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Recordings have been made of afferent activity from spindles of the jaw-closing muscles, together with jaw movement and e.m.g. from temporalis and masseter in conscious, unrestrained cats. 2. In the twenty-nine units studied, the pattern of spindle behaviour observed during eating and lapping was generally what might be expected of stretch receptors. Maximal firing frequencies were found during opening of the mouth (lengthening), while during active closing the discharge was progressively reduced or abolished. Nevertheless, changes in the relation of stretch to firing frequency in different movements indicated that fusimotor drive was not constant. 3. spindle afferents could be divided into two groups on the basis of their maximal firing frequency during eating. "High-frequency" units (range 240-600 impulses/sec) showed pronounced velocity sensitivity, which supports the proposal that they correspond to spindle primaries. 'Low-frequency" units (range 80-200 impulses/sec) showed predominantly length sensitivity and probably correspond to secondary endings. 4. Length sensitivity of low-frequency units was considerably greater in lapping movements than in eating, indicating increased static fusimotor drive in the former. Sensitivity in the opening phase of eating was indistinguishable from that recorded in deeply anaesthetized animals. 5. High-frequency units were generally silenced immediately active shortening commenced. 6. No simple relationship existed between temporalis or masseter e.m.g. and spindle firing. 7. These results imply that normal masticatory movements are not initiated or driven to any appreciable extent via the fusimotor route. Close alpha-gamma co-activation is not a feature of this situation. On the other hand, in some other movements, such as licking the lips, fusimotor drive could fluctuate so as largely to cancel the unloading effects of active muscle shortening.

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