Medullary Projections Research Articles

Medullary expiratory neurons in the decerebrate rat: an intracellular study

Intracellular recordings and labelings with horseradish peroxidase (HRP) of expiratory (E) neurons were performed in decerebrate, paralyzed, and ventilated rats. A total of 37 neurons were recorded, from which 4 cells and 1 axon were labeled. They were located in two regions of the ventrolateral medulla. One was in the rostral portion of the nucleus ambiguus just caudal to the facial nucleus, and the other in the nucleus retroambiguus at the level of the caudal medulla. These expiratory neurons had rhythmical changes in membrane potential similar to those reported in cat, i.e., a depolarization in the intervals between phrenic bursts which evolved in an augmenting (E-aug, n = 15), or bell-shaped or ‘plateau’ (E-all, n = 22) pattern until a rapid hperpolarization at the start of inspiration. Both types were hyperpolarized during inspiration by chloride-dependent, inhibitory postsynaptic potenials (IPSPs) which were demonstrated in 17 neurons (10 E-aug and 7 E-all) from which reversal was obtained. Such IPSPs also existed during post-inspiration (stage I of expiration) in 4 of the 10 augmenting E neurons. They were identified by antidromic stimulation or HRP labeling, or both, as bulbospinal neurons ( n = 2), cranial motoneurons ( n = 4), or not antidromically activated (NAA) neurons ( n = 31). All the identified bulbospinal neurons and the motoneurons bulbospinal neuron with ipsilateral axon giving off intramedullary collaterals, and NAA neurons with rostral medullary projections or with axons crossing the midline. These results indicate that the distribution and the electrophysiological properties of the expiratory neurons in the rat medulla are similar to those described in the cat.

Caudal ventrolateral medullary projections to the nucleus of the solitary tract in the cat

The projections of neurons, in and around the A1 noradrenergic cell group of the caudal ventrolateral medulla (VLM), to nucleus of the solitary tract (NTS) were studied in the cat using the anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L). PHA-L was microiontophoresed into the region of the A1 noradrenergic cell group and after a 7–17 day survival period animals were sacrificed and brainstem sections were processed for PHA-L or tyrosine hydroxylase (TH) immunoreactivity, PHA-L injections within the region of the A1 cell group resulted in labelled fibers with their presumptive terminal boutons primarily in the ipsilateral commissural and medial subnuclei of NTS. A light projection to the ipsilateral parvocellular, lateral and ventrolateral subnuclei of the NTS complex was also observed. These data demonstrate that neurons in the region of the A1 noradrenergic cell group project to regions of NTS that receive cardiovascular afferent inputs and suggest that VLM may influence the activity of neurons in NTS involved in the reflex regulation of the circulation.

Ultrastructural evidence for an olfactory-autonomic pathway through the rat central amygdaloid nucleus

The innervation of medullary projection neurons in the central amygdaloid nucleus (Ce) by afferents from the ventral taenia tecta (VTT) was investigated using combined lesion-induced axonal degeneration and retrograde transport of horseradish peroxidase-conjugated wheat germ agglutinin (HRP-WGA). Injections of HRP-WGA into the nucleus tractus solitarii resulted in retrograde labeling of neurons in the medial Ce. Ultrastructurally HRP-WGA reaction product was identifiable in the perikarya and proximal dendrites of Ce neurons. Degenerating terminals, probably due to damage at the HRP-WGA injection site, were few and confined to the ventral Ce. Electrolytic coagulation of the VTT resulted in approximately 9% of terminals in medial Ce showing signs of degeneration at 5 days post-lesion. Of the terminals sampled, slightly more than 40% were in contact with the dendrites of retrogradely labeled neurons. Where evident, these terminals formed exclusively symmetrical synaptic contacts. These data provide evidence for an oligosynaptic olfactory-autonomic pathway in the rat that may mediate olfactory influences on gastric and cardiovascular aspects of autonomic function.

The medullary projections of afferent bronchopulmonary C fibres in the cat as shown by antidromic mapping.

