The control of muscle contraction

Skeletal muscle is also known as voluntary muscle and, as its name implies, it is under volitional control, unlike smooth or cardiac muscle. Adult skeletal muscle fibres are each innervated by a single branch of the axon arising from an α-motoneuron in the spinal cord. The α-motoneuron and all the fibres it innervates constitute a motor unit, and this is the functional unit of the muscle. α-Motoneurons differ in size and excitability and it is the recruitment of these cell bodies in the spinal cord that determines which fibres within the muscle are active during a movement. The axonal branches generally make contact with their muscle fibre about halfway along its length, this being the ‘motor point’ at which the muscle can most easily be stimulated by an external electrical stimulus during muscle testing. Correct functioning of the neuromuscular junction is clearly critical for muscle action and it is a site at which many drugs affecting muscle have their action.