential is the normal state of an axon. It's like the neuron is at rest, and therefore it's called the resting potential. During resting potential, there are lots of ions that are traveling in and out of the axon, kind of like the lobby of Angela's building. Some of the ions travel easily, and some have it a little harder. During a resting potential, there is a higher concentration of potassium ions inside the axon and a higher concentration of sodium ions outside the axon. This makes the inside of the axon have a more negative charge at rest than the outside of the axon.

But what happens when an action potential begins? If a neuron has a message that needs to be sent, sodium channels in the axon open, and sodium rushes into the axon. This makes the inside of the axon more positive relative to the outside. This sets off the chain reaction, as more sodium channels open up a little further down the axon, causing that part of the axon to become more positive, and so on.

It's kind of like Angela deciding to go up to the roof and pass her message to Jodie. She can't just magically teleport to the roof. Instead, she has to climb up one flight of stairs at a time. She's getting closer and closer to the point where she can pass the message on to Jodie.
Cells that can be stimulated electrically are said to be excitable. This means that if an electrical signal is applied or delivered to these cells, they will respond by becoming functional. This process is driven by electrochemical gradients.

Electrochemical Gradients
Electrochemical gradients are differences in ion concentration on the inside and outside of a cell. Generally speaking, if a cell membrane has more charges on one side of the membrane than the other, then there is an electrical difference along that membrane. In most excitable cells, this gradient is largely based on the concentration of sodium ions and potassium ions. Typically, when a cell is not actively excited, there will be more positively charged ions outside of the membrane, which makes the charge at rest inside the membrane negative. These electrochemical gradients are maintained by sodium-potassium pumps, which use ATP to remove positive charges from the cell.

Depolarization
When it is time to become active, voltage-gated ion channels (sodium channels) will open once the membrane charge reaches the threshold voltage. This is the minimum charge necessary for a cell to become active. When the threshold voltage is reached (which is typically -55 mV) the ion channels open in order to allow positive charges to rush inside the cell. This is known as depolarization, which is the process of becoming positively charged.

After a cell depolarizes, it will eventually reach a maximum positive charge. At this point, the cell will try to return to rest by removing positive charges. This is usually done by potassium ions leaving the inside of the cell to cause the cell to become more negative. Eventually, after the charge gets closer to the original resting membrane level, the sodium potassium pump will re-establish the -70 mV charge.
Transmembrane Domains

In this section, we examine the voltage-gated sodium channel as a specific example of a protein embedded in the plasma membrane. This type of protein is found in the nerve and muscle cells and is used in the rapid electrical signalling found in these cells. The principle subunit of the voltage-gated sodium channel is a polypeptide chain of more than 1800 amino acids.

When the amino acid sequence of any protein embedded in a membrane is examined, typically one or more segments of the polypeptide chain are found to be comprised largely of amino acids with nonpolar side chains. Each of these segments coils is what is called a transmembrane domain, with a length approximately the width of the membrane. Moreover, within a transmembrane domain the side chains necessarily face outward where they readily interact with the lipids of the membrane. By contrast, the peptide bonds, which are quite polar, face inward, separated from the lipid environment of the membrane.

In the case of the voltage-gated sodium channel, there are 24 such transmembrane domains in the polypeptide chain, as shown to the right.




For clarity in the figure to the right, the alpha-helices are shown spread out and in a row. Also, they are shown divided into four groups. Each of these is an homologous domain with a similar sequence of amino acids.

In an actual membrane, of course, the alpha-helices are not in a line, but clustered. This is shown in top view in the figure to the left. At the center of the four domains is the channel through which the sodium ions move.





Opening of Channel by Voltage Sensor

The voltage-gated sodium channel has several functional parts. One portion of the channel determines its ion selectivity. This particular channel is quite selective for sodium ions. Even the chemically similar potassium ions cannot pass through the channel.

Another portion of the channel serves as a gate that can open and close. For many ion channels, the gate opens in response to regulatory molecules that specifically bind to either the inside or outside of the channel. But in the case of the voltage-gated sodium channel, the gate is controlled by a voltage sensor, which responds to the level of the membrane potential.

The membrane potential is designated at the left of the figure by the net excess of positive and negative changes. As shown, cells in general have a small net excess of negative ions clustered under the plasma membrane. In a resting neuron or muscle cell the inside is approximately 70 to 90 millivolts (mV) negative with respect to outside.

In this diagram and those that follow, a single transmembrane domain is shown as the voltage sensor that operates the gate. This is for diagrammatic simplicity. Actually, several voltage sensors must respond before the gate opens.

Finally, an inactivation gate is shown. This limits the period of time the channel remains open, despite steady stimulation. But many other types of ion channels do not have an inactivation gate.

The figure to the left shows the movement of the voltage sensor during changes in the membrane potential. The voltage sensor is represented as a transmembrane domain with fixed positive charges. Each of the homologous domains, in fact, has one transmembrane domain in which a positively charged amino acid is found at every third position, giving a total of four to eight positive charges per transmembrane domain. These transmembrane domains are likely to be the actual voltage sensors.

At a typical resting membrane potential (for example, -70 mV) the channel is closed. Then should any factor depolarize the membrane potential sufficiently (for example, to -50 mV), the voltage sensor moves outward and the gate opens. (Figures of channel based on figures by B. Hille and B. Zagotta.)
refractory period is a period of time during which an organ or cell is incapable of repeating a particular action, or (more precisely) the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation. It most commonly refers to electrically excitable muscle cells or neurons. Absolute refractory period corresponds to depolarisation and repolarisation, whereas relative refractory period corresponds to hyperpolarization.






After orgasm, both men and women experience a resolution stage. At this time, their bodies "recover" from sexual excitement and return to their normal states. For men, the penis becomes flaccid again and he goes through a refractory period.

During the refractory period, a man doesn't think about sex or get aroused. His body does not respond to sexual stimulation and he is unable to reach orgasm again until the period is over. The length of the refractory period is different for every man. It may take a half hour or more for his body to perform sexually again.

Younger men may need only a few minutes of recovery time, but older men usually have a longer refractory period, sometimes between 12 to 24 hours. For some men, the refractory period can last a few days.

Experts aren't sure why the length of refractory periods varies so much among men. But they do know that the length of time needed is not related to potency or testosterone levels.

Some men wonder how they can shorten their refractory period. No drugs have been approved for this purpose, but research has shown that Viagra and Cialis - two drugs used to treat erectile dysfunction - may reduce recovery time.

Women do not have refractory periods the way men do. But fatigue after orgasm can make them lose interest in sex temporarily. This can happen after one orgasm or multiple orgasms.
The only information you get about the world comes through your senses: your eyes, your ears, your balance sense, your nose, etc. The fundamental question is: How do the senses provide you with such accurate information about the world? An early idea -- proposed before anyone knew anything about how nerves worked -- said that tiny replicas (~models) of the things we see or hear travel up the peripheral nerves to the brain. We know now that neurons in the peripheral and central nervous systems use specialized neural signals. How can these neural signals represent the objects and events we perceive in the world about us? This is the problem of coding:

taking physical processes like light, sound pressure, chemicals, etc., converting them into signals that the mind (or brain) can use, and
interpreting that as accurate representations of the world.
Individual neurons have two kinds of codes

Dendrites code excitation and inhibition by graded depolarization and hyperpolarization.
Axons code excitation you the number of all-or-nothing impulses (action potentials) per second (rate).
Anatomical or labeled line coding tells the mind/brain where a stimulus is (for example, how you can tell a singing bird is up and to your left, or where is itches on your back) It also tells (usually) which kind it is (a small, red bird, singing two notes, or that it's an itch and not a touch or pain). J. Muller proposed this idea early in the 19th century code, calling it the Law of Specific Nerve Energies. Muller proposed that we see light when visual areas of the brain become active; we hear sound when auditory areas of the brain become active; we feel touch when somatosensory (~ touch) areas of the brain become active, etc. This idea opened the modern study of how the sense work.

This idea turned into labeled line (anatomical) coding when it was extended to explain the different qualities you experience within each sense: different colors of light, pitches in sound, touch, vibration, warmth, etc. on the skin, etc. Anatomical coding states that you experience different qualities when different parts of a sensory area become active.