1. The activity of eighty-seven bronchopulmonary vagal afferent neurones with unmyelinated axons (C fibres) was recorded extracellularly in the nodose ganglia of decerebrate, paralysed and artificially ventilated cats. On the basis of their response latencies following the right atrial injection of capsaicin or phenyldiguanide, the cells were classified as having their receptor endings within the reach of pulmonary (latency less than 3.5 s) or bronchial (latency above 3.5 s) circulation. 2. Pulmonary and bronchial receptor cells differed only slightly in their response characteristics (firing rate, burst duration) and the conduction velocity of their peripheral axons. Bronchial C fibres represented about 70% of the population studied. 3. The medullary distributions of the central branches of six pulmonary and six bronchial C fibres were determined by means of the antidromic mapping technique. The two receptor subtypes did not differ in their central projection patterns. 4. Rostral to the obex, the central branches of the bronchopulmonary C fibres were localized within the medial portions of the nucleus tractus solitarii (NTS) and area postrema, and were most densely distributed along the borders of the parvicellular subnucleus of the NTS. Caudal to the obex, the most dense branching was found in the dorsal portion of the commissural subnucleus. Projections to the contralateral NTS were found, but these were of a much lower density. 5. The central distribution of bronchopulmonary C fibres is compared to the projection patterns of vagal and glossopharyngeal afferents of other modalities that are involved in respiratory and cardiovascular control. This is discussed in relation to the concept of a modality-specific organization of the NTS.

Medullary projection of nonaugmenting inspiratory neurons of the ventrolateral medulla in the cat

In Nembutal-anesthetized, immobilized, and artificially ventilated cats, we studied the morphological characteristics of inspiratory neurons with nonaugmenting firing patterns. HRP was injected intracellularly into a total of 22 neurons of the Bötzinger complex (BOT) and the ventral respiratory group (VRG). In 20 cases somata with their axonal trajectories were stained, and in two cases only axons were stained. None of the neurons stained could be antidromically activated by stimulation of the cervical cord. The somata of 20 neurons were located in the vicinity of the nucleus ambiguus or the retrofacial nucleus (RFN) between 600 microns and 2,800 microns caudal to the rostral end of the RFN. Their axons could be traced for a distance of several millimeters on the side of the somata, and showed various projection patterns. According to these projection patterns, the 20 neurons were tentatively classified into four groups: A (8/20), B (4/20), C (6/20), and motoneurons (2/20). Group A neurons gave off extensive axon collaterals that arborized and distributed boutons predominantly in the BOT and the VRG areas. Group B neurons had less extensive axon collaterals with various projection patterns, projecting rarely to the BOT or the VRG area. Group C neurons sent their stem axons, without issuing any axonal collaterals, to the contralateral side in five cases and to the ipsilateral pons in one case. The two motoneurons had axons leaving the brainstem without any intramedullary collaterals. Thus, the nonaugmenting inspiratory neurons showed morphological variations, which may play different roles in neural control of respiration.

Projections from the medial agranular cortex to brain stem visuomotor centers in rats

Projections from medial agranular cortex to brain stem in rat were determined by use of the anterograde tracers Phaseolus vulgaris leucoagglutinin, or wheat germ agglutinin conjugated horseradish peroxidase. Axonal trajectories were also followed by means of the Wiitanen modification of the Fink-Heimer degeneration technique. AGm was identified on the basis of its cytoarchitectonics. AGm projected to the anterior pretectal nucleus, the rostral interstitial nucleus of the medial longitudinal fasciculus, the medial accessory oculomotor nucleus of Bechterew, the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, the nucleus cuneiformis and subcuneiformis, intermediate and deep superior collicular layers, the paramedian pontine reticular formation (reticularis pontis oralis and caudalis, and reticularis gigantocellularis), and raphe centralis superior. Differences in connections between rostral and caudal injections were observed: pontine and medullary projections were lighter from the rostral portion of AGm than from the more caudal portions of AGm. The heaviest projections to the anterior pretectal nucleus were from the caudal portion of AGm. The subcortical projections were very similar to those described for the frontal eye field in monkeys, and the majority of them targeted areas thought to be involved in coordination of gaze with head and neck movements. Thus AGm in rats may contain the homologue of the primate frontal eye fields.

Convergence of serotonin-, enkephalin- and substance P-like immunoreactive afferent fibers onto cat medullary dorsal horn projection neurons: a triple immunocytochemical staining technique combined with the retrograde HRP-tracing method

Convergence of serotonin (5-HT)-, enkephalin (ENK)-, and substance P (SP)-like immunoreactive (LI) afferent fibers onto cat medullary dorsal horn projection neurons was examined by retrogradely labeling trigeminothalamic neurons and immunocytochemically labeling 5-HT, ENK and SP fibers in the same tissue section using distinct chromogens. Convergence of 5-HT-, ENK- and SP-LI fibers upon single projection neurons was often observed in laminae I and V.

