An evaluation of retrograde tracing methods for the identification of chemically distinct cochlear efferent neurons.

We have compared retrograde labelling of rat olivocochlear neurons after unilateral cochlear injections of wheatgerm agglutinin conjugated horseradish peroxidase (WGA-HRP) and free HRP. After cochlear injection of WGA-HRP, labelling of nerve cell bodies in the brainstem can be explained not only as conventional retrograde labelling resulting from uptake by efferent nerve terminals synapsing on or near hair cells, but also as spurious labelling originating from tracer leakage, through the periotic duct and over the eighth nerve sheaths, into the cerebral-spinal fluid. Depending on the length of survival time, spurious labelling can involve small portions of the nucleus of the trapezoid body or the entire auditory brainstem and other non-auditory centers. On the contrary, moderate amounts of free HRP delivered to the cochlea do not lead to spurious labelling. With free HRP as the tracer of choice, we found that cochlear efferent cells were located not only in the ipsilateral LSO body and bilaterally within MVPO and RPO as already described by White and Warr, but also surrounding the ipsilateral LSO and in the ipsilateral LVPO. The allocation of these newly described olivocochlear neurons to the medial large cell or lateral small cell system is uncertain because they are located laterally in the brainstem and project ipsilaterally but are large spherical to fusiform or multipolar cells. A zinc salicylate-formol fixative and a metal intensified DAB reaction were found to be effective in visualizing retrogradely transported HRP in neurons and allowed immunocytochemical staining of the same sections with antisera to glutamic acid decarboxylase and choline acetyltransferase. This double label protocol can be used to produce a neurochemical map of the OC systems.     

Primary projections of the trigeminal nerve in two species of sturgeon: Acipenser oxyrhynchus and Scaphirhynchus platorynchus

Horseradish peroxidase histochemical studies of afferent and efferent projections of the trigeminal nerve in two species of chondrostean fishes revealed medial, descending and ascending projections. Entering fibers of the trigeminal sensory root project medially to terminate in the medial trigeminal nucleus, located along the medial wall of the rostral medulla. Other entering sensory fibers turn caudally within the medulla, forming the trigeminal spinal tract, and terminate within the descending trigeminal nucleus. The descending trigeminal nucleus consists of dorsal (DTNd) and ventral (DTNv) components. Fibers of the trigeminal spinal tract descend through the lateral alar medulla and into the dorsolateral cervical spinal cord. Fibers exit the spinal tract throughout its length, projecting to the ventral descending trigeminal nucleus (DTNv) in the medulla and to the funicular nucleus at the obex. Retrograde transport of HRP through sensory root fibers also revealed an ascending bundle of fibers that constitutes the neurites of the mesencephalic trigeminal nucleus, cell bodies of which are located in the rostral optic tectum. Retrograde transport of HRP through motor root fibers labeled ipsilateral cells of the trigeminal motor nucleus, located in the rostral branchiomeric motor column.     

Bronchomotor vagal preganglionic cell bodies in the dog: an anatomic and functional study.

A previous study in our laboratory demonstrated that the stimulation with microinjection of DL-homocysteic acid of cell bodies in the rostral portion of the external formation of the nucleus ambiguus (Aext) increased total lung resistance in dogs. In the present study anatomic experiments were conducted in dogs to determine if the rostral Aext contains vagal preganglionic cell bodies that give rise to axons in the pulmonary branches of the vagus nerve. The application of horseradish peroxidase (HRP) to either the pulmonary branches or the vagus at a point between the pulmonary branches and the cardiac branches resulted in retrograde labeling of cell bodies in both rostral Aext and the dorsal motor nucleus of the vagus (DMN). On the other hand, application of HRP to the vagus at a point below the pulmonary branches did not result in any retrogradely labeled cell bodies in rostral Aext but did result in labeled cell bodies in DMN. In another series of experiments DL-homocysteic acid (2.5 nmol in 25 nl) was microinjected at sites in rostral Aext and DMN. As we previously reported the injection of DL-homocysteic acid in rostral Aext increased total lung resistance. In contrast, in the same animals, the injection of DL-homocysteic acid in DMN did not change total lung resistance. We conclude that bronchomotor vagal preganglionic cell bodies are located in rostral Aext but not in DMN. The functional significance of vagal preganglionic cell bodies in DMN whose axons contribute to the pulmonary branches of the vagus nerve remains to be determined.     

