The structure of neurones, the resting and action potentials, conduction of nerve impulses, and synaptic transmission.

What this dot point is asking

WJEC wants you to describe the structure of neurones, explain the resting potential and how an action potential is generated and conducted, and describe transmission across a synapse.

Neurones and the resting potential

A neurone has dendrites that receive signals, a cell body, and a long axon that carries the impulse. Many axons are wrapped in a fatty myelin sheath made by Schwann cells, with gaps called nodes of Ranvier.

The action potential

The action potential is all-or-nothing: it only fires if the threshold (around $-55 \text{ mV}$) is reached, and it is always the same size. A stronger stimulus increases the frequency of impulses, not their size. The impulse is propagated as each depolarised patch triggers the next. In myelinated neurones the action potential jumps between nodes of Ranvier in saltatory conduction, which is much faster. The refractory period (when sodium channels are recovering) ensures impulses travel one way and limits their frequency.

Synaptic transmission

At a synapse, the arriving action potential opens voltage-gated calcium channels; calcium entering triggers vesicles of neurotransmitter (for example acetylcholine) to fuse with the presynaptic membrane and release their contents by exocytosis. The neurotransmitter diffuses across the synaptic cleft and binds receptors on the postsynaptic membrane, opening sodium channels and starting a new action potential if threshold is reached. The transmitter is then broken down (by acetylcholinesterase) and recycled, so the response is brief and controlled.

Examples in context

Example 1. Multiple sclerosis. In MS the immune system destroys the myelin sheath around neurones in the central nervous system. Without myelin, saltatory conduction fails and impulses slow or stop, causing muscle weakness and numbness. This shows directly why myelin matters for fast conduction.

Example 2. Nerve agents and acetylcholinesterase. Organophosphate nerve agents inhibit acetylcholinesterase, so acetylcholine is not broken down and keeps stimulating the postsynaptic membrane. The result is continuous, uncontrolled firing of muscles, a clinical illustration of why transmitter breakdown is essential for normal synaptic control.

Try this

Q1. State the approximate value of the resting potential of a neurone. [1 mark]

• Cue. About $-70 \text{ mV}$ (inside negative).

Q2. Explain why conduction is faster in a myelinated than an unmyelinated neurone. [2 marks]

• Cue. The action potential jumps between nodes of Ranvier (saltatory conduction) rather than depolarising the whole membrane in sequence.

Q3. An impulse covers $1.2 \text{ m}$ in $0.02 \text{ s}$. Calculate its conduction speed. [2 marks]

• Cue. $\frac{1.2}{0.02} = 60 \text{ m s}^{-1}$.