Amygdaloid projections to commissural interneurons for masticatory motoneurons

Injections of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into the trigeminal motor nucleus resulted in retrograde labeling of neurons, called commissural interneurons for masticatory motoneurons, in the contralateral supratrigeminal region. Further HRP studies, in which WGA-HRP injections were made into the amygdala and supratrigeminal region, indicated that the supratrigeminal region receives fibers from the central nucleus of the amygdala ipsilaterally. These findings raised the possibility of direct connections between the amygdala and commissural interneurons. In order to confirm the connections, electrolytic lesions in the central nucleus of the amygdala and WGA-HRP injections into the contralateral trigeminal motor nucleus were made on the same animal and electron microscopic observation was carried out on the supratrigeminal region. Of particular interest was that degenerating amygdalotegmental fibers synapsed upon HRP-labeled neurons.     

GABAergic and glycinergic neurons projecting to the trigeminal motor nucleus: A double labeling study in the rat

The distribution of GABAergic and glycinergic premotor neurons projecting to the trigeminal motor nucleus (Vm) was examined in the lower brainstem of the rat by a double labeling method combining retrograde axonal tracing with immunofluorescence histochemistry. After injection of the fluorescent retrograde tracer, tetramethylrhodamine dextran amine (TRDA), into the Vm unilaterally, neurons labeled with TRDA were seen ipsilaterally in the mesencephalic trigeminal nucleus, and bilaterally in the parabrachial region, the supratrigeminal and intertrigeminal regions, the reticular formation just medial to the Vm, the principal sensory and spinal trigeminal nuclei, the pontine and medullary reticular formation, especially the parvicellular part of the medullary reticular formation, the alpha part of the gigantocellular reticular nucleus, and the medullary raphe nuclei. Some of these neurons labeled with TRDA were found to display glutamic acid decarboxylase (the enzyme involved in GABA synthesis)-like or glycine-like immunoreactivity. Such double-labeled neurons were seen mainly in the supratrigeminal region, the reticular region adjacent to the medial border of the Vm, and the dorsal part of the lateral reticular formation of the medulla oblongata; a number of them were further scattered in the intertrigeminal region, the alpha part of the gigantocellular reticular nucleus, the nucleus raphe magnus, the principal sensory trigeminal nucleus, and the interpolar subnucleus of the spinal trigeminal nucleus. These neurons were considered to be inhibitory (GABAergic or glycinergic) neurons sending their axons to motoneurons in the Vm, or to local interneurons within and around the Vm. © 1996 Wiley-Liss, Inc.     

Involvement of reticular neurons located dorsal to the facial nucleus in activation of the jaw-closing muscle in rats

The location of excitatory premotor neurons for jaw-closing motoneurons was examined by the use of electrical and chemical stimulation and extracellular single-unit recording techniques in the anesthetized rat. Single-pulse electrical stimulation of the supratrigeminal region (SupV) and the reticular formation dorsal to the facial nucleus (RdVII) elicited masseter EMG response at mean (+/-SD) latencies of 2.22 +/- 0.59 ms and 3.10 +/- 1.14 ms, respectively. Microinjection (0.1-0.3 microl) of glutamate (50 mM) or kainate (0.5-100 microM) into RdVII increased masseter nerve activity in artificially ventilated and immobilized rats by 30.2 +/- 40.5% and 50.7 +/- 46.8% compared to baseline values, respectively. Forty reticular neurons were antidromically activated by stimulation of the ipsilateral trigeminal motor nucleus (MoV). Twenty neurons were found in RdVII, and the remaining 20 neurons were located in SupV, or areas adjacent to SupV or RdVII. Eleven neurons in RdVII responded to at least either passive jaw opening or light pressure applied to the teeth or tongue. Nine neurons responded to passive jaw opening. Five of the nine neurons responded to multiple stimulus categories. A monosynaptic excitatory projection from one neuron in RdVII was detected by spike-triggered averaging of the rectified masseter nerve activity. We suggest that reticular neurons in RdVII are involved in increasing masseter muscle activity and that excitatory premotor neurons for masseter motoneurons are likely located in this area. RdVII could be an important candidate for controlling activity of jaw-closing muscles via peripheral inputs.     

