Action Potential

Definition:

It is an electrochemical change occurring in the excitable tissue in response to the threshold stimulus.

It has phases of depolarization and repolarization.

Ionic Basis of Action Potential

Stimulation of the motor nerve with a stimulus increases permeability of the membrane to the sodium ions

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Sodium ions enter inside the cell. The inside of the cell move toward the positive side

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At around -55 mV the permeability of the membrane increases to sodium ions significantly resulting in it inward movement of sodium ions in a large extent.

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Sudden influx of sodium makes the inside of the cell relatively more positive which is reversal of polarity. This phase is known as Depolarization

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On reaching a maximum potential of +35 mV the sodium channels are inactivated and the potassium channels open.

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The potassium ions move out of the cell. There is restoration of potential. This phase is known as Repolarization

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The potassium ions continue to move out of the cell in spite of reaching the resting level. The inside of the cell becomes relatively more negative. This phase is known as Hyperpolarization

Note:

• Subminimal stimulus leads to slight decrease in RMP potential causing increased potassium efflux and chloride influx restoring RMP
• During depolarization, difference across the membrane becomes zero which is known as isoelectric potential
• Maximum potential or spike potential is at +35mV
• Hyperpolarization reduces excitability of the cell

Types of action potential

1) Monophasic action potential:

The action potential is recorded by placing one electrode within and the other electrode outside the cell. The recording is in one direction.

2) Biphasic action potential:

This is recorded when both the recording electrodes are placed over the surface of the cell Thus, a biphasic action potential shows an upward deflection followed by an isoelectric line and then a downward deflection. The duration of the isoelectric potential depends on the speed of conduction.

3) Compoun action potential:

It is the action potential recorded from a group of nerve fibers

The mixed nerve is made up of groups of fibers with different conduction velocities.

When all the fibers are stimulated simultaneously, impulses reach the recording electrode at different times.

The recording shows multiple peaks. The number and size of peaks vary with the type of fiber stimulated in the nerve.

Graded Potential

When there is a sudden change in the environment around the excitable tissue, there is alteration in the permeability of the membrane to different ions.

This changes the membrane potential over a small area of the membrane. The response dies down exponentially at the site of stimulation. The magnitude of response depends on the intensity of stimulation. This type of local potential without the period of latency is termed the graded potential.

The graded potentials in different regions are named the receptor potential, pacemaker potential, and synaptic potential.

The graded potential and action potential are the important ways by which the nerve cells process and transmit the information.

The graded potentials transmit the signals for a short distance whereas the action potentials transmit them over long distances.

 Differences Between Graded and Action Potential