Cardiac Branches Research Articles

The Location of Cardiac Vagal Preganglionic Neurones in the Brain Stem of the Dogfish Scyliorhinus Canicula

The locations, within the brain stem, of vagal efferent preganglionic neurones with axons in the two pairs of cardiac vagal rami of the dogfish have been defined by the retrograde intra-axonal transport of horseradish peroxidase (HRP). HRP was applied to the cardiac rami in one of two ways: either as crystals placed on the cut central end of the nerve or as a dried concentrated solution administered into the nerve on the tip of a fine pin. No difference was observed in the number of labelled cell bodies identified using either method. Labelled branchial cardiac vagal motoneurones were found ipsilaterally in the medial division of the vagal motor column, in the lateral division of the vagal motor column, and scattered between these two locations. In contrast, visceral cardiac vagal motoneurones were confined to the ipsilateral medial division of the vagal motor column. We suggest that the dual location of cell bodies supplying axons to the branchial cardiac branch of the vagus may represent a separation of function with respect to the two types of activity conducted by this nerve. Cardiac efferent fibres are confined in their exit from the brain to a middle group of vagal rootlets. This corresponds to the topographical representation of cardiac efferent somata within the extent of the vagal motor column.

Synaptic mechanisms involved in the inspiratory modulation of vagal cardio-inhibitory neurones in the cat.

The respiratory modulation of the activity of vagal cardio-inhibitory neurones of the nucleus ambiguus of the cat has been investigated by electrophysiological and neuropharmacological techniques. All twenty-four vagal efferent neurones studied had axons with conduction velocities indicative of B fibres and projected to the right cardiac branches of the vagus. Their spontaneous or DL-homocysteic acid (DLH)-evoked activity showed a marked reduction during the phase of inspiration and all showed signs of receiving a baroreceptor input. Ionophoretic application of DLH always excited cardiac vagal motoneurones (c.v.m.s). Application of acetylcholine to these same cells provoked a decrease in firing rate in twelve of the fifteen neurones tested. In ten of these twelve cells simultaneous application of atropine antagonized the effect of acetylcholine. Atropine applied alone enhanced neuronal firing, particularly in inspiration. Stable intracellular recordings have been made from two c.v.m.s. These were inhibited during inspiration. Input resistance fell markedly during inspiration and injection of chloride reversed this wave of hyperpolarization to a wave of depolarization indicating that this resulted from chloride-mediated inhibitory post-synaptic potentials (i.p.s.p.s). These c.v.m.s were activated during Stage I expiration, and showed a weak and variable wave of inhibition in Stage II expiration. Pulse-rhythmic depolarizing potentials were reduced in their amplitudes during the periods of decreased neurone input resistance. It is concluded that c.v.m.s receive an excitatory input during post-inspiration and a powerful inhibitory synaptic input during inspiration. The implications of these observations for the physiology of cardiorespiratory reflexes are discussed.

Afferent innervation of the lower esophageal sphincter of the cat. Pathways and functional characteristics

The sensory vagal and ‘sympathetic’ innervation of the lower esophageal sphincter (LES) has been investigated in cats using both electrophysiological and histochemical techniques. Histochemical studies produced evidence that ‘sympathetic’ afferents run in: (i) the splanchnic nerves; (ii) the sympathetic thoracic nerves; and (iii) the sympathetic cardiac branch. Electrophysiological studies allowed us to describe different kinds of LES receptors: (i) mucosal vagal receptors acting as rapidly adapting mechanoreceptors or as slowly adapting chemoreceptors; (ii) vagal and ‘sympathetic’ endings located in the muscular layers, adapting slowly and mainly activated by LES contractions; and (iii) vagal and ‘sympathetic’ endings located in the serous membrane behaving as rapidly adapting receptors sensitive to stretching.

Accuracy of regeneration of vagal parasympathetic axons.

