The organization of the gastric efferent projections in the brainstem of monkey: an HRP study

Signal processing in the vagus nerve: Hypotheses based on new genetic and anatomical evidence

Each organism must regulate its internal state in a metabolically efficient way as it interacts in space and time with an ever-changing and only partly predictable world. Success in this endeavor is largely determined by the ongoing communication between brain and body, and the vagus nerve is a crucial structure in that dialogue. In this review, we introduce the novel hypothesis that the afferent vagus nerve is engaged in signal processing rather than just signal relay. New genetic and structural evidence of vagal afferent fiber anatomy motivates two hypotheses: (1) that sensory signals informing on the physiological state of the body compute both spatial and temporal viscerosensory features as they ascend the vagus nerve, following patterns found in other sensory architectures, such as the visual and olfactory systems; and (2) that ascending and descending signals modulate one another, calling into question the strict segregation of sensory and motor signals, respectively. Finally, we discuss several implications of our two hypotheses for understanding the role of viscerosensory signal processing in predictive energy regulation (i.e., allostasis) as well as the role of metabolic signals in memory and in disorders of prediction (e.g., mood disorders).

Neuronal supernumerary and dendritic sprouting of the nucleus ambiguus after chronic alteration of peripheral targets in cats

Anatomic changes of neuronal profiles in response to chronic alteration of peripheral targets were investigated in the nucleus ambiguus (NA) of cats. Unilateral vagal-hypoglossal nerve anastomosis was performed by suturing the transected proximal stump of the vagus nerve to the transected distal stump of the hypoglossal nerve. After comparing horseradish peroxidase (HRP)-labeled neurons on the ipsilateral operated side of the NA with the contralateral unoperated NA and the NA following transection and reuniting to the vagus itself, a remarkable ramification and elongation of the dendritic trees was observed in the HRP-positive neurons on the ipsilateral NA. Quantitative analysis of neuronal profiles revealed that the number of the medium and large neurons on the ipsilateral NA was greater than the contralateral NA and the NA following autologous suturing of the vagus. Comparisons of variable dendritic lengths of the medium and large neurons on the ipsilateral NA revealed longer distances and more branches of the tertiary and perisomatic dendrites than those of the contralateral NA and the NA ipsilateral to autologous reunion. Our results suggest that remarkable sprouting and elongation of the dendritic trees as well as cell supernumerary occurred in the dominant NA motoneurons ipsilateral to the nerve anastomosis. In conclusion, there is a trophic influence in the tongue musculature, which was retrogradely transported to the NA neurons via the regenerating axons and caused the morphological changes in the NA in response to the rerouting of efferents from the vagus nerve to the hypoglossal nerve to innervate intimate tongue musculature.

Vagal innervation of the gastrointestinal tract arises from dorsal motor nucleus while that of the heart largely from nucleus ambiguus in the cat

The origin of medullary cells that form the cardiac vagal branch and the vagal branches in the lower thorax innervating the gastrointestinal (GI) tract was studied using horseradish peroxidase (HRP), a retrograde transport tracer in the cat. The distributions of parasympathetic postganglionic neurons of the heart were studied with acetylcholinesterase histochemistry. Intracardiac ganglionic neurons were found mainly in the connective tissue surrounding the base of the pulmonary arteries and in an area in and dorsal to the interatrial septum. Following injection of HRP into the subepicardum where most of the cardiac postganglionic neurons reside, 91% of the labelled neurons were found bilaterally distributed in the nucleus ambiguus (NA). A small population of labelled neurons was found in the dorsal motor nucleus of the vagus (DMV) and an intermediate zone (IZ) between the two nuclei. When HRP was injected into the left or right cardiopulmonary vagus branch, labelled neurons were found exclusively in the ipsilateral NA, DMV and IZ with a predominance in the NA. In the thorax, after they course around the heart, the left and right thoracic vagus nerves divides into a left and a right branch, respectively. The left branch of the left thoracic vagus joins the left branch of the right thoracic vagus to form the anterior vagus nerve at 3 cm above the diaphragm. The right branch of the right thoracic vagus nerve joins the right branch of the left thoracic vagus to form the posterior vagus nerve. After application of HRP into the right or the left branch of the left thoracic vagus, HRP labelled cells were found in the left DMV. Similarly, after application of HRP into the left or the right branch of the right thoracic vagus, labelled cells were found in the right DMV. On the other hand, when HRP was injected into the anterior vagus, labelled neurons were found bilaterally in the DMV. This suggests that all rostral branches of the thoracic vagus have their origin in the ipsilateral DMV, and intermixing occurs only at the caudal level near the diaphragm. Findings of the present experiments suggest that parasympathetic preganglionic neurons innervating the GI tract are located exclusively in the DMV while those of the heart are located mainly in the NA. Within the DMV, GI vagal neurons were found medially from the level 0-2.5 mm rostral to the obex. In contrast, cardiac vagal neurons were found in the lateral edge of the DMV at the level 0-1 mm rostral to the obex.

