Good control of muscle tone and stretch is essential for the maintenance of posture and for accuracy of movements. This control is mediated by stretch receptors in the skeletal muscles, called 'muscle spindles'. These fusiform structures are scattered throughout the fibres of a skeletal muscle. They are small specialized structures composed of 4-20 intrafusal (within spindle) fibres and are supplied by both sensory (Ia and IIa afferents) and motor (y efferent) nerves.
In the control of muscle tone and movement the spindles provide a feedback signal which tends to maintain a skeletal muscle at a desired length, or controls the rate at which a muscle lengthens or shortens. Thus the spindles provide a static signal (via IIa afferents) helping to maintain posture, and a dynamic component (via Ia afferents) controlling the rate of contraction and, hence, smoothness of movements. When the position of a muscle is disturbed by stretching, the spindles are also stretched which increases their feedback signal to the spinal cord. This increases a motor output and skeletal muscle tone opposing the original disturbance (Figure MP.17).
The y efferent motor nerves to the spindles pre-tension the intrafusal fibres, which effectively sets their sensitivity. y efferent tone is under the influence of higher centres in the central nervous system such as the cortex, basal ganglia and cerebellum. If y efferent tone is high the spindles are taut and slight disturbances in skeletal muscle length elicit reflex contraction of the skeletal muscle. The skeletal musculature then appears to be clinically hypertonic. This can occur with upper motor neurone lesions such as those following cerebrovascular accidents. When y efferent tone is low, muscular hypotonia results, as may occur with some cerebellar lesions.
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