For example,

if one end of the auditory area of the cerebral cortex becomes active, you experience a high-pitched tone.
If the other end of the auditory area of the cortex becomes active, you experience a low- pitched tone.
if the middle of the auditory area of the cortex becomes active, you experience intermediate pitched tones.
Another, less common kind of kind,

temporal or (time) pattern code

signals the mind/brain the kind of stimulus by the pattern of nerve impulses. For example, one model of the code for itch is bursts of impulses on certain kinds of neurons from the skin interrupted by periods of no impulses.
Labeled line (anatomical) coding is based on two ideas:

Each sense (vision, touch, taste, etc,) has neurons, called receptors, that are especially sensitive to (tuned to) a narrow range of stimuli: their adequate stimulus
In the nervous system, a synapse[1] is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron. Some authors generalize this concept to include the communication from a neuron to any other cell type,[2] such as to a motor cell, although such non-neuronal contacts may be referred to as junctions (a historically older term). Santiago Ramón y Cajal proposed that neurons are not continuous throughout the body, yet still communicate with each other, an idea known as the neuron doctrine.[3] Synapses (at least chemical synapses) are stabilized in position by synaptic adhesion molecules (SAMs) projecting from both the pre- and post-synaptic neuron and sticking together where they overlap; SAMs may also assist in the generation and functioning of synapses.

The word "synapse" - from the Greek synapsis (συνάπσις), meaning "conjunction", in turn from συνάπτεὶν (συν ("together") and ἅπτειν ("to fasten")) - was introduced in 1897 by English physiologist Michael Foster at the suggestion of English classical scholar Arthur Woollgar Verrall.[4][5]

Synapses are essential to neuronal function: neurons are cells that are specialized to pass signals to individual target cells, and synapses are the means by which they do so. At a synapse, the plasma membrane of the signal-passing neuron (the presynaptic neuron) comes into close apposition with the membrane of the target (postsynaptic) cell. Both the presynaptic and postsynaptic sites contain extensive arrays of molecular machinery that link the two membranes together and carry out the signaling process. In many synapses, the presynaptic part is located on an axon, but some postsynaptic sites are located on a dendrite or soma. Astrocytes also exchange information with the synaptic neurons, responding to synaptic activity and, in turn, regulating neurotransmission.[6]
Exocytosis is a form of active transport in which a cell transports molecules (such as proteins) out of the cell (exo- + cytosis) by expelling them in an energy-using process. Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane by passive means.

In exocytosis, secretory vesicles carry their contents across the cell membrane and into the extracellular space. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane. However, the mechanism of the secretion of intravesicular contents out of the cell is very different from the incorporation in the cell membrane of ion channels, signaling molecules, or receptors. While for membrane recycling and the incorporation in the cell membrane of ion channels, signaling molecules, or receptors complete membrane merger is required, for cell secretion there is transient vesicle fusion with the cell membrane in a process called exocytosis, dumping its contents out of the cell's environment. Examination of cells following secretion using electron microscopy demonstrate increased presence of partially empty vesicles following secretion. This suggested that during the secretory process, only a portion of the vesicular content is able to exit the cell. This could only be possible if the vesicle were to temporarily establish continuity with the cell plasma membrane, expel a portion of its contents, then detach, reseal, and withdraw into the cytosol (endocytose). In this way, the secretory vesicle could be reused for subsequent rounds of exo-endocytosis, until completely empty of its contents.[3]
An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential.[1] The opposite of an inhibitory postsynaptic potential is an excitatory postsynaptic potential (EPSP), which is a synaptic potential that makes a postsynaptic neuron more likely to generate an action potential. They can take place at all chemical synapses, which use the secretion of neurotransmitters to create cell to cell signalling. Inhibitory presynaptic neurons release neurotransmitters that then bind to the postsynaptic receptors; this induces a postsynaptic conductance change as ion channels open or close. An electric current that changes the postsynaptic membrane potential to create a more negative postsynaptic potential is generated. Depolarization can also occur due to an IPSP if the reverse potential is between the resting threshold and the action potential threshold. Another way to look at inhibitory postsynaptic potentials is that they are also a chloride conductance change in the neuronal cell because it decreases the driving force.[2]Microelectrodes can be used to measure postsynaptic potentials at either excitatory or inhibitory synapses.

In general, a postsynaptic potential is dependent on the type and combination of receptor channel, reverse potential of the postsynaptic potential, action potential threshold voltage, ionic permeability of the ion channel, as well as the concentrations of the ions in and out of the cell; this determines if it is excitatory or inhibitory. IPSPs always want to keep the membrane potential more negative than the action potential threshold and can be seen as a "transient hyperpolarization".[3] EPSPs and IPSPs compete with each other at numerous synapses of a neuron; this determines whether or not the action potential at the presynaptic terminal will regenerate at the postsynaptic membrane. Some common neurotransmitters involved in IPSPs are GABA and glycine.
Could you identify the source of a section if you had nothing to compare it to? In general, you should be able to differentiate cervical from thoracic from lumbar from sacral. Here is a series of cross sections:


The first thing to notice is overall shape. Cervical sections tend to be wide and squashed looking, like an oval. Compare the cervical section to the round lumbar section.
The second thing to check for is a ventral horn enlargement. At segments that control a limb, the motor neurons are large and numerous. This causes enlarged ventral horns in two places: the lower cervical sections (C5-C8) and the lumbar/sacral sections. If you see an enlargement, you just need to differentiate cervical from lumbar. This can be done by shape (see above) or by proportion of white matter.

The amount of white matter relative to grey matter decreases as you move down the cord. This is logical - in the white matter of the cervical cord you have all of the axons going to or from the entire body, more or less. In sacral cord the white matter contains only

those axons going to or from the last couple of dermatomes - all other axons have "gotten off" at higher levels. This is why sacral cord looks like it has so much grey matter - really it has just lost all of the white.
So, in summary, here are the level cues so far: wide flat cord, lots of white matter, ventral horn enlargements = cervical. Round cord, ventral horn enlargements = lumbar. Small round cord, almost no white matter = sacral. And the remaining level, thoracic, is the easiest of all. Notice the pointed tips which stick out between the small dorsal and ventral horns. This extra cell column is called the intermediate horn, or the intermediolateral cell column. It is the source of all of the sympathetics in the body, and occurs only in thoracic sections.
A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into the corresponding cervical, thoracic, lumbar, sacral and coccygeal regions of the spine.[1] There are eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves. The spinal nerves are part of the peripheral nervous system.



Typical spinal nerve location
Each spinal nerve is formed from the combination of nerve fibers from its posterior and anterior roots. The posterior root is the afferent sensory root and carries sensory information to the brain. The anterior root is the efferent motor root and carries motor information from the brain. The spinal nerve emerges from the spinal column through an opening (intervertebral foramen) between adjacent vertebrae. This is true for all spinal nerves except for the first spinal nerve pair (C1), which emerges between the occipital bone and the atlas (the first vertebra). Thus the cervical nerves are numbered by the vertebra below, except spinal nerve C8, which exists below vertebra C7 and above vertebra T1. The thoracic, lumbar, and sacral nerves are then numbered by the vertebra above. In the case of a lumbarized S1 vertebra (aka L6) or a sacralized L5 vertebra, the nerves are typically still counted to L5 and the next nerve is S1.
nerve - a group of fibers (axons) outside the CNS. The spinal nerves contain the fibers of the sensory and motor neurons. A nerve does not contain cell bodies. They are located in the ganglion (sensory) or in the gray matter (motor).

tract - a group of fibers inside the CNS. The spinal tracts carry information up or down the spinal cord, to or from the brain. Tracts within the brain carry information from one place to another within the brain. Tracts are always part of white matter.

The spinal cord does not run through the lumbar spine (lower back). After the spinal cord stops in the lower thoracic spine, the nerve roots from the lumbar and sacral levels come off the bottom of the cord like a "horse's tail" (cauda equina) and exit the spine (view the spinal nerve roots with Figure 1).