The organization of dorsal medullary projections to the central amygdaloid nucleus and parabrachial nuclei in the rabbit

The amygdaloid central nucleus and the pontine parabrachial nucleus receive direct, ascending projections from autonomic regulatory nuclei of the dorsal medulla and are recognized as important components of a forebrain system which contributes to autonomic regulation. The present study was designed to provide more detailed information on the anatomical organization of this ascending system in the rabbit by determining (a) the extent to which separate populations of neurons within the solitary complex project to the central nucleus and parabrachial nucleus, (b) the topographical distribution of the projections of the solitary complex within the amygdaloid central nucleus and parabrachial nucleus and (c) the extent to which projections from the solitary complex to the parabrachial nucleus terminate in the region of origin of projections from the parabrachial nucleus to the amygdaloid central nucleus. A fluorescent dye, double retrograde-labeling technique demonstrated that separate populations of neurons in the solitary complex projected to the amygdaloid central nucleus and parabrachial nucleus. Neurons of both populations were more heavily concentrated within the caudal two thirds of nucleus of the solitary tract and were most numerous within the commissural, medial and dorsomedial subnuclei. Labeled neurons were also located within the dorsal motor nucleus of the vagus nerve. Autoradiographic experiments demonstrated that injections of amino acids into the solitary complex resulted in terminal labeling in the central nucleus. This labeling extended rostrally into the adjacent sublenticular substantia innominata and lateral component of the bed nucleus of the stria terminalis. Label was also observed within the lateral, medial, and Kolliker-Fuse regions of the parabrachial nucleus. A particularly dense field was observed overlying cells located within the ventrolateral region of the lateral parabrachial nucleus. This region contained the majority of labeled neurons within the parabrachial nucleus following fluorescent dye injections into the central nucleus. Furthermore, injections of amino acids into this region resulted in terminal labeling within the central nucleus, with a particularly dense area observed within the medial aspect of the nucleus. The results demonstrate that separate populations of neurons within the solitary complex of the rabbit project to the central amygdaloid and parabrachial nuclei and that the majority of these are located within the caudal two-thirds of the complex. Furthermore, the results suggest that the solitary complex projects both directly and indirectly, primarily via the lateral parabrachial nucleus, to the central amygdaloid nucleus. These projections offer an anatomical substrate by which visceral afferent information may influence the limbic forebrain.

Bulbospinal thyrotropin-releasing hormone projections to the intermediolateral cell column: A double fluorescence immunohistochemical-retrograde tracing stdy in the rat

Whereas the neurochemistry of the peripheral autonomic nervous system has been well characterized, less is known concerning the neurotransmitters utilized by medullary projections onto sympathetic preganglionic neurons residing in the thoracolumbar spinal intermediolaleral cell column. Retrograde transport of rhodamine-labeled fluorescent microspheres following discrete microinjection into the thoracic intermediolateral cell column was combined with immunohistochemistry to determine neuroanatomic location of thyrotropin-releasing hormone-immunoreactive neurons which project to the intermediolateral cell column in the rat. The ventromedial group of raphe nuclei including the nucleus raphe pallidus. obscurus, and magnus possessed the greatest number of medullary thyrotropin-releasing hormone-immunoreactive neurons which also contained rhodamine-labeled microspheres. High numbers of intermediolateral cell column-projecting thyrotropin-releasing hormone-immunoreactive neurons were also observed in nucleus reticularis paragigantocellularis lateralis and magnocellularis, the lateral reticular nucleus, and the superficial ventral (arcuate) medullary surface. Despite the observations that nucleus reticularis gigantocellularis, paramedianus, and ventralis pars beta project to the intermediolateral cell column, double-labeled cells were not observed in these nuclei. Furthermore, whereas the nucleus reticularis magnocellularis and gigantocellularis, and the lateral reticular nucleus displayed strong ipsilateral predominance in projecting to the intermediolateral cell column, other medullary reticular and raphe nuclei displayed bilateral projections. The present findings support the hypothesis that thyrotropin-releasing hormone-containing neurons in the ventral medulla project to the intermediolateral cell column and may influence sympathetic preganglionic neurons.