The degree of accuracy with which regenerating preganglionic parasympathetic fibers can restore their original connections was examined in the frog, Rana pipiens. The normal motor pool of the vagus nerve was determined by labeling the vagal cardiac branch fibers with horseradish peroxidase (HRP). Cardiac neurons form a relatively compact subgroup within the rostral half of the vagal motor column and represent less than 5% of the total vagal motor pool. Five to 14 weeks after crushing the vagus nerve, HRP labeling revealed that a more extensive population of vagal motor neurons has reinnervated the cardiac branch. The regenerated cardiac motor neuron pool is about twice as numerous as normal but is still centered in the rostral half of the vagal motor column. These experiments show that regeneration of the cardiac branch of the vagus nerve is neither a random process nor one which is completely accurate. Some degree of accuracy is maintained during regeneration following a crush lesion. Neurons nearer the original cardiac motor pool are more likely to reinnervate the cardiac branch than are vagal motor neurons located at some distance from the original cardiac pool.

Afferent innervation of the lower oesophageal sphincter of the cat. An HRP study

Labeling of afferent neurons by the retrograde axonal transport of horseradish peroxidase (HRP) was performed on anaesthetized cats in order to examine the afferent innervation of the lower oesophageal sphincter (LOS), involving both the vagal and the sympathetic nerves. The labeled cells, whose fibres follow the sympathetic pathways were found in dorsal root ganglia from T1 to L2. Nerve section experiments indicated that the main pathways involved were the splanchnic nerves, as expected from classical data. Additional pathways passing through the sympathetic cardiac branch emerging from the stellate ganglion and the thoracic sympathetic branches were also evidenced. This work corroborated the electrophysiological data showing the richness of the LOS sensory vagal innervation. Nevertheless, in this case the difficulties related to the HRP technique are particularly enhanced since the abdominal sensory vagal fibres can be affected by HRP injections.

Reversible truncal vagotomy in conscious dogs

Prior attempts at in vivo reversible vagal denervation of the gastrointestinal tract have been limited to cervical cooling techniques that also denervate both sympathetic and vagal pulmonary and cardiac branches. Denervation of vagal efferent fibers at this level has produced results that are inconsistent with those obtained after surgical truncal vagotomy. We have, therefore, developed a technique to provide reversible vagal denervation below the pulmonary and cardiac branches for the study of gastric motility. Five dogs, previously equipped with gastric strain gages and electrodes, underwent implantation of a tubular cooling jacket around a distal thoracic vagal trunk with contralateral vagotomy (4 dogs), or around both vagal trunks (1 dog). The jacket was made of stainless steel tubing in a “J” design. Its inside channel was lined with a sterling silver sheet, and a thermistor was attached to record temperature change. Silicone tubing coursed externally to a pump and flask to which 95% ethanol at −70 °C was circulated at variable speeds. Thoracic vagal cooling, extending up to 5 h, reversibly blocked gastric contractions induced by insulin hypoglycemia. Contractile waves were terminated at device temperatures of 2 °–6 °C but promptly returned with warming. Dogs were tranquil during denervation, and enclosed nerves remained functional for >40 days.

Differential pattern of sympathetic outflow during upper airway stimulation with smoke.

This study investigates directly the possibility that sympathetic discharge to the heart is decreased while it is increased to other organs during upper respiratory perfusion with cigarette smoke. Blood pressure (BP), heart rate, ECG, and respiratory movements were monitored in urethane-anesthetized rabbits. Insertion of two cannulas allowed respiration of room air while passing smoke across the upper respiratory irritant receptors and out through the nares. Through a retroplural incision, the left stellate ganglion was exposed and a cardiac branch isolated. Similarly, a left renal nerve was isolated. Multiunit nerve recordings were obtained from both nerves. In four control animals, cigarette smoke (50 ml) caused apnea, bradycardia (-116 beats/min) and increased BP (33 mmHg). Activity in the renal nerve increased (248% of control [C]) and activity in the cardiac nerve was reduced (62% C). In these animals after Flaxedil and artificial respiration, nerve activity responses were still pronounced (renal, 178% C; cardiac, 66% C). In four other barodenervated animals neural responses to smoke were similar to those observed with baroreceptors intact (renal, 211% C; cardiac, 51% C). In these animals after artificial ventilation and Flaxedil, responses were not significantly changed. These results indicate that smoke stimulation causes a differential pattern of sympathetic discharge. The responses observed cannot be accounted for by secondary adjustments through arterial baroreceptors, chemoreceptors, or pulmonary stretch receptors.

