Structural units of the nervous system. Composed of a body, axon and dendrites. Long lived, amitotic, and have a high metabolic rate.
What is the Nerve Cell Body?
Perikaryon or Soma. Contains the nucleus and nucleolus, has well developed nissl-bodies (rough er), contains an axon hillock.
What is an Axon Hillock?
Cone shaped area from which axons arise.
What are the processes of the nervous system?
Axons and dendrites. Armlike extentions from the Soma.
What are processes called in the CNS?
What are processes called in the PNS?
What are Dendrites?
Short, tapering, and diffusing branced processes. They are the receptive, or input regions of the neuron.
What are Axons?
Slender processes of uniform, diameter arising from the hillock. Generate and transmit actions potentials. Secrete neurotransmitters from axonal terminals.
What are long axons called?
What is the Axonal Terminal?
Branched terminus of an axon.
What is the Myelin Sheath?
Whittish, fatty (protein-lipoid), segmented sheath around most long axons. Protects axons. Electrically insulates fibers from one another. Increases speed of nerve impulse transmissions. Formed by Schwann cells.
What are Action Potentials?
Nerve Impulses. Electrical impulses that are carried along the lengths of the axons. Always the same regardless of the stimulus.
What are Passive, or Leakage Channels?
Type of plasma membrane ion channel that is always open.
What is a Chemically Gated Channel?
Type of plasma membrane ion channel that opens with a binding of a specific neurotransmitter.
What is a Voltage Gated Channel?
Type of plasma membrane ion channel that opens and closes in response to membrane potential.
What is a Mechanically Gated Channel?
Type of plasma membrane ion channel that opens and closes in response to physical deformation of receptors.
How does a Gated Channel Work?
EX: Na-/K+ Gated Channel. Closed when a neurotransmitter is not bound to the extracellular receptor. Na- cannot enter the cell and K+ cannot exit the cell. Open when a neurotransmitter is attached to the receptor. Na- enters the cell and K+ exits the cell.
How does a Voltage Gated Channel Work?
EX: Na- Channel. Closed when intracellular environment is negative. Na- cannot enter the cell. Open when the intracellular environment is positive. Na- can enter the cell.
What is Resting Membrane Potential?
The potential difference (-70mV) across the membrane of a resting neuron.
What is Depolarization?
The inside of the membrane becomes less negative.
What is Hyperpolerization?
The inside of the membrane becomes more negative then the resting potential.
What is Repolarization?
The membrane returns to its resting membrane potential.
What is the Absolute Refractory Period?
Time from the opening of the Na+ activation gates until the closing of inactivation gates. Prevents the neuron from generating an action potential. Ensures that each action potential is separate. Enforces one way transmission of nerve impulses.
What is the Relative Refractory Period?
The interval following the absolute refractory period when sodium gates are closed, potassium gates are open, repolarization is occuring. The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events.
What is EPSP?
Excitatory Postsynaptic Potentials.
What is IPSP?
Inhibitory Postsynaptic Potentials.
What are Neurotransmitters?
Chemicals used for neuronal communication with the body and the brain. Over 50 have been indentified.
First neurotransmitter identified and best understood. Released at the neuromuscular junction. Synthesized and enclosed in the synaptic vesicles. Degraded by the enzyme acetylcholinesterase (ACheE). Released by all neurons that stimulate skeletal muscle and some neurons of the ANS.
What are Biogenic Amines?
Catecholamine - dopamine, norepinephrine (NE), and epinephrine. Indolamines - seratonin and histamine. Broadly distributed in the brain. Play roles in emotional behaviors and our biological clock.
What are Amino Acids?
Include: GABA - Gamma aminobutyric acid, Glycine, Aspartate, Glutamate. Is found only in the CNS.
What are Peptides?
Include: Substance P - mediator of pain signals. Beta Endorphin, dynorphin, and enkephalins. Act as a natural opiate; reduce pain perception. Binds to the same receptors as opiates and morphine. Gut-brain peptides - Somatostain, and cholectokinin.
