Neuromuscular system: Motor units.

The idea

A motor unit is one motor neurone together with all the muscle fibres it stimulates. Its fibres lie scattered through a muscle rather than clumped together, so one unit's contraction is spread across the whole muscle belly. The size principle governs recruitment: small units, which contain slow-twitch fibres and generate little force, are activated first, and larger units with fast-twitch fibres are added as force demand rises. All-or-none behaviour applies within a unit: when its motor neurone fires, every fibre in that unit contracts fully; stronger contraction comes from recruiting more units and raising firing frequency, not from partial firing of one unit. Fine-control muscles such as the eye muscles have few fibres per unit, while powerful muscles such as the quadriceps have many.

What earns the marks

• A motor unit is one motor neurone and all the muscle fibres it innervates.
• The fibres of a single motor unit are dispersed throughout the muscle, not grouped in one region.
• Small motor units are recruited before large ones as force demand increases (size principle).
• Within a motor unit the response is all-or-none: stimulation triggers contraction of all its fibres.
• Greater overall force is produced by recruiting additional motor units and by increasing the frequency of impulses.
• Muscles requiring fine control have few fibres per motor unit; muscles producing large force have many.

Common mistakes and how to put them right

• Thinking a motor unit is a single muscle fibre.

  Put it right: It is one motor neurone plus every fibre that neurone supplies, which may be many fibres.

• Believing a motor unit can contract partially.

  Put it right: The unit behaves all-or-none, so graded force comes from recruiting more units and altering firing rate.

• Assuming all motor units in a muscle are switched on together.

  Put it right: Recruitment follows the size principle, with small units active first.

• Confusing the motor neurone with a sensory neurone.

  Put it right: The motor unit is defined by the motor neurone that carries impulses from the central nervous system to skeletal muscle fibres.

Exam tips

• Define the term precisely before applying it, naming both the motor neurone and the fibres it innervates.
• When asked how force increases, link recruitment of additional motor units with increased frequency of stimulation rather than saying fibres contract harder.
• Use a contrasting example, such as eye muscles for fine control and quadriceps for high force, to show the relationship between unit size and function.