Brainstem topology of the vagal motoneurons projecting to the esophagus and stomach in the house musk shrew, Suncus murinus

The central origin of vagal efferents innervating the esophagus and stomach in the house musk shrew, Suncus murinus, was studied using the retrograde tracing technique. The animals were perfused with fixative 48-72 h after HRP injection and sections were processed by HRP histochemistry. HRP application into the gastroesophagus resulted in bilateral labelling of neurons in the dorsal motor nucleus of the vagus nerve (DMX) and ambiguous nucleus (AN). Labelled neurons in the DMX were observed from all regions except from the cervical esophagus, while ones in the AN were seen from the esophagus and cardia. The more labelled neurons were observed on the right DMX from subdiaphragmatic esophagus, cardia, lesser curvature and ventral corpus, while on the left DMX from the dorsal corpus labelled neurons in the longitudinal extent of the DMX were generally located at the dorsal and dorsomedial part, and those in the middle part were scattered. Labelled neurons in the AN were located restricted in the rostral part. Our results suggest that in the Suncus murinus the rostrocaudal site-specific localization within the DMX was not found, but it was prominent in the AN. In addition, while the majority of neurons which supply the esophagus and stomach were located in the DMX, only a small number was found in the AN.

Brainstem cells of origin of physiologically identified cardiopulmonary nerves in the rhesus monkey (Macaca mulatta)

The distributions of brainstem cells of origin of the cervical vagus nerve, its cervical and thoracic branches, and of neurons projecting to the cricothyroid muscle and the stomach wall were identified and compared following injections of horseradish peroxidase (HRP) in 18 Rhesus monkeys. Physiologically and/or anatomically identified cardiopulmonary nerves were injected with 3-20 microl of HRP to identify the locations of vagal preganglionic cardioinhibitory neurons in 10 of these monkeys. After injections into cardiopulmonary nerves, retrogradely labelled cells were concentrated ipsilaterally in the most lateral parts of the dorsal motor nucleus of the vagus nerve (DMV) and in the ventrolateral nucleus ambiguus (NA). Fewer labelled neurons were identified close to or in the principal (dorsal) division of the NA and in the intermediate zone between the DMV and NA. The results indicate that monkey cardiopulmonary nerves have multiple origins; their somata are located primarily in the ventrolateral NA and to a lesser extent in the lateral DMV. In monkeys, there is a stronger representation in the lateral DMV than in cat, dog and pig. The viscerotopic organization of the cells of origin of primate vagal nerves is similar to that in other species. The cells of origin of vagal projections to the superior laryngeal nerve and cricothyroid muscle were located in the NA rostrally to those of the inferior laryngeal nerve. Injections into the superior laryngeal nerve also resulted in significant labelling in the DMV and intermediate zone (IZ). The cells of origin of projections to the anterior stomach wall were restricted to the DMV with a bilateral distribution of labelled cells, concentrated medially in the nucleus.

