Generation of the Action Potential, Propagation and Velocity of the Action Potential,
The depolarization phase of an action potential results from the opening of which channels?
The repolarization phase of an action potential results from __________.
How is an action potential propagated along an axon?
Why does the action potential only move away from the cell body?
The velocity of the action potential is fastest in which of the following axons?
The membranes of neurons at rest are very permeable to _____ but only slightly permeable to _____.
During depolarization, which gradient(s) move(s) Na+ into the cell?
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?
The concentrations of which two ions are highest outside the cell.
During the action potential of a neuron, which ion is primarily crossing the membrane during the depolarization phase, and in which direction is the ion moving?
Na+ is entering the cell.
(During the depolarization phase of the action potential, open Na+ channels allow Na+ ions to diffuse into the cell. This inward movement of positive charge makes the membrane potential more positive (less negative). The depolarization phase is a positive feedback cycle where open Na+ channels cause depolarization, which in turn causes more voltage-gated Na+ channels to open.)
Na+ channels are inactivating, and K+ channels are opening.
(As voltage-gated Na+ channels begin to inactivate, the membrane potential stops becoming more positive This marks the end of the depolarization phase of the action potential. Then, as voltage-gated K+ channels open, K+ ions rush out of the neuron, following their electrochemical gradient. This exit of positively-charged ions causes the interior of the cell to become more negative, repolarizing the membrane.)
During what part of the action potential do voltage-gated Na+ channels begin to inactivate (their inactivation gates close)?
at the end of the depolarization phase, as the membrane potential approaches its peak value
(Voltage-gated Na+ channels are opened by depolarization and then quickly inactivated. Once inactivated, these channels cannot pass Na+ ions. At the peak of the action potential, a large number of Na+ channels are open, but they are rapidly inactivating. As the action potential enters the repolarization phase, the number of open Na+ channels continues to decrease because more and more inactivation gates close. The number of inactivated Na+ channels is greatest towards the end of the repolarization phase.)
The repolarization phase of the action potential, where voltage becomes more negative after the +30mV peak, is caused primarily by __________.
K+ ions leaving the cell through voltage-gated channels
(The opening of voltage-gated K+ channels allows K+ ions to exit the cell, repolarizing the membrane. In other words, the exit of K+ ions makes the membrane potential more negative. K+ also exits through leakage channels during this phase because leakage channels are always active. However, most of the membrane permeability to K+ during this phase is due to voltage-gated channels. Voltage-gated K+ channels make the action potential more brief than it would otherwise be if only leakage channels were available to repolarize the membrane.)
During an action potential, hyperpolarization beyond (more negative to) the resting membrane potential is primarily due to __________.
K+ ions diffusing through voltage-gated channels
(The large number of voltage-gated K+ channels opening during the repolarization phase quickly makes the membrane potential more negative as positively-charged K+ ions leave the cell. K+ ions continue to leave through open channels as the membrane potential passes (becomes more negative than) the resting potential. This hyperpolarization phase of the action potential is therefore due to K+ ions diffusing through voltage-gated K+ channels. The membrane potential remains more negative than the resting potential until voltage-gated K+ channels close. This period of hyperpolarization is important in relieving voltage-gated Na+ channels from inactivation, readying them for another action potential.)
During the hyperpolarization phase of the action potential, when the membrane potential is more negative than the resting membrane potential, what happens to voltage-gated ion channels?
K+ channels close. Na+ channels go from an inactivated state to a closed state.
(Voltage-gated K+ channels are opened by depolarization. This means that as the membrane potential repolarizes and then hyperpolarizes, these K+ channels close. With the closing of voltage-gated K+ channels, the membrane potential returns to the resting membrane potential via leakage channel activity. Resetting voltage-gated Na+ channels to the closed (but not inactivated) state prepares them for the next action potential.)
Tetraethylammonium (TEA) blocks voltage-gated K+ channels such that K+ cannot pass even when the channels are open. However, TEA leaves K+ leakage channels largely unaffected. How would you expect the action potential to change if you treated a neuron with TEA?
The action potential would depolarize as usual, but the repolarization phase would take longer, causing the action potential to be more broad in time.
(The passage of K+ ions through open voltage-gated K+ channels is an important component of the repolarization phase of the action potential. However, repolarization would still occur (albeit more slowly) in the presence of TEA. Once voltage-gated Na+ channels open during the depolarization phase, those same Na+ channels quickly inactivate. In other words, open Na+ channels inevitably inactivate regardless of whether K+ channels open or not. This means that the depolarization phase of the action potential stops on its own. Once Na+ channels inactivate, the membrane potential is set by other open channels. If voltage-gated K+ channels are blocked by TEA, then the membrane will be (slowly) set, by leakage channels, to the resting membrane potential.)
