An unrelayed projection of jaw-muscle spindle afferents to the cerebellum.

Jaw-muscle spindle afferents were physiologically characterized and stained with horseradish peroxidase. Two types of afferent responses were observed during jaw-muscle stretches; one group showed considerable dynamic sensitivity indicative of primary muscle spindle afferents while the other showed little dynamic responsiveness indicative of secondary spindle afferents. Axon collaterals from both types were observed coursing within the ipsilateral superior cerebellar peduncle, indicating that some jaw-muscle spindle afferents project directly to the cerebellum without relay.