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MUSCLE-FIBER TYPES AND MUSCLE-SPINDLES IN THE JAW MUSCULATURE OF THE RAT

The fibre composition and occurrence of muscle spindles was studied in the masticatory, the suprahyoid and the infrahyoid muscles of the rat. Muscle fibres were typed as fast-white, fast-intermediate, fast-red and slow-red according to their ATPase and SDH activity. Fibre type appeared to be closely related to fibre diameter. In most of the muscles, all four fibre types were found. Slow-red fibres were absent in the superficial masseter, the transverse mandibular and the omohyoid muscles; fast-white fibres were absent in the mylohyoid muscle. The masticatory muscles were mainly composed of the three fast-fibre types; the jaw-opener muscles (the anterior digastric, the posterior digastric, the posterior digastric, the stylohyoid and the lateral pterygoid muscle) showed more slow-red fibres. In the masticatory and most of the suprahyoid muscles, the slow-red fibres were restricted to an area with high SDH activity. In the infrahyoid muscles, the fibre types were evenly distributed. Many muscle spindles, often clustered, were found in the masticatory muscles, except in the lateral pterygoid. In most of the suprahyoid muscles, these sensory structures were absent. In the infrahyoid muscles, solitary muscle spindles were found.

Arrangement and Connections of Mesencephalic Trigeminal Neurons in the Rat

The morphology of the mesencephalic trigeminal nucleus was examined microscopically in serial frozen sections. The nucleus extends over a length of about 4.5 mm, and its cell number was calculated to range from 1,000 to 1,600. 60% of the cells were located in the caudal third of the nucleus. Clustering of large unipolar cells was seen throughout the nucleus. Small spindle-shaped multipolar cells were found in the pontine part of the nucleus. The efferent connections of the mesencephalic trigeminal neurons were investigated by means of iontophoretically delivered Phaseolus vulgaris leuco-agglutinin or horseradish peroxidase after electrophysiological identification of mesencephalic trigeminal neurons. All projections were found ipsilateral to the injection site; they were confined to the trigeminal motor nucleus, especially to its lateral part, and to the dorsolateral reticular formation. The latter projection area included the supratrigeminal nucleus, the nucleus of Probst, and the parvocellular reticular zone. There were no direct projections to the facial or hypoglossal motor nuclei. It is concluded that proprioceptive input from one side is mediated polysynaptically to the bilateral oral final common-path neurons, with the exception of the ipsilateral trigeminal motoneurons.

Bilateral Brainstem Connections of the Rat Supratrigeminal Region

Efferent and afferent connections of the supratrigeminal region were studied in the rat using iontophoretically delivered horseradish peroxidase and Phaseolus vulgaris leuco-agglutinin. Projections of supratrigeminal efferents were found to the contralateral supratrigeminal region, to the ipsi- and contralateral trigeminal motor nuclei and the medullary reticular formation, and to the ipsilateral facial and hypoglossal motor nuclei. Neurons projecting to the supratrigeminal region were located in the contralateral supratrigeminal nucleus, in the ipsilateral mesencephalic trigeminal nucleus and bilaterally in the medullary reticular formation. This organization is discussed with respect to bilateral oral motor control mechanisms.

AFFERENT-PROJECTIONS TO THE MESENCEPHALIC TRIGEMINAL NUCLEUS IN THE RAT - ANTEROGRADE TRACING WITH PHASEOLUS-VULGARIS LEUKOAGGLUTININ

The anatomical pathways from the parvocellular reticular formation and medulla oblongata to the mesencephalic trigeminal nucleus (Mes V) were investigated in the rat by means of the anterograde tracer Phaseolus vulgaris leukoagglutinin. P. vulgaris was injected into the rostral ventromedial medulla, rostral and caudal ventrolateral medulla, the parvocellular reticular formation and surrounding regions. Rostral medullary regions and the parvocellular reticular formation projected to Mes V although the bilaterality, extent and density of the projections originating in these reticular structures showed clear variations. In experiments in which dorsolateral reticular areas were involved, we found the projections to the bilateral trigeminal nuclei to be most prominent. More caudal areas, including the spinal trigeminal nucleus caudalis, showed no projections to Mes V. The present experiments show, for the first time, the existence of ipsilateral and crossed projections from the reticular formation to the Mes V. These pathways are possibly involved in direct modulation of trigeminal reflexes.

Efferent projections of the parvocellular reticular nucleus to the mesencephalic trigeminal nucleus in rat

Field potentials recorded in the caudal part of the mesencephalic trigeminal nucleus have been studied in response to stimulation of the parvocellular reticular nucleus (PCRt). Besides discharges of spindle afferent units evoked by antidromic stimulation of Probsts tract, relayed and direct afferent volleys of PCRt fibres were recorded in this nucleus. The results indicate the existence of two populations of PCRt fibres projecting on different regions of the mesencephalic trigeminal nucleus.

Identification of Alpha and Gamma Trigeminal Motoneurons by the Vibratome Paraplast Technique for HRP Histochemistry

A morphometric analysis of the masseteric motoneuron pool of the trigeminal motor nucleus was performed in the rat using horseradish peroxidase as a marker. Thick (40 microns) cryosections and thin (7 microns) Paraplast sections were compared. Two types of motoneurons related to the masseter muscle were observed. Small motoneurons, which had a high nuclear index, were found interspersed between large motoneurons, which had more cytoplasm. Evidence is provided that the small trigeminal motoneurons are gamma neurons that innervate the intrafusal muscle fibers of the masseteric muscle spindles.

Tracing of lateral pterygoid muscle-related neurons in the trigeminal brainstem nuclei in the rat.

Retrograde transport of fluorescent tracers (diamidino yellow and true blue) was used to study the arrangement of brainstem neurons innervating the lateral pterygoid muscle in the rat. The lateral pterygoid motoneurons were located in the dorsolateral (jaw-closing) part of the trigeminal motor nucleus with clear somatotopy in the caudal part of the nucleus. No muscle-related neurons were present in the mesencephalic trigeminal nucleus. Histological examination of serial sections of lateral pterygoid muscles confirms the notion that, at least in the rat, this muscle is devoid of muscle spindles.

Arrangement of Supramandibular and Suprahyoid Motoneurons in the Rat; A Fluorescent Tracer Study

The arrangement of the motoneurons innervating the supramandibular and suprahyoid muscles was studied in Wistar albino rats using two fluorescent tracers: nuclear yellow and true blue. All supramandibular motoneurons were found within the trigeminal motor nucleus; they appeared to be somatotopically arranged. The suprahyoid motoneurons were located in an accessory trigeminal-facial motor complex. No overlap of the motoneuron pools of the supramandibular and suprahyoid muscle group was observed. Only motoneurons ipsilateral to the treated muscles were labeled. It was shown that a one-to-one relationship always exists between motoneuron and muscle.

HISTOCHEMICAL DETECTION OF HORSERADISH-PEROXIDASE ACTIVITY IN THE RAT BY THE VIBRATOME-PARAPLAST METHOD

A method is described for the histochemical detection of horseradish peroxidase in Paraplast Plus embedded brain sections. The procedure uses 150-micron-thick Vibratome-cut slices of glutaraldehyde-paraformaldehyde-fixed brain tissue. Tetramethylbenzidine stabilized by diaminobenzidine/cobalt/H2O2 is used as chromogen. The Vibratome-cut slices are dehydrated through a graded series of acetone, cleared in toluol and flat-embedded in Paraplast Plus embedding medium. Serial sections can be cut as thin as 5-7 micron. The method is universal in its application and permits optimal visualization of labeled neurons with great morphological detail at the light-microscopic level.