(b) the structure and functions of sensory, relay and motor neurones

The idea

Neurones share a basic structure: a cell body containing the nucleus, dendrites that receive impulses, and an axon that carries impulses away, often wrapped in a myelin sheath formed by Schwann cells with nodes of Ranvier between them. Sensory neurones carry impulses from receptors to the central nervous system; they have a cell body in a dorsal root ganglion and a long dendron. Relay (intermediate) neurones lie within the central nervous system and connect sensory to motor neurones, often with many short dendrites and a short axon. Motor neurones carry impulses from the central nervous system to effectors such as muscles or glands; they have a large cell body in the spinal cord and a long myelinated axon. Myelination allows saltatory conduction, speeding transmission.

What earns the marks

• All neurones have a cell body with a nucleus, dendrites that receive impulses and an axon that transmits impulses away.
• Sensory neurones carry impulses from receptors to the central nervous system and typically have a cell body in a dorsal root ganglion with a long dendron.
• Relay neurones are found within the central nervous system and connect sensory neurones to motor neurones, often with many short dendrites and a short axon.
• Motor neurones carry impulses from the central nervous system to effectors such as muscles or glands, with a cell body in the spinal cord and a long axon.
• The myelin sheath, formed by Schwann cells with nodes of Ranvier between them, enables saltatory conduction and faster transmission.

Common mistakes and how to put them right

• Reversing the direction of sensory and motor neurones.

  Put it right: Sensory neurones carry impulses towards the central nervous system and motor neurones carry impulses away from it to effectors.

• Placing relay neurones outside the central nervous system.

  Put it right: Relay neurones are located within the central nervous system.

• Confusing the dendron of a sensory neurone with the axon.

  Put it right: The sensory neurone has a long dendron carrying impulses towards its cell body.

• Ignoring the role of Schwann cells and nodes of Ranvier.

  Put it right: These structures produce the myelin sheath and allow saltatory conduction.

Exam tips

• Match each neurone type to its direction of impulse travel and its typical cell-body position before selecting an option.
• Use the terms dendron, axon, myelin sheath, Schwann cell and node of Ranvier precisely, as distractors often swap them.
• Link structure to function, for example long myelinated axons to rapid conduction, to eliminate plausible but incorrect options.