Corpuscular bodies in the palate of the rat. 2. Innervation and central projection.

The source of innervation of the corpuscular bodies in the palate and the central projections of the afferent fibres of the entire palate was studied in rats by transganglionic transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) and with substance P (SP) immunohistochemistry. WGA-HRP injected into the incisal papilla was taken up by the nerve fibres that terminated in the corpuscles. Retrogradely labelled neurons were observed in the trigeminal ganglion as well as anterogradely labelled terminals in the dorsolateral part of the spinal trigeminal nucleus and in the lateral part of the nucleus of the solitary tract. No labelling could be found in the geniculate ganglion, the facial nerve and the hypoglossal nucleus. Following WGA-HRP injection in the intermolar area and in the soft palate, labelling was only restricted to the trigeminal ganglion. The lamina propria of the entire palate and the corpuscle-enriched area of the incisal papilla and the soft palate were richly innervated by SP-containing fibres. Numerous SP-containing fibres were also observed in the nerve plexus at the base of the corpuscle. In addition, SP-positive neurons were identified in the trigeminal ganglion and SP-labelled terminals in the sensory trigeminal nuclear complex and in the solitary tract nucleus. On the basis of our morphological observations we conclude that the palatal corpuscular bodies are involved in taste perception which is of trigeminal origin.     

Desheathing the nodose ganglion is not a reliable method of de-efferentation in the ferret

The present study reports the results of physiological and anatomical experiments in which the purpose was to determine whether desheathing the nodose ganglion is a reliable method of vagal de-efferentation in the ferret. In physiological studies, the effects of electrically stimulating the treated and untreated vagal nerves on cardiovascular and intestinal responses were examined and compared with previously obtained data after left supranodose vagotomy. The anatomical studies illustrated the effects of desheathing the left nodose ganglion on the transport of horseradish peroxidase (HRP) within a thoracic vagal communicating branch. These data were compared to data from control animals and animals that had undergone left supranodose vagotomy. The results demonstrated that severing the fascicles overlying the left nodose ganglion and allowing the nerve fibers to degenerate, caused no reduction in labeled efferent cell bodies in the left dorsal motor nucleus of the vagus as compared to controls. However, after left supranodose vagotomy there were no efferent cell bodies labeled in the left dorsal motor nucleus of the vagus. Following degeneration of the fascicles, electrical stimulation of the peripheral cut end of this nerve did not abolish the efferent responses in 7 out of 9 animals studied, whereas supranodose vagotomy abolished the responses in all animals. These findings demonstrate that desheathing the nodose ganglion and thereby removing the nerve bundles overlying the nodose ganglion is not a guaranteed method of destroying the efferent fibers in the vagus nerve of the ferret. Supranodose vagotomy, therefore, is a more reliable method of de-efferentation in this species.     

Innervation and cytochemistry of the neuroepithelial bodies in the ciliated epithelium of the toad lung (Bufo marinus)

Summary Neuroepithelial bodies (NEB) were identified in the lung of Bufo marinus. The characteristics of the cells and their innervation were studied with electron and fluorescence microscopy before and after close vagosympathetic denervation. The bodies consist of low columnar cells which rest on the epithelial basal lamina. The majority of the cells do not reach the lumen of the lung (basal cells); the few which do (apical cells) are bordered by microvilli and possess a single cilium. The neuroepithelial cell cytoplasm contains a variety of organelles the most characteristic of which are dense cored vesicles. Microspectrofluorometry and electron microscopic cytochemistry indicate significant quantities of 5-hydroxytryptamine in these cells. The neuroepithelial bodies could be divided into three groups on the basis of their innervation: 1) About 60% of the NEBs are innervated solely by nerve fibres containing agranular vesicles which form reciprocal synapses; 2) about 20% are innervated solely by adrenergic nerve fibres which form distinct synaptic contacts; and 3) the remaining 20% are innervated by both types of nerve fibres. It is proposed that the NEBs are receptors monitoring intrapulmonary P_{CO 2} and so leading to modulation of activity in afferent nerve fibres (type containing agranular vesicles). The presence of NEBs solely with an adrenergic (efferent) innervation poses a problem with this interpretation.     

