Generation of an Action Potential

Action Potential: 1. Resting Phase

-All voltage- gated channels are closed. -Axon plasma membrane is at resting membrane potential: -small buildup of negative charges along side the surface of the membrane and equal buildup of positive charges along outside surface of membrane.

What are two types of propagation?

1. continuous conduction. 2. saltatory conduction.

relative refractory period

The period immediately after an action potential when the sodium channels are in their original position, but the transmembrane potential has not yet stabilized at resting levels

suprathreshold stimulus

-Several action potentials will form in response to this because it is a stimulus that is strong enough to depolarize the membrane above threshold. -Each of the action potentials caused by a suprathreshold stimulus has the same (size) as an action potential caused by a threshold stimulus.

the threshold of an action potential

-The action potential that occurs in the membrane of the axon of a neuron when depolarization reaches about −55 mV in many neurons. -Different neurons may have different thresholds for generation of an action potential, but the threshold in a particular neuron usually is constant.

Action Potential: 2. Depolarizing phase.

-When membrane potential of axon reaches threshold, sodium channel activation gates open. -as sodium ions move through these channels into the neuron, a buildup of positive charges forms along inside surface of membrane. -membrane becomes depolarized. - -55 mV to +30mV

propagation

-a mode of conduction. (action potential) -depends on positive feedback. -In contrast to the graded potential, an action potential is not decremental (it does not die out). Instead, an action potential keeps its strength as it spreads along the membrane.

action potential (AP) or impulse

-a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.

Saltatory conduction

-action potential propagates much farther along the myelinated axon in the same period of time. -occurs along myelinated axons. -occurs because of the uneven distribution of voltage‐gated channels. -Few voltage‐gated channels are present in regions where a myelin sheath covers the axolemma. -at the nodes of Ranvier (where there is no myelin sheath), the axolemma has many voltage‐gated channels.

the all‐or‐none principle.

-an action potential is generated in response to a threshold stimulus but does not form when there is a subthreshold stimulus. -an action potential either occurs completely or it does not occur at all.

Continuous conduction

-involves step‐by‐step depolarization and repolarization of each adjacent segment of the plasma membrane. -ions flow through their voltage‐gated channels in each adjacent segment of the membrane. -Note that the action potential propagates only a relatively short distance in a few milliseconds. -Continuous conduction occurs in unmyelinated axons and in muscle fibers.

Action Potential: 3. Repolarizing phase.

-sodium channels close. -potassium channels open. -membrane starts to become repolarized as some potassium ions leave neuron and few negative charges begin to build up along inside surface of membrane. - +30 mV to −70 mV

Action Potential: 4. repolarizing phase continues.

-sodium outflow continues. -as more potassium ions leave the neuron, more negative charges build up along the inside surface of membrane. -potassium outflow eventually restores resting membrane potential. -sodium channel inactivation gates open. -return to resting state when potassium gates close.

The speed of propagation of an action potential is affected by three major factors:

1. Amount of myelination: action potentials propagate more rapidly along myelinated axons than along unmyelinated axons. 2. Axon diameter: Larger‐diameter axons propagate action potentials faster than smaller ones due to their larger surface areas. 3. Temperature: Axons propagate action potentials at lower speeds when cooled.

The flow of current across the membrane only at the nodes of Ranvier has two consequences:

1. Because an action potential leaps across long segments of the myelinated axolemma as current flows from one node to the next, it travels much faster than it would in an unmyelinated axon of the same diameter. 2. Because only small regions of the membrane depolarize and repolarize, minimal inflow of Na+ and outflow of K+ occurs each time an action potential passes by. Thus, less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

An action potential has two main phases:

1. a depolarizing phase. 2. a repolarizing phase.

What are the names of the channels that open in an action potential?

1. voltage‐gated Na+ channels, allow Na+ to rush into the cell, which causes the depolarizing phase. 2. voltage‐gated K+ channels open, allowing K+ to flow out, which produces the repolarizing phase. 3. The after‐hyperpolarizing phase occurs when the voltage‐gated K+ channels remain open after the repolarizing phase ends.

Will an action potential occur in response to a hyperpolarizing graded potential that spreads from the dendrites or cell body to the trigger zone of the axon of a neuron? Why or why not?

An action potential will not occur in response to a hyperpolarizing graded potential because a hyperpolarizing graded potential causes the membrane potential to become inside more negative and, therefore, farther away from threshold (−55 mV).

Which channels are open during the depolarizing phase? During the repolarizing phase?

Voltage‐gated Na+ channels are open during the depolarizing phase, and voltage‐gated K+ channels are open during the repolarizing phase.

After‐hyperpolarizing Phase

While the voltage‐gated K+ channels are open, outflow of K+ may be large enough to cause an after‐hyperpolarizing phase of the action potential. -the voltage‐gated K+ channels remain open and the membrane potential becomes even more negative (about −90 mV). -As the voltage‐gated K+ channels close, the membrane potential returns to the resting level of −70 mV.

subthreshold stimulus

a stimulus that is a weak depolarization that cannot bring the membrane potential to threshold.

threshold stimulus

a stimulus that is just strong enough to depolarize the membrane to threshold.

During the repolarizing phase

the membrane potential is restored to the resting state of −70 mV.

Following the repolarizing phase there may be an after‐hyperpolarizing phase

the membrane potential temporarily becomes more negative than the resting level.

During the depolarizing phase

the negative membrane potential becomes less negative, reaches zero, and then becomes positive.

An action potential will only occur once the membrane potential reaches___________________.

threshold