Therefore, because the lumbar spine has no spinal cord and comprises a large amount of space for the nerve roots, even serious conditions (such as a large disc herniation) are unlikely to cause paraplegia (loss of motor function in the legs).



The white matter of the spinal cord contains tracts which travel up and down the cord. Many of these tracts travel to and from the brain to provide sensory input to the brain, or bring motor stimuli from the brain to control effectors. Ascending tracts, those which travel toward the brain are sensory, descending tracts are motor. Figure 12.30 shows the location of the major tracts in the spinal cord. For most the name will indicate if it is a motor or sensory tract. Most sensory tracts names begin with spino, indicating origin in the spinal cord, and their name will end with the part of the brain where the tract leads. For example the spinothalamic tract travels from the spinal cord to the thalamus. Tracts whose names begin with a part of the brain are motor. For example the corticospinal tract begins with fibers leaving the cerebral cortex and travels down toward motor neurons in the cord.
How does information travel between body and brain? In this lesson, we'll explore somatic sensory pathways, including ascending and descending tracts, afferent and efferent nerves, and how they work together in the body.
Somatic Senses
Patrick has a problem. He had an accident, and he can't feel anything from his right leg or foot. If you tickle his left foot, he laughs, but his right foot doesn't get any reaction from him. Likewise, if his left leg brushes up against something hot, he pulls it away and says, 'Ouch!' But, if his right leg brushes up against something hot, he doesn't feel it and so doesn't pull it away.

Somatic senses are the senses that have to do with touch. Tickling and pain, like on Patrick's legs, are somatic senses, but so are other things that you might not think of right away, like temperature and movement. Somatosensory pathways relay information between the brain and nerve cells in the skin and organs. For example, these pathways are how Patrick knows that someone is tickling his left foot. But, why doesn't he feel tickling on his right foot?

To find out, let's look closer at somatosensory pathways, including the difference between ascending and descending pathways.

Ascending Pathways
Patrick can't really feel anything in his right leg or foot. As we've seen, this can be a problem, like when his right leg brushes up against something hot, and Patrick doesn't feel it, so he doesn't pull his right leg away, and risks injury.

The problem that Patrick is experiencing is with his ascending somatosensory pathway, which is sometimes called the afferent pathway. This is a series of nerves that send information to the brain from the body. Think about the word ascending, which means going up, and you can remember that the ascending pathway sends information up to the brain.

The nerves the connect the body to the spinal cord and the spinal cord to the brain are called afferent nerves, and they send information from the body to the brain. You can remember afferent pathway and afferent nerves by thinking about the letter a: ascending and afferent both start with a, and they are the same somatosensory pathway.

Let's look at an example of the ascending pathway. In most people, an afferent pathway might send sensory information from the right leg to the brain so that they understand what their right leg is experiencing. Of course, for Patrick, that particular ascending pathway isn't working correctly, even though the afferent pathway for his left leg is working fine.

How does the ascending pathway normally work? Information goes from the body part to the spinal cord. From there, it goes up the spinal cord to the brain. As it enters the brain, it shifts to the opposite side, and then goes all the way up to the top of the brain where it settles in the somatosensory cortex, or the part of the brain dedicated to somatic sensory information.

Let's look at that in Patrick's body. His left leg (the one that works normally) might send information to his spinal cord about how scratchy his wool pants are. This information then travels up Patrick's spinal cord. Just as it enters his brain, it crosses over to the right side of his brain and then goes up into the somatosensory cortex, where his brain registers that the sensation he's feeling is scratchiness because of the wool pants.

Descending Pathways
As we've seen, ascending pathways send information from the body up to the brain, but it wouldn't be a very good system if that's the only direction information could flow. Descending somatosensory pathways, also sometimes called efferent pathways, send information from the brain down to the body. Think of the word descending, and you can remember that the descending pathway is sending information down to the body from the brain. Like afferent nerves in afferent pathways, there are efferent nerves in efferent pathways, which are nerves that send information from the brain to the body.

Descending somatosensory pathways work in kind of the opposite way that ascending pathways do. Information goes from the motor cortex in the brain (which is right next to the somatosensory cortex) and shifts to the opposite side in the spinal cord. It travels down the spinal cord and out to the muscles and organs of the body.

Let's look at an example. Remember that Patrick's ascending pathway has made him aware of the fact that his wool pants are making his left leg itch. Now, the right side of Patrick's brain sends a message from the motor cortex down to the spinal cord. It moves from the right side to the left side in the spinal cord, and then travels down into his left arm. The arm then moves down to scratch his itchy left leg.

Notice that the ascending pathways--from the body to the brain--communicate somatic sensory information, whereas the descending pathways--from the brain to the body--communicate motor movement information. In other words, The afferent pathways are about taking sensory information in, and the efferent pathways are about sending motor movement information out.

Lesson Summary
The somatic senses are senses that have to do with the experience of touch. Somatosensory pathways relay information between the brain and nerves in the skin and organs. Ascending pathways, also called afferent pathways, send somatosensory information from the body up to the brain through a series of afferent nerves. Meanwhile, descending pathways, also called efferent pathways, send motor movement information from the brain down to the body through a series of efferent nerves.
The cerebellum, which stands for "little brain", is a structure of the central nervous system. It has an important role in motor control, with cerebellar dysfunction often presenting with motor signs. In particular, it is active in the coordination, precision and timing of movements, as well as in motor learning.

During embryonic development, the anterior portion of the neural tube forms three parts that give rise to the brain and associated structures:

Forebrain (prosencephalon)
Midbrain (mesencephalon)
Hindbrain (rhombencephalon)
The hindbrain subsequently divides into the metencephalon (superior) and the myelencephalon (inferior). The cerebellum develops from the metencephalon division.

This article will focus on the anatomy of the cerebellum. It will provide a brief overview of its functions and development, and finally it will highlight the clinical relevance of cerebellar disorders.

Anatomical Location
The cerebellum is located at the back of the brain, immediately inferior to the occipital and temporal lobes, and within the posterior cranial fossa. It is separated from these lobes by the tentorium cerebelli, a tough layer of dura mater.

It lies at the same level of and posterior to the pons, from which it is separated by the fourth ventricle.

© 2015-2016 TeachMeAnatomy.com [CC-BY-NC-ND 4.0]Fig 1.0 - Anatomical position of the cerebellum. It is inferior to the cerebrum, and posterior to the pons. Fig 1.0 - Anatomical position of the cerebellum. It is inferior to the cerebrum, and posterior to the pons.
Anatomical Structure and Divisions
The cerebellum consists of two hemispheres which are connected by the vermis, a narrow midline area. Like other structures in the central nervous system, the cerebellum consists of grey matter and white matter:

Grey matter - located on the surface of the cerebellum. It is tightly folded, forming the cerebellar cortex.
White matter - located underneath the cerebellar cortex. Embedded in the white matter are the four cerebellar nuclei (the dentate, emboliform, globose, and fastigi nuclei).
There are three ways that the cerebellum can be subdivided - anatomical lobes, zones and functional divisions

Anatomical Lobes

There are three anatomical lobes that can be distinguished in the cerebellum; the anterior lobe, the posterior lobe and the flocculonodular lobe. These lobes are divided by two fissures - the primary fissure and posterolateral fissure.

© 2015-2016 TeachMeAnatomy.com [CC-BY-NC-ND 4.0]Fig 1.1 - Anatomical lobes of the cerebellum. Fig 1.1 - Anatomical lobes of the cerebellum.
Zones

There are three cerebellar zones. In the midline of the cerebellum is the vermis. Either side of the vermis is the intermediate zone. Lateral to the intermediate zone are the lateral hemispheres. There is no difference in gross structure between the lateral hemispheres and intermediate zones

By Nrets [CC-BY-SA-3.0], from Wikimedia CommonsFig 1.2 - Superior view of an "unrolled" cerebellum, placing the vermis in one plane. Fig 1.2 - Superior view of an "unrolled" cerebellum, placing the vermis in one plane.
Functional Divisions

The cerebellum can also be divided by function. There are three functional areas of the cerebellum - the cerebrocerebellum, the spinocerebellum and the vestibulocerebellum.

