5.1.3 Neuronal communication: the structure of a neurone; the establishment of the resting potential by the sodium-potassium pump; the generation of an action potential by voltage-gated channels (depolarisation and repolarisation); the all-or-nothing principle and the refractory period; saltatory conduction in myelinated neurones; and synaptic transmission by acetylcholine at a cholinergic synapse.

What this dot point is asking

OCR wants you to describe the structure of a neurone, explain the resting potential, explain the generation of an action potential through voltage-gated channels (depolarisation and repolarisation), explain the all-or-nothing principle and the refractory period, explain saltatory conduction, and describe synaptic transmission at a cholinergic synapse.

The answer

Neurone structure

A neurone carries electrical impulses. A motor neurone has a cell body with dendrites (receive impulses), a long axon (carries the impulse), and an axon terminal. Many neurones are wrapped in a myelin sheath of Schwann cells, with gaps called nodes of Ranvier; myelin acts as an electrical insulator.

The resting potential

At rest the inside of the axon is negative relative to the outside (about $-70 \text{ mV}$), the resting potential. It is maintained by:

• the sodium-potassium pump, which actively pumps 3 sodium ions out for every 2 potassium ions in (using ATP);
• the membrane being more permeable to potassium than to sodium (potassium leaks back out).

The action potential

A stimulus changes the membrane potential. If it reaches the threshold (about $-55 \text{ mV}$), an action potential fires:

1. Depolarisation. Voltage-gated sodium channels open and sodium ions diffuse in, making the inside positive (about $+40 \text{ mV}$).
2. Repolarisation. The sodium channels close and voltage-gated potassium channels open; potassium ions diffuse out, returning the inside to negative.
3. Hyperpolarisation. The potential briefly overshoots below the resting value before the sodium-potassium pump restores the resting potential.

The impulse propagates because depolarisation at one point triggers the channels at the next point (local currents).

All-or-nothing and the refractory period

• The all-or-nothing principle: an action potential only fires if the stimulus reaches threshold, and it is always the same size regardless of how strong the stimulus is. A stronger stimulus produces a higher frequency of impulses, not bigger ones.
• The refractory period is the short time after an action potential when the sodium channels are recovering and another impulse cannot fire. It ensures impulses are discrete (do not merge), travel in one direction, and limits the frequency.

Saltatory conduction

In a myelinated neurone, the myelin insulates the axon so depolarisation can only happen at the nodes of Ranvier. The action potential therefore jumps from node to node (saltatory conduction), which is much faster than the continuous conduction in an unmyelinated axon.

Synaptic transmission

At a cholinergic synapse (using acetylcholine):

1. The action potential reaches the presynaptic membrane and opens voltage-gated calcium channels; calcium ions diffuse in.
2. Vesicles of acetylcholine fuse with the presynaptic membrane and release it into the synaptic cleft by exocytosis.
3. Acetylcholine diffuses across and binds to receptors on the postsynaptic membrane, opening sodium channels so the postsynaptic membrane depolarises; if threshold is reached a new action potential fires.
4. Acetylcholinesterase breaks acetylcholine down so the response stops, and the products are reabsorbed and recycled.

Synapses ensure one-way transmission (only the presynaptic neurone has vesicles), and allow summation (adding stimuli) and integration of signals.

Examples in context

Example 1. Multiple sclerosis. In MS the myelin sheath is damaged, so saltatory conduction is disrupted and impulses travel slowly or fail, causing muscle weakness and numbness, a clear demonstration of myelin's role.

Example 2. Nerve agents and insecticides. Organophosphates inhibit acetylcholinesterase, so acetylcholine is not broken down and the postsynaptic neurone is continuously stimulated, causing uncontrolled muscle contraction, which shows why the enzyme is essential.

Try this

Q1. Explain how the sodium-potassium pump helps maintain the resting potential. [2 marks]

• Cue. It actively pumps 3 sodium ions out for every 2 potassium ions in (using ATP), so the inside becomes negative relative to the outside.

Q2. Explain why saltatory conduction is faster than conduction in an unmyelinated axon. [2 marks]

• Cue. Myelin insulates the axon, so depolarisation only occurs at the nodes of Ranvier and the impulse jumps from node to node rather than depolarising the whole membrane.

Q3. Name the enzyme that breaks down acetylcholine in the synaptic cleft. [1 mark]

• Cue. Acetylcholinesterase.