Actions on gamma‐motoneurones elicited by electrical stimulation of cutaneous afferent fibres in the hind limb of the cat.

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Abstract
The reflex actions elicited by graded electrical stimulation of hind-limb cutaneous (sural, superficial peroneal and tibial) nerves were investigated with intra- and extracellular micro-electrode recordings in .gamma.-motoneurones projecting to hind-limb muscles in twenty-four cats anaesthetized with .alpha.-chloralose. In total, reflex responses of 100 .gamma.-motoneurones were analysed, 82 of the .gamma.-cells were classifid as dynamic (43) or static (39) using the method of mesencephalic stimulation (Appelberg, Hulliger, Johansson and Sojka, 1982). The general responsiveness (i.e., number of input nerves with effect/number of input nerves tested) of the whole sample of .gamma.-cells to stimulation of skin nerves was extremely high (94.8%). All negative observations were encountered among static and non-classified .gamma.-cells. Generally, the stimulation strengths needed for evoking effects in the .gamma.-cells were very low. A majority of the excitatory effects in the dynamic cells appeared with stimulation intensities below 1.5 threshold (T), while most static cells were excited with stimulation strengths between 1.5 and 2 T. Also a statistical comparison of the populations of stimulation strength thresholds for the excitatory effects revealed a significant difference (P < 0.0009) between dynamic and static .gamma.-cells. By contrast, the thresholds for inhibitory effects in dynamic cells were slightly higher than for excitatory effects (P < 0.009). As regards excitation of static cells, inhibition of dynamic cells and inhibition of static cells, no statistically significant threshold differences were found. A strong dominance of excitation over inhibition was found in both dynamic and static flexor (posterior biceps and semitendinosus) .gamma.-motoneurones from all input nerves. In comparison to flexor .gamma.-motoneurones, there was a much higher incidence of inhibitory and mixed (excitatory and inhibitory) responses in extensor (triceps) .gamma.-motoneurones, from all nerves tested. For dynamic cells there was an about even balance between excitation and inhibition, while for static cells inhibition seemed to prevail. The latencies for excitatory effects in dynamic .gamma.-cells differed from those in static .gamma.-cells (P < 0.027). The shortest latencies of excitatory effects found for dynamic .gamma.-cells indicate a disynaptic coupling, while for static cells the shortest route seemed to involve at least three synapses. Inhibitory effects on both static and dynamic .gamma.-cells were generally found to be polysynaptically mediated, but a single case of evidence for trisynaptically coupled inhibition was observed for a dynamic gastrocnemius et soleus .gamma.-cell. The general differences between the subpopulations of .gamma.-motoneurones were found to mask a remarkable amount of variation among the individual .gamma.-cells, as regards thresholds, latencies and types of effects from the different skin nerves. A comparative survey is given where the effects on .gamma.-motoneurones are discussed in relation to the known effects on .alpha.-motoneurones. It is concluded that the data from this investigation further corroborate the view that reflexes to .gamma.- and .alpha.-motoneurones are differently organized. It is furthermore tentatively proposed that the complex and potent influences from muscle, joint and skin receptors on the messages in the primary muscle spindle afferents (via integration in the .gamma.-motor system) partly might explain why the primary muscle spindle afferents can provide the central nervous system with information relevant for movement and position sense.