Efferent control of human muscle spindles

It is usually assumed that muscle spindles function in essentially the same way in humans and cats. Indeed, there was disquiet when, on comparing afferent activity from spindles during voluntary movements in awake humans and cats, human spindles could not be made to behave like cat spindles (Hagbarth, 1981; Vallbo, 1981). Primary and secondary afferent activity during passive muscle stretching is similar in humans and cats (Poppele & Kennedy, 1974; Newsom Davis, 1975). The efferent supply of human spindles can be stimulated in intramuscular nerves while monitoring the movement of the intrafusal muscle fibres (Gladden et al., 1982) by using techniques developed to study the effects of fusimotor stimulation on cat intrafusal fibres in vitro (Boyd & Ward, 1975). The results suggest that human spindles have dynamic and static fusimotor axons which the central nervous system can activate independently.