Table of Contents
Introduction
Action Potential:
In human physiology ,the action potential is the fundamental electrical signals through which excitable cells -neurons and muscle fibers communicate, contract and transmit rapid signals across the body.
An action potential is a rapid, brief sequence of changes in the membrane potential that occurs after excitation.
A stimulation that causes a rapid sequence of changes in the resting membrane potential of an excitable cell is known as an action potential.
Do you know types of excitable cells?
Yes, there are two types of excitable cellsà Muscle cells and Nerve cells.
RMP (Resting Membrane Potential) is between -70 and -100 millivolts.
1. The Resting Membrane Potential
When stimulated, the resting membrane potential changes due to a disturbance in ionic equilibrium across the receptor zone of the cell membrane.
Action potential generation occurs in Phases.
The Phases are:
The latent period is the time taken by the impulses to travel along the axon from the site of stimulation to the recording electrode. It is expressed in milliseconds.
The Resting Membrane Potential is between -70 and -100 millivolts. This is because the interior of the cell membrane is negative relative to the exterior. Potassium ion permeability is greater than that of sodium ions.
The size and duration of an action potential vary and depend on the nature of the tissue. For example, in nerves, it is no more than a few milliseconds, while in cardiac muscle, it is 200 to 300 milliseconds.
Outer positive and inner negative near the cell membrane.
Point of stimulus
The point of stimulus is the point or place on the cell membrane where the stimulus is applied.
When a stimulus is applied, it produces a brief, irregular baseline deflection due to current leakage from the stimulating electrode to the recording electrode. This is the point of stimulus. At the point of stimulation, a slight decrease in the RMP is due to an increase in K+ influx, which is restored by K+ efflux.
2. Latent Period
The latent period depends on-
- Speed of conduction of impulses in the axon-character of the axon. And
- Distance between the stimulus and recording electrode.
We can calculate the speed of conduction in a nerve by applying a stimulus and recording the time to response, and measuring the distance.
The stimulus artifact is followed by an iso potential interval –without any obvious change -which ends with the next potential change.
Depolarization
Depolarization is the first indication the approaching impulse. Depolarization is the change from a negative to a positive membrane potential.
At the point of stimulation, a slight decrease in the RMP is due to increased K+ influx, which is counteracted by K+ efflux.
When stimulation persists sodium leaky channel opens, sodium enters the cell, and the membrane potential rises up to +7 m V. But potassium efflux increases through the leaky channels and restore membrane potential. If another stimulus reaches, it will increase the membrane potential, say up to 12 m V. In this way, if it approaches +15 mV, it will open voltage-gated sodium channels. Sodium ions will enter rapidly and voltage will shoot up to +60 m V. This is depolarization.it is acute and sharp.
Repolarization
At this point, voltage-gated sodium channels close, blocking sodium ion entry; voltage-gated potassium ion channels open, allowing potassium efflux, restoring the membrane potential. The voltage-gated potassium ion channels are slow but more prolonged. This is also steep.
After Depolarization
When it approaches the baseline, K+ efflux through voltage-gated potassium ion channels slows for a few milliseconds before returning to baseline. There is a slight delay in returning to the baseline.
Hyperpolarization
RMP is achieved after afterdepolarization, but resting ionic composition is not restored. It is achieved by slow prolonged K+ efflux and Na+-K+ pump.
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