Trigeminal proprioceptive projections to the hypoglossal nucleus and the cervical ventral gray column

Trigeminal proprioceptive projections to the hypoglossal nucleus and the cervical ventral gray column in the cat were investigated by means of neuroanatomical and neurophysiological methods. Degeneration studies (Nauta and Fink-Heimer methods) involved circumscribed electrolytic lesions of the trigeminal mesencephalic nucleus and/or the supratrigeminal nucleus. Degenerated fibers in Probst's tract, which is composed of the central processes of trigeminal mesencephalic neurons, terminated in the ventrolateral portion of the ipsilateral hypoglossal nucleus and in the medial part of the ventral gray column of C₁–C₄. The descending juxtatrigeminal fascicle, a separate bundle of degenerated fibers, originated from the supratrigeminal region, which is known to receive processes from trigeminal mesencephalic neurons. This descending fascicle contributed fibers to the spinal trigeminal nucleus and the juxtatrigeminal reticular formation, from which region interneurons connect to the hypoglossal nucleus. Probst's tract, as well as the descending juxtatrigeminal fascicle, could be considered as parts of two separate polysynaptic pathways from trigeminal proprioceptors to those motoneurons responsible for the innervation of the tongue and infrahyoid musculature. Electrophysiological experiments revealed that proprioceptive muscle afferents from the masseter muscle project directly to the ipsilateral hypoglossal nucleus and to the ipsilateral upper cervical ventral column.     

Jaw-muscle spindle afferent pathways to the trigeminal motor nucleus in the rat.

Neural pathways conveying proprioceptive feedback from the jaw muscles were studied in rats by combining retrograde and intracellular neuronal labeling. Initially, horseradish peroxidase was iontophoresed unilaterally into the trigeminal motor nucleus (Vmo). Two days later, 1-5 jaw-muscle spindle afferent axons located in the mesencephalic trigeminal nucleus were physiologically identified and intracellularly stained with biotinamide. Stained mesencephalic trigeminal jaw-muscle spindle afferent axon collaterals and boutons were predominantly distributed in the supratrigeminal region (Vsup), Vmo, dorsomedial trigeminal principal sensory nucleus (Vpdm), parvicellular reticular formation (PCRt), alpha division of the parvicellular reticular formation (PCRtA), and dorsomedial portions of the spinal trigeminal subnuclei oralis (Vodm), and interpolaris (Vidm). Numerous neurons retrogradely labeled with horseradish peroxidase from the trigeminal motor nucleus were found bilaterally in the PCRt, PCRtA, Vodm, and Vidm. Retrogradely labeled neurons were also present contralaterally in the Vsup, Vpdm, Vmo, peritrigeminal zone, and bilaterally in the dorsal medullary reticular field. Putative contacts between intracellularly stained mesencephalic trigeminal jaw-muscle spindle afferent boutons and trigeminal premotor neurons retrogradely labeled with horseradish peroxidase were found in the ipsilateral Vodm, PCRtA, and PCRt, as well as the contralateral Vsup, Vmo, Vodm, PCRt, and PCRtA. Thus, multiple disynaptic jaw-muscle spindle afferent-motoneuron circuits exist. These pathways are likely to convey long-latency jaw-muscle stretch reflexes and may contribute to stiffness regulation of the masticatory muscles.     

Trigeminal mesencephalic neurons innervating functionally identified muscle spindles and involved in the monosynaptic stretch reflex of the lateral pterygoid muscle of the guinea pig