What is the All Or None Phenomenon?
Action potentials either happen completely or not at all.
What is the role of the Sodium Potassium Pump?
Ionic redistribution back to resting conditions is restored by this. Restores at hyperpolerization.
Nervous System Functions
Sensory input - monitoring stimuli Integration - interpretation of sensory input Motor output - response to stimuli
Central nervous system (CNS)
Brain and spinal cord Integration and command center
Peripheral nervous system (PNS)
Paired spinal and cranial nerves Carries messages to and from the spinal cord and brain
PNS Functional Divisions
Sensory (afferent) division Motor (efferent) division
Sensory (afferent) division
In the PNS, sensory afferent fibers carry impulses from skin, skeletal muscles, and joints to the brain (aka CNS)
Motor (efferent) division
In the PNS, transmits impulses from the CNS to effector organs
Motor Division: Two Main Parts
Somatic nervous system Autonomic nervous system (ANS)
Somatic nervous system
Conscious control of skeletal muscles
Autonomic nervous system (ANS)
Regulates smooth muscle, cardiac muscle, and glands
Autonomic nervous system (ANS) divisions
sympathetic and parasympathetic
excitable cells that transmit electrical signals structural unit of the nervous system
cells that surround and wrap neurons
supporting cells of the nervous system also called neuralgia or glial cells
Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth
most abundant, versatile, and highly branched glial cells cling to one or more neurons and their synaptic endings, and cover capillaries
Support and brace neurons Anchor neurons to their nutrient supplies Guide migration of young neurons Control the chemical environment
small, ovoid cells with spiny processes
Phagocytes that monitor the health of neurons
ciliated cells that range in shape from squamous to columnar line the central cavities of the brain and spinal column
Ependymal cells functions
filters for CSF
branched cells that wrap CNS nerve fibers help regulate how neuron fires
aka neurolemmocytes surround fibers of the PNS help regulate how neuron fires
body, axon & dendrites long, lived, high metabolic rate do not undergo mitosis
Neuron plasma membrane functions
electrical signaling cell-to-cell signaling during development
Nerve Cell Body (Perikaryon or Soma)
Contains the nucleus and a nucleolus Is the major biosynthetic center Is the focal point for the outgrowth of neuronal processes Has no centrioles (hence its amitotic nature) Has well-developed Nissl bodies (rough ER) Contains an axon hillock - cone-shaped area from which axons arise
Armlike extensions from the soma
CNS neuron processes
PNS neuron processes
two types of neuron processes
axons and dendrites
Dendrites of Motor Neurons
'messenger' cells that receive stimulus anchor the cell body relay signal to cell body
slender processes that arise from hillock one unbranched per neuron
rare branching from the axon
generate & transmit action potentials secrete neurotransmittters from the axon terminals
whitish, fatty, segmented sheath around most long axons composed of 80% fat & 20% protein
myelin sheath functions
Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission
Formed by Schwann cells in the PNS The Schwann cell: -Envelopes an axon in a trough -Encloses the axon with its plasma membrane -Has concentric layers of membrane that make up the myelin sheath
Nodes of Ranvier (Neurofibral Nodes)
Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge
grey matter neurons Schwann cells are present but myelin sheath is not b/c they do not coil around axons
in the PNS, dense collections of myelinated fibers 'faster' than gray matter
in the CNS, mostly soma and unmyelinated fibers 'slower' than white matter
aka nerve impulses Electrical impulses carried along the length of axons Always the same regardless of stimulus the underlying functional feature of the nervous system
Reflects the flow of ions rather than electrons
potential on either side of membranes when:
The number of ions is different across the membrane The membrane provides a resistance to ion flow
open and close in response to physical deformation of receptors
Operation of a Gated Channel
Example: Na+-K+ pump Closed when a neurotransmitter is not bound to the extracellular receptor Na+ cannot enter the cell and K+ cannot exit the cell Open when a neurotransmitter is attached to the receptor Na+ enters the cell and K+ exits the cell
