Morphology of jaw-muscle spindle afferents in the rat.

The morphology of jaw-muscle spindle afferents in the rat has been studied by intra-axonal injection of horseradish peroxidase. All stained axons were located in the motor root of the trigeminal nerve and could be traced dorsomedially to the vicinity of the trigeminal motor nucleus, where they divided into an ascending branch in the tract of the mesencephalic nucleus and a descending branch in the tract of Probst. Axon collaterals and swellings on fine collateral branches presumed to be synaptic boutons were located in the following regions: the trigeminal motor nucleus, the region dorsal to the trigeminal motor nucleus including the supratrigeminal nucleus, the parvicellular reticular formation immediately caudal to the trigeminal motor nucleus, the reticular formation at the level of the facial nucleus, and the caudal portion of the mesencephalic nucleus. No evidence of a projection to the cerebellum was observed. Boutons were most numerous in the region surrounding the trigeminal motor nucleus, especially dorsally. Here they were not demonstrated in close proximity to counterstained cells, and therefore it was not possible to determine how many of these contacts are located on cells in this region and how many are on the distal dendrites of trigeminal motorneurons. Boutons located within the trigeminal motor nucleus were always confined to a small portion of the nucleus and were significantly larger than those located dorsally. Some boutons were found in close apposition to trigeminal motorneurons and presumably make somatic contacts. These results suggest that jaw-muscle spindle afferents make somatic and proximal dendritic contacts with only a limited number of trigeminal motorneurons and also project to masticatory interneuronal regions dorsal and caudal to the motor nucleus.