Motor unit

(anatomy) The axon of an anterior horn cell, or the motor fiber of a cranial nerve, together with the striated muscle fibers innervated by its terminal branches.

The entity consisting of the lower motor neuron, motor end-plate, and muscle fibers supplied by the end-plate. The final motor activity resulting from a sequence of stimulations to the lower motor neuron is considered a function of the motor unit. The proportion of nerve fibers to the muscle fibers in motor units is designated as the innervation ratio. Motor units may have ratios ranging from 1:4 to 1:150. The closer the ratio approximates unity, the greater the finesse of specificity of the muscular action. The eye muscles have the highest ratio of striated muscles, and the tongue, facial, masticatory, and pharyngeal muscles succeed in that order.

A single motor neurone and all the muscle fibres it stimulates. Each motor unit supplies from four to more than a hundred muscle fibres. Generally, small muscles capable of precise actions (e.g. intrinsic hand muscles) are composed of motor units with few muscle fibres, whereas trunk and proximal limb muscles contain motor units with a large number of muscle fibres. Each motor unit obeys the all-or-none law programmes to bring about particular movements.

Motor unit
(Click to enlarge)

A single somatic motor neuron and the group of muscle fibers innervated by it.

A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibres will be of the same type (either fast twitch or slow twitch). When a motor unit is activated, all of its fibers contract. Groups of motor units often work together to coordinate the contractions of a single muscle; all of the motor units that subserve a single muscle are considered a motor unit pool. Larger motor units have stronger twitch tensions.[1]

The number of muscle fibers within each unit can vary: thigh muscles can have a thousand fibers in each unit, eye muscles might have ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle's need for refined motion. The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers.

Nerve cell axons are very thin, about 1 micrometer. However, they are extraordinarily long. For many motor neurons the axon is over a meter long, extending from the spinal column to a muscle cell. They stretch the spinal column to increase height.

In medical electrodiagnostic testing for a patient with weakness, careful analysis of the "motor unit action potential" (MUAP) size, shape, and recruitment pattern can help in distinguishing a myopathy from a neuropathy.

See also

• motor unit recruitment
• tetanic contraction
• myopathy (muscle pathology/diseases)
• motor unit number estimation
• Tonic and Phasic Inputs
• Motor pool

External links

• Neurons and Support Cells

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