Substance P-immunoreactive neurons in the brainstem of the cat related to cardiovascular centers

Summary The distribution of substance P-immunoreactivity (SP-IR) in the brainstem and spinal cord of normal and colchicine-pretreated cats was analysed using the peroxidase-antiperoxidase (PAP) technique. Numerous SP-IR fibers are present in the nucleus solitarius, nucleus dorsalis nervi vagi and nucleus spinalis nervi trigemini, various parts of the formatio reticularis, substantia grisea centralis mesencephali, locus coeruleus and nucleus parabrachialis. SP-IR perikarya occur in the substantiae gelatinosa and intermedia of the spinal cord, the nucleus spinalis nervi trigemini-pars caudalis, the nucleus dorsalis nervi vagi, and the nucleus solitarius, as well as in the adjacent formatio reticularis and the medullary nuclei of the raphe. In addition, SP-IR cell bodies are located in the nuclei raphe magnus and incertus, ventral and dorsal to the nucleus tegmentalis dorsalis (Gudden), nucleus raphe dorsalis, substantia grisea centralis mensencephali, locus coeruleus, nucleus parabrachialis and colliculus superior.The results indicate that SP-IR neurons may be involved in the regulation of cardiovascular functions both at the central and peripheral level. A peripheral afferent portion seems to terminate in the nucleus solitarius and an efferent part is postulated to originate from the nucleus dorsalis nervi vagi and from the area of the nuclei retroambiguus, ambiguus and retrofacialis.     

Sensory projections to the nucleus basalis prosencephali of the pigeon

Summary The afferent pathways to the nucleus basalis prosencephali of the pigeon were studied by use of the horseradish peroxidase (HRP) technique. It was confirmed that this nucleus receives a direct pathway from the nucleus sensorius principalis nervi trigemini and that, as in the starling, it receives a direct input from the nucleus lemnisci lateralis, pars ventralis, an auditory relay. Totally novel is the finding that the nucleus basalis prosencephali is the target of a direct pathway originating in the medullary nucleus vestibularis superior. All three pathways bypass the thalamus. From within the telencephalon the nucleus basalis prosencephali also receives fibres from the tuberculum olfactorium and the peri-ectostriatal belt, suggestive of olfactory and visual input. Marked cell bodies were also found in the neostriatum frontolaterale. It is assumed that these arose from HRP uptake by axons of the tractus fronto-archistriatalis that course through the nucleus basalis prosencephali to the anterodorsal archistriatum. Marked fibres and bouton-like formations were observed in the latter structure. The afferents to the nucleus basalis prosencephali are discussed in conjunction with the probable role of the nucleus as a sensorimotor coordinator of the pecking/feeding behaviour of the pigeon.     

Distribution of serotonin and FMRF-amide in the brain of Lymnaea stagnalis with respect to the visual system

Despite serotonin’s and FMRF-amide’s wide distribution in the nervous system of invertebrates and their importance as neurotransmitters,the exact roles they play in neuronal networks leaves many questions.We mapped the presence of serotonin and FMRF-amide-immunoreactivity in the central nervous system and eyes of the pond snail Lymnaea stagnalis and interpreted the results in connection with our earlier findings on the central projections of different peripheral nerves.Since the chemical nature of the intercellular connections in the retina of L.stagnalis is still largely unknown,we paid special attention to clarifying the role of serotonin and FMRF-amide in the visual system of this snail and compared our findings with those reported from other species.At least one serotonin-and one FMRF-amidergic fibre were labeled in each optic nerve,and since no cell bodies in the eye showed immunoreactivity to these neurotransmitters,we believe that efferent fibres with somata located in the central ganglia branch at the base of the eye and probably release 5HT and FMRF-amide as neuro-hormones.Double labelling revealed retrograde transport of neurobiotin through the optic nerve,allowing us to conclude that the central pathways and serotonin-and FMRF-amideimmunoreactive cells and fibres have different locations in the CNS in L.stagnalis.The chemical nature of the fibres,which connect the two eyes in L.stagnalis,is neither serotoninergic nor FMRF-amidergic.     

Immunohistochemical evidence for cholecystokinin-like peptides in neuronal cell bodies of the rat spinal cord

Summary Cholecystokinin-like immunoreactivity has been demonstrated by radioimmunoassay and immunocytochemistry in the spinal cord of various mammals, in particular in nerve fibers of the superficial layers of the posterior column, but had not been detected in neuronal cell bodies. We report immunohistochemical evidence for the presence of a group of cholecystokinincontaining neuronal cell bodies in the lumbar spinal cord of the rat. This group of cells is only visualized after direct injection of colchicine into the spinal cord and is located near the central canal in the intermedio-medial nucleus of area X of Rexed.     

