Proprioceptive reflex connections of head musculature and the mesencephalic trigeminal nucleus in the carp.

The location of the sensory cells concerned with the proprioception of respiratory and extraocular musculature in the carp was studied by retrograde transport of horseradish peroxidase. Sensory cell labeling after intramuscular HRP injection was exclusively found in the trigeminal-facial-anterior lateral line ganglion complex. The muscles innervated by the trigeminal system are represented in the more rostral ganglion areas, the muscles innervated by the facial system in the more caudal ganglion parts. Nearly all labeled cells were situated on the ipsilateral side. Sensory cells labeled after extraocular muscle injection were also found all over the V-VII ganglion, however, to a considerable degree also on the contralateral side. All muscle injections failed to give mesencephalic trigeminal cell labeling. The resluts of intranerve HRP injections in peripheral trigeminal nerve branches strongly suggests a perioral mechanoreceptive function for the mes.V neurons. A bisynaptic proprioceptive reflex model is described for respiratory musculature consisting of a sensory cranial ganglion component connected to the descending trigeminal nucleus, which on its turn links the proprioceptive ganglion cells to the trigeminal and facial motorneurons. Monosynaptic proprioceptive reflex circuits are discussed on neurophysiological grounds.