Cerebrocerebellum - the largest division, formed by the lateral hemispheres. It is involved in planning movements and motor learning. It receives inputs from the cerebral cortex and pontine nuclei, and sends outputs to the thalamus and red nucleus. This area also regulates coordination of muscle activation and is important in visually guided movements.
Spinocerebellum - comprised of the vermis and intermediate zone of the cerebellar hemispheres. It is involved in regulating body movements by allowing for error correction. It also receives proprioceptive information.
Vestibulocerebellum - the functional equivalent to the flocculonodular lobe. It is involved in controlling balance and ocular reflexes, mainly fixation on an target. It receives inputs from the vestibular system, and sends outputs back to the vestibular nuclei.
The diencephalon is a part of the brain that is responsible for many functions in the human body. In this lesson, you will learn about the diencephalon, including its location, parts, and functions.
Importance of the Diencephalon
What do blood pressure, water balance, childbirth, appetite, and sleep all have in common? Besides the fact that all are bodily functions, each is controlled in part by the diencephalon. The diencephalon helps control many different functions of the body, which is why it is important to understand this organ.

What, Where and How Big is the Diencephalon?
The diencephalon is a part of the brain that includes the thalamus and the hypothalamus. It is the link between the nervous system and the endocrine system. The diencephalon receives signals from the nerves (the nervous system) and interprets the signals, then the pituitary gland (which largely controls the endocrine system) responds by excreting hormones.

The thalamus is the size of a walnut, whereas the hypothalamus is the size of an almond; in total, the size of the diencephalon is about the size of an apricot. The diencephalon is located deep in the brain underneath the cerebrum and above the pituitary gland.

Now that we know the general function, size, and location of the diencephalon, let's discuss the specific functions of the thalamus and hypothalamus.

Function of the Thalamus and Hypothalamus
The thalamus sends and receives signals to and from the brain and body. The brain sends a signal to the thalamus, which relays the signal to the body. Similarly, the body sends a signal to the thalamus, and the thalamus relays the signal to the brain. You can think of the thalamus as a mediator; it receives messages then sends the messages to the intended destination.

The hypothalamus is responsible for triggering the pituitary gland to release hormones. In conjunction with the pituitary gland, it regulates bodily functions and has many effects. Let's examine five important functions affected by the hypothalamus.

1. Sleep inhibition: When you feel awake, it is in part due to the hypothalamus. The hypothalamus sends signals to other parts of the brain to keep you alert.

2. Appetite: The hormones ghrelin and leptin are produced in the gastrointestinal tract and indicate hunger and fullness. Ghrelin is excreted , and receptors in the hypothalamus receive the ghrelin, thereby causing the hunger feeling. After we eat, the amount of ghrelin decreases, while leptin increases. Receptors in the hypothalamus receive the leptin, indicating fullness.

3. Oxytocin: The hypothalamus signals the pituitary gland to release oxytocin, resulting in contractions during childbirth.

4. Water balance: The hypothalamus helps to maintain water balance when there is a lack of water in the body. The hypothalamus causes the pituitary gland to secrete an anti-diuretic hormone, prompting more water to be absorbed by the kidneys. Also, the hypothalamus signals the thirst sensation so that we know when we need to drink water.

5. Blood pressure: The hypothalamus helps control blood pressure by controlling the heart beat and the dilation of the blood vessels.

Lesson Summary
The diencephalon is located deep in the brain underneath the cerebrum, and it is the link between the nervous system and the endocrine system. It includes the thalamus and hypothalamus. The thalamus relays signals to and from the brain and body. The hypothalamus triggers the pituitary gland to secrete hormones, and it also helps to control sleep, appetite, oxytocin, water balance and blood pressure.
This lesson will describe the hypothalamus as it relates to the endocrine system. It will examine the anatomic features of the hypothalamus and how it uses the pituitary gland to communicate with the rest of the body. A short quiz will follow.
Introduction
Every organization needs some form of structure in order for messages to be delivered and received. Without methods of communication, order can break down into chaos in a short matter of time. Just try to imagine how much work the employees of an office would be able to get done if they lost access to their phones and email!

In the case of the endocrine system, the hypothalamus plays a super-sized role by making decisions about what actions need to be taken by various endocrine glands throughout the body. Its primary purpose is to make sure that the body stays in a continual state of balance, known as homeostasis.

Features of the Hypothalamus
The hypothalamus is only about the size of a pearl, and is located in the middle part of the brain. It monitors the state of the body through the circulatory and nervous systems, and effectively links these two systems to the endocrine system through the pituitary gland.

Location of the hypothalamus inside the brain
Hypothalamus inside the brain
The hypothalamus communicates with the anterior portion of the pituitary gland by way of hormonal messages. Neurosecretory cells in the hypothalamus create hypothalamic-releasing and hypothalamic-inhibiting hormones, which tell the anterior pituitary to start or stop an action. Located in the pituitary stalk, a unique arrangement of capillaries and veins, called a portal system, allows the hypothalamic hormones to pass directly to the anterior pituitary without circulating through the body.

The hypothalamus uses the posterior portion of the pituitary gland like a warehouse and distribution center. Neurosecretory cells in the hypothalamus create anti-diuretic hormone (ADH) and oxytocin, which are sent through axons in the pituitary stalk to be stored in the posterior pituitary. When the hypothalamus detects that either of these hormones are needed, they are released from the posterior pituitary into the circulatory system to do their jobs.

Role of the Hypothalamus
A good way to visualize the relationship between the hypothalamus and the pituitary gland is like the President and his Chief of Staff. While the hypothalamus, or President, makes the decisions, the pituitary gland, or Chief of Staff, executes those decisions by sending out commands to the rest of the body.

When the hypothalamus detects that something is out of balance, its sends a message to the pituitary gland that a corrective action is needed. When the pituitary gland gets this message from the hypothalamus, it releases specific hormones into the bloodstream that can stimulate other endocrine glands, organs or tissues depending on what action is needed. It's kind of a like a game of telephone. Instead of the hypothalamus communicating directly with the body, it relies on the pituitary gland to send out the messages. The hypothalamus continues to monitor the state of the body, and when it detects that balance has been restored, it tells the pituitary gland to stop sending out stimulating hormones, thereby stopping the corrective action.

An example of this process is when we become dehydrated. The hypothalamus is able to detect the increased blood concentration caused by the loss of water. To correct the situation, it uses the posterior pituitary to release anti-diuretic hormone (ADH) into the circulatory system. When ADH reaches the kidneys, it causes more water to be reabsorbed into the bloodstream, diluting the blood. When the hypothalamus detects the return to a normal blood concentration, it stops the release of ADH from the pituitary gland and the kidneys return to normal functioning.

Summary
The hypothalamus is responsible for maintaining homeostasis. It monitors the state of the body through the circulatory and nervous systems, and effectively links these two systems to the endocrine system through the pituitary gland. When the hypothalamus detects that something is out of balance, its sends a message to the pituitary gland that a corrective action is needed. Depending on the need, either hormones produced in the anterior pituitary, or hypothalamic hormones stored in the posterior pituitary, are released into the body. When balance has been restored, the hypothalamus tells the pituitary gland to stop sending out stimulating hormones, thereby stopping the corrective action.
In this lesson, we'll discuss the functions of the anterior and posterior portions of the pituitary gland, the hormones they release and the relationship with the hypothalamus.
Pituitary Gland
Located beneath the brain, the pituitary gland is a pea-sized endocrine gland that sits in a bony pocket in the base of the skull called the pituitary fossa. The pituitary fossa is also known as the 'sella turcica,' which translates to 'Turkish saddle' because it resembles a saddle with supports in the front and back used by the Turkish people. Despite its small size, the pituitary gland plays such an important role in controlling the body that it is often called the 'master gland.'

There are actually two main parts of the pituitary gland. The front portion, commonly referred to as the anterior pituitary, is also known as the adenohypophysis. The back portion, or posterior pituitary, is called the neurohypophysis. We can keep these two names straight by noting that the words 'anterior' and 'adenohypophysis' both start with the letter 'A.'

The pituitary gland is attached to the hypothalamus by the pituitary stalk, which contains nerves and a unique circulatory system, which enables communication between the two. Let's take a closer look at the way the hypothalamus and the pituitary gland work together.

