Functional roles of fusimotor and skeletofusimotor neurons studied in the decerebrate cat
Although the existence of skeletofusimotor innervation of muscle spindles in mammalian muscles is now amply confirmed, the physiological effects of this innervation have not been established. In an attempt to determine such effects, we recorded the discharge of triceps surae primary and secondary spindle receptor afferents, isolated from small dorsal root fascicles in decerebrate cat preparations. Afferent discharge was recorded in quiescent states, and during reflex excitation of the receptor-bearing muscle.
Mechanical or electrical stimulation of contralateral skin or deep tissues consistently evoked increased discharge in soleus and medial gastrocnemius (MG) afferents. During stimulation, afferent discharge rates were reliably observed to exceed those recorded in quiescent states, either when the receptor-bearing muscle was held at constant length, or during muscle stretch and release. These rate increases arose at stimulus intensities insufficient to provoke an EMG response, and the patterns of response were consistent with the activation of both static and dynamic fusimotor neurons. We argue that the absence of EMG activity or active force implies that neither skeletomotor nor skeletofusimotor neuronal activity could have been responsible for the initial rate increases. It follows that fusimotor neurons were responsible, and that they were activated at stimulus intensities insufficient to drive skeletomotor (or skeletofusimotor) neurons.
Approximately 50% of primary and secondary afferents showed some further rate increase with increasing force and in 3/49 endings this discharge consistently increased in a very abrupt manner, as if a new efferent fibre were being recruited. Recordings from MG fusimotor (gamma) fibres confirmed that an early increase in discharge rate occurred at stimulus intensities insufficient to activate skeletomotor fibres, and showed that the discharge rate appeared to saturate during stimulation sufficient to activate the muscle. We obtained no evidence supporting continuing recruitment of fusimotor neurons at high force levels.
We propose that increases in spindle receptor discharge arising above the extrafusai threshold are mediated predominantly by skeletofusimotor neurons. More direct support for this hypothesis is now being sought with the aid of spike-triggered averaging techniques, which may permit us to identify the existence of skeletofusimotor innervation under more physiological conditions.
KeywordsMedial Gastrocnemius Afferent Discharge Efferent Fibre Contralateral Stimulation High Force Level
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