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How are action potentials propagated along a neuron?

Electical impulses usually arrive at the dendrites of a neuron. These then pass through the cell body and a propagated down the longest part of the neuron, the axon.

Resting potential

This is the potential difference across the membrane of the axon when there is no action potnential being carried. It is established by sodium-potassium pumps and membrane channels. The pumps pump out sodium from the cell and pump potassium into it. This, in iteslef, does not change membrane potential as an equal number of ions are pumped in an out, and sodium and potassium ions have equal yet oppostie charges. However, there are also membrane channels which allow sodium or potassium to move passively across the membrane. At resting potential, these are usually closed but the potassium channel is slightly leaky. This means that some of the potassium ions inside the cell are able to move down the concentration gradient to outside the cell. This establishes a negative charge inside the cell, of about -70mV. This is the resting potential.

Action potential

The sodium and potassium channels in the membrane are voltage-gated. Therefore, when an action potential arrives from further up the axon, it stimulates the sodium channels, which open. This allows the sodium ions to move from outside to inside the cell. This causes a relatively higher charge inside the cell compared to outside, and thus an action potential of around +30mV. The cell is now depolarised.

The potassium pumps are slightly slower to respond, but are eventually activated by this voltage. They open, allowing potassium ions to flow out of the cell. This causes the membrane potential to fall to just below the original resting potential, as the potassium ions flow down the concentration gradient outside the cell. This is known as hyperpolarisation.

The resting potential is eventually re-established by the sodium-potassium pumps.

This sequence is repeated by adjacent channels further down the axon and in this way the action potnential is propagated.

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