Hypothalamus and Pituitary Gland
A good way to visualize the relationship between the hypothalamus and the pituitary gland is like the president and his chief of staff. While the hypothalamus, or president, makes the decisions, the pituitary gland, or chief of staff, executes those decisions by sending out commands to the rest of the body.

The hypothalamus monitors the body through the circulatory and nervous systems. When it detects that something is out of balance, it sends a message to the pituitary gland that a corrective action is needed. When the pituitary gland gets this message from the hypothalamus, it releases specific hormones into the bloodstream that can stimulate other endocrine glands, organs or tissues depending on what action is needed.

It's kind of a like a game of telephone. Instead of the hypothalamus communicating directly with the body, it relies on the pituitary gland to send out the messages. The hypothalamus continues to monitor the state of the body, and when it detects that balance has been restored, it tells the pituitary gland to stop sending out stimulating messages, thereby stopping the corrective action.

An example of this process is when we become dehydrated. The hypothalamus is able to detect the increased blood concentration caused by the loss of water. To correct the situation, it uses the posterior pituitary to release anti-diuretic hormone (ADH) into the circulatory system. When ADH reaches the kidneys, it causes more water to be reabsorbed into the bloodstream, diluting the blood. When the hypothalamus detects the return to a normal blood concentration, it stops the release of ADH from the pituitary gland, and the kidneys return to normal functioning.

Anterior Pituitary Gland
The hypothalamus communicates with the anterior portion of the pituitary gland by way of hormonal messages. These messages come in the form of hypothalamic-releasing and hypothalamic-inhibiting hormones, which tell the anterior pituitary to start or stop an action. Located in the pituitary stalk, a unique arrangement of capillaries and veins, called a portal system, allows the hypothalamic hormones to pass directly to the anterior pituitary without circulating through the body.

The anterior pituitary contains glands that produce and store a number of different hormones that control many different functions throughout the body. When a hormone message comes down from the hypothalamus, the anterior pituitary releases its own hormones into the main circulatory system to control the needed action.

These pituitary hormones can stimulate other endocrine glands, such as the thyroid, the adrenal cortex and the gonads. The anterior pituitary also sends growth hormone to the bones and muscles and prolactin to the mammary glands to stimulate milk production during pregnancy.

Posterior Pituitary Gland
The hypothalamus uses the posterior pituitary like a warehouse and distribution center. Anti-diuretic hormone (ADH) and oxytocin are both produced in the hypothalamus and sent through axons to be stored in the posterior pituitary. When the hypothalamus detects that either of these hormones are needed, they are released from the posterior pituitary into the circulatory system to do their jobs. As mentioned earlier, ADH works on the kidneys to increase the reabsorption of water into the bloodstream, but it also causes the constriction of blood vessels to increase blood pressure. Oxytocin is responsible for stimulating uterine contractions during childbirth and the release of milk during nursing.

The posterior pituitary differs from the anterior in two distinct ways. First, it interacts with the hypothalamus through direct axon connections, not by hormonal messages. Secondly, it does not produce any of its own hormones but rather stores and releases hormones produced in the hypothalamus. That means that this part of the pituitary gland contains no glands at all.

Lesson Summary
The pituitary gland is a pea-sized endocrine gland located beneath the hypothalamus in a bony pocket called the pituitary fossa, or sella turcica. The anterior pituitary is called the adenohypophosis, and the posterior pituitary is called the neurohypophosis.

The hypothalamus communicates with the anterior pituitary by sending hypothalamic-releasing and hypothalamic-inhibiting hormones through a portal system located in the pituitary stalk. These hypothalamic hormones tell the anterior pituitary to start or stop the release of its own hormones into the bloodstream. Some hormones of the anterior pituitary control other endocrine glands, such as the thyroid, the adrenal cortex and the gonads. It also sends growth hormone to the bones and muscles and prolactin to the mammary glands.

The hypothalamus produces anti-diuretic hormone (ADH) and oxytocin and sends them through axons to be stored in the posterior pituitary, where they can be released into the circulatory system when needed. The posterior pituitary does not produce any of its own hormones and does not contain any glands.
The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the medial longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres has an outer layer of grey matter, the cerebral cortex, that is supported by an inner layer of white matter. In eutherian (placental) mammals, the hemispheres are linked by the corpus callosum, a very large bundle of nerve fibers. Smaller commissures, including the anterior commissure, the posterior commissure and the fornix also join the hemispheres and these are also present in other vertebrates. These commissures transfer information between the two hemispheres to coordinate localized functions.

The central sulcus is a prominent fissure which separates the parietal lobe from the frontal lobe and the primary motor cortex from the primary somatosensory cortex.

Macroscopically the hemispheres are roughly mirror images of each other, with only subtle differences, such as the Yakovlevian torque seen in the human brain, which is a slight warping of the right side, bringing it just forward of the left side. On a microscopic level, the cytoarchitecture of the cerebral cortex, shows the functions of cells, quantities of neurotransmitter levels and receptor subtypes to be markedly asymmetrical between the hemispheres.[1][2] However, while some of these hemispheric distribution differences are consistent across human beings, or even across some species, many observable distribution differences vary from individual to individual within a given species.
The cerebral cortex is the cerebrum's (brain) outer layer of neural tissue in humans and other mammals. It is divided into two cortices, along the sagittal plane: the left and right cerebral hemispheres divided by the medial longitudinal fissure. The cerebral cortex plays a key role in memory, attention, perception, awareness, thought, language, and consciousness. The human cerebral cortex is 2 to 4 millimetres (0.079 to 0.157 in) thick.[1]

In large mammals, the cerebral cortex is folded, giving a much greater surface area in the confined volume of the skull. A fold or ridge in the cortex is termed a gyrus (plural gyri) and a groove or fissure is termed a sulcus (plural sulci). In the human brain more than two-thirds of the cerebral cortex is buried in the sulci.

The cerebral cortex is composed of gray matter, consisting mainly of cell bodies (with astrocytes being the most abundant cell type in the cortex as well as the human brain in general) and capillaries. It contrasts with the underlying white matter, consisting mainly of the white myelinated sheaths of neuronal axons. The most recent part of the cerebral cortex to develop in the evolutionary history of mammals is the neocortex (also called isocortex), which differentiated into six horizontal layers; the more ancient part of the cerebral cortex, the hippocampus, has at most three cellular layers. Neurons in various layers connect vertically to form small microcircuits, called cortical columns. Different neocortical regions known as Brodmann areas are distinguished by variations in their cytoarchitectonics (histological structure) and functional roles in sensation, cognition and behavior.
The cerebral cortex is classified into four lobes, according to the name of the corresponding cranial bone that approximately overlies each part. Each lobe contains various cortical association areas - where information from different modalities are collated for processing. Together, these areas function to give us a meaningful perceptual interpretation and experience of our surrounding environment.

Frontal Lobe

The frontal lobe is located beneath the frontal bone of the calvaria and is the most anterior region of the cerebrum. It is separated from the parietal lobe posteriorly by the central sulcus and from the temporal lobe inferoposteriorly by the lateral sulcus.

The association areas of the frontal lobe are responsible for: higher intellect, personality, mood, social conduct and language (dominant hemisphere side only).

Parietal Lobe

The parietal lobe is found below the parietal bone of the calvaria, between the frontal lobe anteriorly and the occipital lobe posteriorly, from which it is separated by the central sulcus and parieto-occipital sulcus, respectively. It sits superiorly in relation to the temporal lobe, being separated by the lateral sulcus.

Its cortical association areas contribute to the control of: language and calculation on the dominant hemisphere side, and visuospatial functions (e.g. 2-point discrimination) on the non-dominant hemisphere side.

Temporal Lobe

The temporal lobe sits beneath the temporal bone of the calvaria, inferior to the frontal and parietal lobes, from which it is separated by the lateral sulcus.

The cortical association areas of the temporal lobe are accountable for memory and language - this includes hearing as it is the location of the primary auditory cortex.

Occipital Lobe

The occipital lobe is the most posterior part of the cerebrum situated below the occipital bone of the calvaria. It rests inferiorly upon the tentorium cerebelli which segregates the cerebrum from the cerebellum. The parieto-occipital sulcus separates the occipital lobe from the parietal and temporal lobes anteriorly.

