Nerve impulses (A-level only): The structure of a myelinated motor neurone.

The idea

A motor neurone carries impulses from the central nervous system to an effector. Its cell body, containing the nucleus and much rough endoplasmic reticulum for making proteins such as neurotransmitter enzymes, lies in the spinal cord, and many short dendrites carry impulses towards it. One long axon carries impulses away, ending in synaptic knobs at the effector. Schwann cells wrap repeatedly around the axon, and the many layers of their plasma membrane form the myelin sheath, which is rich in lipid and acts as an electrical insulator. Between adjacent Schwann cells are short gaps, the nodes of Ranvier, where the axon membrane is exposed to tissue fluid and voltage-gated sodium ion channels are concentrated. Depolarisation can occur only at these nodes, so the action potential jumps from node to node by saltatory conduction, which is faster than conduction along an unmyelinated axon.

What earns the marks

• Identify the axon, or its membrane, as the process carrying impulses away from the cell body towards the effector.
• Identify the Schwann cell, or the myelin sheath formed from its layered plasma membranes, wrapped around the axon.
• Name the nodes of Ranvier as gaps in the myelin sheath where the axon membrane is exposed to tissue fluid.
• State that the cell body contains the nucleus and lies in the central nervous system, with dendrites carrying impulses towards it.
• Link the myelin sheath to electrical insulation of the axon membrane, restricting depolarisation to the nodes.
• Explain that depolarisation occurring only at nodes allows saltatory conduction, so the impulse jumps from node to node and travels faster.

Common mistakes and how to put them right

• Treating the myelin sheath as a separate substance coating the axon, when it is the many layers of the Schwann cell's own plasma membrane wrapped around it.

• Calling the dendrites axons, which reverses the described direction of conduction.

• Drawing nodes of Ranvier as breaks in the axon itself rather than gaps in the sheath around it.

• Describing the myelin sheath as protein when it is largely lipid.

• Confusing a neurone with a nerve.

  Put it right: A nerve is a bundle of many axons (nerve fibres), not a bundle of whole neurones.

Exam tips

• On a labelling question, check that your label line points precisely to the structure named; a correct term on the wrong structure gains nothing.
• Give a function alongside every name, since structure questions on neurones usually carry a function mark as well as a label mark.
• Add an arrow for the direction of the impulse if you are asked to annotate a diagram.