Operation of a Voltage-Gated Channel
Example: Na+ channel Closed when the intracellular environment is negative Na+ cannot enter the cell Open when the intracellular environment is positive Na+ can enter the cell
When gated channels are open:
Ions move quickly across the membrane Movement is along their electrochemical gradients An electrical current is created Voltage changes across the membrane
electrical and chemical gradients taken together Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration Ions flow along their electrical gradient when they move toward an area of opposite charge
Resting Membrane Potential (Vr)
potential difference (-70 mV) across the membrane of a resting neuron
Membrane Potentials: Signals
Used to integrate, send, and receive information
Membrane potential changes are produced by:
Changes in membrane permeability to ions Alterations of ion concentrations across the membrane
Types of signals
graded potentials and action potentials
3 events that cause Changes in Membrane Potential
depolarization repolarization hyperpolarization
the inside of the membrane becomes less negative
the membrane returns to its resting membrane potential
the inside of the membrane becomes more negative than the resting potential
Magnitude varies directly with the strength of the stimulus Short-lived, local changes in membrane potential Decrease in intensity with distance Sufficiently strong graded potentials can initiate action potentials Current is quickly dissipated due to the leaky plasma membrane Only travel over short distances
Action Potentials (APs)
'neuron firing' +35mV Action potentials are only generated by muscle cells and neurons They do not decrease in strength over distance They are the principal means of neural communication
Action Potential: Resting State
Na+ and K+ channels are closed Leakage accounts for small movements of Na+ and K+ Each Na+ channel has two voltage-regulated gates
Action Potential: Depolarization Phase
Na+ gates are opened; K+ gates are closed Cell becomes more positive
Depolarization Phase Threshold
a critical level of depolarization (-55 to -50 mV) At this point, the process becomes self-generating
Action Potential: Repolarization Phase
sodium gates close, voltage-sensitive K+ gates open K+ exits the cell and cell becomes more negative, as usual
Action Potential: Hyperpolarization
Potassium gates remain open, causing an excessive efflux of K+ The neuron is insensitive to stimulus and depolarization during this time
Action Potential: Role of the Sodium-Potassium Pump
Repolarization -Restores the resting electrical conditions of the neuron -Does not restore the resting ionic conditions
wave of impulse moving down the axon to the axon terminals
Action Potential Threshold
membrane is depolarized membrane potential goes from resting state -70mV to -55mV
the CNS determines stimulus intensity by the frequency of impulse transmission
absolute refractory period
neuron cannot fire time from the Na+ activation gates until the closing of inactivation gates
relative refractory period
Na+ gates are closed, K+ gates are open repolarization is occurring threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events
the larger the diameter, the faster the impulse
myelin sheath presence
dramatically increases impulse speed
Current passes through a myelinated axon only at the nodes of Ranvier avoiding fats Action potentials are triggered only at the nodes and jump from one node to the next Much faster than conduction along unmyelinated axons Na+ channels are concentrated at these nodes
scar tissue in myelin sheath causes too long of a jump resulting in muscle control spreads to diaghram & causes respiratory failure
Central Nervous System (CNS)
This nervous system consists of the brain and spinal cord.
Peripheral Nervous System (PNS)
The section of the nervous system lying outside the brain and spinal cord
Sensory or Afferent
Part of the (PNS) that carry impulses towards the (CNS)
Motor or Efferent
Part of the (PNS) that carry impulses away from the (CNS)
Somatic Nervous System
This nervous system conduct impulses from the (CNS) to skeletal muscles.
Autonomic Nervous System (ANS)
This nervous system regulate the activity of smooth muscles.
