Muscle spindle function during normal movement.

1. Discharge of muscle spindles during voluntary movement Recent recordings from afferents in awake animals have re-emphasized the function of spindle endings as stretch receptors. The available evidence suggests that, in voluntary movements involving muscle velocities above 0.2 RL sec-1 the modulation of firing rate of both primary and secondary endings is closely related to the length variations. Below 0.2 RL sec-1, phasic changes in fusimotor action may sometimes dominate such modulation. The modulatory strength of the component of fusimotor action strictly linked to skeletomotor activity is generally low. On the other hand, there is good evidence that in most movements, at least in cats and monkeys, there is steady, low level fusimotor action, independent of homonymous skeletomotor activity. It is therefore probable that fusimotor neurons are controlled largely independently of skeletomotor neurons, in such a way as to set the sensitivity and bias of spindle endings to levels appropriate for whole sequences of movements. 2. Reflex excitation of motoneurons Skeletomotor reflex responses to length perturbations can have a powerful linearizing effect on muscle stiffness. Spindle afferents often play an important role in mediating such reflexes and probably also contribute significantly to skeletomotor excitation during tonic contractions. Transmission in the pathways from spindles to motoneurons, including those through supraspinal structures, can be modified by a number of identified segmental mechanisms. These mechanisms could well be involved in the volitional control of the amount of spindle afferent excitation reaching the motoneuron pool, and may act as a form of gain control at different levels of arousal. 3. Proprioceptive cues for coordination Stretch-evoked activity of spindle system to modify the rhythm of the spinal locomotor generator. In particular, the onset of flexion or extension depends in part on spindle afferent activity. 4. Kinesthesia There is now evidence that muscle spindles contribute to position and velocity sense, although deficits nevertheless occur if skin and joint afferents are paralyzed.