Location of the neurons in the trigeminal mesencephalic nucleus innervating stretch receptors of the lateral pterygoid muscle and the mode of their synaptic connection on the lateral pterygoid motoneurons of the guinea pig were studied physiologically as well as morphologically, in comparison with the trigeminal mesencephalic neurons innervating muscle spindles in the superficial masseter muscle, with the following results: stimulation of the caudal half of the trigeminal mesencephalic nucleus evoked monosynaptic excitatory postsynaptic potentials in the ipsilateral lateral pterygoid motoneurons. Stimulation of the lateral pterygoid nerve directly evoked spike potentials in the neurons located in the caudal half of the ipsilateral trigeminal mesencephalic nucleus, which responded with increased firing to stretch, and with silent period to twitch, of the ipsilateral lateral pterygoid muscle. Averaging of intracellular potentials of the lateral pterygoid motoneurons with extracellular spike potentials of these trigeminal mesencephalic neurons revealed excitatory postsynaptic potentials after a monosynaptic latency, but no inhibitory postsynaptic potentials. Injection of horseradish peroxidase into the lateral pterygoid muscle labeled 15-20 cells in the caudal half of the ipsilateral trigeminal mesencephalic nucleus, while 174-228 cells retrogradely labeled by horseradish peroxidase were found throughout the whole rostrocaudal extent of the ipsilateral trigeminal mesencephalic nucleus following injection of horseradish peroxidase into the masseter muscle. It was concluded that neurons in the caudal half of the trigeminal mesencephalic nucleus send their peripheral processes to stretch receptors, presumably muscle spindles, in the ipsilateral lateral pterygoid muscle and that their central processes have excitatory synapses on ipsilateral lateral pterygoid motoneurons, thus comprising the afferent limb of a monosynaptic stretch reflex arc of the lateral pterygoid muscle of the guinea pig.     

Synaptic actions of tectofugal pathways on abducens motoneurons in the cat.

Organization of pathways between the superior colliculus (CS) and abducens motoneurons (VI-MNs) was studied in cats under pentobarbital anesthesia using intracellular recordings from VI-MNs and adjacent reticular neurons. Latencies of EPSPs elicited by contralateral CS stimulation indicate that a small fraction of the excitatory pathway may be monosynaptic while its major part is disynaptic. As suggested by an analysis of synaptic responses to microstimulation of the paramedian pontine region, excitatory impulses descend in the tectobulbospinal tract after crossing at midbrain levels. An attempt was made to identify interneurons of the excitatory tectoabducens pathway in the region just ventral and rostroventral to the VI-nucleus. About one-quarter of the reticular neurons in this region received monosynaptic excitation specifically from the contralateral CS. They were acceptable as interneurons with regard to other response characteristics too. Axonal projection to, or through, the abducens nucleus was demonstrated for some of them by intranuclear microstimulation or by tracing axons after Procion yellow injections. It is suggested that "premotor" interneurons of the excitatory tectoabducens pathway are concentrated in the vicinity of the abducens nucleic. A similar investigation of inhibitory responses to ipsilateral CS-stimulation indicates that inhibitory pathways are at least disynaptic and, for the most part, contain three or more synapses. In its initial trajectory the inhibitory pathway appears to be identical with the tectobulbospinal tract,but it decussates for the second time at caudal pontine levels to reach ipsilateral VI-MNs.     

The physiological and morphological characteristics of interneurons caudal to the trigeminal motor nucleus in rats

In this study we have characterized the membrane properties and morphology of interneurons which lie between the caudal pole of the trigeminal motor nucleus and the rostral border of the facial motor nucleus. Previous studies suggest that many of these interneurons may participate in the genesis of rhythmical jaw movements. Saggital brainstem slices were taken from rats aged 5-8 days. Interneurons lying caudal to the trigeminal motor nucleus were visualized using near-infrared differential interference contrast (DIC) microscopy, and were recorded from using patch pipettes filled with a K-gluconate- and biocytin-based solution. The 127 neurons recorded could be categorized into three subtypes on the basis of their responses to injection of depolarizing current pulses, namely tonic firing (type I), burst firing (type II) and spike-adaptive (type III) neurons. Type I interneurons had a higher input resistance and a lower rheobase than type II neurons. All three neuron subtypes showed 'sag' of the voltage response to injection of large-amplitude hyperpolarizing current pulses, and, in addition, also showed rectification of the voltage response to injection of depolarizing current pulses, with type II neurons showing significantly greater rectification than type I neurons. The axonal arborizations were reconstructed for 44 of 63 neurons labelled with tracer. Neurons of each subtype were found to issue axon collaterals terminating in the brainstem nuclei, including the parvocellular reticular nucleus (PCRt), the trigeminal motor nucleus (Vmot), the supratrigeminal nucleus or the trigeminal mesencephalic nucleus. Twenty-five of the 43 neurons issued collaterals which terminated in the Vmot and the other brainstem nuclei. When viewed under 100x magnification, the collaterals of some interneurons were seen to give off varicosities and end-terminations which passed close to the somata of unidentified neurons in the trigeminal motor nucleus and in the area close to the interneuron soma itself. This suggests that the interneurons may make synaptic contacts both on motoneurons and also on nearby interneurons. These results provide data on the membrane properties of trigeminal interneurons and evidence for their synaptic connections both with nearby interneurons and also with motoneurons. Thus, the interneurons examined could play roles in the shaping, and possibly also in the generation, of rhythmical signals to trigeminal motoneurons.     