Organization of central adrenergic pathways: I. Relationships of ventrolateral medullary projections to the hypothalamus and spinal cord.

We studied the organization of projections from the C1 adrenergic and A1 noradrenergic cell groups in the ventrolateral medulla (VLM) to the hypothalamus and the spinal cord by using a combination of retrograde transport of fluorescent tracers and immunocytochemistry. Three issues were addressed. Neurons in the VLM that stain immunohistochemically for phenylethanolamine N-methyltransferase (PNMT) have been assumed to be adrenergic. However, the presence of PNMT-immunoreactive neurons in the hypothalamus that do not stain for tyrosine hydroxylase (TH) prompted us to re-evaluate the VLM by an elution-restaining immunohistochemical procedure. We confirmed that nearly all of the rostral medullary PNMT-immunoreactive neurons also stained for TH. By contrast, in the caudal medulla, very few TH-positive neurons stained for PNMT. Neurons of the C1 group in the rostral VLM project both to the thoracic spinal cord and to the hypothalamus. To determine whether individual C1 neurons send collaterals to the hypothalamus and spinal cord, we injected different-colored fluorescent dyes (diamidino yellow or fast blue) into the thoracic spinal gray matter and either the median preoptic (MnPO) or paraventricular (PVH) nuclei of the hypothalamus. Very few double-labeled neurons were found in the VLM, indicating that hypothalamic and spinal cord projections arise from almost completely independent populations of cells. Approximately half of the neurons projecting to the spinal cord from rostral VLM were not immunoreactive for TH or PNMT, indicating that a substantial part of this projection is noncatecholaminergic. The MnPO and the PVH both receive extensive catecholaminergic inputs from the VLM. We also used fluorescent retrograde tracers to determine whether individual VLM neurons send collaterals to both hypothalamic sites. Approximately 20% of neurons projecting to the MnPO in the rostral two thirds of the VLM also sent collaterials to the PVH, nearly all of these neurons being TH-positive. The collateralization of the VLM catecholaminergic projection to the hypothalamus may provide an anatomical substrate for integration of fore-brain participation in cardiovascular regulation. In contrast, the adrenergic projection from the VLM to the intermediolateral column of the spinal cord arises from a separate population of neurons.

Autoradiographic study of descending pathways from the pontine reticular formation and the mesencephalic trigeminal nucleus in the rat.

Descending projections were studied in autoradiographically prepared material after injections of tritiated leucine in the pontine tegmentum of rats. Injections involving the medial pontine reticular formation resulted not only in labeling commissural fibers, the medial reticulospinal tract, and the dorsal cap of the inferior olive, but also, in two cases, in labeling a cerebellar projection that originated from a region near the midline and clearly dorsal to the nucleus reticularis tegmenti pontis. The labeled fibers passed ventral in the midline to the pontine gray, then laterally through the gray and into the middle cerebellar peduncle to terminate as mossy fibers primarily in the flocculus, lobulus simplex, and Crus I of the ansiform lobule. Injections involving the mesencephalic nucleus of the trigeminal nerve (Vmes), resulted in labeling of Probst's tract, which descends in the dorsolateral reticular formation. Probst's tract gave off extensive terminal branches to the lateral medullary reticular formation and weaker projections to restricted portions of the descending trigeminal nucleus, the solitary nucleus, and the hypoglossal nucleus. In one case, fibers could be traced into the dorsal horn of the upper cervical cord.

Pulmonary stretch receptor relay neurones of the cat: location and contralateral medullary projections.

1. The activity of pump (p.) cells, second-order neurones in the pulmonary stretch receptor pathway, was recorded extracellularly in the nucleus of the tractus solitarius (n.t.s.) of the decerebrate cat. Their firing was proportional to changes in lung volume but unrelated to the centrally determined respiratory rhythm. A systematic search of the n.t.s. for the location of p. cells was made and an assessment of their efferent projection to the contralateral n.t.s. was determined electrophysiologically by the antidromic mapping technique. 2. P. cells were located around, and in close proximity to, the solitary tract. The two sites of greatest density were ventromedial and dorsolateral to the tract, with lower concentrations found laterally and ventrolaterally. 3. For twelve of the thirty p. cells tested, evidence of a projection to the contralateral n.t.s. was obtained; in seven of these cells, axonal arborizations within the projection area were identified. Almost all the cells that sent axons to the contralateral n.t.s. were located dorsolateral to the tract; there was no evidence that cells in the ventromedial region had contralateral projections. 4. No evidence that R beta neurones project to the contralateral commissural and ventrolateral subnuclei was found. 5. No p. cells projected to the contralateral ventrolateral n.t.s. The site of projection and branching was consistently localized just caudal to the obex and medial to the solitary tract, in the caudal medial, and commissural subnuclei of the n.t.s. This same region has been shown to receive a dense, direct projection from pulmonary rapidly adapting receptors.