Vagal cardiac preganglionic neurons: distribution, cell types, and reflex discharges.

In chloralose- and urethan-anesthetized rats, the cardiac branch (CB) of the vagus nerve was electrically stimulated, and antidromic responses of medullary cells were recorded. The cells identified as the vagal cardiac preganglionic neurons (VCPN) were localized in the dorsal motor nucleus (ND group, 8 cells), a region in and around the nucleus ambiguus (NA group, 7 cells) and an intermediary zone (IM group, 2 cells) lying in between. Latencies of the antidromic responses were distinctly different among the three groups, and calculated conduction velocities indicated that the VCPN of the ND group possess C-fiber axons whereas those of the NA group and probably of the IM group, B-fiber axons. In another series of experiments, the right carotid sinus nerve (CSN) or the left cervical vagus nerve was stimulated, and efferent fiber group(s) mediating reflexly evoked discharges to the CB was determined by means of two-point recordings. Among reflex discharges evoked by stimulation of the CSN the shortest latency reflex was proved to be mediated by B-efferent fibers. In contrast, among reflex discharges evoked by stimulation of the vagus nerve, the greatest reflex component was found to be conveyed by C-efferent fibers. It was concluded that the VCPN consist of two types of cells, each located in a different region of the medulla oblongata and contributing to vagal cardiac reflex mechanisms in a different manner.

Reinnervation of denervated parasympathetic neurones in cardiac ganglia from Rana pipiens.

1. The sequence of events during reinnervation of the cardiac ganglion in the frog following interruption of the vagosympathetic nerve supply was studied with both electrophysiological and morphological techniques. 2. When cardiac ganglia were denervated by crushing the vagosympathetic nerve supply to the heart all synaptic endings on parasympathetic ganglion cells degenerated. Vacated post-synaptic densities were detected on denervated neurones for periods of at least 7 weeks. 3. The earliest signs of reinnervation were subthreshold responses evoked by stimulating the regenerating vagosympathetic trunks 2 1/2-3 weeks after crushing the cardiac branches of the vagus nerves. Analysis of the reversal potentials of these responses indicated that these synapses were distant from the cell body. 4. At slightly longer times (4-5 weeks), regenerating synapses could be recognized on post-ganglionic axons; no synapses were detected on the neuronal perikarya at these times. 5. By 6-7 weeks following denervation, vagal synapses reinnervated neuronal perikarya as well as post-ganglionic axons. At the same time, vacated post-synaptic densities declined in number. Furthermore, vagal stimulation at this stage evoked large, suprathreshold post-synaptic potentials. 6. These studies indicate that post-ganglionic axons are the initial sites for reinnervation of parasympathetic neurones in the heart. Only some time later are neuronal perikarya reinnervated and ganglionic transmission completely restored.

Influence of the vagus nerve on changes in heart rate during sleep apnoea in man.

