Transmission of action potential

Action Potential

An action potential is an electrical signal that travels along an axon when a nerve impulse is transmitted.
It is the rapid and substantial change in the electrical potential across the membrane of a neurone cell, causing it to depolarise and then repolarise.
This depolarisation and repolarisation process is due to ions moving in and out of the neurone.

Stages of Action Potential

Resting potential: In its resting state, the inside of a neurone is negatively charged compared to the outside. This is maintained by sodium-potassium pumps and leak channels.
Depolarisation: When a stimulus reaches a resting neurone, the gated sodium channels in the cell membrane open. Sodium ions rush into the neurone, generating a positive charge inside the cell.
Threshold: If the potential change reaches a certain level known as the threshold, a full action potential will be initiated. Below this level, no action potential will occur.
Repolarisation: The sodium channels close, and potassium channels open. Potassium ions rush out of the neuron, returning the cell to its original negative charge.
Hyperpolarisation and Refractory period: After an impulse has passed, a neurone cannot immediately fire another impulse due to a refractory period where the cell is slightly more negative than at rest (hyperpolarized). This resets the neurone, preparing it for the next impulse.

Propagation of the Nerve Impulse

The action potential represents a single nerve impulse.
It moves along the axon as local currents between regions of differing charge stimulate the next section of the neurone to depolarise.
This ‘wave’ of depolarisation (and subsequent repolarisation) continues down the length of the axon.
This is called propagation of the nerve impulse, and is a rapid process.

Saltatory Conduction

In myelinated neurones, action potentials only occur where the axon is exposed at the nodes of Ranvier.
The current jumps from node to node in a process called saltatory conduction, which increases the speed of nerve impulse transmission.
This is why myelinated neurones transmit impulses faster than unmyelinated ones.

Role of Synapses

When an action potential reaches the end of a neurone (the synaptic terminal), it triggers the release of neurotransmitters into the synapse.
These chemicals diffuse across the synaptic gap and bind to receptors on the next neurone, generating a new action potential.
This is how nerve impulses are transmitted from one neurone to the next.