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The ________ nervous system controls the skeletal muscles
somatic
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The efferent division of the peripheral nervous system innervates:
glandular cells, heart muscle cells, skeletal muscle cells, smooth muscle cells
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The axoplasm of the axon contains which of the following?
vesicles, mitochondria, neurofibrils, neurotubules
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The cytoplasm that surrounds the nucleus of a neuron is called the
perikaryon
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Axons terminate in a series of fine extensions known as
telodendria
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Neurons that are rare, small, and lack features that distinguish dendrites from axons are called
anaxonic
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Deteriorating changes in the distal segment of an axon as a result of a break between it and the soma is called ________ degeneration.
Wallerian
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The myelin sheath that covers many CNS axons is formed by
oligodendrocytes
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Damage to ependymal cells would most likely affect the
formation of cerebrospinal fluid
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Ions are unequally distributed across the plasma membrane of all cells. This ion distribution creates an electrical potential difference across the membrane. What is the name given to this potential difference?
Resting membrane potential (RMP)
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Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
Leak channels
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On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?
The inside surface of the plasma membrane is much more negatively charged than the outside surface.
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The plasma membrane is much more permeable to K+than to Na+. Why?
There are many more K+ leak channels than Na+ leak channels in the plasma membrane.
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The resting membrane potential depends on two factors that influence the magnitude and direction of Na+and K+diffusion across the plasma membrane. Identify these two factors.
The presence of concentration gradients and leak channels
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What prevents the Na+and K+gradients from dissipating?
Na+-K+ ATPase
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The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
K+; Na+
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During depolarization, which gradients move Na+ into the cell?
both the electrical and chemical gradients
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What is the value for the resting membrane potential for most neurons?
–70 mV
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The Na+–K+ pump actively transports both sodium and potassium ions across the membrane to compensate for their constant leakage. In which direction is each ion pumped?
Na+ is pumped out of the cell and K+is pumped into the cell.
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The concentrations of which two ions are highest outside the cell.
Na+ and Cl–
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In a neuron, sodium and potassium concentrations are maintained by the sodium-potassium exchange pump such that __________.
the sodium concentration is higher outside the cell than inside the cell and the potassium concentration is higher inside the cell than outside the cell.
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The sodium-potassium exchange pump transports potassium and sodium ions in which directions?
Sodium ions are transported out of the cell. Potassium ions are transported into the cell.
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Leak channels allow the movement of potassium and sodium ions by what type of membrane transport?
channel-mediated diffusion
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The electrochemical gradient for potassium ions when the transmembrane potential is at the resting potential (-70 mV) is caused by what?
a chemical gradient going out of the cell and an electrical gradient going into the cell
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What is the electrochemical gradient of an ion?
the sum of the electrical and chemical gradients for that ion
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In a typical neuron, what is the equilibrium potential for potassium?
-90 mV
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The electrochemical gradient for sodium ions in a neuron when the transmembrane potential is at the resting potential is caused by what?
chemical and electrical gradients both going into the cell
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Compared to the electrical gradient for sodium at rest, the electrical gradient for potassium at rest is __________.
in the same direction and of the same magnitude.
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In a typical neuron, what is the equilibrium potential for sodium?
+66 mV
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At rest, why is the transmembrane potential of a neuron (-70 mV) closer to the potassium equilibrium potential (-90 mV) than it is to the sodium equilibrium potential (+66 mV)?
The membrane is much more permeable to potassium ions than to sodium ions.
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If the axolemma becomes more permeable to potassium ion:
a stronger stimulus will be required to cause an action potential
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Where do most action potentials originate?
Initial segment
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What opens first in response to a threshold stimulus?
Voltage-gated Na+ channels
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What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value.
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What characterizes repolarization, the second phase of the action potential?
Once the membrane depolarizes to a peak value of +30 mV, it repolarizes to its negative resting value of -70 mV.
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What event triggers the generation of an action potential?
The membrane potential must depolarize from the resting voltage of-70 mV to a threshold value of -55 mV.
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What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
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What type of conduction takes place in unmyelinated axons?
Continuous conduction
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An action potential is self-regenerating because __________
depolarizing currents established by the influx of Na+ flow down the axon and trigger an action potential at the next segment
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Why does regeneration of the action potential occur in one direction, rather than in two directions?