The primary visual cortex (V1) is located within the occipital lobe and hence its cortical association area is responsible for vision.
The cerebral cortex is connected to various subcortical structures such as the thalamus and the basal ganglia, sending information to them along efferent connections and receiving information from them via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus. Olfactory information, however, passes through the olfactory bulb to the olfactory cortex (piriform cortex). The vast majority of connections are from one area of the cortex to another rather than to subcortical areas; Braitenberg and Schüz (1991) put the figure as high as 99%.[15]

The cortex is commonly described as comprising three parts: sensory, motor, and association areas.

[edit] Sensory areas
The sensory areas are the areas that receive and process information from the senses. Parts of the cortex that receive sensory inputs from the thalamus are called primary sensory areas. The senses of vision, audition, and touch are served by the primary visual cortex, primary auditory cortex and primary somatosensory cortex. In general, the two hemispheres receive information from the opposite (contralateral) side of the body. For example the right primary somatosensory cortex receives information from the left limbs, and the right visual cortex receives information from the left visual field. The organization of sensory maps in the cortex reflects that of the corresponding sensing organ, in what is known as a topographic map. Neighboring points in the primary visual cortex, for example, correspond to neighboring points in the retina. This topographic map is called a retinotopic map. In the same way, there exists a tonotopic map in the primary auditory cortex and a somatotopic map in the primary sensory cortex. This last topographic map of the body onto the posterior central gyrus has been illustrated as a deformed human representation, the somatosensory homunculus, where the size of different body parts reflects the relative density of their innervation. Areas with lots of sensory innervation, such as the fingertips and the lips, require more cortical area to process finer sensation.

[edit] Motor areas
The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine fragmented movements performed by the hand. The right half of the motor area controls the left side of the body, and vice versa.

Two areas of the cortex are commonly referred to as motor:

Primary motor cortex, which executes voluntary movements
Supplementary motor areas and premotor cortex, which select voluntary movements.
In addition, motor functions have been described for:

Posterior parietal cortex, which guides voluntary movements in space
Dorsolateral prefrontal cortex, which decides which voluntary movements to make according to higher-order instructions, rules, and self-generated thoughts.
[edit] Association areas
Association areas function to produce a meaningful perceptual experience of the world, enable us to interact effectively, and support abstract thinking and language. The parietal, temporal, and occipital lobes - all located in the posterior part of the cortex - organize sensory information into a coherent perceptual model of our environment centered on our body image. The frontal lobe or prefrontal association complex is involved in planning actions and movement, as well as abstract thought. In the past it was theorized that language abilities are localized in the left hemisphere in areas 44/45, the Broca's area, for language expression and area 22, the Wernicke's area, for language reception. However, language is no longer limited to easily identifiable areas. More recent research suggests that the processes of language expression and reception occur in areas other than just the perisylvian structures, such as the prefrontal lobe, basal ganglia, cerebellum, pons, caudate nucleus, and others.
The basal ganglia (or basal nuclei) comprise multiple subcortical nuclei, of varied origin, in the brains of vertebrates, which are situated at the base of the forebrain. Basal ganglia nuclei are strongly interconnected with the cerebral cortex, thalamus, and brainstem, as well as several other brain areas. The basal ganglia are associated with a variety of functions including: control of voluntary motor movements, procedural learning, routine behaviors or "habits" such as bruxism, eye movements, cognition[1] and emotion.[2]

The main components of the basal ganglia - as defined functionally - are the dorsal striatum (caudate nucleus and putamen), ventral striatum (nucleus accumbens and olfactory tubercle), globus pallidus, ventral pallidum, substantia nigra, and subthalamic nucleus.[3] It is important to note, however, that the dorsal striatum and globus pallidus may be considered anatomically distinct from the substantia nigra, nucleus accumbens, and subthalamic nucleus. Each of these components has a complex internal anatomical and neurochemical organization. The largest component, the striatum (dorsal and ventral), receives input from many brain areas beyond the basal ganglia, but only sends output to other components of the basal ganglia. The pallidum receives input from the striatum, and sends inhibitory output to a number of motor-related areas. The substantia nigra is the source of the striatal input of the neurotransmitter dopamine, which plays an important role in basal ganglia function. The subthalamic nucleus receives input mainly from the striatum and cerebral cortex, and projects to the globus pallidus.

Currently, popular theories implicate the basal ganglia primarily in action selection; that is, it helps determine the decision of which of several possible behaviors to execute at any given time. In more specific terms, the basal ganglia's primary function is likely to control and regulate activities of the motor and premotor cortical areas so that voluntary movements can be performed smoothly.[1][4] Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The "behavior switching" that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.[2][5]
the elongated ridges on the floor of each lateral ventricle of the brain, thought to be the center of emotion, memory, and the autonomic nervous system.

The hippocampus plays a very important role in storing our memories and connecting them to our emotions, among other roles. This lesson explains all the roles of the hippocampus and its structure in the brain.
The Hippocampus and Limbic System
The hippocampus is a part of the limbic system. The limbic system is the area in the brain that is associated with memory, emotions, and motivation. The limbic system is located just above the brain stem and below the cortex. The hippocampus itself is highly involved with our memories.

The limbic system plays a huge part in our survival roles. It is responsible for our fight or flight responses. This is when a person feels like he is in danger and either needs to fight his way out or run away from the situation. The limbic system also gives us that 'gut feeling.' The limbic system, including the hippocampus, is located in a very protected area of the brain.

The hippocampus is a horseshoe-shaped structure. There are actually two pieces that are mirrored in pairs. One of the pairs is located in the right hemisphere. Its mirrored other half of the horseshoe is located in the left hemisphere.

The Role of the Hippocampus in Memory
The hippocampus plays a very important role in our memories. It attaches memories to the emotions and senses that go with them. For instance, it will take a memory of being happy and calm while in a field and link it with the smell of the flowers. It will associate both the feeling of being happy and the sense of smell to the memory of being in a field.

Once the emotions and senses are attached, the hippocampus sends the memory off to be stored. It will actually file the memory in the appropriate part of the cerebral cortex. Here the memory will be put in long term storage, where it can later be retrieved.

Brain Damage
Most research of the brain has to be done by studying the effects of patients with brain damage. In patients with Alzheimer's disease the hippocampus is one of the first areas that experiences damage. These patients usually have symptoms of memory loss and disorientation. This suggests that the hippocampus is not only responsible for memories, but also plays a role in spatial orientation.

Even though the hippocampus and limbic system are placed in the middle of the skull in a protective area, there are still other ways besides Alzheimer's disease for the hippocampus to become damaged. It can become damaged if it is deprived of oxygen or by encephalitis or medial temporal lobe epilepsy. Extensive damage to the hippocampus can result in amnesia, which is the loss of memory.

Lesson Summary
The hippocampus is part of our limbic system, a vital section of the brain that plays a key role in our survival. The hippocampus itself is responsible for many things, including connecting senses and emotions to our memories. It connects them all together and then sends the memory off to the proper area of the cerebral cortex. The memory is stored and will be able to be retrieved when called upon. Although the hippocampus is in the middle of the brain it can still be damaged, which causes the loss of memories and disorientation.
The amygdala is a section of the brain that is responsible for detecting fear and preparing for emergency events. This lesson discusses the amygdala, its functions, and its role in our perception of fear and other emotions.
The Role of Fear
Do you have any fears? For some people, their biggest fear may be death. For others, it may be public speaking. In fact, most humans will have at least one or more things that they fear in life, no matter how dangerous or innocent the object of that fear may be.

But there is a reason for that. Fear often helps us with self-preservation. We feel fear, as well as related emotions, in order to protect ourselves from danger and to heighten our awareness. This awareness is thought to be controlled by a section of the brain known as the amygdala. Let's discuss the amygdala and how it functions in the well-being of the human body.

Definition and Function of the Amygdala
The amygdala is an almond-shaped section of nervous tissue located in the temporal (side) lobe of the brain. There are two amygdalae per person normally, with one amygdala on each side of the brain. They are thought to be a part of the limbic system within the brain, which is responsible for emotions, survival instincts, and memory. However, this inclusion has been debated heavily, with evidence that the amygdalae function independently of the limbic system.


