Takahiko Nishiguchi

Location of cell somata and the peripheral course of axons of the geniohyoid and thyrohyoid motoneurons: A horseradish peroxidase study in the rat

The location of cell somata of the motoneurons supplying the rat geniohyoid and thyrohyoid muscles and the peripheral course of their axons were studied by intramuscular injection of horseradish peroxidase in combination with severing the innervating nerves of these muscles. Labeled geniohyoid motoneurons were found ipsilaterally in the ventrolateral subnucleus of the hypoglossal nucleus, which separates at midlevel of this nucleus from the main hypoglossal nucleus lying near the central canal and finally projects to the dorsomedial part of the ventral horn in the transition level of the brain/spinal cord. Geniohyoid motoneuron axons mostly pass through the hypoglossal nerve, whereas axons of the motoneurons in the transition level, though much fewer in number, pass through the first cervical nerve. Labeled thyrohyoid motoneurons were found ipsilaterally to form a cell column that extends from the caudal end of the main hypoglossal nucleus in the transition level of the brain/spinal cord to the dorsomedial part of the ventral horn in the caudal first cervical segment. All their axons passed through the first cervical nerve, and thus, the rostral portion of the thyrohyoid motoneuron column, which appears to be part of the hypoglossal nucleus, is not included in this nucleus but constitutes the supraspinal nucleus, which connects the hypoglossal nucleus to the ventral horn motoneuron group. The mean cell body sizes of the geniohyoid and thyrohyoid motoneurons were 28.7 +/- 4.4 and 29.3 +/- 4.6 micrometers, respectively, and their cell body size distributions were unimodal.

A horseradish peroxidase study of rat lingual motoneurons with axons passing through the cervical nerve

The peripheral course of axons of rat lingual motoneurons was studied by HRP injection into the hypoglossal nerve in combination with transecting of the hypoglossal and/or cervical nerve components of the hypoglossocervical plexus. Furthermore, soma sizes of labeled lingual motoneurons were compared in transverse section with those of labeled geniohyoid and thyrohyoid motoneurons, which are situated adjacent to the lingual motoneurons. We found that axons of the majority of lingual motoneurons lying in the main hypoglossal nucleus passed through the hypoglossal nerve throughout their course to the tongue. In a remaining small number of lingual motoneurons lying in a medial portion of the ventromedial subnucleus in the caudal fourth of the main hypoglossal nucleus, their axons passed through the first cervical nerve to the upper root of the ansa cervicalis to the hypoglossal nerve and then to its medial branch. The labeled lingual motoneurons with axons passing through the cervical nerve were intermingled with those whose axons passed through the hypoglossal nerve. The latter motoneurons, however, diminished in number while being traced caudally, and finally in the most caudal main hypoglossal nucleus the former motoneurons occupied a major part of this nucleus. The lingual motoneurons with axons passing through the cervical nerve were smaller in soma size than those with axons passing through the hypoglossal nerve. These two types of lingual motoneurons were both smaller in soma size than the geniohyoid and thyrohyoid motoneurons, and their soma shape was not as flat as that of the latter types of motoneurons.

SYNAPTIC CONNECTIONS OF A LOW-THRESHOLD MECHANORECEPTIVE PRIMARY NEURON WITHIN THE TRIGEMINAL SUBNUCLEUS ORALIS

The central axon of a primary afferent neuron that responded to indentation of the glabrous skin of the lower lip in a slowly adapting fashion was intra-axonally injected with horseradish peroxidase. The labeled terminal within the subnucleus oralis was examined electron microscopically. The labeled ending had a pale axoplasm and contained clear spherical synaptic vesicles. The labeled ending formed a synaptic triad with a dendrite and an unlabeled axonal ending with pleomorphic vesicles (a mixture of oval, flattened and dense core vesicles). The labeled primary ending was presynaptic only to the dendrite, while the unlabeled ending was presynaptic to both the dendrite and the labeled primary ending.

An HRP study of the location of the motoneurons supplying the tensor veli palatini muscle of the rabbit

Location of the motoneurons supplying the tensor veli palatini muscle of the rabbit was examined with the retrograde labeling technique following intramuscular injection of HRP. Labeled motoneurons were ipsilaterally located in the ventral or ventromedial portion of the rostral two-thirds of the motor trigeminal nucleus at the level of about 6.0 to 8.5 mm rostral to the obex. The location of the labeled motoneurons was ventromedial to the region supplying the masseter, the temporalis, and the medial pterygoid muscles and ventral to the region supplying the anterior digastric and the mylohyoid muscles, the location which coincided with the lateral pterygoid region. The labeled motoneurons were scattered around or in this region.

An HRP study of the motoneurons supplying the rat hypobranchial muscles

The locations of motoneurons (MNs) supplying the rat hypobranchial muscles (lingual, geniohyoid, and infrahyoid) and the peripheral courses of axons of these MNs were investigated by using a method of HRP injection into the hypoglossal nerve or these muscles in combination with severing of the hypoglossal and/or cervical components of the plexus hypoglossocervicalis. Moreover, some sizes of the MNs were investigated in both transverse and horizontal sections. The hypobranchial MNs formed a sequence of cell columns extending caudally from the hypoglossal nucleus, via the supraspinal nucleus, to the medial and then the ventrolateral subnuclei of the ventral horn of C1 to C3. The lingual and geniohyoid MNs were located in the hypoglossal nucleus. The majority of their axons passed solely through the hypoglossal nerve, whereas a small number of axons, whose somata lay in the caudal hypoglossal nucleus, passed through the first cervical nerve and then through the ansa cervicalis to reach the hypoglossal nerve. The infrahyoid MNs were located in the supraspinal nucleus and the ventral horn, and their axons passed through the first to third cervical nerves. The hypobranchial MNs were divided into three groups according to their size: the lingual, the geniohyoid and thyrohyoid, and lastly the other infrahyoid MNs, in order of smaller to larger size. The sizes of lingual, geniohyoid, and thyrohyoid MNs displayed a unimodal distribution both in transverse and horizontal sections, whereas the other infrahyoid MNs showed a bimodal distribution.