Extraocular muscles from representative species of vertebrate groups ranging from amphibians to the higher mammals were examined in serial histological sections for the presence of muscle spindles. These observations and data from the literature indicate that extraocular muscles of the pig, calf, sheep and other even-toed ungulates are richly supplied with well-defined spindles having a generous complement of intrafusal fibers distinguishable as nuclear bag and chain fibers. Spindles in human eye muscles are also numerous. In macaque and chimpanzee muscles a few poorly developed spindles were present in some, but not all, muscles. No encapsulated receptors were found in 20 other mammalian and submammalian species examined in this study. When present, spindles tended to be located in the zone of small muscle fibers found along the orbital surface of the muscle. Rectus and oblique muscles in all species had such a zone, so that its existence did not determine whether spindles would occur.
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. Since the initial study by Parhon and Nadejde ('061, which suggested a somatotopic organization of skeleto-motor neurons comprising the trigeminal motor nucleus, two fundamentally different patterns have been reported. Motor neurons innervating jaw-closing muscles have been described as being more ventral (Szentagothai, '49 In view of the unresolved questions and since many of the previous studies were on mammals other than rodents, we are reporting on the cell body locations and axonal pathways of neurons innervating several masticatory muscles in the rat.
MATERIALS AND METHODSExperiments were done on Sprague-Dawley rats of either sex, weighing 150-500 gm. They were anesthetized before surgery with Innovar Vet@ (0.2 mg fentanyl/kg, intramuscular) or pentobarbital sodium (50 mgkg, intraperitoneal). Three types of data were collected on trigeminal neurons: (1) population measures of the number and diameter of somata comprising the motor and mesencephalic nuclei, and of myelinated fibers in the corresponding motor root of unoperated rats; (2) the size of cell bodies in the motor nucleus and the number of neurons in the mesencephalic nucleus undergoing retrograde change following the destruction of specific nerves; and (3) the location, number and size of somata labeled by retrograde transport following injection of horseradish peroxidase (HRP) into selected muscles (Kristensson and Olsson, '71; Furstman et al., "75). All counts refer to a nucleus or nerve on one side only.
Normal animalsIn two rats the trigeminal nerve roots were exposed unilaterally and fixed in si...
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