The central connections of the vagus nerve in the ferret

The vagus nerve mediates emesis due to gastric irritation. The central representation of the vagus in the ferret was studied to establish how the nerve is connected to areas important in the regulation of emesis. In a series of 10 ferrets, WGA-HRP injections (10 microliters) were made into the nodose ganglion. After 24-48 h, animals were reanesthetized and perfused transcardially. A block extending from the pons to upper cervical spinal cord was cut at 50 microns and sections reacted. Nodose ganglion injections of WGA-HRP produced labeling of vagal preterminal segments in the ipsilateral dorsal vagal complex including all subnuclei of the solitary complex where the medial and subgelatinous subnuclei received the densest input, the area postrema (AP), which contained a modest amount of terminal label, and the dorsal motor nucleus of the vagus (DMX). Contralateral terminal label, quantitatively much less, was similarly distributed except that within the solitary complex it was limited to the medial and subgelatinous subnuclei. Retrogradely labeled cells formed ipsilateral dorsomedial and ventrolateral columns, corresponding, respectively, to the DMX and the nucleus ambiguus (including retrofacial and retroambiguus).

Localization of the stomach-projecting neurons in the dorsal motor nucleus of the vagus nerve in the guinea pig

The medullary origin of cells of the cervical vagus nerve and the vagal innervation of the stomach in the guinea pig were studied using the retrograde transport of horseradish peroxidase. The horseradish peroxidase was injected into the cervical portion of the vagus nerve, and also into the greater or lesser curvature of the stomach. The animals were perfused with fixative two days after the injection. The medulla oblongata containing the dorsal motor nucleus of the vagus nerve was sectioned and processed histochemically with the tetramethyl benzidine method. The injection of horseradish peroxidase in the cervical vagus nerve resulted in heavy retrograde labelling of neurons in the ipsilateral dorsal motor nucleus and in the nucleus ambiguous. Following the injection of horseradish peroxidase into the greater curvature of the stomach, the stomach-projecting neurons which were bilaterally labelled were localized in the dorsal and dorsolateral part of the dorsal motor nucleus. Although also bilaterally labelled in the dorsal and dorsolateral part of the dorsal motor nucleus, the neurons projecting to the lesser curvature of the stomach were predominantly (approx. 70%) located in the left dorsal motor nucleus. Our study suggests that the parasympathetic preganglionic neurons innervating the greater and lesser curvatures of the stomach are organized viscerotopically in the dorsal motor nucleus in the guinea pig.

Fine distribution of substance P-like immunoreactivity in the dorsal nucleus of the vagus nerve in cats

The ultrastructure of substance P (SP)-immunoreactive elements in the cat dorsal motor nucleus of the vagus nerve was examined using pre- and post-embedding immunocytochemical procedures. Substance P-like immunoreactivity was observed in axon terminals and axon fibres which were mostly unmyelinated. Quantitative data showed that at least 16% of axon terminals contained SP. Their mean diameter was larger than that of their non-immunoreactive counterparts. Most (83%) SP-containing terminals were seen to contact dendrites but some were observed adjoining soma or entirely embedded in the cytoplasm of vagal neurons (4.5%). Only 0.5% were observed to contact soma of internuerons. A few immunoreactive axon terminals (4%) were observed in contact with non-immunoreactive axon terminals. Round agranular vesicles and numerous dense core vesicles were visible in most SP-containing axon terminals (84.6%). The immunogold procedure showed the preferential subcellular location of SP to be dense core vesicles. In 32.4% of cases, SP-containing terminals were involved in synaptic contacts that were generally of the asymmetrical Gray type 1 and mainly apposed dendrites. The theoretical total of synaptic contacts was 74.5% and this suggests the existence of weak non-synaptic SP innervation involving approximately 25% of SP-containing axon terminals. No axo-axonic synapses were observed in the dorsal vagal nucleus. These results support the hypothesis that SP found in the dorsal vagal nucleus originates partly from vagal afferents and is involved in direct modulation of visceral functions mediated by vagal preganglionic neurons.