Immunoreactive pancreatic polypeptide (PP) occurs in the central and peripheral nervous system: Preliminary immunocytochemical observations

Summary Pancreatic polypeptide (PP) is a candidate hormone of unknown physiological significance. It is produced by a population of endocrine cells in the pancreas. In the present study a PP-like peptide was found to occur in the mammalian and avian central and peripheral nervous systems. Immunoreactive nerve fibres and nerve cell bodies were widely distributed in the brain. Dense accumulations of nerve fibres occurred in the following areas: nucleus accumbens, interstitial nucleus of the stria terminalis, para- and periventricular hypothalamic nuclei, and medial preoptic area. In addition, nerve fibres were regularly seen in cortical areas. Immunoreactive perikarya were observed in the following regions: cortex, nucleus accumbens, neostriatum and septum. In the gut, immunoreactive nerve fibers were distributed in the myenteric plexus, in smooth muscle, around blood vessels, and in the core of the villi. Immunoreactive perikarya occurred in the submucosal and myenteric plexus, suggesting that PP immunoreactive nerves are intrinsic to the gut.In the species examined, the neuronal PP-like peptide could be demonstrated with an antiserum raised against avian PP, but not with those raised against bovine or human PP. Thus, neuronal PP is distinct from the PP that occurs in pancreatic endocrine cells.     

5—HT能纤维向猫脊髓膈神经核及延髓膈肌前运动神经元的投射

实验在6只成年猫上进行,将WGA－HRP微量注入C5膈神经核内,通过逆行追踪及5－HT免疫组织化学FITC荧光双重标记方法,研究了中缝核5－HT能神经元向脊髓膈神经核的投射,同时观察了延髓膈肌产运动神经元接受5－HT能纤维投射的情况,结果在中缝苍白核观察到较多的HRP－5－HT双标记神经元,在中缝大核,中缝隐核观察到少数散在的双标记神经元,在延髓疑核,孤束核腹外侧区域的HRP单核记神经元（即膈肌前运动神经元）周围观察到5－HT能轴突末梢,结构表明：发自中缝苍白核5－HT能神经元的传出纤维可投射到脊髓膈神经核,延髓膈肌前运动神经元接受5－HT能纤维的传入投射。     

Galanin-like immunoreactivity in the chicken brain

The anatomical distribution of neurons containing galanin has been studied in the central nervous system of the chicken by means of immunocytochemistry using antisera against rat galanin. Major populations of immunostained perikarya were detected in several brain areas. The majority of galanin-immunoreactive cell bodies was present in the hypothalamus and in the caudal brainstem. Extensive groups of labeled perikarya were found in the paraventricular, periventricular, dorsomedial and tuberal hypothalamic nuclei, and in the nucleus of the solitary tract in the medulla oblongata. In the telencephalon, immunoreactive perikarya were observed in the preoptic area, in the lateral septal nucleus and in the hippocampus. The mesencephalon contained only a few galanin-positive perikarya located in the interpeduncular nucleus. Immunoreactive nerve fibers of varying density were detected in all subdivisions of the brain. Dense accumulations of galanin-positive fibers were seen in the preoptic area, periventricular region of the diencephalon, the ventral hypothalamus, the median eminence, the central gray of the brainstem, and the dorsomedial caudal medulla. The distributional pattern of galanin-immunoreactive neurons suggests a possible involvement of a galanin-like peptide in several neuroregulatory mechanisms.     

Brainstem neurons projecting to the ampullae of anterior,lateral, and posterior semicircular canals in the guinea pig

Neurons projecting to the ampullae of anterior, lateral, and posterior semicircular canals were identified in the guinea pig brainstem using horseradish peroxidase labeling techniques. Two groups of neurons forming bilateral connections were found, one located dorsally and the other ventrally to facial nerve trajectories. The dorsal group of vestibular efferent neurons projecting to all three canals was detected in the subependymal granular layer of the floor of the 4th ventricle lateral to the facial nerve genu and in the abducent nerve nucleus. Efferent neurons belonging to the ventral group were unevenly distributed through different areas of the parvocellularis nucleus and the rostral portion of the pontine caudal reticular nucleus. The morphological characteristics and distribution pattern of these cells are taken as confirmation of their heterogeneity of neuronal and functional organization in the vestibular efferent system of semicircular canal ampullae.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 4, 1988, pp. 526–532.     