Afferent projections to the orbicularis oculi motoneuronal cell group. An autoradiographical tracing study in the cat

The motoneurons innervating the orbicularis oculi muscle form a subgroup within the facial nucleus, called the intermediate facial subnucleus. This makes it possible to study afferents to these motoneurons by means of autoradiographical tracing techniques. Many different injections were made in the brainstem and diencephalon and the afferent projections to the intermediate facial subnucleus were studied. The results indicated that these afferents were derived from the following brainstem areas: (1) the dorsal red nucleus and the mesencephalic tegmentum dorsal to it; (2) the olivary pretectal nucleus and/or the nucleus of the optic tract; (3) the dorsolateral pontine tegmentum (parabrachial nuclei and nucleus of Kölliker-Fuse) and principal trigeminal nucleus; (4) the ventrolateral pontine tegmentum at the level of the motor trigeminal nucleus; (5) the caudal medullary medial tegmentum; (6) the lateral tegmentum at the level of the rostral pole of the hypoglossal nucleus and the ventral part of the trigeminal nucleus and (7) the nucleus raphe pallidus and caudal raphe magnus including the adjoining medullary tegmentum. These latter projections probably belong to a general motoneuronal control system. The mesencephalic projections are mainly contralateral, the caudal pontine and upper medullary lateral tegmental projections are mainly ipsilateral and the caudal medullary projections are bilateral. It is suggested that the different afferent pathways subserve different functions of the orbicularis oculi motoneurons. Interneurons in the dorsolateral pontine and lateral medullary tegmentum may serve as relay for cortical and limbic influences on the orbicularis oculi musculature, while interneurons in the ventrolateral pontine and caudal medullary tegmentum may take part in the neuronal organization of the blink reflex.

The pattern of spinal and medullary projections from a cutaneous nerve and a muscle nerve of the forelimb of the cat: a study using the transganglionic transport of HRP.

The transport of HRP into the spinal cord and medulla in the cat has been examined from a forelimb cutaneous nerve, the lateral superficial radial nerve (LSR), and from the muscle nerves supplying both heads of the forelimb muscle, extensor carpi radialis (ECR). HRP transported by the LSR was widely distributed in the spinal cord throughout laminae I-IV in the vicinity of the root entry zone and from spinal segments T1 to C5. HRP was also transported from the LSR to the medulla where there was intense patchy, discontinuous labelling in the main cuneate nucleus. The pattern of labelling in the cuneate nucleus did not follow any simple somatotopic plan. Exposure of the muscle nerve to HRP led to labelling in the spinal dorsal horn in lamina I, in the deep dorsal horn on the lamina V/VI border, and in lateral and medial lamina VI at sites that contain cells of origin of spinocerebellar tracts. The medial lamina VI label was contiguous with a deposit that extended medially to the central canal. The label in lateral lamina VI was patchy and formed a discontinuous column from T1 to C5. HRP transported by the muscle nerve also produced label in the more ventral regions of the cuneate nucleus where it had a lacy appearance, in part due to its extensive distribution around dendrites. A relatively dense, patchy, and discontinuous deposit of reaction product was also present in the external cuneate nucleus after muscle nerve exposure. This deposit was most intense on the dorsomedial surface of this nucleus, but another, less intense, deposit was also present ventrally.

