Modulating Processes of Single Fusimotor Fibre Discharge in Man
Experiments were performed on awake human subjects in which single nerve fibre activity was recorded in the lateral peroneal nerve using tungsten microelectrodes as described by Hagbarth & Vallbo (1967).
This discharge of thirteen single efferent fibres innervating the tibialis anterior muscle (TA) or the extensor digitorum longus muscle (EDL) was recorded. On the basis of their functional activity, seven fibres were identified as fusimotor fibres.
Their efferent nature was demonstrated by the fact that the various tests used to identify afferent fibres elicited no response of these fibres. These efferent fibres were considered as fusimotor because their discharges were uncorrected with any activation of extrafusal muscle fibres. Several means were used to detect activation of extrafusal fibres: surface EMG electrodes, tungsten electrodes deeply implanted in the muscle and especially the use of a high-sensitivity tension transducer (0.1 mN) placed on muscle tendons. Fusimotor fibres were generally spontaneously active with some fluctuation in the discharge frequency.
The activity in fusimotor fibres could be either elicited or modulated under the following conditions: clenching of the fists, pinna twisting, voluntary isometric contraction, passive phasic stretch of the muscle, mental computation, environmental disturbances, subject laughing, the sound of hand clapping, and subject listening to manoeuvre instructions. Moreover, during spontaneous fusimotor fiber activity two subjects were 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 motoneuron activities, and to another available data concerning the effects of various stimulations on muscle spindle afferent activities in man.
KeywordsTibialis Anterior Extensor Digitorum Longus Muscle Spindle Muscle Nerve Sympathetic Activity Mental Computation
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