Dendritic ramifications of trigeminal motor neurons innervating jaw-closing muscles of rats

A cholera toxin subunit conjugated horseradish peroxidase (CTHRP) was found to be very useful in labelling the dendrites of motoneurons. CTHRP was injected individually to jaw-closing muscles (temporalis, masseter, and medial pterygoid) of rats, and their motoneurons including the dendrites were labelled and studied. The results show that the motoneuron cell bodies innervating temporalis, masseter, and medial pterygoid muscles are located in the trigeminal motor nucleus in dorsal, ventromedial and ventrolateral position. The dendrites of these motoneurons extend radially into mesencephalic nucleus, supratrigeminal nucleus, pontine reticular formation, trigeminal sensory nucleus and even into the bundles of the ascending root of the facial nerve. These dendrites may serve as an extended surface for various synaptic contacts to the jaw closing motoneurons. The possibility that they may also have presynaptic influence on the input to the trigeminal motoneurons is also discussed.     

Afferents to the trigeminal and facial motor nuclei in pigeon (Columba livia L.): Central connections of jaw motoneurons

Trigeminal and facial motor nuclei innervating the pigeon's jaw muscles were identified using a combination of microstimulation and EMG recording and HRP injections were made iontophoretically. The trigeminal motor nucleus receives an ipsilateral projection from sensory neurons in the trigeminal mesencephalic nucleus which forms the afferent limb of the monosynaptic stretch reflex of the jaw-closers. Both the trigeminal and facial motor nuclei receive bilateral projections from interneurons in the intertrigeminal area and the lateral (parvocellular) reticular formation of the pons and medulla. These neurons serve as premotor elements in the control of jaw movements, mediating ascending, descending and internuclear connections. The similarity of inputs to the trigeminal and facial nuclei may reflect their common function as jaw motoneurons in this species.     

Properties of secondary vestibular neurons fired by stimulation of ampullary nerve of the vertical, anterior or posterior, semicircular canals in the cat

Experiments on cats were performed to study the pathway and location of the secondary vestibulo-ocular neurons in response to stimulation of the ampullary nerves of the vertical, anterior or posterior, semicircular canals. Experiments on the medial longitudinal fasciculus transection disclosed that vertical canal-evoked, disynaptic excitation and inhibition were transmitted to the extraocular motoneurons through the contra- and ipsilateral medial longitudinal fasciculus respectively. Secondary vestibular neurons, which receive input from the ampullary nerve of the vertical semicircular canals and send their axons to contralateral medial longitudinal fasciculus, were intermingled in the rostral half of the descending and lateral part of the medial vestibular nuclei. A direct excitatory connection of some of these neurons to the target extraocular motoneurons was confirmed by means of a spike-triggered signal averaging technique. It was also found that neurons activated by antidromic stimulation of ipsilateral medial longitudinal fasciculus were located in the superior vestibular nucleus, some of which made direct inhibitory connections to the target extraocular motoneurons. Both excitatory and inhibitory vestibuloocular neurons made synaptic contact in about half of the impaled target motoneurons.     

Cervical afferent pathway to inferior oblique motoneuron in the cat

The neuronal pathway implicated in the vertical cervico-ocular reflex (COR) was investigated electrophysiologically in chloralose anesthetized cats. The effect of bilateral C2 dorsal root afferent stimulation on inferior oblique motoneurons (IO-MN) was investigated by intracellular recording. Control disynaptic excitatory postsynaptic potentials elicited in IO-MNs following stimulation of the contralateral anterior semicircular canal nerve (ACN) were invariably facilitated by conditioning stimulation to both ipsilateral and contralateral C2 dorsal roots (DR) in all motoneurons tested. This result indicates that inputs from the C2 DR of both sides and the contralateral ACN converge onto secondary vestibular neurons (‘common interneurons’) which project directly to the IO-MN. Common interneurons mediating vestibular and cervical excitation to the IO-MNs were studied in the vestibular nuclei on the side opposite to the motoneurons by extracellular recording. Nineteen vestibular neurons were identified as common interneurons; they were distributed in the caudal half of the lateral nucleus and the rostral half of the descending nucleus. The present experiment provides electrophysiological evidence of the projection of upper cervical afferents to the ipsilateral vestibular nuclei. The difference in neuronal organization between the horizontal and vertical COR is also briefly discussed.     