Lateral hypothalamic dynorphinergic efferents to the amygdala and brainstem in the rat

Dynorphin is present within perikarya of the lateral hypothalamus (LH) and perifornical nucleus (PeF), and within nerve terminals of the central nucleus of the amygdala, central grey, parabrachial nucleus, and the dorsal vagal complex (nucleus of the solitary tract and dorsal motor nucleus of the vagus). Each of these nuclei receive efferent projections from the LH and PeF. In this study, the possibility that dynorphin cells with LH and PeF innervate each of these nuclei was investigated using a combined retrograde tracing-immunofluorescence technique. As enkephalinergic perikarya have also been localized to LH and PeF, peptide E (an enkephalin precursor fragment) was also studied for comparison. Following injections of fast blue into the central nucleus, parabrachial nucleus, central grey, and dorsal vagal complex, numerous retrogradely-labeled dynorphin-immunoreactive neurons were present within the LH and PeF. In comparison, retrogradely-labeled peptide E-immunoreactive cells were infrequently observed. These results suggest the LH and PeF to be a major source of dynorphin to the forebrain and brainstem.     

Substance P in the suprachiasmatic nucleus of the rat: an immunohistochemical and in situ hybridization study

Using a biotin-streptavidin-horseradish peroxidase (HRP) immunohistochemical technique the distribution of substance P-immunoreactive neuronal elements was investigated in the rat suprachiasmatic nucleus (SCN). Substance P-immunoreactive nerve fibres and varicosities were distributed throughout the suprachiasmatic nucleus, with the largest accumulation in its ventral part. Because this location overlaps with the innervation of retinal afferents, the distribution and density of substance P-immunoreactive fibres in bilaterally enucleated rats were compared to normal rats. The density of substance P-immunoreactive fibres and nerve terminals in the ventral part of the suprachiasmatic nuclei was reduced in the rats with bilateral destruction of the optic nerves, whereas the density of fibres and nerve terminals in the dorsal part as well as other retinal target areas in the thalamus and mesencephalon was unaffected. In rats pretreated with an intraventricular injection of colchicine several substance P-immunoreactive perikarya were identified in the suprachiasmatic nucleus. The immunoreactive neurons, measuring 9.7 m±1.1 m in diameter, were frequently observed in the central core of the nucleus and to a lesser extent in the dorsomedial and ventrolateral subparts. Using in situ hybridization histochemistry pre-protachykinin-A mRNA was found in the same part of the SCN indicating that synthesis of substance P takes place in SCN neurons. Using a double immunohistochemical approach applying diaminobenzidine and benzidinedihydrochloride as chromagens substance P-, vasoactive intestinal peptide (VIP)-, and vasopressin/neurophysin-immunoreactivities were identified in the same brain section. The substance P-immunoreactive perikarya constituted a separate population of SCN neurons, which were not vasopressin-, neurophysin- or VIP-immunoreactive. Taken together, these observations show that substance P is contained in the retinohypothalamic pathway and within a group of SCN cell bodies, indiating that substance P may play a role in the generation and entrainment of circadian rhythmicity.     

正常豚鼠耳蜗核一氧化氮合酶阳性神经元的投射

本文采用辣根过氧化物酶（ＨＲＰ）逆行追踪技术结合硫辛酰胺脱氨酸（ＮＡＤＰＨ－ｄ）组织化学方法,研究正常豚鼠耳蜗核一氧化氮合酶（ＮＯＳ）阳性神经元的上行投射特点。探讨耳蜗核ＮＯＳ阳性神经元在听觉信号传递中的可能作用。结果表明,一侧上橄榄复合体加压注射ＨＲＰ后,两侧耳蜗核均出现ＨＲＰ标记细胞,同侧耳蜗核ＮＯＳ－ＨＲＰ双标细胞较多占８２．６３％,并可见ＨＲＰ阳性纤维和终末包绕ＮＯＳ阳性胞体,对侧耳蜗核ＮＯＳ－ＨＲＰ双标细胞相对较少,仅占１４．８７％。一侧下丘加压注入ＨＲＰ后两侧耳蜗核均无ＨＲＰ－ＮＯＳ双标细胞。结果提示,耳蜗核ＮＯＳ阳性神经元向上橄榄复合体投射,可能具有调节听觉声信号传递的作用     

