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Experiments were performed on awake human subjects in which single nerve fibre activity was recorded in the lateral peroneal nerve using tungsten micro-electrodes as described by Hagbarth & Vallbo (1967, 1968a). The discharge of twelve single efferent fibres innervating the tibialis anterior muscle (t.a.) or the extensor digitorum longus muscle (e.d.l.) was recorded. On the basis of their functional activity, six fibres were identified as skeletomotor and six as fusimotor fibres. Skeletomotor fibres, which were completely silent in relaxed subjects, discharged when subjects performed voluntary isometric or isotonic contractions, they also fired during Jendrassik's manoeuvre and tonic vibration reflex (t.v.r.) induced by mechanical vibration applied to the distal muscle tendon. Units considered as fusimotor fibres were generally spontaneously active with some fluctuation in the discharge frequency. Various tests used to identify afferent fibres elicited no response of these fibres (nor of the skeletomotor fibres). Efferent fibres were considered as fusimotor because their discharges were uncorrelated with any activation of extrafusal muscle fibres. Several means were used to detect activation of extrafusal fibres: surface electromyogram (e.m.g.) electrodes, tungsten electrodes deeply implanted in the muscle and especially the use of a high-sensitivity tension transducer (0 X 1 mN) placed on muscle tendons. The activity in fusimotor fibres could be either elicited or modulated under the following conditions: clenching of the fists, pinna twisting, mental computation, voluntary isometric contraction, passive phasic stretch of the muscle, environmental disturbances, subject laughing, the sound of hand clapping, and subject listening to manoeuvre instructions. Moreover, during spontaneous fusimotor fibre activity the subject was able to voluntarily stop the unit discharge. The results are compared to those obtained in animal studies and discussed with reference to the notion of alpha-gamma linkage, static and dynamic gamma-motoneurone activities, and to other available data concerning the effects of various stimulations on muscle spindle afferent activities in man.

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