Effects of temperature on the discharges of muscle spindles and tendon organs

1. In anaesthetized cats the effects of temperature on the nervous outflow from skeletal muscle via thick myelinated afferent fibres were studied. Single unit recordings were made from afferents of muscle spindles and tendon organs during slow and fast temperature changes of the medial gastrocnemius muscle which was deefferented by ventral root section and prestretched to a tension of 100 p.
2. Group I afferent units from muscle spindles were activated by warming and depressed by cooling, the effect of warming being much more pronounced than that of cooling. Afferents from secondary spindle endings with a high background discharge behaved similar to Ia fibres, whereas those with a low initial discharge rate showed an activation by cooling and a depression (mostly to cessation of firing) by warming. The discharges of group I afferents from tendon organs varied; an activation by warming was the most frequently observed reaction.
3. Some of the afferents from muscle spindles and tendon organs showed signs of a dynamic sensitivity to thermal stimulation, but in general the dynamic component in the responses to temperature changes was only small.
4. The results suggest that the afferent outflow via thick myelinated fibres from a resting, moderately prestretched muscle strongly depends on temperature. At raised intramuscular temperatures (about 42°C) the nervous outflow is characterized by an increased activity in all of the I a and many of the I b afferents, while the majority of group II spindle afferents will be depressed. In contrast, in a cold muscle (about 29°C) the nervous outflow via afferents from primary spindle endings will be reduced, while the net activity from secondary spindle endings will be increased and no marked changes are expected to occur in the discharges of I b fibres.