Efferent neurons of the lateral-line system and the VIII cranial nerve in the brainstem of anurans

Summary The octavo-lateral efferent system of several anuran species was studied by means of retrograde transport of horseradish peroxidase. This system is organized similarly in all larval anurans and in all adult aglossids. All have two groups of efferent neurons in the nucleus reticularis medialis between the VIIIth and the IXth motor nucleus. The caudal group consists of efferent neurons that supply the posterior lateral-line nerve (NLLp) and a considerably smaller group of neurons supplying both the NLLp and the anterior lateral-line nerve (NLLa). The rostral group is composed of efferent neurons supplying the NLLa, neurons projecting to the inner ear and neurons supplying both the inner ear and the NLLa. Efferent neurons of the VIIIth cranial nerve exhibit a rostrocaudal cytoarchitectonic differentiation. Caudal perikarya, which are rounder in shape than those of the rostral part, have a dendritic projection to the superior olive. It is suggested that this differentiation reflects a functional differentiation of acoustic and vestibular efferent neurons.Labeled neurons were ipsilateral to the site of application of HRP. None were found in the vestibular nuclei or in the cerebellum.Efferent axons projecting to neuromasts of the NLLa leave the medulla with the VIIth nerve, axons projecting to neuromasts of the NLLp exit via the IXth nerve. Cell counts and the observation of axonal branching revealed that efferent units of both the lateral-line and the VIIIth-nerve system supply more than one receptor organ. In contrast to the lateral-line system, dendrites of efferent neurons of the VIIIth nerve project dorsally onto its nuclei, and afferents of the VIIIth nerve project onto efferent neurons. These structures most probably represent a feedback loop between the afferent and efferent systems of the VIIIth cranial nerve.     

Distribution of dopamine immunoreactive systems in brain stem and spinal cord of the chameleon

An immunohistochemical method, using glutaraldehyde fixation and a highly specific monoclonal antibody recently synthetized against dopamine (DA)-glutaraldehyde protein conjugate, permitted direct visualization of DA structures in the brainstem and spinal cord of a reptile (Chameleon). DA-immunoreactive cell bodies occurred in some contiguous areas of the midbrain tegmentum. The first one was located in the ventral tegmental area. Some somata intermingled with the oculomotor nucleus. The second group was the large round or oval DA-Immunostained neurons located in the substantia nigra. More caudally, a third group of round or fusiform DA-cell bodies was seen in an homologous area of so called mammalian A8 and were continuous with the substantia nigra group. In the medulla oblongata, the DA-containing cells were shown in the nucleus of solitary tract and in the dorsal lateral part of the dorsal motor nucleus of the vagus. The density of this DA-Immunoreactive neurons decreased more caudally. At the medullo-spinal level and upper cervical spinal cord, a few labelled cells were distinguished near the central canal. In the spinal cord DA-immunopositive cell bodies were observed in the vicinity of the central canal and formed a continuous column that extended throughout the rostral spinal cord. The apical processes of these neurons seemed to be in contact with the lumen of the central canal. This study constitute the first visualization of the immunoreactive DA-cell bodies at the medullo-spinal level which were already described, as TH immunoreactive in other species of reptiles.     

Spinal projections from the rhombencephalon in the toad

Rhombencephalic cell groups projecting to the spinal cord are demonstrated following single pressure injections and/or iontophoretic ejections of HRP solution in either cervical or lumbar enlargements of the toad spinal cord. A group uptake and transport of HRP were obtained with both application techniques, when sufficiently long survival times (8-11 days) were used. Following injections in the cervical cord labeled cells are located mostly in the ventral nucleus of the VIIIth nerve and in the medial zone of the rhombencephalic reticular formation, i.e. the nucleus reticularis inferior, medius and superior. Following injections in the lumbar enlargement the majority of labeled cells are situated in the caudalmost portion of the ventral nucleus of the VIIIth nerve and in the nucleus reticularis inferior. These observations indicate that in the toad the main supraspinal descending pathways from the rhombencephalon originate in the ventral nucleus of the VIIIth nerve and the medial zone of the reticular formation, and that both these pathways are somatotopically organized.