Chapter 48 - Neurons, Synapses, and Signaling

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48.1 - Neuron Organization and structure reflect function in info transfer 48.2 - Ion pumps and ion channels establish the resting potential of a neuron 48.3 - Action Potentials are the signals conducted by axons 48.4 - Neurons communicate with other cells at synapses

Ganglia

A structure containing a number of nerve cell bodies, typically linked by synapses, and often forming a swelling on a nerve fiber

Brain

part of the central nervous system that includes all the higher nervous centers

Central Nervous System

consists of the brain and spinal cord, organizes and integrates info

Peripheral Nervous System

PNS connects the central nervous system CNS to sensory organs (such as the eye and ear), other organs of the body, muscles, blood vessels and glands. The peripheral nerves include cranial nerves, the spinal nerves and roots, and the autonomic nerves that are concerned specifically with the regulation of the heart muscle, the muscles in blood vessel walls, and glands.

Nerves

fiber composed of bundled axons of PNS neurons

Sensory Neurons

a nerve cell that conducts impulses from a sense organ to the central nervous system; transmit info from sensors that detect external stimuli (light, sound, touch, heat, smell, and taste) or internal conditions (blood pressure, CO2 lvl, and muscle tension)

Interneurons

local circuits connecting neurons to the brain

Motor Neurons

transmit signals from brain or spinal cord tomuscle cells or glands, causing them to contract

Cell body

The part of a neuron containing the nucleus but not incorporating the axon and dendrites

Dendrites

branched extensions that receive signals from other neurons.

Axon

the impulses are transmitted to other nerve cells or to effector organs. Larger axons are covered by a myelin sheath;
the extension of a neuron, ending in branching terminal fibers, through which messages pass to other neurons or to muscles or glands

Synapse

specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie perception and thought.

Neurotransmitters

The neurotransmitter diffuses across the gap to bind with receptors on the postsynaptic cell membrane and cause electrical changes in that neuron (depolarization/excitation or hyperpolarization/inhibition)

Glial Cells; Glia

non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for the brain's neurons. Regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and nutrify neurons. During early embryogenesis, glial cells direct the migration of neurons and produce molecules that modify the growth of axons and dendrites. Recent research indicates that glial cells of the hippocampus and cerebellum participate in synaptic transmission, regulate the clearance of neurotransmitters from the synaptic cleft, release factors such as ATP, which modulate presynaptic function, and even release neurotransmitters themselves.

Describe the basic pathway of information flow through neurons that causes you to turn your head when someone calls your name

Sensors in your ear transmit info to your brain. There the activity of interneurons in processing centers enables you to recognize your name. In response, signals transmitted via motor neurons cause contraction muscles that turn your neck,

How might increased branching of an axon help coordinate responses to signals communicated y the nervous system?

Increased ranching would allow control of a greater number of postsynaptic cell,s enhancing coordination of responses to nervous system signals.

Consider how communication occurs in a colony of bacteria. In what general ways is that communication similar to and different from transmission of a nerve impulse by a neuron?

Communication by bacteria involves all the cells in a colony, whereas communication by neurons involves just a few cells in the animal body. Also, neurons direct signals from one location to another, but bacterial cells communicate in all directions.

Presynaptic Cell

transmitting neuron

Postsynaptic cell

receiving signal
ex) neuron, muscle, or gland cell

Membrane Potential

The difference in electrical charge (voltage) across a cell's plasma membrane due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances.

Resting potential

the potential difference between the two sides of the membrane of a nerve cell when the cell is not conducting an impulse, or when a neuron is in polarization; more negative ions are inside the neuron cell membrane with a positive ions on the outside, causing a small electrical charge; release of this charge generates a neuron's impulse (signal/message).

Basis of Membrane Potential

The sodium potassium pump generates and maintains the ionic gradients of Na+ and K+ into the cell. The pump uses ATP to actively transport Na+ out of cell and K+ into cell. Although there is a substantial concentration gradient of sodium across the membrane very little net diffusion of Na+ occurs because there are few open sodium channels. Large number of open potassium channels allow net outflow of K+. Membrane is only weakly permeable to chloride and large anions, this outflow of K+ results in a net negative charge inside the cell.

Under what circumstances could ions flow through ion channels from regions of low ion concentration to regions of high ion concentrations?

Ions can flow against a chemical concentration gradient if there is an opposing electrical gradient of greater magnitude.

