Action potential can be defined as a sequence of changes in the membrane potential of an excitable cell due to the opening and closure of different ion channels after the application of a threshold stimulus.

Phases

Resting Membrane Potential Stage

Here, the inside of the nerve is negative, and the outside of the nerve is positive. RMP = –70 mV. The membrane is maintained in a polarized state by:

• Due to the distribution of ions across the cell membrane. Some K⁺ ions diffuse out of the cell along their concentration gradient, while non-diffusable anions (e.g., proteins) stay in the cell.
• Na⁺ influx does not compensate for K⁺ efflux, because the membrane at rest is less permeable to Na⁺ than K⁺.
• Na⁺-K⁺ pump causes continuous pumping of 3Na⁺ to the outside for each 2K⁺ ion pumped inside the membrane. This creates an additional degree of negativity.

Stimulus Artifact

When a stimulus is applied, there is a brief irregular deflection of the baseline due to a current leakage from the stimulating electrode to the recording electrode.

Latent Period

It corresponds to the time it takes to travel along the axon from the site of stimulation to the recording electrode.

Depolarization

When the membrane potential reaches a voltage between –70 mV and –50 mV, a sudden conformational change occurs in the activation gate of sodium channels, flipping it into an open state (voltage-gated sodium channels have two separate gates—activation and inactivation gate).

This is called the activated state, and sodium ions move in. The tracing rises rapidly till +35 mV. Now, the inside of the nerve becomes positive, and the outside of the nerve becomes negative.

Repolarization

After the sodium channel has remained open for 10,000 of a second, it suddenly closes. At this point, the membrane potential begins to recover back to RMP.

• Repolarization starts with K⁺ efflux due to the opening of voltage-gated K⁺ channels.
• A very important characteristic of the sodium channel inactivation process is that the inactivated gate will not reopen until the membrane potential returns to RMP.

After Depolarization

At the termination of spike potential, K⁺ conduction is slowed down, and thus a few milliseconds are delayed in restoring the membrane potential.

This last phase of slow K⁺ efflux is called after depolarization.

After Hyperpolarization

Resting membrane potential is achieved by the active Na⁺–K⁺ pump mechanism, which transports 3Na⁺ out and 2K⁺ in.

Properties of Action Potential

Threshold Stimulus

It is the minimum intensity of stimulating current that, after acting for a given duration, will just produce an action potential.

Note: The relationship between the strength and duration of a stimulus has been studied by varying the duration of the stimulus and finding out the threshold strength for each duration.

Following information can be gathered from the strength-duration curve:

Rheobase and Chronaxie

• The weakest current strength which can excite a tissue, if allowed to flow through it for an adequate time, is called rheobase.
• The time for which it is applied is called utilization time.
• The length of time for which a current of twice rheobase intensity must be applied to produce a response is called chronaxie.

All or None Response

The action potential is an all or none response to stimuli, i.e., if the stimulus is subthreshold (not adequate), no action potential is produced. Once threshold is reached, a full-fledged action potential is produced.

Further increase in the intensity of a stimulus produces no increment in the height of the action potential, provided the other experimental conditions remain the same.

Accommodation

If an excitable cell is applied with a constant strength of current, the site of the membrane under stimulation fails to produce an AP, i.e., the membrane adapts to the applied stimulus. This is due to slower opening and delayed closing of the voltage-gated K⁺ channels.

Conductivity

The action potential developed at one location on the excitable cell acts as a stimulus for the production of an AP in the adjacent region of the membrane.

The excitation impulse is conducted along the cell membrane as a wave of depolarization.

Refractory Period

If two successive stimuli of more than threshold intensity are applied to an excitable cell, it is found that for some time after the first stimulus, the cell becomes refractory. There are two types of refractory periods:

a. Absolute Refractory Period (ARP)

• Begins from the time the firing level is reached until the repolarization is approximately one-third complete.
• During this period, no strong stimulus can initiate a fresh impulse.

b. Relative Refractory Period (RRP)

• Begins at the end of ARP to the start of the after depolarization.
• During this period, stimuli stronger than normal can cause excitation.

Saltatory Conduction

Depolarization in myelinated axons jumps from one node of Ranvier to the next. This jumping of depolarization from node to node is called saltatory conduction.

• It is a rapid process.
• Myelinated axons conduct up to 50 times faster than the fastest unmyelinated fibers.

Monophasic Action Potential

• In this type, one microelectrode is placed inside the nerve fiber, and the other electrode is placed on the outer surface.
• These electrodes are then connected to the cathode ray oscilloscope (CRO).
• Monophasic record has the following components:
  • Depolarization
  • Repolarization

Biphasic Action Potential

One negative deflection below the baseline

Here, both electrodes are placed on the outer surface of the nerve fibers, and they are connected to a CRO.

When the nerve is stimulated, a record of alternate deflections is observed:

One positive deflection above the baseline

Compound Action Potential

Compound action potential is the monophasic recording of an action potential from a mixed nerve, which contains different types of nerve fibers with varying diameters.

Therefore, the compound action potential represents an algebraic summation of the all-or-none action potentials of many axons.

Causes of Compound Action Potential

A mixed nerve is made up of families of fibers with varying speeds of conduction.

1. Different Conduction Speeds

• When all the fibers are stimulated, the activity in fast-conducting fibers arrives at the recording electrodes sooner than the activity in slower fibers.

2. Distance from the Stimulating Electrodes

• The farther away from the stimulating electrodes the action potential is recorded, the greater the separation between the fast and slow fiber peaks.

3. Contribution of Different Fiber Groups

Group ‘C’ fibers contribute the least.

Group ‘A’ fibers contribute maximum to the compound action potential.