Monosynaptic projections of single muscle spindle afferents to neck motoneurons in the cat

Monosynaptic connections to dorsal neck motoneurons of the cat from single afferents supplying primary endings of neck muscle spindles were studied using spike-triggered averaging techniques. Single-fiber EPSPs were detected in only 11 of the 112 afferent-motoneuron pairs examined. The average amplitude of single-fiber EPSPs recorded in motoneurons with a membrane potential of greater than -40 mV was 49 μV. Motoneurons receiving functional contacts from a single afferent were confined to a small rostrocaudal zone within the motor nucleus. The low frequency of single-fiber EPSPs in neck motoneurons could not be attributed to the absence of projections to the ventral horn or to damage to either the afferents or motoneurons. Our results suggest, therefore, that single afferents from neck muscle spindles make functional contacts with a small fraction of neck motoneurons, unlike the arrangement seen in more commonly studied hindlimb muscle systems (Henneman and Mendell, 1981).