Catecholamine varicosities in cat dorsal root ganglion and spinal ventral roots

Convergence upon reticulospinal neurons which mediate disynaptic, contralateral pyramidal EPSPs to neck motoneurons has been examined in cats with contralateral pyramidal transection at the obex. Conditioning stimuli in the contralateral tectum and ipsilateral mesencephalic tegmentum produced monosynaptic facilitation of the disynaptic pyramidal EPSP, whereas facilitation evoked from the ipsilateral pyramid showed a disynaptic time course. These results show that contralateral pyramidal, tectal and ipsilateral tegmental fibers converge onto common reticulospinal neurons which have direct excitatory connections with neck motoneurons.     

Central oculomotor circuits

Recent data and hypotheses concerning the central oculomotor pathways are reviewed. Lateral and vertical eye movements are discussed successively, beginning in each case with the final common pathway and then progressing step by step along the main supranuclear tracts selectively involved in the 3 types of eye movements: vestibular movements, saccades and smooth pursuit. It is now established that the final common pathway of lateral eye movements in frontal-eyed species is the abducens nucleus, which controls not only the ipsilateral lateral rectus, but also, through the internuclear neurons, almost all the conjugate lateral activity of the opposite medial rectus. The ascending tract of Deiters, providing direct excitatory vestibular signals to the medial rectus motoneurons, could either have totally regressed in man or would play only a minor functional role. Likewise, a direct inhibition of the medial rectus motoneurons now seems unlikely or ineffective, the relaxation of this muscle resulting essentially from the disfacilitation mediated by the abducens internuclear neurons. This particular mechanism could be explained by the fact that the medial rectus motoneurons also receive messages of convergence, a slow disjunctive movement independent of lateral eye movements. Convergence is performed by excitatory reticular neurons near the oculomotor nucleus and by inhibitory pathways projecting onto the abducens motoneurons, perhaps passing through the internuclear neurons of the oculomotor nucleus. The premotor relay of horizontal reflex eye movements is the medial vestibular nucleus (M.V.N.) which contains excitatory and inhibitory neurons projecting onto the contralateral and ipsilateral abducens nuclei respectively. Afferences of the M.V.N. arise from: the labyrinth, through the vestibular nerve (vestibulo-ocular reflex); the neck, through the dorsal part of the medullary tegmentum (cervico-ocular reflex); the peripheral retina and the visual pathways (for the vestibular contribution of optokinetic nystagmus), perhaps via the pretectum, the nucleus reticularis tegmenti pontis (N.R.T.P.) and/or the nucleus prepositus hypoglossi (N.P.H.) (visuo-ocular reflex). The premotor relay for all ipsilateral saccades is the paramedian pontine reticular formation (P.R.F.) which excites the ipsilateral abducens nucleus and inhibits the contralateral abducens nucleus, via the burst inhibitory neurons located ventrally to the ipsilateral abducens nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hypoglossal and reticular interneurons involved in oro-facial coordination in the rat

Chewing, swallowing, breathing, and vocalization in mammals require precise coordination of tongue movements with concomitant activities of the mimetic muscles. The neuroanatomic basis for this oro-facial coordination is not yet fully understood. After the stereotaxic microinjection of retrograde and anterograde neuronal tracers (biotin-dextran, Fluoro-Ruby, Fluoro-Emerald, and Fluoro-Gold) into the facial and hypoglossal nuclei of the rat, we report here a direct bilateral projection of hypoglossal internuclear interneurons onto facial motoneurons. We also confirm the existence of a small pool of neurons in the dorsal part of the brainstem reticular formation that project ipsilaterally to both facial and hypoglossal nuclei. For precise tracer injections, both motor nuclei were located and identified by the electrical antidromic activation of their constituent motoneurons. Injections of retrograde tracers into the facial nucleus consistently labeled neurons in the hypoglossal nucleus. These neurons prevalently lay in the ipsilateral side, were small in size, and, like classic intrinsic hypoglossal local-circuit interneurons, had several thin dendrites. Reverse experiments - injections of anterograde tracers into the hypoglossal nucleus - labeled fine varicose nerve fiber terminals in the facial nucleus. These fiber terminals were concentrated in the intermediate subdivision of the facial nucleus, with a strong ipsilateral prevalence. Double injections of different tracers into the facial and the hypoglossal nuclei revealed a small, but constant, number of double-labeled neurons located predominantly ipsilateral in the caudal brainstem reticular formation. Hypoglossal internuclear interneurons projecting to the facial nucleus, as well as those neurons of the parvocellular reticular formation that project to both facial and hypoglossal nuclei, could be involved in oro-facial coordination.     