The amygdala is responsible for the perception of emotions such as anger, fear, and sadness, as well as the controlling of aggression. The amygdala helps to store memories of events and emotions so that an individual may be able to recognize similar events in the future. For example, if you have ever suffered a dog bite, then the amygdalae may help in processing that event and, therefore, increase your fear or alertness around dogs. The size of the amygdala is positively correlated with increased aggression and physical behavior.

The amygdala in humans also plays a role in sexual activity and libido, or sex drive. It can change in size and shape based on the age, hormonal activity, and gender of the individual. For example, males who have low testosterone, or who may have been castrated, (had their testicles removed), tend to have smaller amygdalae, and, in turn, may also have a lower sex drive.

Fear and the Amygdala
It is important to state that the amygdalae are most functional in immediate fear situations. Whenever our senses detect a change in our surroundings that could be dangerous, the amygdalae are responsible for preparing the body for escape or defense. This is part of what is known as the startle circuit of the brain, which controls our response to being startled.

The amygdalae, however, can cause problems if they are over-active. Panic is often a result of increased activity of the amygdalae. Usually, the initial response of the amygdalae is brief, particularly if someone is startled, but the situation is not a real threat. Imagine your friend sneaking up behind you and yelling 'BOO'! You will be startled, but the response will be brief once you realize it is just a prank. But in the case of panic, the physiological changes that prepare for emergency situations do not turn off as quickly, which can lead to prolonged fear, regardless of an actual threat.

Effects of Damaged Amygdalae
Scientists have also noted that damage to the amygdalae may result in various psychological and behavioral changes. Lesions in the amygdalae have been linked to the loss of emotion, loss of fear, hypersexuality, and depression. Compulsive behaviors, such as binge drinking and alcoholism, may occur. In animals, such as monkeys, damage to the amygdalae may result in a loss of maternal and parenting instincts after birth.

Lesson Summary
The amygdalae are found in the temporal lobes of the brain, and are responsible for the perception of emotions, with fear being the most noticeable. They help to store memories of events for future recognition and protection. The primary response of the amygdala is to prepare for immediate action, but this response is usually short-lived. Prolonged amygdala activity can lead to panic and increased fear. Damage to this region may lead to many negative psychological and social behaviors, such as loss of emotion, increased sexual activity, and compulsive habits.
The neurohypophysial hormones form a family of structurally and functionally related peptide hormones. Their main representatives are oxytocin and vasopressin. They are named for being secreted by the neurohypophysis, i.e. the posterior pituitary gland (hypophysis refers to the pituitary gland), itself a neuronal projection from the hypothalamus.

Most of the circulating oxytocin and vasopressin hormones are synthesized in magnocellular neurosecretory cells of the supraoptic nucleus and paraventricular nucleus of the hypothalamus. They are then transported in neurosecretory granules along axons within the hypothalamo-neurohypophysial tract by axoplasmic flow to axon terminals forming the pars nervosa of the posterior pituitary. There, they are stored in Herring bodies and can be released into the circulation on the basis of hormonal and synaptic signals with assistance from pituicytes.[1][2][3]

Vasopressin and oxytocin are also synthesized in the parvocellular neurosecretory cells of the paraventricular nucleus of the hypothalamus, which project to the median eminence, where they are transported and secreted into the hypophyseal portal system to stimulate the anterior pituitary.[4] Thus they can also be considered as hypophysiotropic hormones.[5]

Oxytocin mediates contraction of the smooth muscle of the uterus and mammary gland, while vasopressin has antidiuretic action on the kidney, and mediates vasoconstriction of the peripheral vessels.[6] Due to the similarity of the two hormones, there is cross-reaction: oxytocin has a slight antidiuretic function, and high levels of AVP can cause uterine contractions.[7][8] In common with most active peptides, both hormones are synthesised as larger protein precursors that are enzymatically converted to their mature forms.
he thyroid gland is a butterfly-shaped organ located in the base of your neck. It releases hormones that control metabolism—the way your body uses energy. The thyroid's hormones regulate vital body functions, including:

Breathing
Heart rate
Central and peripheral nervous systems
Body weight
Muscle strength
Menstrual cycles
Body temperature
Cholesterol levels
Much more!


The thyroid gland is about 2-inches long and lies in front of your throat below the prominence of thyroid cartilage sometimes called the Adam's apple. The thyroid has two sides called lobes that lie on either side of your windpipe, and is usually connected by a strip of thyroid tissue known as an isthmus. Some people do not have an isthmus, and instead have two separate thyroid lobes.

How the Thyroid Gland Works
The thyroid is part of the endocrine system, which is made up of glands that produce, store, and release hormones into the bloodstream so the hormones can reach the body's cells. The thyroid gland uses iodine from the foods you eat to make two main hormones:

Triiodothyronine (T3)
Thyroxine (T4)
It is important that T3 and T4 levels are neither too high nor too low. Two glands in the brain—the hypothalamus and the pituitary communicate to maintain T3 and T4 balance.

The hypothalamus produces TSH Releasing Hormone (TRH) that signals the pituitary to tell the thyroid gland to produce more or less of T3 and T4 by either increasing or decreasing the release of a hormone called thyroid stimulating hormone (TSH).

When T3 and T4 levels are low in the blood, the pituitary gland releases more TSH to tell the thyroid gland to produce more thyroid hormones.
If T3 and T4 levels are high, the pituitary gland releases less TSH to the thyroid gland to slow production of these hormones.


Why You Need a Thyroid Gland
T3 and T4 travel in your bloodstream to reach almost every cell in the body. The hormones regulate the speed with which the cells/metabolism work. For example, T3 and T4 regulate your heart rate and how fast your intestines process food. So if T3 and T4 levels are low, your heart rate may be slower than normal, and you may have constipation/weight gain. If T3 and T4 levels are high, you may have a rapid heart rate and diarrhea/weight loss.

Listed below are other symptoms of too much T3 and T4 in your body (hyperthyroidism):

Anxiety
Irritability or moodiness
Nervousness, hyperactivity
Sweating or sensitivity to high temperatures
Hand trembling (shaking)
Hair loss
Missed or light menstrual periods
The following is other symptoms of too little T3 and T4 in your body (hypothyroidism):

Trouble sleeping
Tiredness and fatigue
Difficulty concentrating
Dry skin and hair
Depression
Sensitivity to cold temperature
Frequent, heavy periods
Joint and muscle pain
The liver is a vital organ that is responsible for many of the processes that keep us alive. This lesson will discuss the key functions of the liver, its location in the body, and the diseases that can affect it.
The Liver: A Vital Organ
The human body is a fascinating structure, composed of many different parts working together for the purpose of keeping us alive. Within the human body are multiple organs, which are large structures designed to perform certain functions. Many of the organs in the body are familiar to most people. The heart, for example, is used to pump blood throughout the body. The lungs are used to breathe in oxygen and remove carbon dioxide.

However, one of the most important organs in the body is also one of the least understood. It is an organ that is vital for digestion. It is an organ that protects us from harmful substances. It's one of the organs that we cannot live without. That organ is the liver, and in this lesson, we will take a look at this valuable part of our bodies.

Characteristics of the Liver
The liver is located superolateral, or above and to the side, of the stomach. It is found in the abdominal cavity of the body, which is where many of the internal organs reside, and is inferior to, or below, the lungs.

The adult liver weighs between 3 and 4 pounds. It is the largest internal organ in the body, and is second to the skin as the largest organ overall. It has a rubbery texture and is reddish-brown in color. One of the most unique characteristics of the liver is that it has the ability to, in some cases, regenerate, or regrow, different sections of itself in the event of damage.

The liver has a triangular shape and is divided into four lobes. The left and right anatomical lobes are visible when viewing the liver from the front, while the quadrate and caudate lobes are visible when viewed from the underside. These lobes are divided further into smaller functional sections, called lobules. These lobules have various functions and contribute to processes we'll discuss next.

