The functional role of the muscle spindles--facts and hypotheses.

A review is given of recent work on the functional role of muscle spindles in the control of movement. The fusimotor neurons (gamma motoneurons) maintain the spindles in a state of responsiveness to length and to rate of change of length of muscle. Centrifugal control of muscle spindles takes two forms: first, a steady or slowly fluctuating tonic firing of fusimotor neurons, as a part of general states of arousal or readiness-to-move, independent of the firing of skeletomotor neurons (alpha motoneurons), and not related in time to specific movements; secondly, a precise coactivation of skeletomotor and fusimotor neurons (alpha-gamma linkage) which is related to the time-course of specific movements. Both types are likely to be important in man. Recent work on the connexions at the segmental level between spindle inputs, descending pathways, interneurons, gamma motoneurons and alpha motoneurons is reviewed and discussed, with special attention to work on man. These segmental circuits, rather than their individual components, are the units which are operated by reflexes and by central programmes for movemenst.