Jaw-muscle spindle afferent pathways to the trigeminal motor nucleus in the rat

Neural pathways conveying proprioceptive feedback from the jaw muscles were studied in rats by combining retrograde and intracellular neuronal labelin g. Initially, horseradish peroxidase was iontophoresed unilaterally into th e trigeminal motor nucleus (Vmo). Two days later, 1-5 jaw-muscle spindle af ferent axons located in the mesencephalic trigeminal nucleus were physiolog ically identified and intracellularly stained with biotinamide. Stained mes encephalic trigeminal jaw-muscle spindle afferent axon collaterals and bout ons were predominantly distributed in the supratrigeminal region (Vsup), Vm o, dorsomedial trigeminal principal sensory nucleus (Vpdm), parvicellular r eticular formation (PCRt), alpha division of the parvicellular reticular fo rmation (PCRtA), and dorsomedial portions of the spinal trigeminal subnucle i oralis (Vodm), and interpolaris (Vidm). Numerous neurons retrogradely lab eled with horseradish peroxidase from the trigeminal motor nucleus were fou nd bilaterally in the PCRt, PCRtA, Vodm, and Vidm. Retrogradely labeled neu rons were also present contralaterally in the Vsup, Vpdm, Vmo, peritrigemin al zone, and bilaterally in the dorsal medullary reticular field. Putative contacts between intracellularly stained mesencephalic trigeminal jaw-muscl e spindle afferent boutons and trigeminal premotor neurons retrogradely lab eled with horseradish peroxidase were found in the ipsilateral Vodm, PCRtA, and PCRt, as well as the contralateral Vsup, Vmo, Vodm, PCRt, and PCRtA. T hus, multiple disynaptic jaw-muscle spindle afferent-motoneuron circuits ex ist. These pathways are likely to convey long-latency jaw-muscle stretch re flexes and may contribute to stiffness regulation of the masticatory muscle s. (C) 2001 Wiley-Liss, Inc.