Functions of the Liver
The liver has several major functions in the body. First, the liver is responsible for producing enzymes and solutions necessary for digestion. This includes the production of bile, which helps with the breakdown of fat from our food. The liver is also responsible for the storage of sugars for energy use. Glucose, a simple sugar used by the body for energy, is stored as glycogen in the liver until needed. During emergency situations, our bodies will tap into the stored glucose to provide additional energy for survival.

Another major function of the liver is to detoxify and remove harmful substances in the bloodstream. Drinking alcohol, for example, is poisonous to the human body. However, you probably wouldn't be able to tell it based on how much we, as humans, consume it. The liver is responsible for processing alcohol so that it does not cause harm to the rest of our bodies. Additionally, the liver will also break down and process other drugs that enter our system, including medications and recreational drugs.

Other functions of the liver include:

Production of cholesterol, which is a lipid necessary for hormone production
Vitamin storage, such as vitamins A and K
Digestion and recycling of red blood cells and components when they become old
There are also several other functions of the liver. In fact, the liver has so many functions that we cannot live without it, and scientists have not been able to produce an artificial version of this organ.

Diseases of the Liver
Many diseases can affect the function of the liver. These diseases can range in severity, depending on how much of the liver is damaged and which functions are affected. These diseases include:

Hepatitis, which is inflammation of the liver due to infection or irritation
Cirrhosis, which is scarring of the liver and is usually a result of heavy alcohol consumption
Liver cancer, which is an overgrowth of cells within the liver
It's important to remember that the liver can regenerate in certain conditions. So, for some of these diseases, once the disease has been controlled, the liver may regain function.

Lesson Summary
The liver is one of the most important organs in the body. It's located in the abdominal cavity, below the lungs and to the side of the stomach. It has many functions, including detoxification of drugs, digestive enzyme production, and fat digestion. Finally, many diseases may affect the liver, including hepatitis, cirrhosis, and liver cancer. In certain cases, the liver can regenerate, or regrow, damaged tissues.
Five classes of steroid hormones are produced in the adrenal cortex: glucocorticoids, mineralocorticoids, progestins, androgens, and estrogens. However, the amount of progestin, androgen, and estrogen produced by the adrenal is a minor fraction of the total amount of these steroids produced in the body. By contrast, glucocorticoids and mineralocorticoids are produced almost exclusively in the adrenal cortex. Glucocorticoids have a broad physiologic role that includes both regulation of glucose metabolic pathways and modulation of the immune system. Mineralocorticoids are key regulators of mineral and water balance.


Glucocorticoid, any steroid hormone that is produced by the adrenal gland and known particularly for its anti-inflammatory and immunosuppressive actions.

The adrenal gland is an organ situated on top of the kidney. It consists of an outer cortex (adrenal cortex) and an inner medulla (adrenal medulla). The hormones secreted from the cortex are steroids, generally classified as glucocorticoids (e.g., cortisol) and mineralocorticoids (e.g., aldosterone, which causes sodium retention and potassium excretion by the kidney). Those substances emanating from the medulla are amines, such as epinephrine and norepinephrine.

Glucocorticoids together with mineralocorticoids are used in replacement therapy in acute or chronic adrenal insufficiency (Addison disease). Glucocorticoids, including a range of synthetic analogs (e.g., prednisolone, triamcinolone, and dexamethasone), are also used as anti-inflammatory and immunosuppressant agents. As anti-inflammatory agents, they are used in the treatment of bronchial asthma. Glucocorticoids indirectly inhibit the activity of phospholipase A2, an enzyme that plays an essential role in the synthesis of prostaglandins and leukotrienes; its inhibition by lipocortin-1 underlies part of the anti-inflammatory effects of glucocorticoids. Glucocorticoids also reduce the synthesis of some proteins that directly mediate the inflammatory response.



Corticotropin-releasing hormone (CRH) also known as corticotropin-releasing factor (CRF) or corticoliberin is a peptide hormone and neurotransmitter involved in the stress response. It is a releasing hormone that belongs to corticotropin-releasing factor family. In humans, it is encoded by the CRH gene.[1]

Its main function is the stimulation of the pituitary synthesis of ACTH, as part of the HPA Axis.

Corticotropin-releasing hormone (CRH) is a 41-amino acid peptide derived from a 196-amino acid preprohormone. CRH is secreted by the paraventricular nucleus (PVN) of the hypothalamus in response to stress. Increased CRH production has been observed to be associated with Alzheimer's disease and major depression,[2] and autosomal recessive hypothalamic corticotropin deficiency has multiple and potentially fatal metabolic consequences including hypoglycemia.[1] In addition to being produced in the hypothalamus, CRH is also synthesized in peripheral tissues, such as T lymphocytes, and is highly expressed in the placenta. In the placenta, CRH is a marker that determines the length of gestation and the timing of parturition and delivery. A rapid increase in circulating levels of CRH occurs at the onset of parturition, suggesting that, in addition to its metabolic functions, CRH may act as a trigger for parturition.[1]


Adrenocorticotropic hormone (ACTH), also known as corticotropin (INN, BAN) (brand names Acortan, ACTH, Acthar, Acton, Cortigel, Trofocortina),[1][2] is a polypeptide tropic hormone produced and secreted by the anterior pituitary gland.[3] It is an important component of the hypothalamic-pituitary-adrenal axis and is often produced in response to biological stress (along with its precursor corticotropin-releasing hormone from the hypothalamus). Its principal effects are increased production and release of cortisol by the cortex of the adrenal gland. Primary adrenal insufficiency, also called Addison's disease, occurs when adrenal gland production of cortisol is chronically deficient, resulting in chronically elevated ACTH levels; when a pituitary tumor is the cause of elevated ACTH (from the anterior pituitary) this is known as Cushing's disease and the constellation of signs and symptoms of the excess cortisol (hypercortisolism) is known as Cushing's syndrome. Conversely, deficiency of ACTH is a cause of secondary adrenal insufficiency, often as a result of hypopituitarism. ACTH is also related to the circadian rhythm in many organisms.[4]



Cortisol is a steroid hormone, in the glucocorticoid class of hormones, and is produced in humans by the zona fasciculata of the adrenal cortex within the adrenal gland.[1] It is released in response to stress and low blood-glucose concentration.

It functions to increase blood sugar through gluconeogenesis, to suppress the immune system, and to aid in the metabolism of fat, protein, and carbohydrates.[2] It also decreases bone formation.[3]

Hydrocortisone (INN, USAN, BAN) is a name for cortisol when it is used as a medication. Hydrocortisone is used to treat people who lack adequate naturally generated cortisol. It is on the World Health Organization's List of Essential Medicines needed in a basic health system.[4]
We have emphasized that once the depolarization caused by the stimulus is above threshold, the resulting neuronal action potential is a complete action potential (i.e., it is all-or-nothing). If the stimulus strength is increased, the size of the action potential does not get larger (see figure). If the size (i.e., amplitude) of the action potential is always the same and independent of the size of the stimulus, how then does the nervous system code the intensity of the stimulus? The trick that the nervous system uses is that the strength of the stimulus is coded into the frequency of the action potentials that are generated. Thus, the stronger the stimulus, the higher the frequency at which action potentials are generated (see Figs. 1 and 2 below). Therefore, we say that our nervous system is frequency-modulated and not amplitude-modulated. The frequency of action potentials is directly related to the intensity of the stimulus.
Given that the frequency of action potentials is determined by the strength of the stimulus, a plausible question to ask is what is the frequency of action potentials in neurons? Another way of asking this question is how many action potentials can a neuron generate per unit time (e.g., action potentials per second)? Physiologically, action potential frequencies of up to 200-300 per second (Hz) are routinely observed. Higher frequencies are also observed, but the maximum frequency is ultimately limited by the absolute refractory period. Because the absolute refractory period is ~1 ms, there is a limit to the highest frequency at which neurons can respond to strong stimuli. That is to say that the absolute refractory period limits the maximum number of action potentials generated per unit time by the axon. As described previously, the strength of the stimulus must be very high in oder to ensure that the duration of the action potential is as short as the duration of the absolute refractory period. A stronger than normal stimulus is required to overcome the relative refracctory period (see Refractory Periods for a review).