Spinal cord substance P mediates bicuculline-induced activation of cardiovascular responses from the ventral medulla

Substance P (SP) and serotonin are contained in ventral medullary projections to the intermediolateral cell column (IML) of the spinal cord, and both neurotransmitters excite sympathetic preganglionic neurons upon injection into the IML. Since γ-aminobutyric acid (GABA) in the ventral medulla inhibits, and GABA-receptor antagonists excite sympathetic outflow to the cardiovascular system, experiments were done to determine if SP and serotonin in the spinal cord were responsible for mediating these GABAergic effects. Anesthetized rats were either given intrathecal injections of SP antagonists acutely, or pretreated with intrathecal injections of the serotonin neurotoxin, 5,7-dihydroxytryptamine. The effects of these drugs on mean arterial pressure (MAP) and heart rate (HR), as well as their ability to block the responses to topical application of GABA or the GABA antagonist, bicuculline, at the ventral medulla were assessed. Three SP antagonists (50 μg) decreased MAP to 2/3 baseline values, but did not change HR. They also blocked the characteristic increases in MAP and HR elicited by application of bicuculline to the ventral medulla. A lower dose (5 μg) of a SP antagonist also decreased MAP and blocked the bicuculline-induced increases in MAP and HR, an effect which was reversed in 1–2 h. Neonatal capsaicin treatment reduced the SP content in the dorsal horns of the thoracic spinal cord, but did not affect the cardiovascular responses to intrathecal injection of SP antagonist nor the blockade of bicuculline-induced responses. Intrathecal injection of 5,7-dihydroxytryptamine two weeks prior to the experiments resulted in 56% depletion of serotonin in the thoracic spinal cord, but did not change either basal MAP and HR, nor the responses to bicuculline and GABA applied to the ventral surface of the medulla. These data provide evidence for a role of spinal cord SP in cardiovascular regulation.

The medullary projection from the mesencephalic trigeminal nucleus. An experimental study with comments on the intrinsic trigeminal connections

The medullary projection from the mesencephalic trigeminal nucleus has been studied in cats where the wheat germ agglutinin-horseradish peroxidase complex has been used as a retrograde and anterograde tracer. All injections were made at the level of the caudal pole of the inferior olive and show that it is especially the lateral part of nucleus parvicellularis of the reticular formation which is the main area for termination of the fibres. In addition, it can not be excluded that the descending fibres also reach the medialmost part of the spinal trigeminal sensory nucleus (pars magnocellularis and the adjoining pars gelatinosa). All three cell types of the mesencephalic trigeminal nucleus are labelled, the large globular, the small globular and the polygonal cells. Some of these cells are only faintly labelled. The observations confirm previous studies of the intrinsic trigeminal connections, but show that when the wheat germ agglutinin-horseradish peroxidase complex is used as a tracer, the origin as well as the termination for the intrinsic fibres (also the commissural) can be studied in the same section. The use of this sensitive tracer also shows new details within the intrinsic connections. The findings are discussed with special reference to the recent observations by Ruggiero et al. (1982). These authors found that in rabbit and rat there is a more restricted termination for the descending mesencephalic trigeminal fibres. The discrepancies between the observations seem to indicate that there are species differences as regards the connections between the mesencephalic trigeminal nucleus and the nuclei in the lower part of the brain stem.

The central projections of carotid baroreceptors and chemoreceptors in the cat: a neurophysiological study.

The medullary projections of afferent neurons with cell bodies in the petrosal ganglion have been investigated using an antidromic mapping technique. Of the ninety-three units studied, fifty-eight were shown to have patterns of discharge indicating that they were baroreceptors and thirty-five showed responses to stimuli indicating that they were arterial chemoreceptors. Twelve baroreceptor and thirteen chemoreceptor afferents had sufficiently stable unitary discharges to permit a detailed estimation of some of their central projections using stimulation through monopolar tungsten micro-electrodes to evoke antidromic spikes. In order to estimate their pattern of projection, depth-threshold contours for each penetration through the dorsomedial medulla and the values of antidromic latency were considered. Baroreceptor afferent fibres with myelinated (six units) and non-myelinated (six units) axons showed similar patterns of central projection. All could be activated from the ipsilateral nucleus of the tractus solitarius (n.t.s.), most often from its lateral divisions rostral to the obex. The dorsolateral and dorsomedial portions of the n.t.s. were most often innervated, with the commissural subnucleus receiving an innervation in seven of the twelve neurones studied. Stimulation of the ventrolateral subnucleus was effective in activating two afferent fibres whilst stimulation of the ventral subnucleus was effective in only one case. All chemoreceptor afferent fibres had calculated conduction velocities less than 4 m/s and all were activated from the dorsomedial and medial subnuclei of the ipsilateral n.t.s. In twelve of the thirteen neurones investigated in detail there was evidence of an innervation of the commissural nucleus both at the level of the obex and behind it. In three cases this extended into the contralateral portion of the commissural nucleus. In four cases a sparse innervation of the lateral subnucleus, comprising its dorsolateral aspects, was seen. The potential significance of these distinctive patterns of projection of arterial baroreceptors and chemoreceptors is discussed in relation to cardiovascular and respiratory control.

