Nerve impulsesAQA A-Level Biology: Revision notes

A motor neurone carries impulses from the central nervous system to an effector. It has a cell body containing the nucleus, dendrons carrying impulses towards it, and a long axon carrying impulses away.

In a myelinated neurone the axon is wrapped by Schwann cells, which form the myelin sheath, a fatty electrical insulator. Gaps between neighbouring Schwann cells, where the axon membrane is exposed, are the nodes of Ranvier.

At rest the inside of an axon is negative compared with the outside, by about −70 mV: the resting potential.

It is established by:

• the sodium-potassium pump, which actively transports 3 Na⁺ out for every 2 K⁺ in, so there are fewer positive ions inside
• the membrane being more permeable to K⁺ than to Na⁺ at rest, so K⁺ diffuse out along their electrochemical gradient faster than Na⁺ diffuse in

The result is an electrochemical gradient, with the inside negative.

A stimulus changes the membrane permeability and the potential becomes less negative.

1. Depolarisation: if the threshold (about −55 mV) is reached, voltage-gated sodium ion channels open and Na⁺ diffuse in. The potential reverses to about +40 mV.
2. Repolarisation: sodium ion channels close and voltage-gated potassium ion channels open. K⁺ diffuse out, and the potential returns towards negative.
3. Hyperpolarisation: the potential briefly drops below the resting level, and then the pump restores the resting potential.

The all-or-nothing principle: below the threshold nothing happens; once it is reached a full-size action potential occurs, whatever the stimulus size. A stronger stimulus gives a higher frequency, not a bigger action potential.

Non-myelinated axon: depolarisation at one point causes sodium ions to spread sideways (local circuits), which depolarises the next region. The action potential is regenerated along the whole length of the axon.

Myelinated axon: the myelin sheath insulates the axon, so depolarisation occurs only at the nodes of Ranvier. Local circuits carry the depolarisation from node to node: saltatory conduction. This is much faster.

The refractory period is the time after an action potential during which the voltage-gated sodium ion channels are closed and cannot open, so no new action potential can be generated. It ensures that action potentials are discrete, that they travel in one direction only and that there is a maximum frequency of impulses, which is roughly 1 divided by the length of the refractory period.

• Myelination: saltatory conduction means fewer depolarisations along the axon, so conduction is faster.
• Axon diameter: a larger diameter has less resistance to ion flow and less leakage, so conduction is faster.
• Temperature: a higher temperature increases the rate of diffusion of ions and the opening of channels, so conduction is faster, up to the point where proteins denature.

Damage to the myelin sheath, for example in multiple sclerosis, slows conduction.