The target specificity of the extrinsic innervation of the rat small intestine

The target specificity of the extrinsic innervation of the rat small intestine was examined by simultaneously injecting the proximal and distal small intestine with either wheat germ agglutinin-horseradish peroxidase (HRP) or fast blue. The number of single- and double-labeled cells in the nodose, dorsal root and coeliac-superior mesenteric ganglia and the dorsal motor nucleus of the vagus were counted and expressed as percentages of total labeled cells. Cells containing both HRP and Fast blue projected to both regions of the intestine. We found that the nodose and mesenteric ganglia contained significantly fewer double-labeled neurons (approximately 3 and 9% respectively) than the dorsal motor nucleus (19%) or dorsal root ganglion (20%). Presumably, a large number of double-labeled afferent or efferent neurons would limit the ability of a given component of the extrinsic innervation to control the activity of restricted regions of the small intestine (but might be important in overall regulation of intestinal function). In a separate series of experiments we examined the topography of neurons in the dorsal motor nucleus of the vagus labeled with HRP injection into either the proximal or distal small intestine. Both of these injections labeled neurons in the entire rostro-caudal extent of the nucleus, though approximately 75% of the cells were located between 720 microns caudal and 720 microns rostral to the obex. Cells in the rostral regions were found primarily in the lateral pole of the nucleus, whereas caudal regions contained labeled cells in both the medial and lateral poles.

Vagal motor neurons innervating the stomach are site-specifically organized in the dorsal motor nucleus of the vagus nerve in rats

The aim of the present study was to examine the neuroanatomical relationship between the brainstem and the stomach. For this purpose, by means of a horseradish peroxidase retrograde method, we identified the distribution of individual parasympathetic preganglionic neurons projecting to three specific gastric regions that have structural and functional differences in the rat stomach. Preganglionic neurons innervating the ventral forestomach were mainly distributed in the lateral part of the left dorsal motor nucleus of the vagus nerve, while those innervating both ventral corpus and antrum were chiefly located in the medial part of the left dorsal motor nucleus. On the other hand, preganglionic neurons projecting to the dorsal forestomach and the dorsal corpus and antrum were predominantly observed in the lateral and the medial part of the right dorsal motor nucleus, respectively. These results suggest that parasympathetic preganglionic neurons innervating the stomach are site-specifically organized in the dorsal motor nuclei.

The location of extrinsic efferent and afferent nerve cell bodies of the normal canine stomach

The location of the extrinsic efferent and afferent nerve cell bodies to the mucosa, submucosa, and tunica muscularis of the cardiac, gastric, and pyloric gland regions of the ventral stomach and to the mucosa-submucosa alone of these 3 glandular gastric regions was determined using the horseradish peroxidase technique. All animals of the study demonstrated labeling bilaterally in the rostrocaudal extent of the dorsal motor nucleus of the vagus nerve (DMV) although mucosa-submucosa injections resulted in fewer labeled cells in the DMV. There was no evidence of viscerotopic organization within the DMV for the different gastric regions. However, the left nucleus generally contained a greater number of labeled cells than the right nucleus. Injection of the mucosa, submucosa, and tunica muscularis of the cardiac gland region also resulted in labeling in the nucleus ambiguus in 4 of 5 animals. The vast majority of labeled postganglionic sympathetic neurons were found in the celiacomesenteric ganglion. Labeled cells were also located variously in the stellate ganglion, middle cervical ganglion, and sympathetic trunk ganglia for the different groups. There was no discernible pattern of localization of labeled cells within a sympathetic ganglion. For the stomach, afferent labeled cells were located in the range of the first thoracic to fourth lumbar spinal ganglia and the nodose ganglia, bilaterally. As with sympathetic neurons, there was no discernible pattern of localization of labeled cells within a sensory ganglion.

False-positive artifacts of tracer strategies distort autonomic connectivity maps

The widespread use of new axonal transport tracing techniques in the ANS has resulted in substantially revised and amended descriptions of ANS organization. The present review suggests, however, that at least some of the results on which proposed revisions of ANS anatomy have been based have incorporated artifacts and therefore should be cautiously interpreted. The peripheral nervous system and viscera are composed in part of connective and endothelial tissues that are porous or 'leaky' to solutes with appropriate chemical characteristics, including the major tracer compounds. As a result, several extra-axonal routes for redistribution of label from the application site into other tissues are present. These include (1) diffusion through tissue membranes to enter directly adjacent tissues and (2) leakage into extracellular fluids within the body cavity, vasculature, lymphatics, exocrine ducts, or organ lumens to migrate to more distant tissues. As a consequence of the extreme sensitivity of the methods used, such redistribution of even minute amounts of label can produce false positives. Review of autonomic neuroanatomy suggests additional mechanisms, including tracer uptake by fibers of passage, can produce artifactual staining. Based on these surveys of tissue composition, tracer characteristics and sources of artifact, experimental controls and criteria for identifying and avoiding labeling artifacts are described. Since no single procedure is foolproof for ANS experimentation, the routine application of multiple controls, particularly ones which restrict or prevent tracer diffusion, are needed.

