The neuron

Neurones (nerve cells) make up the nervous system. Sensory neurones carry impulses from receptors to the central nervous system, motor neurones carry them out to effectors such as muscles and glands, and relay neurones connect the two inside the spinal cord and brain.

A neuron is highly specialised: it is one of the longest cells in the body, and its shape lets it receive a signal at one end and pass it on, sometimes over a metre, at the other. In an exam you are expected to label its parts, link each part to its job, and compare the three types.

Parts and functions

How structure suits function

The DSKP expects you to link each specialised feature to the job it performs, not just to name the parts.

• Long axon, a single long fibre lets one neuron carry an impulse a great distance, for example from the spinal cord to a muscle in the foot, without the signal being relayed cell to cell.
• Myelin sheath, the fatty insulation stops the impulse leaking sideways and forces it to jump between the nodes of Ranvier (saltatory conduction), which makes conduction much faster than in an unmyelinated fibre.
• Branched dendrites, the fine branches give a large surface area, so a single neuron can receive impulses arriving from other neurones at the same time.
• Axon terminals packed with mitochondria, the endings hold a high density of mitochondria that release the ATP needed to make and release neurotransmitter at the synapse.
• Cell body with a prominent nucleus, the nucleus directs the production of the proteins and enzymes that keep the long fibre working.

Related processes

Every part of a neuron takes part in a process you meet elsewhere in the syllabus. The axon is the site of nerve impulse transmission, where a wave of electrical change (the action potential) travels along the membrane.

At the axon terminal this electrical signal is converted into a chemical one during synaptic transmission: neurotransmitter diffuses across the synapse and starts an impulse in the next neuron.

In a reflex arc all three neuron types work in sequence, a sensory neuron carries the impulse from a receptor to the spinal cord, a relay neuron passes it across, and a motor neuron carries it to an effector. This produces a rapid, automatic response that protects the body before the brain is fully aware of the stimulus.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): Diagram Z shows a motor neuron. Structure R is a long fibre wrapped in a fatty layer that has regular gaps along it.

(a) Name the fatty layer and state one function. [2 marks] (b) Explain how the gaps in this layer affect the speed of the impulse. [2 marks] (c) A person whose fatty layer is damaged reacts more slowly to a pinprick. Explain why. [2 marks]

Model answer: (a) The fatty layer is the myelin sheath; it insulates the axon and speeds up the impulse. (b) The gaps are nodes of Ranvier; the impulse jumps from node to node, so it travels faster than in a fibre with no myelin.

(c) With the myelin sheath damaged, the impulse cannot jump between the nodes and travels more slowly along the axon, so it reaches the effector later and the response is delayed.

Marking note: in part (b) the mark for speed needs the idea of jumping between nodes, not just the phrase the impulse is faster.

Source:SRC-DSKP-EN

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