Suppose a cell's membrane shifts from -70 mV to -50 mV. What changes in the cell's permeability to K+ or Na+ could cause such a shift?

A decrease to permeability to K+, and increase to Na+, or both.

Plant substance disables sodium-potassium pump. What change in resting potential would you expect to see if you treated a neuron with this plant substance?

The activity of the sodium potassium pump is essential to maintain the resting potential. With the pump inactivated, the sodium and potassium concentration gradients would gradually disappear, resulting in a greatly reduced resting potential.

Could diffusion eliminate the concentration gradient of a dye that has a net charge?

Charge dye molecules could equilibrate only if other charged molecules could also cross the membrane. If not, a membrane potential would develop that would counterbalance the chemical gradient.

Gated ion channels

ion channels that open or close in response to stimuli, alters membrane permeability to particular ions = changes membrane potential

Hyperpolarization

change in a cell's membrane potential such that the inside of the membrane becomes more negative relative to the outside. Reduces the chance a neuron will fire a nerve impulse.

Depolarization

a change in a cell's membrane potential such that the inside of the membrane is made less negative relative to the outside. If stimulus causes the gated sodium channels in resting neuron to open, the membrane's permeability to Na+ increases, it diffuses into the cell along its concentration gradient.

Graded potential

in a neuron, shift in the membrane potential that has an amp proportional to signal strength and that decays as it spreads

Action potential

electrical signal that travels along the membrane of a neuron or other excitable cell as a all-or-none depolarization. Triggered by depolarization that reaches the threshold -55mV

Refractory Period

short time immediately after an action potential in which the neuron cannot respond to another stimulus, owing to the inactivation of voltage-gated sodium channels. Ensures all signals in axon travel in one direction

Myelin sheath

produced by two types of glia - oligodendrocytes in the CNS and Schwann cells in the PNS. During development, these two glia cells wrap axons in many layers of membrane. The membrane forming these layers are mostly lipid, which is a poor conductor of electrical current.

Nodes of Ranvier

gaps in myelin sheath (between Schwann cells). Extracellular fluid is in contact with axon membrane only at the nodes, so action potentials are not generated in the regions between nodes.

Schwann Cells

Supporting cells of the peripheral nervous system responsible for the formation of myelin.

Oligodendrocytes

glia cells that surround and insulate certain axons in the vertebrate brain and spinal cord.

Salatory Conduction

rapid conduction of impulses when the axon is myelinated since depolarizations jump from node (of Ranvier) to node.
Current passes through a myelinated axon only at the nodes of Ranvier.
Voltage-gated Na+ channels are concentrated at these nodes.
Action potentials are triggered only at the nodes and jump from one node to the next. Much faster than conduction along unmyelinated axons.

If all Ca2+ in the fluid surrounding a neuron were removed, how would this affect the transmission of info within and between neurons?

The production and transmission of action potentials would be unaffected. However, action potentials arriving at chemical synapse would be unable to trigger release of neurotransmitter. Signal at such synapse would be blocked.

A Chemical Synapse

1) An action potential arrives, depolarizing the presynaptic membrane.
2) The depolarization opens voltage-gated channels, triggering an influx of Ca2+
3) Increase Ca2+ concentration causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.
4) The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane.

How do action potentials and graded potentials differ?

A graded potential has a magnitude that varies with stimulus strength, whereas action potential has an all-or-none magnitude that is independent of stimulus strength.

In MS, myelin sheaths harden and deteriorate. How would this affect nervous sys function?

Loss of insulation provided by myelin sheaths leads to a disruption in action potential propagation along axons. Voltage gated sodium channels are restricted to the nodes of Ranvier, and without the insulating effect of myelin, the inward current produced at one node during an action potential cannot depolarize the membrane to the threshold at the next node.

Suppose mutation cause gated-sodium channels to remain inactivated longer after an action potential. How would this affect the frequency at which action potentials could be generated?

The max frequency would decrease because the refractory period would be extended.

Ligand-gated ion channel

Ionotropic receptor clustered in the membrane of postsynaptic cell. Binding of the neurotransmitter to a certain part of the receptor opens the channel and lets specific ions diffuse across the postsynaptic membrane = postsynaptic potential (graded potential

Synaptic cleft

the gap that separates the presynaptic neuron from the postsynaptic cell. Diffusion time is very short because the gap < 50 nm across.

