Intestinal chemosensitivity.
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Afferent Pathways↗
Biomedical subjects
N Mei
Publications and source records attributed to N Mei.
[Macro and micro-electrode study of hypothalamic projections of periodontal afferents in the rat and cat].
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Animals↗
Recent studies on intestinal vagal afferent innervation. Functional implications.
Data obtained during recent years have completely changed our understanding of the organization and function of visceral sensitivity. The digestive tract--and especially the small intestine--provides a very good example of this evolution. Classically, it was believed that the sensory innervation of the gut is supplied by both the vagus and the splanchnic nerves. The vagus nerves play the major role in the sensory innervation of the first part of the intestine (all layers of the bowel, except mesentery). These vagal sensory fibers are mostly nonmedullated and generally originate from free endings. Microelectrophysiological techniques have disclosed the existence of a great variety of receptors: mechanoreceptors, chemoreceptors, thermoreceptors and osmoreceptors in the intestinal area. Some receptors like glucoreceptors, are specific receptors since they do not respond to any stimuli other than glucose. Others, like osmosensitive receptors, behave as non-specific or multimodal receptors. The mechanism by which an identical structure (free ending) induces various sorts of signals (mechanical, thermal, chemical etc.) is not yet known, but several hypotheses have been proposed. The discovery of a varied and complex mass of information obtained from studies of the gut finally corroborates the behavioural and clinical data which suggest that the intestinal sensory innervation plays an important role in physiological regulation. Now it is possible to distinguish 3 main kinds of mechanisms involved according to whether they concern digestive motility, homeostasis or alimentary behaviour.
Afferent Pathways↗
Thoracic esophageal mechanoreceptors connected with fibers following sympathetic pathways.
The existence of splanchnic mechanoreceptors was demonstrated in the esophagus including the lower esophageal sphincter of anesthetized cats. For this purpose, unitary activities were recorded in T9, T10 and T11 spinal ganglia by means of extracellular glass microelectrodes. Two types of receptors were evidenced according to their location: the muscular and the serosal receptors. The muscular mechanoreceptors usually exhibited a weak spontaneous discharge (0.8-18 imp/sec) and adapted slowly to mechanical stimulations (distension, contraction, digital compression). The potent physiological stimulus resulted in distension for the receptors situated in the thoracic esophagus and contraction for the receptors located in the lower esophageal sphincter. The serosal mechanoreceptors were always silent and belonged to the rapidly adapting type. They responded mainly to touching the serous membrane, but strong distension or stretching was sometimes efficient. A comparison with the vagal receptors already described in this region is drawn, and the role of splanchnic mechanoreceptors is discussed.
Action Potentials↗
Vagal mechanoreceptors located in the lower oesophageal sphincter of the cat.
In anaesthetized cats, sixty-two vagal sensory units with afferent endings in the lower oesophageal sphincter were recorded by means of extracellular glass micro-electrodes implanted in the nodose ganglion. All the receptors had non-medullated fibres, with conduction velocities ranging from 0.8 to 1.2 m/s. From the direct stimulation of the lower oesophageal sphincter, three types of mechanoreceptors were identified. Thirty-one were activated by natural stimuli:tonic contraction of the sphincter and distension elicited by the passage of a bolus. Artificial stimulation effected by digital compression was also effective. These receptors were similar to muscular endings already described in the digestive tract. Their main characteristic, i.e. their slow adaptation, suggests that they act as sensors of sphincter opening and closure. This was corroborated by observations obtained during distension of the cervical or thoracic oesophagus; a maximum decrease occurred in the lower oesophageal sphincter mechanoreceptor discharge when the distension was produced between 9 and 12 cm from the lower oesophageal sphincter. Twenty-nine endings were found in the superficial layers (mucosae). Contrary to the muscular receptors, the mucosal receptors were not affected by normal contractions or distensions of the lower oesophageal sphincter. They were activated only by strong stimuli like digital compression or distension achieved with a balloon. In addition, mucosal stroking was a potent stimulus. Whatever the stimulus used, the mucosal receptors showed rather rapidly adapting discharges. These receptors should be considered to be sensors of bolus consistency. Two mechanoreceptors, located in the serous membrane of the lower oesophageal sphincter, were identified by touching or by stretching. Their discharges showed that they belonged to the rapidly adapting type. A comparison of the three types of receptors found in the lower oesophageal sphincter is made with known digestive endings and their possible role is discussed.