Anatomical regeneration and behavioral recovery following crush injury of the trigeminal root in lamprey

In normal larval lamprey, bilateral application of horseradish peroxidase (HRP) to the dorsal part of the anterior oral hood labeled subpopulations of trigeminal components on both sides of the brain; peripherally projecting motoneurons, medullary dorsal cells (sensory), and spinal dorsal cells (sensory), as well as centrally projecting afferents in the trigeminal descending tracts. Following unilateral crush injury of the right trigeminal root, HRP labeling of sensory and motor trigeminal components on the right side gradually increased with increasing recovery time, between 2 weeks and 12 weeks postcrush (PC). Axons of trigeminal motoneurons appeared to exhibit robust regeneration, whereas restoration of projections in the descending trigeminal tract ipsilateral to the injury was incomplete. Control experiments indicated that motor and sensory axons from the intact side of the oral hood did not sprout across the midline to the denervated side. Several results suggested that regenerated trigeminal sensory fibers made synapses with brain neurons that have direct or indirect inputs to reticulospinal (RS) neurons. Following a unilateral crush injury of the right trigeminal root, escape behavior in response to stimulation of the right side of the oral hood gradually returned to normal. Muscle recordings at various recovery times confirmed that anatomical regeneration of trigeminal sensory axons was functional. In addition, at 8 or 12 weeks PC, brief stimulation of the oral hood ipsilateral or contralateral to the crush injury elicited synaptic responses in RS neurons on either side of the brain, similar to that in normal animals. In the lamprey, compensatory mechanisms probably allow recovery of behavioral function despite incomplete regeneration of trigeminal sensory axons within the central nervous system. J. Comp. Neurol. 396:322–337, 1998. © 1998 Wiley-Liss, Inc.     

A survey of spinal collateral actions of feline ventral spinocerebellar tract neurons

The aim of this study was to identify spinal target cells of spinocerebellar neurons, in particular the ventral spinocerebellar tract (VSCT) neurons, giving off axon collaterals terminating within the lumbosacral enlargement. Axons of spinocerebellar neurons were stimulated within the cerebellum while searching for most direct synaptic actions on intracellularly recorded hindlimb motoneurons and interneurons. In motoneurons the dominating effects were inhibitory [inhibitory postsynaptic potentials (IPSPs) in 67% and excitatory postsynaptic potentials (EPSPs) in 17% of motoneurons]. Latencies of most IPSPs indicated that they were evoked disynaptically and mutual facilitation between these IPSPs and disynaptic IPSPs evoked by group Ia afferents from antagonist muscles and group Ib and II afferents from synergists indicated that they were relayed by premotor interneurons in reflex pathways from muscle afferents. Monosynaptic EPSPs from the cerebellum were accordingly found in Ia inhibitory interneurons and intermediate zone interneurons with input from group I and II afferents but only oligosynaptic EPSPs in motoneurons. Monosynaptic EPSPs following cerebellar stimulation were also found in some VSCT neurons, indicating coupling between various spinocerebellar neurons. The results are in keeping with the previously demonstrated projections of VSCT neurons to the contralateral ventral horn, showing that VSCT neurons might contribute to motor control at a spinal level. They might thus play a role in modulating spinal activity in advance of any control exerted via the cerebellar loop.     

