5.1.3(b) - Sensory, relay and motor neurones

The Route Of A Nerve Impulse

A nerve impulse is an electrical event that travels along a neurone. In a coordinated response, impulses usually move through a route rather than through one isolated cell.

The basic route is:

1. A receptor detects a stimulus.
2. A sensory neurone carries impulses from the receptor towards the central nervous system.
3. A relay neurone passes impulses within the central nervous system.
4. A motor neurone carries impulses from the central nervous system to an effector.
5. An effector, such as a muscle or gland, produces a response.

The central nervous system, or CNS, is the brain and spinal cord. Sensory and motor neurones link the CNS with the rest of the body. Relay neurones are different because they are found within the CNS and connect neurones to each other.

A sensory receptor may be a separate specialised cell or a specialised ending of a sensory neurone. Distinguish detection at the receptor region from transmission along the sensory neurone.

Use the direction carefully. Sensory neurones do not "make the response"; they carry information inwards from receptors. Motor neurones do not detect the stimulus; they carry impulses outwards to effectors.

Three Neurone Types

The names sensory, relay and motor describe function, but each function is supported by structure. A neurone has a cell body containing the nucleus, cytoplasmic extensions that receive or carry impulses, and terminal regions that connect with another cell.

[DIAGRAM: neurone_type_comparison: Lesson 103: Sensory, relay and motor neurones - diagram 01; asset_slug: 103_m05_1_3_sensory_relay_and_motor_neurones__diagram_01; recommended_method: drawn_biology; description: Deterministic comparison diagram showing sensory, relay and motor neurones with direction of impulse, cell body position, dendron or dendrites, axon and receptor/CNS/effector context.]

The sensory neurone is adapted for carrying impulses from a receptor over distance. Its cell body is not usually at the receiving end; it is positioned part-way along the route, commonly off to the side. This helps explain why a sensory neurone is often drawn with a long dendron leading from the receptor and an axon leading into the CNS.

The relay neurone is adapted for connection inside the CNS. Its short branches allow it to receive impulses from one neurone and pass impulses to another. In a simple pathway, this means a relay neurone can connect a sensory neurone to a motor neurone.

The motor neurone is adapted for output. Its cell body is in the CNS, where it can receive input from relay neurones. Its long axon carries the impulse away from the CNS to an effector, such as muscle tissue.