Motor Unit Of A Single Motor Neuron

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Motor unit consists of a single motor neuron together with skeletal muscle fibres that are innervated by the motor neuron. Alpha motor neurons are the lower motor neurons of the brainstem and spinal cord that predominate within motor pools and innervate muscle fibres that cause muscle contraction. A single alpha motor neuron can innervate a large number of skeletal muscle fibres since motor neuron (approximately 420,000) are outnumbered by muscle fibres (approximately 200 million). This is achieved by the branching of a single motor axon in the muscle to synapse on numerous distinct fibres that are well spread over a large area within the muscle so the contractile force would scatter uniformly.

Motor units are sorted according to speed of contractions, amount of tension generated and fatigue resistance. In human, there is one slow-twitch fibre and two types of fast-twitch fibres. Therefore, the three principal types of motor units are slow (S, type-I) motor units, fast fatigable (FF, type-IIB) motor units and fast fatigue-resistant (FR, type-IIA) motor units.

‘Red’ muscles are innervated by small motor neuron of the S type and the muscles slowly contract producing fairly minute force compared to the fast-twitch fibres because S has small and few contractile filaments. S motor unit has large amount of mitochondria and sufficient blood supply to maintain aerobic metabolism because of extensive network of capillaries. Moreover, they are rich of myoglobin that stores oxygen
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