Action Potentials↗
Afferent and efferent components of the bronchial vagal branches in cats.
The composition of the bronchial branches of the vagus nerves was studied in cats using light and electron microscopy. In order to determine the number and the diameter of fibers in the afferent and the efferent components, a unilateral efferent vagotomy was performed. The myelinated and the non-myelinated fibers were counted from the total nerve area and the endoneural area of each one was measured by means of a computer. Composition of "afferent" bronchial nerves (after degeneration following the efferent vagotomy) was compared to that of "entire" nerves. The main results are: (1) efferent fibers represent about 40% of the "entire" bronchial nerve; (2) non-myelinated fibers constitute more than 90% of the total population of the "entire" nerves as well as of the efferent component; (3) the density of myelinated and non-myelinated fibers (i.e. their number per surface unit) was similar in all nerves. However, there were discrepancies between diameter histograms established from different areas of a section. This feature seems to be due to preferential compartmentalization by Schwann cell envelopment of fibers having comparable diameter. This "packing effect" was observed in both efferent and afferent components.
Afferent Pathways↗
Electrophysiologic properties and role of the vagal thermoreceptors of lower esophagus and stomach of cat.
Vagal unitary discharges were elicited in anesthetized cats by thermal stimulation of the lower thoracic esophagus and stomach. Discharges were recorded from the nodose ganglion, using extracellular glass microelectrodes. Three types of receptors were distinguished according to the temperature ranges at which they discharged: the cold receptors (10 degrees-36 degrees C), the warm receptors (39 degrees-50 degrees C), and the mixed receptors (10 degrees-35 degrees C and 40 degrees-50 degrees C). All endings were connected to unmyelinated fibers (conduction velocities around 1 m/s). These receptors are stimulated neither by mechanical (strong distention, localized stroking of mucosa) nor by chemical (acid and glucose solutions) stimuli. Therefore they must be considered as true thermoreceptors. Stimulation of the esophageal and gastric thermoreceptors produced changes both in esophageal motility and in respiratory frequency. It was concluded that they are involved in coordination of digestive activity as well as in thermoregulation.
Animals↗
[Vagal acido- and glucoreceptors in the gastro-duodenal region (author's transl)].
Vagal sensitivity to carbohydrates (mainly glucose) and to acids (hydrochloric and acetic acids) was studied in the gastro-duodenal region of anesthetized cats. Action potentials were recorded extracellularly from the nodose ganglion by means of glass microelectrodes. Receptors responding to glucose perfusion were found at this level as well as receptors stimulated by acid perfusion. It is shown that each type of receptor was activated by only one kind of stimulus (carbohydrates and acids, respectively). These receptors must thus be considered as true glucoreceptors or acido-receptors. Being silent before activation, these two types of chemoreceptors discharged at a frequency that varied from 2 to 14 imp/s for the acido-receptors, and from 2 to 30 imp/s for the glucoreceptors. The response could be irregular or regular and might last several minutes. Its latency was short (between 1 and 20 s for both types of chemoreceptors. It was therefore supposed that they were located in the mucosa or neighboring structures. From the conduction velocities (0.8-1.2 m/s), it was concluded that fibers originating from the acido- and glucoreceptors belonged to the C type. Recording of the electromyographic activity from the gastro-duodenal region demonstrated that the gluco- and acid-receptors are involved in the regulation of motility of the gastric and duodenal regions.
Action Potentials↗
Nervous regulation of insulin release by the intestinal vagal glucoreceptors.