Central origin of vagal nerve fibres innervating the fundus and corpus of the stomach in rat

The origin of vagal nerve fibres innervating the anterior and posterior walls of the fundus and corpus of the rat stomach was investigated using the axon tracing dye, Fast blue. The secretomotor nerve supply to the rat stomach was predominantly ipsilateral. A large majority (98-99%) of the vagal perikarya innervating the anterior fundus and corpus were located on the left side of the brainstem. A large majority (96-99%) of the vagal perikarya innervating the posterior fundus and corpus were located on the right side. Vagal perikarya were arranged in longitudinal, dorsal cell columns which extended beyond the normally accepted cytoarchitectural limits of the dorsal motor nucleus of the vagus (DMV). A few vagal cells innervating the fundus were also found in the nucleus ambiguus. Vagal cell columns innervating the anterior and posterior fundus extended rostrocaudally over a distance of up to 4 mm and projected caudally as far as the cervical spinal cord. Vagal cell columns innervating the anterior and posterior corpus were more compact, extended over a distance of 2-3 mm, and projected rostrally as far as the inferior salivatory nucleus of the glossopharyngeal nerves. Vagal cell columns for the fundus and corpus overlapped in the region of the DMV which lay immediately ventral to the area postrema. Between one-third to one-half of the vagal cells innervating the fundus and corpus were concentrated under the area postrema. A simple form of viscerotopic organisation appears to occur within the vagal cell columns innervating the fundus and corpus.

Preganglionic vagus nerve fibers also enter the greater curvature of the stomach in rats and ferrets

The efferent gastric vagus nerve fibers appear to enter the stomach by several routes. For example, the rate of gastric acid secretion is directly affected by the nerves of the greater curvature of the stomach. Specifically, acid secretion decreases abruptly after division of the gastroepiploic nerve(s). To determine whether efferent vagus nerve fibers are contained in the gastroepiploic nerve(s), horseradish peroxidase, a protein that undergoes retrograde axonal transport, was applied to these nerves; the brainstem locus of the nuclei of the vagus nerves was examined 2 days later. Typical peroxidase labeling was observed in the dorsal motor nucleus of the vagus nerve in 5 of 6 rats and 3 of 3 ferrets; the hypothesis that efferent vagus nerves enter the greater curvature of the stomach was thus supported in two vertebrate species. These previously unrecognized nerves should be considered in the interpretation of experimental and clinical phenomena.

Connections of a vagal communicating branch in the ferret. II. Central projections

There is increasing interest in the central mechanisms involved in the regulation of gastrointestinal function. The ferret is becoming widely used for research in this area. However, knowledge of the brain stem organization of this species is inadequate. As part of an on-going study designed to provide information regarding the site of termination of abdominal afferents, the central connections of a supradiaphragmatic vagal communicating branch were determined in the ferret through the use of the horseradish peroxidase (HRP) tracing technique. The branch was exposed using a thoracotomy and HRP crystals were applied to the cut ventral end of the branch. Following a 72 hour survival period, the animals were reanesthetized and perfused. The brain stem was removed and processed using the tetramethylbenzidine method. Afferent terminals were found bilaterally in the nucleus of the solitary tract (nTS), area postrema (AP), the dorsal motor nucleus of the vagus (DMV), the external cuneate nucleus (ECN) and the principal subnucleus of the inferior olive (IOP). This is the first study of a brain stem projection of a specific vagal branch in this species, and demonstrates the similarities and differences which exist between the ferret and other species.