Excitatory postsynaptic potential

EPSP - a postsynaptic potential that depolarizes the neuronal membrane, making the cell more likely to fire an action potential

Inhibitory postsynpatic potential

IPSP - hyperpolarizes the neuronal membrane, making the cell less likely to fire an action potential (moves further from threshold).

Temporal Summation

Occurs when a single synapse generates EPSPs so quickly that each is generated before the previous decays. This allows the EPSPs to add up to reach a threshold voltage that triggers an action potential.

Spatial Summation

Multiple EPSPs occur at the same time by different synapses at the same postsynaptic neuron and the voltage at the axon hillock reaches threshold

Spatial summation of EPSP and IPSP

Through summation, an IPSP can also counter the effect of an EPSP (Hyperpolarization vs Depolarization)

Axon Hillock

neuron's integrating center, the region where the membrane potential at any instant represents the summed effect of all EPSPs and IPSPs. Whenever the membrane potential here reaches threshold, an action potential is generated and travels along the axon to synaptic terminals.

Metabotropic receptor

Neurotransmitters binding to these receptors do not directly open ion channels; results in the opening/closing of ion channels, which depends on the activation of signal transduction pathway involving a second messenger (Gprotein). The effects of second-messenger are slower but last longer than potentials made by ligand-gated ion channels.

Example of signal transduction pathways in modulating synaptic transmission

Neurotransmitter norepinephrine binds to its metabotropic receptor, and activated a G protein --> turns on adenylyl cyclase, the enzyme that converts ATP to cAMP. Cyclic AMP activates protein kinase A, which phosphorylates specific ion channel proteins in the postsynaptic membrane. Ion channels open or close.

Acetylcholine

muscle stimulation, memory formation, and learning. When released by motor neurons and binds to ligand-gated ion channel receptor at neuromuscular junction, the ion channel opens and produces EPSP. Excitatory activity terminated by acetylcholinesterase, an enzyme in the synaptic cleft that hydrolyzes the neurotransmitter. Inhibitiion of acetylcholinesterase causes a buildup of acetylcholine, which triggers paralysis and typically death.
Metabotropic receptor found in heart muscle. G proteins inhibit adenylyl cyclase and open potassium channels in the muscle cell membrane. Both effects reduce the rate at which the heart pumps.

Glutamate

amino acid neurotransmitter in CNS that binds to ligand-gated ion channels. Has an excitatory effect on postsynaptic cells. Synapses at which glutamate is the neurotransmitter have a key role in formation of long-term memory.

Gamma-aminobutyric acid (GABA)

neurotransmitter at most inhibitory synapses in the brain. Binding of GABA to receptors increases membrane permeability to Cl-, results in IPSP.

Glycine

acts at inhibitory synapses in parts of CNS outside of the brain. Binds to ionotropic receptor that is inhibited by strychnine.

Biogenic Amines

Neurotransmitter derived from amino acids, and include catecholamines (dopamine, epinephrine & norepinephrine) & indolamines (histamine & serotonin)

Norepinephrine

made from tyrosine. noradrenaline; chemical which is excitatory, similar to adrenaline, and affects arousal and memory; raises blood pressure by causing blood vessels to become constricted, but also carried by bloodstream to the anterior pituitary which relaxes ACTH thus prolonging stress response

Epinephrine

adrenaline; activates a sympathetic nervous system by making the heart beat faster, stopping digestion, enlarging pupils, sending sugar into the bloodstream, preparing a blood clot faster

Dopamine

made from tyrosine, neurotransmitter that influences associated with movement, attention and learning and the brain's pleasure and reward system; lack of dopamine linked with Parkinson's disease; too much is linked with schizophrenia.

Serotonin

made from tryptophan, a neurotransmitter that affects hunger,sleep,arousal,and mood. appears in lower than normal levels in depressed persons, serves as the precursor to melatonin.

Neuropeptides

relatively short chains of amino acids, serve as neurotransmitters that operate via metabotropic receptors.

Substance P

A neuropeptide that is involved in the transmission of pain messages to the brain.

Endorphins

"morphine within"- natural, opiatelike neurotransmitters linked to pain control and to pleasure. Decrease urine output, depress respiration, and produce euphoria.

Gases

CO generated by the enzyme heme oxygenase, found in brain and PNS. In brain, regulates release of hypothalamic hormones. In PNS, acts as inhibitory neurotranmistter that hyperpolarizes the plasma membrane of intestinal smooth muscle cells.

How is it possible for a particular neurotransmitter to produce opposite effects in different tissue?