In anesthetized cats and rats, it is demonstrated that glucose perfusion of the small intestine produces a rapid increase of insulin secretion (IRI) which precedes glycemia variation. This mechanism involves the autonomic nervous system and originates from intestinal glucoreceptors, the existence of which was recently reported. The nervous pathways are described in this study:(1) the afferent pathway is represented by vagal fibers coming from the intestinal glucoreceptors; (2) the efferent pathway involves both sympathetic fibers (splanchnic nerves) and chiefly parasympathetic fibers (vagal nerves). These results are established after surgical suppression of afferent and efferent vagal fibers, and pharmacological exclusion of parasympathetic or sympathetic fibers. The role of this nervous regulation of insulin secretion is discussed with special reference to other already known mechanisms.
Animals↗
Assessment of true splanchnic glucoreceptors in the jejuno-ileum of the cat.
In anesthetized cats, the unitary activity of splanchnic sensory neurons were recorded from T9, T10 and T11 spinal ganglia by means of extracellular glass microelectrodes. Twelve neurons, generally silent, were activated by perfusion of jejuno-ileum with glucose solutions (10, 50, 100 or 200 g/l). The responses indicated that the corresponding receptors belonged to the slowly adapting type. The discharge frequencies were always low (less than 10 Imp/sec), but varied according to the glucose concentration. Saline solutions (NaCl having the same osmotic pressure as glucose ones), acid solutions (HCl, pH 2.7), or mechanical stimulation were unable to stimulate them. These neurons must be associated with true glucoreceptors. These receptors were connected via non-myelinated fibres (conduction velocities-0.8-2.4 m/sec). It is concluded that these splanchnic glucoreceptors are similar to vagal glucoreceptors described previously.
Adaptation, Physiological↗
Changes in activity of vagal bronchopulmonary C fibres by chemical and physical stimuli in the cat.
1. In eighteen anaesthetized cats, action potentials in non-myelinated vagal afferent neurones were recorded in the nodose ganglion by means of extracellular micro-electrodes. 2. The pulmonary or bronchial origin of these C fibres was assessed in closed chest preparations by injecting phenyl diguanide into either the right atrium or the ascending aorta (bronchial circulation). This was confirmed in two animals by local mechanical stimulation. 3. Eighty per cent of bronchopulmonary C fibres increased their discharge frequency when the end-tidal CO2 concentration (FA,CO2) increased from 0.02 to 0.10. Most of these C endings showed a maximal response when FA,CO2 reached 0.04. For the others a further increase in discharge occurred when CO2 concentration reached 0.08-0.10. Continuous measurement of C fibre discharge frequency indicated that they detected preferentially changes in the inspired CO2 content, but adapted when a high CO2 level was maintained. Their activation by hypercapnia was followed by an increase in lung resistance. 4. Lowering the O2 content of the inspired gas had no effect on the spontaneous activity of bronchopulmonary C endings. 5. When the stroke volume of the pump was doubled, the spontaneous activity of bronchopulmonary C fibres decreased in intact chest preparations. Inflation of the lungs had the opposite effect after the chest was opened. In both cases hyperdeflation was a potent stimulus to these receptors. 6. In tracheotomized cats, the tracheal temperature was 28-29 degrees C. When normal thermal conditions were restored in the tracheal lumen (33-34 degrees C) the spontaneous discharge frequency of some bronchial C fibres was greatly increased. 7. It is concluded that the spontaneous activity of most of the bronchial or pulmonary C fibres was maximal when chemical and physical physiological conditions were restored in the lungs. It appears that changes in alveolar CO2 concentration constitute the usual stimulus for these fibres.
Action Potentials↗
[Demonstration of mucous mechanoreceptors in the lower esophageal sphincter. Comparison with muscle mechanoreceptors].
In anaesthetized cats, vagal unitary discharges originating from the Lower Oesophageal Sphincter (L.O.S.) were recorded in nodose ganglia by means of glass microelectrodes. Numerous mechanoreceptors located both in mucosa and muscular layers were found in L.O.S. The mucus mechanoreceptors (high threshold receptors) were activated by strong compressions and distensions, by rapid passage of liquid through the oesophagus and by striking the mucosa. The muscular mechanoreceptors (low threshold receptors) responded to contraction and distension of L.O.S. Both receptors were connected to nonmyelinated fibres (conduction velocity: 0.9-1.4 m/sec).
Action Potentials↗
[Visceral sensitivity].