Central distribution of the cervical vagus nerve in Old and New World primates

The central distribution of the cervical vagus nerve was examined in Old and New World primates using anterograde transganglionic and retrograde horseradish peroxide (HRP) histochemistry. Crystals of HRP were applied to the cut central end of the cervical vagus nerve in two Old World (one bonnet, one cynomolgus) and two New World (squirrel) monkeys. Bright- and darkfield examination of coronal sections from the pons, medulla, and upper cervical spinal cord revealed two major concentrations of retrogradely labeled cells in the ipsilateral dorsal motor nucleus (DMX) and nucleus ambiguous (NA). DMX was heavily labeled, containing about 5 times as many labeled cells as NA. The anterograde distribution of reaction product did not extend as far in the rostrocaudal plane as did the retrograde distribution. Labeled afferent fibers entered the medulla at the level of the caudal dorsal cochlear nucleus, joined the solitary tract, and descended to the obex. Ipsilateral terminal label first appeared at the level where the nucleus of the solitary tract (NST) abuts the IVth ventricle. The terminal field grew in extent and density, until at the level of the area postrema (AP), the distribution extended throughout the medial NST, ventrolateral NST, and AP. Contralateral terminal label was sparse and restricted to the medial NST. In the commissural division of the solitary nucleus, sparse reaction product was present bilaterally, with the denser concentration ipsilateral to the treated nerve. Examination of peripheral ganglia revealed labeled somata in the nodose, jugular, and superior cervical ganglia.

Tracer diffusion has exaggerated CNS maps of direct preganglionic innervation of pancreas

Small injections of True Blue (TB) into 4 different segments of the pancreas of the rat resulted in characteristic and different numbers and distributions of labeled cells within the dorsal motor nucleus of the vagus (DMN). Composites of these patterns of labeled cells in the DMN closely matched the distributions previously reported for more extensive injections of retrograde tracers into the pancreas. However, the application of a diffusion barrier (formed with a plastic wound spray) on the outer surface of the stomach and intestines adjacent to the pancreas segment which contained the TB injection depot prevented virtually all of the labeling of DMN cells. Similarly, applying a diffusion barrier directly to the injected pancreas segment itself prevented all or most (mean greater than 99%) of the DMN labeling. In contrast to this effect on DMN label, the barrier reduced more modestly the labeling of celiac ganglion somata after pancreas injections. Several additional control experiments also suggest that the absence of DMN label after the barrier application resulted from interference with tracer diffusion from the injected organ and not from neurotoxic effects. These include the following demonstrations in the presence of the barrier: (1) observation of unimpaired vagally stimulated insulin secretion, (2) uncompromised cell labeling of DMN from other organs treated with TB plus the barrier, and (3) normal hematoxylin and eosin stained pancreas tissue which had received TB injections and barrier application. It was concluded that both the number of parasympathetic preganglionic neurons that project monosynaptically to the pancreas and their distribution in the medulla may have been very significantly overestimated in previous tracer studies.

Observations on the afferent and efferent organization of the vagus nerve and the innervation of the stomach in the squirrel monkey

Horseradish peroxidase was injected into the cervical vagus nerve or stomach wall of adult squirrel monkeys. Following cervical vagus nerve injections, labelled afferent fibres were present in the tractus solitarius and labelled fibres and terminals were present in medial and lateral parts of the nucleus of the tractus solitarius (NTS) ipsilaterally. Afferent labelling was also seen in the ipsilateral commissural nucleus and in the area postrema. Labelling was present contralaterally in caudal levels of the medial parts of the NTS, in the commissural nucleus, and in the area postrema. Afferent projections to the ipsilateral pars interpolaris of the spinal trigeminal nucleus and to the substantia gelatinosa of the C1 segment of the spinal cord were also labelled. Following injections of HRP into the anterior and posterior stomach walls, the tractus solitarius was labelled bilaterally. Afferent labelling was concentrated bilaterally in the dorsal parts of the medial division of the NTS, i.e., in the subnucleus gelatinosus, and in the commissural nucleus. The regions of NTS immediately adjacent to the tractus solitarius were largely unlabelled. Injections of HRP into the cervical vagus nerve resulted in heavy retrograde labelling of neurons in the ipsilateral dorsal nucleus of the vagus (DMX) and in the nucleus ambiguus (NA). In addition a few neurones were labelled in the intermediate zone between these two nuclei. Retrogradely labelled neurons were also present in the nucleus dorsomedialis in the rostral cervical spinal cord and in the spinal nucleus of the accessory nerve. Injections of HRP into the left cricothyroid muscle in two cases resulted in heavy retrograde labelling of large neurons in the left NA. Following stomach wall injections of HRP retrograde labelling of neurons was seen throughout the rostrocaudal and mediolateral extent of the DMX; there was no apparent topographical organization of the projection. In these cases, a group of labelled smaller neurons was found lying ventrolateral to the main part of the NA through its rostral levels. This study in a primate indicates that a large vagal afferent projection originates in the stomach wall and terminates primarily in the subnucleus gelatinosus of the NTS and in the commissural nucleus with a distribution similar to that described previously in studies in several subprimate mammalian species. The present results and those of other studies suggest some degree of segregation of visceral input within different subnuclei of the NTS.(ABSTRACT TRUNCATED AT 400 WORDS)