1. The changes in heart rate surrounding a period of apnoeic asphyxia, produced by either central or obstructive sleep apnoea, are characteristically a progressive bradycardia during the apnoeic period followed by an abrupt reversal to tachycardia with resumption of lung inflation. 2. In a subject who had undergone bilateral pulmonary vagotomy with sparing of the cardiac branches on the left side, the bradycardia is still present during the apnoeic period; however, there is no abrupt response to lung inflation. 3. A subject who had undergone unilateral right-sided pulmonary vagotomy demonstrated a normal response, the increase in heart rate coinciding with resumption of airflow. 4. These findings are consistent with the hypothesis that the bradycardia of sleep apnoea is a hypoxia-induced chemoreflex and that reversal to tachycardia with resumption of lung inflation is a reflex with afferent fibres in the pulmonary vagus nerves.

Pulmonary opiate receptor activation evokes a cardiorespiratory reflex

The administration of [D-Ala 2,Met 5]enkephalinamide (D-AME) and [D-Ala 2,Leu 5]enkephalinamide (D-ALE) into the right atrium of decerebrate rats caused bradycardia, a slight transient biphasic blood pressure response and apnea within 1–2 sec. Apnea was followed by rapid shallow breathing. These effects were dose related (1–1000 μg/kg) and blocked by pretreatment with naloxone. Atropine blocked the bradycardia. Sectioning the vagi at the level of the diaphragm did not affect the responses, whereas bivagotomy below the cardiac branches abolished all responses. The triad of responses was attributed to a reflex action arising from vagal afferents within the lung. These results were confirmed in paralyzed, artificially ventilated animals. In these animals, the enkephalin analogues produced a cessation of phrenic nerve (PN) activity followed by a decrease in the duration of bursts. The recurrent laryngeal nerve (RLN) was concomitantly excited in a continuous decremental fashion. This excitation was independent of PN inhibition. Recordings of single and near single pulmonary vagal affarents demonstrated no effect of D-AME or D-ALE on stretch and irritant receptors. However, type J-receptors were stimulated.

THE NATURE OF THE ATRIAL RECEPTORS RESPONSIBLE FOR A REFLEX INCREASE IN ACTIVITY IN EFFERENT CARDIAC SYMPATHETIC NERVES

In dogs anaesthetized with chloralose, distension of small balloons in the right upper and middle pulmonary vein-atrial junctions, to stimulate left atrial receptors, caused an increase in heart rate and an increase in activity in efferent sympathetic nerve fibers in cardiac branches of the right stellate ganglion. Cooling of the cervical vagi in steps reduced the magnitude of the responses in these sympathetic nerve fibres. In four dogs, the response in six preparations of sympathetic nerves was slightly reduced with the vagi at 18 degrees C and markedly reduced or abolished at 12 degrees C. In these nerves there was no significant response to distension of the balloons when the cervical vagi were cooled to 9 degrees C. The effect of cooling the vagi was the same as the previously shown effect of cooling on the increase in activity in myelinated afferent vagal fibres and the increase in heart rate during stimulation of atrial receptors. It is concluded that the increase in activity in efferent sympathetic cardiac nerve fibres during distension of small balloons in the pulmonary vein-atrial junctions involves receptors discharging into myelinated vagal nerve fibres; receptors which discharge into non-myelinated vagal nerves of afferent sympathetic nerve fibres are not involved in this response. Thus, the efferent sympathetic nerve fibres studied are likely to represent the efferent pathway of the response of an increase in heart rate to distension of the small balloons.

Medullary origin of vagal preganglionic axons to the heart of the cat

It is apparent from the literature that a controversy exists concerning the site of origin of cardiac vagal preganglionic axons. Physiological studies have suggested that the location of these neurons may be different in different species and there has been disagreement between physiological and anatomical findings in the same species. We now present anatomical and neurophysiological studies suggesting that in the cat cardiac vagal preganglionic neurons are located in two medullary regions: the areas of the dorsal motor nucleus of the vagus (DMV) and of the nucleus ambiguus (AMB). This suggestion is based on the following observations. Firstly, after application of horseradish peroxidase to the right cardiac branches of the vagus nerve, labeled neurons were found primarily in the regions of the DMV and AMB. Additional scattered neurons were found in the reticular formation between these two nuclei. Secondly, following injections of tritiated amino acids into either the DMV or AMB, labeled vagal fibers were found in the atrial myocardium. Finally, electrical stimulation of the right cardiac branches of the vagus nerve antidromically activated neurons in the DMV and AMB regions with latencies corresponding to conduction velocities of B-fibers. In addition, these neurons were orthodromically excited by electrical stimulation of the carotid sinus and aortic depressor nerves.

