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Action potential

The nervous system communicates using action potentials.

Action potentials are electric impulses that are transmitted through the axon of a neuron.

Action potentials are generated when neurons receive an electrical stimulus.

Action potential generation follows an "all or nothing" principle: if the stimulus is large enough, an action potential will be produced.

This action potential is always of the same magnitude.

The size of the stimulus is reflected by the frequency of the action potential generation (the number of impulses generated per unit time) rather than the magnitude of the action potential.

All neurons maintain a resting potential or membrane potential.

In most neurons, the resting potential is around $$-70\text{mV}$$ (millivolts).

This resting potential is created by the difference in concentration of the ions $$\ce{Na^+}$$ and $$\ce{ K^+}$$ inside and outside the neuron.

There is a far higher concentration of sodium ions outside the cell compared to within it.

This is known as the electrochemical gradient. It results in a potential difference between the inside and the outside of the neuron.

The electrochemical gradient generates potential difference, which is the cause of action potentials.

The electrochemical gradient is created in two ways:

  • Facilitated diffusion: At the resting potential, the cell has many potassium channels open but only a few sodium channels open. This results in a far higher concentration of sodium ions outside the cell.
  • Active transport: The sodium-potassium pump maintains the resting potential. For every three sodium ions pumped out, two potassium ions are pumped in.

    ATP is required and the sodium-potassium pump alone constitutes $$2/3$$ of a neuron's energy expenditure. A greater number of positive ions leave the cell, resulting in a negative resting potential.

These two processes are required for the restoration and maintenance of the resting potential.

The action potential is generated by altering the membrane potential (voltage across the cell membrane). This change is due to the movement of ions across the membrane.

The cell membrane contains voltage-gated sodium and potassium channels that are closed at rest. These are channels through the membrane that are open or shut depending on the voltage across them.

When the neuron is stimulated, some of the voltage-gated sodium channels open and sodium ions flow in.

If the stimulus is large enough, the neuron threshold voltage (around $$-55\text{ mV}$$) will be met and an action potential will be triggered.

When the action potential is triggered, all the voltage gated sodium channels open, and sodium ions flood into the cell.

This is a positive feedback mechanism. The flow of sodium ions into the cell causes more channels to open, leading to the potential of the cell changing from -70 mV to +40 mV. The change in potential towards a more positive value is called depolarisation.

Once +40 mV is reached, the sodium channels close and the potassium channels open. Potassium ions flow out of the cell. This flow of positive ions repolarises the neuron. This is the falling phase.

The neuron becomes hyperpolarised (undershoots) and the potassium channels close. The potassium-sodium pump then returns the neuron to its resting potential.

Once the action potential is generated, it propagates (flows) down the axon.

The axon acts like a leaky cable. If the electrical impulses were generated only once, the signal would not reach the other end.

The action potential needs to be regenerated along the axon in order for the signal to reach the terminal.

This is done through positive feedback. A local current is created between an area where there is an action potential and its neighbouring area which is at rest.

The flow of some sodium ions into an area of the axon that is at rest causes voltage-gated sodium channels to open.

This generates an action potential and maintains the electrical impulse along the axon.

An action potential is followed by a refractory period. A refractory period is a time when no further action potential can be generated.

The refractory period ensures that the action potential only flows in one direction.

The electrical impulse has to flow into the next section of the axon. It cannot travel backwards as the section it has come from is in its refractory period.

The absolute refractory period is a time when no action potential can be generated.

The relative refractory period is a time when an action potential can be generated, but the stimulus needs to be stronger than usual.

Axons are myelinated in order to increase their conductivity.

The myelination insulates the axon. This reduces the number of times the action potential is regenerated along the axon by increasing its conductivity.

Fatty myelin sheaths surround the axon, insulating it. Myelin is produced by Schwann cells that grow around the axon.

Action potentials are only generated in the gaps in the myelin sheath. These gaps are called nodes of Ranvier. The action potential appears to 'jump' from one node to the next.

This 'jumping' of action potentials is called saltatory conduction.

In reality, the action potential does not jump. The myelin insulation simply allows sodium ions to flow down the axon further before they leak out. The next action potential is therefore generated further away at the next node.

A myelinated neuron.
A myelinated neuron.