Generation of an Action Potential
What is the magnitude (amplitude) of an action potential?
100 mV
The resting membrane potential is maintained by Na+-K+ pumps that actively transport into and+ out of the cell.
K+Na
What characterizes depolarization, the first phase of the action potential?
The membrane potential changes from a negative value to a positive value. -70 to+30mV
In a synapse, neurotransmitters are stored in vesicles located in the __________.
presynaptic neuron
When calcium ions enter the synaptic terminal,
they cause vesicles containing neurotransmitter molecules to fuse to the plasma membrane of the sending neuron.
Which of the following membrane regions would have significant numbers of voltage-gated Na+ and K+ ion channels?
C & D
The membrane is more permeable to
K+
Sodium and potassium ions can diffuse across the plasma membranes of all cells because of the presence of what type of channel?
Leak channels for Na+ and K+ are ubiquitous, and they allow for the diffusion of these ions across plasma membranes.
Part complete The concentrations of which two ions are highest outside the cell?
Na+ and Cl−
What prevents the Na+ and K+ gradients from dissipating?
Na+/K+ ATPase
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.
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)
What opens first in response to a threshold stimulus?
The activation gates of voltage-gated Na+ channels open, and Na+ diffuses into the cytoplasm.
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.
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.
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. This is the minimum value required to open enough voltage-gated Na+ channels so that depolarization is irreversible.
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
Which of the following is expected to occur first if the membrane potential decrease shown in the graph were to reach the threshold value indicated at -55 mV?
The threshold value is the point at which voltage sensitive Na+ channels open. This leads to the characteristic rapid depolarization phase of the action potential.
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.
What is the basic condition when a neuron is described as polarized?
There is a separation of positive and negative charges across a membrane. Correct Polarization describes a condition where there is some sorting into poles. In a neuron, the inside of the cell is slightly negative compared to the positive charge (provided primarily by Na+) outside of the cell.
What is the first change to occur in response to a threshold stimulus?
Voltage-gated Na+ channels change shape, and their activation gates open.
The velocity of the action potential is fastest in which of the following axons?
a small myelinated axon the myelination acts as insulation and the action potential is generated only at the nodes of Ranvier. Propagation along myelinated axons is known as saltatory conduction.
The mechanism by which the neurotransmitter is returned to a presynaptic neuron's axon terminal is specific for each neurotransmitter. Which of the following neurotransmitters is broken down by an enzyme before being returned?
acetylcholine
Where in the neuron is an action potential initially generated?
axon hillock
Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
chemically gated; postsynaptic
What major ion currents occur at the point along the action potential phase labeled D?
diffusion of K+ out of the axon through gated ion channels
Binding of the neurotransmitter to its receptor causes the membrane to __________.
either depolarize or hyperpolarize
Where do most action potentials originate?
initial segment; axon hillock of cell body
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.
A molecule that carries information across a synaptic cleft is a
neurotransmitter
The electrical gradient for Na+ would tend to move Na+ __________ while the chemical gradient for Na+ would tend to move Na+ __________.
out; in
Hyperpolarization results from __________.
slow closing of voltage-gated K+ channels
The small space between the sending neuron and the receiving neuron is the
synaptic cleft
How is an action potential propagated along an axon?
the influx of sodium ions depolarizes adjacent areas, causing the membrane to reach threshold and cause an action potential. Thus, the action potential is regenerated at each new area.
The repolarization phase of an action potential results from __________.
the opening of voltage-gated K+ channels
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.
An action potential releases neurotransmitter from a neuron by opening which of the following channels?
voltage-gated Ca2+ channels
The depolarization phase of an action potential results from the opening of which channels?
voltage-gated Na+ channels