It can bind to different types of receptors, each triggering a specific response in postsynaptic cells. Drugs that target receptor activity rather than neurotransmitter release or stability are likely to exhibit more specificity and potentially have fewer undesirable side effects.

If a drug mimicked the activity of GABA in the CNS, what general effect on behavior might you expect?

Since GABA is an inhibitory neurotransmitter, this drug would be exprected to decrease brain activity. A decrease brain activity might be expected to slow down or reduce behavioral activity.

What membrane activity is common to fertilization and neurotransmitter release?

Membrane Fusion

In what what ways do both positive and negative feedback contribute to the shape of an action potential?

Positive feedback is responsible for the rapid opening of many voltage-gated sodium channels, causing the rapid outflow of sodium ions responsible for the rising phase of the action potential. As the membrane potential becomes more positive, voltage-gated potassium channels open in a form of negative feedback that helps bring about the falling phase of action potentials.

How would severing an axon affect the flow of info in a neuron?

It would prevent information from being transmitted away from the cell body along the axon

Generally, the flow of information in the neuron is in one direction. The dendritic tree (and sometimes the cell body as well) receive information. If the neuron fires an action potential, it travels to a target cell along the axon.

Suppose you placed an isolated neuron in a solution similar to extracellular fluid and later transferred the neuron in a solution lacking any sodium ions. What change would you expect in the resting potential?

There are very few open sodium channels in a resting neuron, so the resting potential either would not change or would become slightly more negative (hyperpolarization).

Why are many drugs used to treat nervous system diseases or affect brain function targeted to specific receptors rather than particular neurotransmitters?

Drugs that target receptor activity rather than neurotransmitter release or stability are likely to exhibit more specificity and potentially have fewer undesirable side effects. Neurotransmitters can bind to different types of receptors that differ in location and activity.

What happens when a resting neuron's membrane depolarizes?

The neuron's membrane voltage becomes more positive. Some sort of signal /chemical (i.e. pain signal) binds to receptors on the neuron, which opens Na+ gates, allowing Na+ to rush into the cell. Since the resting membrane potential is already negative, the inflow of Na+ will neutralize some of this negativity, and the voltage will actually raise up closer to zero. This is what is known as depolarization.

A common feature of action potentials

triggered by a depolarization that reaches the threshold

Where are neurotransmitter receptors located?

Postsynaptic membrane

Temporal Summation always involves what?

multiple inputs at a singly synapse.

Why are action potentials usually conducted in one direction?

The brief refractory period prevents reopening of voltage-gated Na+ channels

A direct result of depolarizing the presynaptic membrane of an axon terminal

voltage-gated calcium channels in the membrane open

What evolutionary advantage might on/off signaling have over a graded kind of signaling?

Much of the current knowledge of action potentials comes from squid axon experiments. One function of action potentials is rapid, long-range signaling within the organism; Despite being slower than graded potentials, action potentials have the advantage of traveling long distances in axons with little or no decrements. The conduction velocity can exceed 110 m/s, which is one-third the speed of sound. For comparison, a hormone molecule carried in the bloodstream moves at roughly 8 m/s in large arteries. Part of this function is the tight coordination of mechanical events, such as the contraction of the heart. A second function is the computation associated with its generation. Being an all-or-none signal that does not decay with transmission distance, the action potential has similar advantages to digital electronics. The integration of various dendritic signals at the axon hillock and its thresholding to form a complex train of action potentials is another form of computation, one that has been exploited biologically to form central pattern generators and mimicked in artificial neural networks.

True or false? The potential energy of a membrane potential comes solely from the difference in electrical charge across the membrane.

False-The potential energy of a membrane potential comes both from the difference in electrical charge and from the concentration gradient of ions across a membrane.

How is an action potential propagated down an axon after voltage-gated sodium channels open in a region of the neuron's membrane?

Sodium ions enter the neuron and diffuse to adjacent areas, resulting in the opening of voltage-gated sodium channels farther down the axon.-The entry of sodium ions into the neuron and their diffusion to adjacent areas of the membrane causes those portions of the membrane to become depolarized and results in the opening of voltage-gated sodium channels farther down the axon, which release potassium ions to the outside, returning the charge to its previous state.

What is the sequence of events that occurs once the membrane potential reaches threshold

-Membrane reaches threshold
-Many voltage-gated Na+ channels open
-Na+ rush into the cell, moving down their electrochemical gradient
-Membrane potential rises (depolorizes) rapidly

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