Action Potentials

Action Potential

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

Action Potential: Main Phases

Depolarizing & Repolarizing Phase

Action Potential: Main Phases: Depolarizing Phase

A stage in which the cell membrane becomes less negative and shifts toward a positive charge.

Action Potential: Main Phases: Repolarizing Phase

A stage in which the cell membrane returns to the resting membrane potential of -70 mV after an electrical spike

Action Potential: Main Phases: After-Hyperpolarizing Phase

There might a chance in which there is an undershoot when the membrane potential drops below its normal resting level

Voltage-Gated Channels: Types

Voltage-gated Na⁺ channels and voltage-gated K⁺ channels are the two types of voltage-gated channels that open and then close during a nerve impulse

Voltage-Gated Channels: Na⁺ Channels

Voltage-gated Na⁺ channels open first, allowing Na⁺ to rush into the neuron and cause depolarization, making the inside less negative.

Voltage-Gated Channels: K⁺ Channels: Function

Voltage-gated K⁺ channels open after the Na⁺ channels, allowing K⁺ to flow out of the neuron and cause repolarization, making the inside more negative again.

Voltage-Gated Channels: K⁺ Channels: After-Hyperpolarizing Phase

Triggers when voltage-gated K⁺ channels remain open after repolarization, allowing extra K⁺ to leave the cell and making the inside more negative than the resting membrane potential.

Threshold

The voltage level (usually around -55 mV) that a neuron's membrane potential must reach to trigger an action potential

Threshold: Neuron Differences

Different neurons may have different threshold values, but the threshold of a particular neuron is usually constant.

Threshold: Subthreshold

A weak stimulus that causes depolarization but does not bring the membrane potential to threshold, so no nerve impulse occurs.

Threshold: Threshold Stimulus

A stimulus that is just strong enough to depolarize the membrane to threshold and produce a nerve impulse.

Threshold: Suprathreshold

A stimulus strong enough to depolarize the membrane above threshold and produce multiple nerve impulses.

Nerve Impulse: Amplitude

The amplitude (size) of a nerve impulse is always the same once it is generated and does not depend on stimulus strength.

Nerve Impulse: All-or-None Principle

A nerve impulse either occurs completely when threshold is reached or does not occur when threshold is not reached; a stronger stimulus does not create a larger nerve impulse.

Stimulus Strength: Frequency Relationship

The greater the stimulus strength above threshold, the greater the frequency of nerve impulses, up to a maximum frequency.

Example Pathway: Resting Membrane Potential

The action potential has yet to occur, resting at -70 mV, with Na⁺ concentrated outside of the cell and K⁺ concentrated inside of the cell— forming a concentration gradient

Example Pathway: Depolarizing Phase

When an action potential reaches the threshold at -55 mV, voltage-gated Na+ channels open, allowing a rapid influx of Na+ into the cell, changing the membrane potential from −55 mV to +30 mV due to the increase in positive ions.

• The concentration gradient will tend to equalize the inequality, creating equal amounts of Na+ inside and outside the cell.

Example Pathway: Repolarizing Phase

The voltage-gated Na+ channels will shortly close, allowing the voltage-gated K+ channels to open and a rapid efflux of K+ from the cell, changing the membrane potential from +30 mV to -70 mV due to the decrease in positive ions.

• The concentration gradient will tend to equalize the inequality, creating equal amounts of K+ inside and outside the cell.

Example Pathway: After-Hyperpolarizing Phase

In some cases, the membrane potential will undershoot when extra K+ leaves the cell, becoming even more negative (about -90 mV); voltage-gated K+ channels will close to restore the resting membrane potential to -70 mV.

Example Pathway: Refractory Period

Because there are no concentration gradient among the Na+ and K+, the sodium-potassium pump will pull 3 Na+ ions out of the cell and 2 K+ ions at the cost of 1 ATP to reestablish concentration gradient to repeat the cycle.

Refractory Period

The period after a nerve impulse begins when an excitable cell cannot generate another nerve impulse in response to a normal threshold stimulus to ensure a nerve impulse can occur

Absolute Refractory Period

The period during which a second nerve impulse cannot be generated, even with a very strong stimulus.

Relative Refractory Period

The period during which a second nerve impulse can occur, but only in response to a stronger-than-normal stimulus.