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Afferent Pathways↗
[Periodontal sensitivity and bone growth].
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Animals↗
Vagal and splanchnic effects at the level of the ventromedian nucleus of the hypothalamus (VMH) in the cat.
A systematic study of vagal and splanchnic projections to the VMH area was undertaken in anaesthetized cats by means of macroelectrodes and microelectrodes. Responses elicited by vagal and splanchnic nerve stimulation were recorded from the VMH nucleus and the region situated above it. The fact that they are identical, whatever the nerve stimulated, indicates that the afferences follow a common central pathway. From the response latency and the stimulation parameters, it is assumed that the small vagal and splanchnic fibres (A delta, B and C) are involved. Evoked potentials are generally constituted of two parts: the earlier implies certainly the VPL nucleus (since it disappears definitively after VPL coagulation), whereas the later concerns a different relay probably located in the neighbouring and associative structure (since it persists after VPL coagulation). Similar late responses were simultaneously recorded in VPL nucleus, suggesting that this structure was the same for both nuclei. Its exact location was discussed. On the other hand, effects of gastric distension on the evoked potentials produced by vagal and splanchnic nerve stimulation were studied. The results obtained (decrease or increase in the responses, occurrence of an additional potential) indicated clearly that the gastric afferences projected to the VMH region. This simple method could be used to determine the importance of the sensory innervation of the different parts of the digestive tract.
Afferent Pathways↗
The composition of the vagus nerve of the cat.
The number and caliber of myelinated and non-myelinated fibers of entire and sensory vagal nerves of cats were studied by means of light and electron miscroscopy. The results obtained with electron microscopy show that the non-myelinated component is particularly rich (about 40,000 elements at the cervical level), with clearly higher numbers of fibers than demonstrated thus far with light microscopy. The ratio of myelinated to non-myelinated fibers is on the average 1:4 for the total vagi and only 1:8 for the sensory vaga component. The comparison of the nerve above and below the level of the nodose ganglion shows that (1) mean fiber diameter is usually greater at the infranodose than at the supranodose level, and (2) some myelinated fibers of small diameter occurring below the nodose ganglion become non-myelinated above it. Additionally, the number of non-myelinated fibers per Schwann cell is greater at the supranodose than at the infranodose level; this speaks in favor of a reorganization of the C-fiber population from one level to the other.
Animals↗
[Role of vagal intestinal glucoreceptors in insulin regulation].
This paper summarizes recent data on the existence of insulin nervous regulation by duodenal glucoreceptors which are connected to non-medullated fibres included in the vagal nerves. These glucoreceptors respond to intestinal perfusion of glucose or other carbohydrates, but other conventional mechanical stimuli are unable to activate them; therefore, they must be considered as true specific glucoreceptors. Their activation produces an immediate increase of insulin which precedes changes in glycemia. From bivagotomy experiments and from experiments which pharmacologically exclude the efferent parasympathetic or sympathetic fibres, it has been shown that this regulation is chiefly mediated by the vagal nerves. On the other hand, it was found that glucoreceptor afferents project on the hypothalamic nuclei (VMH and LHA) like the whole splanchnic and vagal afferents. The physiological importance and the role of insulin nervous regulation are discussed.
Afferent Pathways↗
[Electrophysiology of the peripheral effect of two analgesics: aspirin and dibencozide].
The peripheral effect of two analgesics (aspirin and dibencozide) was studied on anaesthetized cats. Several types of neurons and stimulations were performed in this work: traction for periodontal mechanoreceptors connected to small-sized trigeminal fibres, distension for the muscular intestinal mechanoreceptors connected to non-myelinated vagal fibres, chemical stimulation by means of phenyldiguanide for the non-myelinated vagal fibres, electrical stimulation of the myelinated and non-myelinated vagal fibres. In all cases, unitary activities were recorded into corresponding ganglia (nodose or gasserian) with extracellular glass microelectrodes. After injection of analgesics, a decrease of control responses were observed till 30 minutes but the maximum occurred between 1 and 5 minutes. This effect concerned the non-myelinated neurones as well as the myelinated ones. It can be explained by a direct action of analgesics on the ending excitability.
Animals↗