These neurons are associated closely with much smaller cells., Cells that provide basic support systems for neurons and perform a variety of maintenance functions
Star shaped cells found throughout the CNS, cleaning up debris in the extracellular space and removing neurotransmitters from the synaptic cleft, connects neurons to nearby capilaries, components of the blood-brain barrier
Smallest neuroglial cells; phagocytic cells that engulf cellular debris, waste products and pathogens. increase in number as a result of infection or injury
A glial cell that lines membranes within the brain and spinal cord and helps form cerebrospinal fluid
Provides myelination in CNS, electrically insulates certain axons, & speeds up rate of electrical signals
Surrounds the neuron cell bodies in ganglia of PNS little is known of their function (PNS)
Supporting cells of the peripheral nervous system responsible for the formation of myelin.
Cells specialized for transmitting nerve impulses.
Cytoplasm surrounding the nucleus of a neuron
The branching extensions of a neuron that receive messages and conduct impulses toward the cell body
The extension of a neuron, ending in branching terminal fibers, through which messages pass to other neurons or to muscles or glands
A branch of an axon
Series of fine, terminal extensions branching from the axon tip.
The plasma membrane of the axon
A layer of fatty tissue segmentally encasing the fibers of many neurons; enables vastly greater transmission speed of neural impulses as the impulse hops from one node to the next.
Portion of the Schwann cell which includes the exposed part of its plasma membrane.
Nodes of Ranvier
Small gaps in the myelin sheath of medullated axons
Regions of the brain and spinal cord containing a dense collection of myelinated fibers.
Contains mostly nerve cell bodies and unmyelinated fibers.
Neurons having three or more processes.
Neruons having two processes
Central nervous system neurons that internally communicate and intervene between the sensory inputs and motor outputs
The difference in electrical charge between two points in a circuit expressed in volts.
Sodium rushes into neuron through membrane, potassium ruses out; results in a change in charge
The movement of the membrane potential of a cell away from rest potential in a more negative direction.
Na+ and K+ channels are closed
Increase in Na+ permeability and reversal of membrane potential.
Decrease in Na+ permeability and an increase in K+ permeability
Sodium potassium pump
Helps establish a difference in charge across the membrane (membrane potential) moves ions and molecules against the concentration gradient (takes energy)
The time after a neuron fires or a muscle fiber contracts during which a stimulus will not evoke a response
Synapses between the axon endings of one neuron and the dendrites of other neurons.
Between axon endings of one neuron and cell bodies of other neurons
A neuron conducting impulses toward the synapse
A neuron on the receiving end of a synapse
Made of connexons- tubular proteins that allow passage of an impulse in 2 directions at once- are located in gap junctions where 2 cells come together- more rapid than a chemical synapse
A synapses that is specialized to release and the reception of chemical neurotransmitters.
Tiny sacs in a terminal button that release chemicals into the synapse.
A space between two connecting neurons where neurotransmitters are released.
Simple hormones; water soluble; derived from amino acids; e.g, epinephrine, norepinephrine, dopamine
One incoming fiber triggers responses in ever-increasing numbers of neurons farther along in a circuit.
A cell from which a nerve cell develops.
Growing tip of an axon that has a prickly, fanlike structure that gives an axon the ability to interact with its environment.
Rapid, automatic responses to stimuli, in which a particular stimulus always causes the same response.
Any disease of nervous tissue, but particularly degenerative disease of the nerves.
Central Nervous System
integrates and coordinates incoming and outgoing neural signals and carries our higher learning functions (thinking and learning)
collection of nerve cell bodies in the CNS
unmyelinated axons, cell bodies, and dendrites; found in horns and commissures of spinal cord; surrounds ventricles, in cortex, in nuclei of brain
The portions of the central nervous system that are abundant in axons rather than cell bodies of neurons. The colour derives from the presence of the axon's myelin sheaths
Peripheral Nervous System
-consists of nerve fibers and cell bodies outside the CNS that conduct impulses to or away from the CNS -Also, organized into nerves that connect the CNS with peripheral structures
Nerve fibers consist of:
a. bundle of nerve fibers outside the CNS (or a "bundle of bundled fibers" or fasicles) b. the connective tissue coverings that surround and ind the nerve fibers and fascicles together c. blood vessels (vasa nervorum) that nourish the nerve fibers and their coverings
Types of nerve connective tissue coverings
Endoneurium Perineurium Epineurium
Delicate connective tissue immediately surrounding the neurilemma cells and axons
Layer of dense connective tissue that encloses a fascicle of nerve fibers, providing an effective barrier against penetration of the nerve fibers by foreign substances
a thick connective tissue sheath that surrounds and encloses a bundle of fascicles, forming the outermost covering of the nerve -includes fatty tissue, blood vessels, and lymphatics
Types of nerves
-Cranial: exit the cranial cavity through the foramina in the cranium -Spinal: exit the vertebral column through the intervertebral foramina
Trigger zone (aka threshold)
-55 mV; If the stimulus reaches the trigger zone then the action potential happens.
