Jaw-muscle spindle afferent feedback to the cervical spinal cord in the rat

Putative synaptic contacts between masticatory-muscle spindle afferents and brainstem neurons which project to the cervical spinal cord were studied i n rats by combining retrograde and intracellular neuronal labeling. Spinal cord projecting neurons were retrogradely labeled via injection of horserad ish peroxidase unilaterally or bilaterally into cervical spinal cord segmen ts C2 through C5. Twenty-four hours after the injection of horseradish pero xidase, one to five jaw-muscle spindle afferent axons were physiologically identified and intracellularly stained with biotinamide on each side of the brainstem. Horseradish-peroxidase-labeled neurons were found bilaterally i n the supratrigeminal region, trigeminal principal sensory nucleus, parvice llular reticular nucleus including its alpha division, spinal trigeminal su bnuclei oralis and interpolaris and the medullary reticular formation. Retr ogradely labeled neurons were most numerous in the spinal trigeminal subnuc leus oralis, parvicellular reticular formation and the ventral part of the spinal trigeminal subnucleus interpolaris. A small number of horseradish-pe roxidase-labeled neurons were also present in the trigeminal mesencephalic nucleus and spinal trigeminal subnucleus caudalis. Appositions between jaw- muscle spindle afferent boutons and spinal projecting neurons were found in the supratrigeminal region, dorsomedial portions of the trigeminal princip al sensory nucleus and spinal trigeminal subnuclei oralis and interpolaris, and the parvicellular reticular formation including its alpha division. Pu tative synaptic contacts were most frequent in the parvicellular reticular formation and the dorsomedial portion of the trigeminal subnucleus oralis. These results indicate that some orofacial proprioceptive feedback transmit ted via the mesencephalic trigeminal nucleus reaches the cervical spinal co rd directly and suggests that jaw-muscle spindle afferent feedback reaches the cervical spinal cord predominately via relays in the dorsomedial part o f the spinal trigeminal subnucleus oralis and the parvicellular reticular f ormation. It is hypothesized that these pathways are primarily involved in the coordination of jaw and neck movement during mastication and biting.