K. Tsuru

The distribution of muscle primary afferents from the masseter nerve to the trigeminal sensory nuclei

Transganglionic transport of horseradish peroxidase--wheat germ agglutinin conjugate was used to study the pattern of termination of somatic afferent fibers innervating the masseter muscle within the trigeminal sensory nuclear complex (TSNC) of the cat. The central processes of the masseteric nerve terminated in the caudal third of the pars interpolaris, and laminae I/V through the caudal two-thirds of caudalis and rostral parts of the C1 spinal cord segment. The functional significance of the masseteric afferent projections to the TSNC with a preferential pattern was discussed, particularly with respect to muscle pain.

Physiological and morphological characteristics of cat masticatory motoneurons--intracellular injection of HRP.

The physiology and morphology of masticatory motoneurons of adult cats were examined by the methods of intracellular recording and intracellular injection of horseradish peroxidase. Masseter and jaw-opening motoneurons were identified by intracellular recordings of the antidromic response following stimulation of the masseter and mylohyoid nerves, respectively. An excitatory postsynaptic potential (EPSP) was recorded from masseter neurons by stimulation of the masseter nerve with stimulus intensity below threshold for antidromic response. In contrast, the EPSP was not recorded from jaw-opening motoneurons by stimulation of the mylohyoid nerve with stimulus intensity below threshold for antidromic response. Patterns of postsynaptic potentials (PSPs) in the masseter motoneurons following stimulation of the tooth pulp or periodontal afferents were classified into 4 types: hyperpolarization (n = 40), depolarization-hyperpolarization (n = 9), hyperpolarization-depolarization (n = 5), and depolarization with spike potentials (n = 10). On the other hand, patterns of the PSPs in the jaw-opening motoneurons following stimulation of the same afferents were classified into two types: depolarization with spike potentials (n = 19), and hyperpolarization (n = 5). Twenty-five masseter and 7 jaw-opening motoneurons and an intranuclear neuron were reconstructed from serial sections in the transverse plane. On the basis of dendritic morphology, the masseter motoneurons could be classified into two major groups, type I (n = 15) and type II (n = 9), whereas two neurons were found to constitute a separate category of the masseter motoneuron. The dendritic distributions of all the jaw-opening motoneurons examined were generally similar and there was no indication of the existence of subtypes, whereas there were 2 or 3 subgroups in type I and type II masseter motoneurons. Type I masseter neurons had primary dendrites which extended radially in all directions, and the whole profile of their dendritic trees presented a spherical and an egg-shaped appearance. In type II masseter neurons, the origin of primary dendrites was bipolar or tripolar, and the whole profile of their dendritic trees presented a hemispherical and mirror-imaged, funnel-shaped appearance. The other two masseter motoneurons had a particular dendritic tree which was much simpler in configuration, with less tapering or branching than those of other neurons examined. In contrast, the dendritic profiles of all the jaw-opening motoneurons were similarly organized and showed vertically oriented dendritic trees which were more developed in the dorsomedial than in the ventrolateral direction.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological and morphological characteristics of periodontal mesencephalic trigeminal neurons in the cat — intra-axonal staining with HRP

Intra-axonal recording and horseradish peroxidase (HRP) injection techniques were employed to define the response properties of periodontal mechanoreceptive afferents originating from the trigeminal mesencephalic nucleus (Vmes) and their morphological characteristics. The periodontal Vmes neurons were classified into two types: slowly adapting (SA) and fast adapting (FA) types. The central terminals of 7 SA and 4 FA afferents were recovered for detailed analyses. The whole profile of SA and FA neurons were unipolar in shape and their cell bodies were located in the dorsomedial parts of the Vmes. The united (U) fiber traveled caudally from the soma to the dorsolateral aspect of the trigeminal motor nucleus (Vmo), where it split into the peripheral (P) and C fibers with a T- or Y-shaped appearance. The P fiber joined the trigeminal sensory or motor tract. The C fiber descended caudally within Probsts tract. All 3 stem fibers issued main collaterals. The main collaterals of all neurons examined formed terminal arbors in the supratrigeminal nucleus (Vsup) and all but two SA neurons projected to the intertrigeminal region (Vint), while the projections to other nuclei of the trigeminal motor nucleus (Vmo), juxtatrigeminal region (Vjux), main sensory nucleus (Vp) and oral nucleus (Vo.r) differed between SA and FA afferents and between neurons of the same type. The SA and FA neurons were classified into three and two subgroups, respectively. The major differences in central projections between the two types were that all the FA neurons projected to the Vp or Vo.r but none of SA type and this relation was reversed in the projection to the Vjux, and that more than half of SA neurons projected to Vmo but only one FA neuron to the Vmo. The Vmes neurons which sent their collaterals into the Vmo had the P fiber passing through the tract of the trigeminal motor nerve. The average size of somata and mean diameters of U fibers and main collaterals from C fiber were significantly larger in SA neurons than FA neurons. The average size of fiber varicosities became smaller in the following nuclei, Vmo, Vsup, Vp, Vint and Vo.r, but not significant between the two functional types. The functional role of the periodontal Vmes afferents to jaw reflexes was discussed particularly with respect to their central projection sites in the brainstem nuclei.