Depolarized graded potentials
Becomes more positive.... so is excitatory
Hyperpolarizing graded potentials
Becomes more negative... so is inhibitory
Larger vs Shorter Neurons
Larger neurons conduct APs faster
2 Key Parameters that influence speed of AP
Diameter of the neuron (larger --> faster) and the Resistance of the axon to ion leakage (more myelination --> faster)
a) APs appear to jump from one node of ranvier to the next b) In demyelinating diseases, conduction slows due to current leakage out of the previously insulated regions between the nodes.
Draw an action potential***
- label voltages (-55mV, -70mV, +30mv) - know what channels open and closes at each step
What is the synaptic cleft and what happens at the synapse?
Area between the presynaptic neuron and the postsynaptic membrane. The presynaptic and postsynaptic cells makes the synapse.
The majority of synapses are chemical synapses.
They are proteins or neurohormones that are made in the cell body then travels via axonal transport to a synaptic vesicle
What is the role of calcium at the end of a neuron?
Calcium triggers exocytosis of the synaptic vesicle contents (in that the synaptic vesicle fuses with the membrane) and therefore signaling the release of neurocrine neurotransmitters at the synapse.
What are the 2 major neurocrines secreted by the nervous system, what types of receptors do they have, and where are those receptors?
Acetylcholine (cholinergic; nicotonic- skeletal muscles, autonomic neurons, CNS | muscarinic- smooth and cardiac muscle, endocrine and exocrine glands, CNS) and norephinephrine (adrenergic; smooth and cardiac muscle, endocrine exocrine glands, CNS)
Why are neurotransmitter activities normally rapidly terminated?
Neurotransmitter activity is rapidly terminated 1) by removal or inactivation of neurotransmitter or 2) moved back into presynaptic cell
In order for activity to last longer, stimulation most occur continuously.
What is plasticity?
If a cell body still exists, plasticity allows neurons to make new connections and relearn how to function.
What are the anatomic and functional categories of neurons?
a)unipolar neurons have a single process called the axon. During development, the dendrite fused with the axon. b) Bipolar neurons have 2 relatively equal fibers extending off the central cell body. c) Axaxonic CNS interneurons have no apparent axon. d) Multipolar CNS interneurons are highly branched but lack long extensions. e) A typical multipolar efferent neuron has 5 to 7 dendrites, each branching 4 to 6 times. A single long axon may branch several times and end at enlarged axon terminals.
How are organelles quickly moved down an axonal transport?
What are the different glial cells and their functions?
- Provide support - Outnumber neurons 10-50 to 1
A)PNS has 2 types: 1.Schwann cells- wrap around neuron (speed up electrical impulse) 2. Satellite cells- supportive capsules around nerve cells in ganglia (cluster of nerve bodies found outside of CNS)
B) CNS has 4 types 1. oligodendrocytes- like Schwann cells in PNS 2. microglia- immune cells in CNS 3. astrocytes- take up and release chemicals 4. ependymal cells- one source of stem cells
Schwann and oligodendrocytes support and insulate axon by forming myelin (nodes of ranvier)
What is the difference between the absolute refractory period and the relative refractory period?
During the absolute refractory period, no stimulus can trigger another action potential.
During the relative refractory period, only a large than normal stimulus can initiate a new action potential.