The inactivation gates of voltage-gated Na+ channels close in the node, or segment, that has just fired an action potential.
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What is the function of the myelin sheath?
The myelin sheath increases the speed of action potential conduction from the initial segment to the axon terminals.
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What changes occur to voltage-gated Na+and K+channels at the peak of depolarization?
Inactivation gates of voltage-gated Na+ channels close, while activation gates of voltage-gated K+ channels open.
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In which type of axon will velocity of action potential conduction be the fastest?
Myelinated axons with the largest diameter
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Where in the neuron is an action potential initially generated?
axon hillock
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The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
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The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+channels
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Hyperpolarization results from __________.
slow closing of voltage-gated K+channels
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What is the magnitude (amplitude) of an action potential?
100 mV
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How is an action potential propagated along an axon?
An influx of sodium ions from the current action potential depolarizes the adjacent area.
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Why does the action potential only move away from the cell body?
The areas that have had the action potential are refractory to a new action potential.
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The velocity of the action potential is fastest in which of the following axons?
a small myelinated axon
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In what part of the neuron does the action potential typically initiate?
initial segment of the axon
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During an action potential of a neuron, what directly causes the different channels to open and close?
the transmembrane potential (voltage)
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What is the typical duration of a nerve action potential?
2 ms
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Around what transmembrane potential does threshold commonly occur?
-60 mV
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What ion is responsible for the depolarization of the neuron during an action potential?
Na+ (sodium)
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What type of membrane transport causes the depolarization phase of the action potential in neurons?
diffusion
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During an action potential, after the membrane potential reaches +30 mV, which events primarily affect the membrane potential?
Voltage-gated sodium channels begin to inactivate (close) and voltage-gated potassium channels begin to open.
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What ion causes repolarization of the neuron during an action potential?
K+ (potassium)
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What causes repolarization of the membrane potential during the action potential of a neuron?
potassium efflux (leaving the cell)
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What is primarily responsible for the brief hyperpolarization near the end of the action potential?
voltage-gated potassium channels taking some time to close in response to the negative membrane potential
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Action potential propagation begins (is first generated at) what region of a neuron?
initial segment
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Where are action potentials regenerated as they propagate along an unmyelinated axon?
at every segment of the axon
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The movement of what ion is responsible for the local currents that depolarize other regions of the axon to threshold?
sodium (Na+)
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In an unmyelinated axon, why doesn't the action potential suddenly "double back" and start propagating in the opposite direction?
The previous axonal segment is refractory.
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Approximately how fast do action potentials propagate in unmyelinated axons in humans?
1 meter per second
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In contrast to the internodes of a myelinated axon, the nodes __________.
have lower membrane resistance to ion movement
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Where are action potentials regenerated as they propagate along a myelinated axon?
at the nodes
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The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called __________.
saltatory propagation
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How do action potential propagation speeds in myelinated and unmyelinated axons compare?
Propagation is faster in myelinated axons
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Multiple sclerosis (MS) is a disease that stops action potential propagation by destroying the myelin around (normally) myelinated axons. Describe how MS stops action potential propagation?
Without myelin, the internode membrane resistance decreases, preventing local currents from reaching adjacent nodes.
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What type of nerve fiber possesses the fastest speed of impulse propagation?
type A
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The small space between the sending neuron and the receiving neuron is the
synaptic cleft.
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A molecule that carries information across a synaptic cleft is a
neurotransmitter
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When calcium ions enter the synaptic terminal,
they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.
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When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron,
ion channels in the plasma membrane of the receiving neuron open.
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If a signal from a sending neuron makes the receiving neuron more negative inside,
the receiving neuron is less likely to generate an action potential.
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In a synapse, neurotransmitters are stored in vesicles located in the __________.
presynaptic neuron
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An action potential releases neurotransmitter from a neuron by opening which of the following channels?
voltage-gated Ca2+ channels
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Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
chemically gated; postsynaptic
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Binding of the neurotransmitter to its receptor causes the membrane to __________.
either depolarize or hyperpolarize
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The mechanism by which the neurotransmitter is returned to a presynaptic neuron’s axon terminal is specific for each neurotransmitter. Which neurotransmitter is broken down by an enzyme before being returned?
acetylcholine
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