Pathway of the blink reflex in the brainstem of the cat: Interneurons between the trigeminal nuclei and the facial nucleus

Blink reflex responses evoked by electrical stimulation of the supraorbital nerve were examined using cats and the pathway of the blink reflex in the brainstem was elucidated. Both early response (ER) and late response (LR) were mediated by the main sensory trigeminal nucleus and the spinal trigeminal nucleus. However, a lesion of the main sensory trigeminal nucleus had less effect on the blink reflex than a lesion of the spinal trigeminal nucleus. The ER was mediated not only by the shorter disynaptic pathway of 3 neurons through the trigeminal nerve, the trigeminal nuclei and the facial nucleus but also by a polysynaptic pathway of 4 neurons. The interneurons were located between the trigeminal nuclei and the facial nucleus. Some of these interneurons participated in the production of both ER and LR. The area of the brainstem responsible for ER and LR of the blink reflex was the reticular formation from the rostral part of the medulla to the pons except the medial area around the median sulcus. The LR interneurons were distributed more widely than the ER interneurons.     

Evidence for glycine as an inhibitory neurotransmitter of vestibular, reticular, and prepositus hypoglossi neurons that project to the cat abducens nucleus

The localization and distribution of brain-stem afferent neurons to the cat abducens nucleus has been examined by high-affinity uptake and retrograde transport of 3H-glycine. Injections of 3H-glycine selectively labeled (by autoradiography) only neurons located predominantly in the ipsilateral medial vestibular and contralateral prepositus hypoglossi nuclei, and in the contralateral dorsomedial reticular formation, the latter corresponding to the location of inhibitory burst neurons. The specificity of uptake and retrograde transport of 3H-glycine was indicated by the absence of labeling of the dorsomedial medullary reticular neurons ipsilateral and in close proximity to the injection site, where local uptake by diffusion could have occurred. The selectivity of uptake and transport was demonstrated by the absence of retrograde labeling following injections of 3H-GABA or 3H-leucine into the abducens nucleus. The immunohistochemical localization of glycine and GABA revealed a differential distribution of the 2 inhibitory neurotransmitter candidates in the extraocular motor nuclei. Glycine-immunoreactive staining of synaptic endings in the abducens nucleus was dense with a widespread soma-dendritic distribution but was sparse in the trochlear and oculomotor nuclei. By contrast, GABA-immunoreactive staining within the oculomotor and trochlear nuclei was associated with synaptic endings that were particularly prominent on the somata of motoneurons. GABA-immunoreactive staining in the abducens nucleus, however, was sparse. These differences between glycine- and GABA-immunoreactive staining in the extraocular motor nuclei were correlated with differences in the immunoreactivity of axons in the descending (glycine) and ascending (GABA) limbs of the medial longitudinal fasciculus. Glycine-immunoreactive neurons, furthermore, were observed in the same locations as neurons that were labeled autoradiographically by retrograde transport of 3H-glycine from the abducens nucleus. Electrophysiological recordings from abducens motoneurons and internuclear neurons revealed a marked reduction in the slow positivity of the orthodromic extracellular potential elicited by ipsilateral vestibular nerve stimulation following systemic administration of strychnine, an antagonist of glycine. Intracellular recordings demonstrated that the vestibular-evoked disynaptic inhibitory postsynaptic potentials in abducens neurons were effectively blocked by strychnine but were unaffected by picrotoxin, an antagonist of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)     

Afferent projections to the oral motor nuclei in the rat

Projections to the trigeminal, facial, ambiguus, and hypoglossal motor nuclei were determined by using horseradish peroxidase histochemistry. Most of the afferent projections to these motor nuclei were from the brainstem reticular formation, frequently in areas adjacent to other synergetic motor nuclei. The reticular formation lateral to the hypoglossal nucleus and reticular structures surrounding the trigeminal motor nucleus projected to each of these other brainstem motor nuclei involved in oral-facial function. Afferent projections to these motor nuclei also were organized along the rostrocaudal axis. Within the reticular formation most of the afferent projections to the trigeminal motor nucleus originated rostral to the majority of neurons projecting to the hypoglossal and ambiguus nuclei, which in turn were rostral to the primary source of reticular afferents to the facial nucleus. In comparison, projections from the sensory trigeminal nuclei and nucleus of the solitary tract were sparse. The interneuron pools that project to the orofacial motoneurons provide one further link in understanding the brainstem substrates for integrating oral and ingestive behaviors.