The nervous system: the structure of neurones; the resting and action potentials; the propagation of the nerve impulse; saltatory conduction; synaptic transmission; and the reflex arc.

What this dot point is asking

Eduqas wants you to describe neurone structure, explain the resting and action potentials, explain how the impulse is propagated (including saltatory conduction), explain synaptic transmission, and describe the reflex arc. This completes Component 3's core content.

Neurone structure

A motor neurone has a cell body, dendrites (receiving signals), a long axon (carrying the impulse), and an axon often wrapped in a myelin sheath (made by Schwann cells) with gaps called nodes of Ranvier. Sensory neurones carry impulses from receptors to the central nervous system; relay (intermediate) neurones connect them.

The resting and action potentials

Propagation and saltatory conduction

The action potential is propagated along the axon: the local depolarisation causes sodium ions to flow sideways, depolarising the next section to threshold, and so on, so the impulse moves in one direction (the refractory period behind it prevents it going backwards). In a myelinated neurone, the myelin insulates the membrane, so the action potential jumps from one node of Ranvier to the next (saltatory conduction), which is much faster than continuous conduction in an unmyelinated axon.

Synaptic transmission

At a synapse, neurones do not touch; the impulse is carried by a chemical neurotransmitter across the synaptic cleft:

1. The action potential depolarises the presynaptic membrane, opening voltage-gated calcium channels so calcium ions enter.
2. Calcium causes synaptic vesicles to fuse with the membrane and release neurotransmitter (for example acetylcholine) by exocytosis.
3. The neurotransmitter diffuses across the cleft and binds receptors on the postsynaptic membrane, opening sodium channels so the postsynaptic neurone depolarises (an action potential if threshold is reached).
4. Acetylcholinesterase breaks down the acetylcholine so the response stops, and the products are recycled.

Synapses ensure one-way transmission (receptors are only on the postsynaptic side) and allow summation (adding up of signals).

The reflex arc

A reflex arc gives a rapid, automatic, protective response without conscious thought: receptor, sensory neurone, relay neurone (in the spinal cord), motor neurone, effector. The brain is informed but the response does not wait for it, which is why reflexes are fast.

Examples in context

Example 1. Multiple sclerosis. In MS the myelin sheath is damaged, so saltatory conduction is lost and impulses travel slowly or fail, directly linking the role of myelin to a disease.

Example 2. Some pesticides and nerve gases. They inhibit acetylcholinesterase, so acetylcholine is not broken down and the postsynaptic neurone is overstimulated, applying synaptic transmission to toxicology.

Try this

Q1. State the approximate value of the resting potential and what maintains it. [2 marks]

• Cue. About minus 70 millivolts; maintained by the sodium-potassium pump and the membrane being more permeable to potassium.

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

• Cue. Myelin insulates the axon, so the action potential jumps between nodes of Ranvier (saltatory conduction), which is faster than continuous conduction.

Q3. State the role of calcium ions in synaptic transmission. [1 mark]

• Cue. They enter the presynaptic neurone and trigger the synaptic vesicles to fuse and release neurotransmitter.