The cytoplasm that surrounds the nucleus of a neuron is called the
Axons terminate in a series of fine extensions known as
Neurons that are rare, small, and lack features that distinguish dendrites from axons are called
Deteriorating changes in the distal segment of an axon as a result of a break between it and the soma is called ________ degeneration.
The myelin sheath that covers many CNS axons is formed by
Damage to ependymal cells would most likely affect the
formation of cerebrospinal fluid
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)
Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
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.
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.
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
What prevents the Na+and K+gradients from dissipating?
The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
During depolarization, which gradients move Na+ into the cell?
both the electrical and chemical gradients
What is the value for the resting membrane potential for most neurons?
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.
The concentrations of which two ions are highest outside the cell.
Na+ and Cl–
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.
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.
Leak channels allow the movement of potassium and sodium ions by what type of membrane transport?
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
What is the electrochemical gradient of an ion?
the sum of the electrical and chemical gradients for that ion
In a typical neuron, what is the equilibrium potential for potassium?
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
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.
In a typical neuron, what is the equilibrium potential for sodium?
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.
If the axolemma becomes more permeable to potassium ion:
a stronger stimulus will be required to cause an action potential
Where do most action potentials originate?
What opens first in response to a threshold stimulus?
Voltage-gated Na+ channels
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value.
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.
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.
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
What type of conduction takes place in unmyelinated axons?
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
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.
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.
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.
In which type of axon will velocity of action potential conduction be the fastest?
Myelinated axons with the largest diameter
Where in the neuron is an action potential initially generated?
The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+channels
Hyperpolarization results from __________.
slow closing of voltage-gated K+channels
What is the magnitude (amplitude) of an action potential?
How is an action potential propagated along an axon?
An influx of sodium ions from the current action potential depolarizes the adjacent area.
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.
The velocity of the action potential is fastest in which of the following axons?
a small myelinated axon
In what part of the neuron does the action potential typically initiate?
initial segment of the axon
During an action potential of a neuron, what directly causes the different channels to open and close?
the transmembrane potential (voltage)
What is the typical duration of a nerve action potential?
Around what transmembrane potential does threshold commonly occur?
What ion is responsible for the depolarization of the neuron during an action potential?
What type of membrane transport causes the depolarization phase of the action potential in neurons?
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.
What ion causes repolarization of the neuron during an action potential?
What causes repolarization of the membrane potential during the action potential of a neuron?
potassium efflux (leaving the cell)
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
Action potential propagation begins (is first generated at) what region of a neuron?
Where are action potentials regenerated as they propagate along an unmyelinated axon?
at every segment of the axon
The movement of what ion is responsible for the local currents that depolarize other regions of the axon to threshold?
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.
Approximately how fast do action potentials propagate in unmyelinated axons in humans?
1 meter per second
In contrast to the internodes of a myelinated axon, the nodes __________.
have lower membrane resistance to ion movement
Where are action potentials regenerated as they propagate along a myelinated axon?
at the nodes
The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called __________.
How do action potential propagation speeds in myelinated and unmyelinated axons compare?
Propagation is faster in myelinated axons
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.
What type of nerve fiber possesses the fastest speed of impulse propagation?
The small space between the sending neuron and the receiving neuron is the
A molecule that carries information across a synaptic cleft is a
When calcium ions enter the synaptic terminal,
they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.
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.
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.
In a synapse, neurotransmitters are stored in vesicles located in the __________.
An action potential releases neurotransmitter from a neuron by opening which of the following channels?
voltage-gated Ca2+ channels
Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
chemically gated; postsynaptic
Binding of the neurotransmitter to its receptor causes the membrane to __________.
either depolarize or hyperpolarize
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?