Cell body locations and axonal pathways of neurons innervating muscles of mastication in the rat

Abstract

Abstract In the rat the size distributions of cell bodies in the trigeminal motor nucleus and of myelinated fibers in the portio minor were bimodal. The size distribution of mesencephalic nuclear somata was unimodal. The number of fibers in the portio minor was approximated by the number of somata in the motor and mesencephalic nuclei. Within the motor nucleus large and small cells were chromatolytic after destruction of the portio minor. Injections of horseradish peroxidase (HRP) into the masseter and temporalis muscles labeled both the large and small neurons in the dorsolateral part of the nucleus. Similar injections into the anterior belly of the digastric and mylohyoid muscles labeled neurons in the ventrolateral part of the nucleus, but these neurons were not among the smallest in the nucleus. Within the mesencephalic nucleus chromatolytic somata occurred only ipsilaterally after unilateral lesions of the portio minor, and in lesser numbers after destroying the portio major. Unilateral injections of HRP into the masseter and temporalis muscles labeled cells in the ipsilateral mesencephalic nucleus and trigeminal ganglion. Labeling of mesencephalic neurons was abolished by lesions of the portio minor. Similar injections into the anterior belly of the digastric and mylohoid muscles labeled cells in the ipsilateral trigeminal ganglion, but not in the mesencephalic nucleus. These results are discussed in light of previous work on the trigeminal system.