The Human Muscle Spindle and its Fusimotor Control
It is now 25 years since Hagbarth and Vallbo (1968) described the technique of microneurography, with the first direct recordings of the activity of muscle afferents in human subjects. It is appropriate to take stock of what has been learnt about fusimotor control of human muscle spindles since then in order to define the course of future experiments.
Muscle spindle discharge is used in microneurographic recordings to infer the action of the fusimotor system. This Chapter considers some of the fusimotor and non-fusimotor mechanisms which can influence the behaviour of human muscle spindle afferents. Voluntary contractions under isometric (or lengthening) conditions activate the fusimotor system such that overall spindle discharge increases. Evidence is presented that this can lead to considerable reflex “support” to motoneuronal output. This contribution may diminish during muscle fatigue and be absent during voluntary shortening contractions of sufficient speed.
Further studies will continue to focus on possible flexibility in the activation of fusimotor neurones produced by reflex and voluntary drives. Little data exist on possible abnormalities in fusimotor-induced spindle feedback or in reflexes acting on fusimotor neurones in patients with motor disorders.
KeywordsMuscle Spindle Supportive Excitation Spindle Ending Extrafusal Muscle Spindle Discharge
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