The diffusion of what ion, across the neuronal membrane, is responsible for the local currents that depolarize regions of the axon to threshold?
An action potential in one segment of axon causes adjacent sections of axon membrane to reach threshold through what mechanism?
During action potential propagation in an unmyelinated axon, why doesn't the action potential suddenly "double back" and start propagating in the opposite direction?
In a myelinated axon, how do the nodes of Ranvier differ from other segments of the same axon?
Where are action potentials regenerated as they propagate along a myelinated axon?
How do action potential propagation speeds compare in myelinated and unmyelinated axons?
The node-to-node "jumping" regeneration of an action potential along a myelinated axon is called __________.
The myelin on myelinated neurons can be degraded or destroyed in diseases such as multiple sclerosis-a process called demyelination. If a myelinated neuron was affected by demyelination, how would this affect action potentials in that neuron?
Which of the following best describes the Na+ and K+ concentrations across a neuron's plasma membrane?
What is the major role of the Na+-K+ pump in maintaining the resting membrane potential?
Which of the following is the clearest example of a neuronal membrane's selective permeability?
Suppose a drug is developed that blocks K+ leakage channels. The drug prevents ions from passing through those channels. If this drug was applied to a neuron, what would be the most immediate effect on that neuron?
Imagine you changed the concentration of K+ outside a neuron such that the resting membrane potential changed to -80 mV (from the normal resting value of -70 mV). What have you changed?
What is the electrochemical gradient of an ion?
Hypothetically, what would be the most immediate effect of doubling the number of Na+ leakage channels in the plasma membrane?
1. An action potential arrives at the synaptic terminal.
2. Calcium channels open, and calcium ions enter the synaptic terminal.
3. Vesicles containing neurotransmitter fuse with the plasma membrane of the sending neuron.
4. Neurotransmitter molecules diffuse across the synaptic cleft.
5. The neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron, causing ion channels there to open.
When calcium ions enter the synaptic terminal,
When neurotransmitter molecules bind to receptors in the plasma membrane of the receiving neuron,
If a signal from a sending neuron makes the receiving neuron more negative inside,
Arrange these parts in order, from left to right, of a successful direct depolarization path within one neuron.
Which of the following best characterizes depolarization?
When an action potential arrives at the end of the axon terminal, a series of events take place that result in the release of neurotransmitter from the presynaptic axon. Select the answer that correctly describes the primary stimulus for vesicles to move towards the cell membrane and eventually release their contents.
Which statement best describes exocytosis?
What conditions will increase the diffusion of molecules, such as neurotransmitters?
If the membrane of a postsynaptic dendrite is setting up a graded potential, what must have happened after neurotransmitter was released by the presynaptic terminal?
Drag and drop the descriptive labels of events into the correct sequence at the chemical synapse.
Which best represents synaptic transmission?
Predict the possible effect of a drug that totally blocks the neurotransmitter receptor on the postsynaptic membrane.
For example, curare is a neurotoxin used by several South American cultures. The primary effect of curare is that acetylcholine, a major neuromuscular neurotransmitter, cannot bind at its receptor because curare is blocking it. Predict the possible effects of curare on the postsynaptic membrane and muscle.
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?
Binding of a neurotransmitter to its receptors opens __________ channels on the __________ membrane.
Binding of the neurotransmitter to its receptor causes the membrane to __________.
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?
A postsynaptic cell can be a neuron, a muscle cell, or a secretory cell. What is an example of a presynaptic cell?
Which component has a role in the postsynaptic cell during synaptic activity?
What is the role of calcium in synaptic activity?
What is the direct role of neurotransmitter at a chemical synapse?
What type of channel on the postsynaptic membrane binds neurotransmitter?
In addition to diffusion, what are two other mechanisms that terminate neurotransmitter activity?
Events that occur during synaptic activity are listed here, but they are arranged in an incorrect order. Choose the correct order of these events below. (a) Voltage-gated calcium channels open (b) Neurotransmitter binds to receptors (c) Action potential arrives at axon terminal (d) Neurotransmitter is removed from the synaptic cleft (e) Neurotransmitter released into synaptic cleft (f) Graded potential generated in postsynaptic cell