The central organization of the vagus nerve innervating the stomach of the rat

We employed the neural tracers cholera toxin-horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase to examine the organization of the afferent and efferent connections of the stomach within the medulla oblongata of the rat. The major finding of this study is that gastric motoneurons of the dorsal motor nucleus (DMN) possess numerous dendrites penetrating discrete regions of the overlying nucleus of the solitary tract (NTS). In particular, dendritic labelling was present in areas of NTS which also received terminals of gastric vagal afferent fibers such as the subnucleus gelatinosus, nucleus commissuralis, and medial nucleus of NTS. This codistribution of afferent and efferent elements of the gastric vagus may provide loci for monosynaptic vagovagal interactions. A small number of dendrites of DMN neurons penetrated the ependyma of the fourth ventricle and a few others entered the ventral aspect of the area postrema, thus making possible the direct contact of preganglionic neurons with humoral input from the cerebrospinal fluid and/or the peripheral plasma. Nucleus ambiguus neurons projecting to the stomach predominantly innervate the forestomach. The dendrites of these cells, when labelled, were generally short, and extended beyond the compact cluster of ambiguus neurons in a ventrolateral direction, parallel to the fascicles of vagal efferent fibers traversing the medulla.

Brainstem origins and projections of the cervical and abdominal vagus in the golden hamster: a horseradish peroxidase study

Vagal afferent projections, and preganglionic parasympathetic neurones contributing to the vagus nerve in golden hamsters were traced following application of horseradish peroxidase (HRP) to the proximal end of the cervical or abdominal nerve stump. Efferents in the cervical vagus were traced to their perikarya or origin in the dorsal motor nucleus (DMN) of the vagus, the commissural gray of the cervical spinal cord, the nucleus ambiguus, the nucleus of the accessory spinal nerve (NASN), and in the ventral horn dorsolateral to the NASN. Perikarya in the NASN and the region dorsolateral to it did not contribute efferent fibres to the abdominal vagus. In the remaining cell groups, fewer labelled perikarya were labelled in the abdominal cases than in the cervical cases. Extraperikaryal labelling (presumptive terminals) in the cervical cases was seen primarily in the nucleus of the solitary tract. A modest distribution of extraperikaryal grains was also noted along the inner rim of the area postrema and the ventral border of the DMN. Anterograde labelling was sparser and had a more restricted distribution in the abdominal cases. A detailed description of brainstem pathways of vagal efferent and afferent fibres is provided, as is a comparison of the present observations with those in similar studies of other species.

Sensation in the gastrointestinal tract

There are similarities between sensation in the gastrointestinal tract (GI tract) and somatic sensation. This review concentrates on parasympathetic (vagal) components of GI sensation rather than the sympathetic (splanchnic) elements. A wide range of enteroceptors have been described over the whole length of the gut which subserve several different sensory modalities. Fibres from these enteroceptors project to the medulla, primarily to the nucleus of the solitary tract. In the medulla there is considerable integration of afferent information from different parts of the GI tract. Regulatory peptides are present both in the brain and in the GI tract. It is likely that these peptides may play a role in the modulation of sensory information in the medulla. Parallels may be drawn at a receptor level between somatic sensation and sensation in the GI tract. More centrally, sensory mechanisms relating to the gut seem less highly organized than in somatic sensation. This reduced influence of the central nervous system in GI tract sensation may be explained by the presence in the gut of a highly sophisticated intrinsic nervous system, the enteric nervous system, which pre-programmes many of the functions of the GI tract.