A cobalt study of medullary sensory projections from lateral line nerves, associated cutaneous nerves, and the VIIIth nerve in adult Xenopus

The medullary projections of the anterior lateral line nerve, dorsal branch (Alln.d), the posterior lateral line nerve, dorsal branch (PLLn.d), associated cutaneous nerves, and the VIIIth nerve in Xenopus laevis have been delineated by axonal infusion of cobalt chloride and silver intensification. The peripheral innervation of the posterior lateral line sense organs has also been traced. From wholemount and sectioned preparations, we describe three central projections, extending the length of the ipsilateral medulla but occupying distinct zones: lateral line afferents dorsomedially, stato-acoustic dorsolaterally, and cutaneous ventrolaterally. Arborizations of ALLn.d and PLLn.d afferents are morphologically similar, intermingling throughout the lateral line lobe. Each divides into ascending and descending limbs bearing collaterals, which terminate in the lateral line neuropile and nucleus. Evidence is presented for directional and positional mapping in the branching of individual PLLn.d afferents and for topography in the ALLn.d projection. Second-order neurones have been identified by transneuronal staining and their axons traced into the contralateral torus semicircularis. The morphology of efferent neurones is also described. Rostral branches of PLLn.d also contain cutaneous afferents which run through the medulla into the spinal cord, similar to the nerve V (cutaneous) projection. In nerve VIII preparations, the projection to the compact cochlear nucleus and the massive vestibular projection are identified. Cutaneous and vestibular but not lateral line afferents extend into the cerebellum. The separation of VIIIth nerve and lateral line afferents in Xenopus medulla is considered as evidence against the validity of the acousticolateralis concept. Information processing in the lateral line lobe is discussed in relation to connectivity patterns between first- and second-order neurones.

Peripheram origins of human evoked spinal cord potentials : H. Shimizu, Y. Maruyama, H. Endo, S. Urano and K. Shimoji (Niigata, Japan)

This study has combined physiological pressure stimulation of carotid baroreceptors via a vascularly isolated carotid sinus and anodal block of baroreceptor afferent fibers in the carotid sinus nerve to examine the medullary projections of type I vs. type II (large A- vs. small A- and C-fiber afferent) baroreceptors. The control distribution of cells in the nucleus tractus solitarius expressing c-fos in response to physiological activation of carotid baroreceptors in the isolated sinus was compared to that during anodal block of large A-fibers in the carotid sinus nerve. Carotid baroreceptor stimulation primarily activated neurons in the ipsilateral commissural and medial subnuclei of the caudal nucleus tractus solitarius and the dorsolateral, dorsomedial and medial subnuclei in the intermediate and rostral levels of the nucleus tractus solitarius. Elimination of large A-fiber carotid baroreceptor afferents, during similar carotid baroreceptor stimulation resulted in a decrease in the nmber of cells expressing c-fos in the dorsomedial subnucleus of the rostral nucleus tractus solitarius. These data indicate that projections of larger A-fiber (type I) carotid baroreceptors are localized primarily to the rostral dorsomedial subnucleus, while those of smallr A- and C-fiber baroreceptors are more widely distributed to the commissural, medial and dorsal subnuclei of the nucleus tractus solitarius.

Electrophysiological study of hepatic vagal projection to the medulla

Medullary projection of the hepatic branch of vagus was examined electrophysiologically. Analysis of compound field potentials elicited by the hepatic vagal stimulation revealed that this nerve branch projects dominantly in the left side of the medulla. Afferent neurons in the solitary tract nucleus were identified by the electrical stimulation of the nerve branch. A collision test was also employed to separate the afferent from the efferent. Some afferent neurons which innervate the liver were responsive to the portal infusion of isotonic glucose solution, hypertonic saline and water. Their response patterns were almost similar to those of the hepatic afferent nerves.