Central terminations of periodontal mechanoreceptive and tooth pulp afferents in the trigeminal principal and oral nuclei of the cat.

Intra-axonal recording and horseradish peroxidase (HRP) injection techniques were employed to define the response properties of low-threshold mechanoreceptive periodontal afferents and of the tooth pulp afferents and the morphological characteristics of their axon arbors in the nucleus principalis (Vp) and rostrodorsomedial (Vo.r) and dorsomedial parts (Vo.dm) of the nucleus oralis (Vo). The central terminals of 3 fast adapting (FA) and 4 slowly adapting (SA) periodontal afferents and 4 tooth pulp (TP) afferents were recovered for detailed analyses. Stained axons in the trigeminal sensory tract ascended and descended (bifurcating fibers), or descended without bifurcation (descending non-bifurcating fibers). The ratio of the bifurcating fibers to the descending non-bifurcating fibers was about three to one for each type of afferents. The main collaterals given off from the ascending branches terminated in the Vp. Most collaterals given off from the descending branches terminated in the Vo with the exception of few instances. In case of the FA afferents, the ascending branches gave off all main collaterals into the Vp with rostrocaudal and dorsoventral continuities in their arbors, whereas the descending branches gave off all main collaterals, except two collaterals, into the Vo with rostrocaudal discontinuities. The projections from the FA afferents to the Vo.dm was predominant in terms of the number of boutons and the length of preterminal and terminal branches. In case of the SA afferents, the collaterals from the ascending and descending branches formed rostrocaudally and dorsoventrally discontinuous terminal arbors. In terms of the density of boutons the SA afferents were divided into two subtypes. One had a preferential projection into the Vp or Vo, whereas others lacked a selective projection. In case of the TP afferents, the main collaterals of the ascending branches formed partially overlapping terminal arbors, but the terminal arbors formed by the collateral of the descending branches did not overlap. The frequency of collaterals of the TP afferents was less than that of the other types of afferents. The terminal arbors including the density of boutons of the pulpal afferents were less extensive than those of the other types of afferents. The average size of varicosities became smaller in the following subdivisions. Vp, Vo.r and Vo.dm for SA and TP afferents. The size of varicosities of the TP afferents was smaller and that of the FA afferents was larger than that of the SA afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological and functional properties of trigeminal nucleus oralis neurons projecting to the trigeminal motor nucleus of the cat

Horseradish peroxidase (HRP) was injected into the somata located in the rostrodorsomedial part (Vo.r) of the trigeminal nucleus oralis; an axonal projection to the trigeminal motor nucleus (Vmo) was demonstrated in two Vo.r neurons. The two neurons differed in their morphological and functional properties. The first Vo.r neuron responded to stimulation of low-threshold mechanoreceptors and its stem axon gave off massive axon collaterals that issued terminal branches to the dorsolateral subdivision of Vmo, Vo.r, and the medial and lateral parts of the lower brainstem reticular formation. The second Vo.r neuron was activated by stimulation of the tooth pulp or lingual nerve at twice longer latency than that of the first neuron. This stem axon was divided into two main ascending and one descending branches, and one of the main ascending branches was further bifurcated into two branches. The main non-bifurcated ascending branch gave off 4 collaterals, two of which sent terminal branches into the dorsolateral subdivision of Vmo and others into the Vo.r and juxta-trigeminal regions. The somato-dendroarchitectonic differences were also described in the two Vo.r neurons stained.

Morphology of masticatory motoneurons stained intracellularly with horseradish peroxidase

Masticatory motoneurons were identified electrophysiologically and stained with horseradish peroxidase (HRP). The masseter motoneurons could be divided into 3 groups on the basis of their dendritic morphology. In contrast, the digastric or mylohyoid motoneurons showed a similar dendritic configuration. These neurons had much developed dendritic trees in the dorsomedial than ventrolateral direction. The first group of the masseter motoneurons had their dendritic trees which extended radially in all directions with a slight preference to project rostrally. These somata were located in the center of the subdivision containing the masseter motoneurons. In the second group, their dendritic arbores had a polarity extending hemispherically. These neuronal somata were located in the medial, ventral, and lateral regions of the subdivision. For the masseter motoneurons in the two groups and jaw-opening motoneurons, the dendritic swellings were frequently observed in the distal branches. The third group had their dendritic trees which were much simpler in configurations with less tapering or branching than those of other neurons examined. Furthermore, a wide variety of dendritic spines and appendages, and no dendritic swellings, observed in the third group were distinct from other neurons stained. The dendritic trees of the jaw-closing and -opening motoneurons were confined to the individual subdivisions. There were no instances in which axon collaterals were observed for well-stained 16 axons.