The distribution in the cat brain stem of neurones activated by vagal nonmyelinated fibres from the heart and lungs.

In anaesthetized cats, right cardiac vagal branches were electrically stimulated and recordings of evoked 'slow wave' and single neurone activity were made in the brain stem. Short-latency 'slow wave' and multi-neuronal activity evoked by excitation of myelinated vagal afferent fibres were recorded in the medial and lateral subnuclei of the nucleus tractus solitarius, the area postrema, the dorsal vagal motor nucleus, the lateral reticular formation and the nucleus ambiguus. Long-latency responses evoked by vagal non-myelinated fibres were recorded in the medial subnucleus of the nucleus tractus solitarius, the area postrema, dorsal vagal motor nucleus, the parahypoglossal area and the lateral reticular formation dorsal to the nucleus ambiguus. A specific study was made of seventy-two single neurones activated by non-myelinated afferent fibres in the cardiac branch. Thirty-four were shown to be synaptically activated, twenty-one were activated nonsynaptically and seventeen could not be classified. One neurone was also activated by myelinated cardiac afferent fibres, and two by thoracic vagal (including pulmonary) afferent fibres. Neurones were not spontaneously active. Indirect evidence suggests that the majority of the recordings of nonsynaptically activated neurones were likely to be from cell bodies. Neurones were located from the level of the obex to 3.0 mm rostral to it in the medial subnucleus of the nucleus tractus solitarius (45), and in the lateral subnucleus (2), the area postrema and its border with the medial subnucleus of the nucleus tractus solitarius (13), the dorsal vagal motor nucleus (9), the parahypoglossal area (1) and the lateral reticular formation dorsal to nucleus ambiguus (2). Recordings were made from fifteen neurones activated by myelinated fibres in the cardiac vagal branches, and twelve were excited synaptically. The neurones were located in the medial (8) and lateral (3) subnuclei of the nucleus tractus solitarius, the dorsal vagal motor nucleus (1) and the lateral reticular formation (1). Four neurones were also excited by vagal afferent fibres in the thoracic vagal nerve immediately caudal to the caudal cardiac branch.

Suppression of the depressor cardiac reflex during coronary occlusion by lidocaine--evidence for a central nervous site of action.

The effect of lidocaine (0.35 and 0.7 mg/kg/min i.v.) on reflex cardiovascular depression was studied during coronary occlusion in 10 chloralose-anesthetized cats. Twenty to 60 s occlusion of the left anterior descending coronary artery decreased heart rate, efferent cardiac sympathetic nerve activity and blood pressure by 33.8 ± 8.8%, 56.1 ± 31.8% and 48.4 ± 13.1% of control values, respectively. During infusion of 0.35 (0.7) mg/kg/min lidocaine the heart rate was decreased by 17.6±6.1% (10.2 ±6.9), the sympathetic activity by 46.2 ± 25.5% (30.2 ± 13.4) and blood pressure by 44.7 ± 13.6% (36.4 ± 16.6) compared to controls. The same reflex was elicited in 12 cats by afferent stimulation of cardiac vagal nerve branches, and the bradycardia and decrease of blood pressure were also inhibited by lidocaine. It is concluded, that the suppression of the reflex is due to a central nervous effect of lidocaine. A possible anticholinergic effect of the drug seems to play no major role, since the bradycardia during efferent vagal stimulation (eight animals) was not affected by 0.35 mg/kg/min lidocaine.

A horseradish peroxidase study of vagal motoneurones with axons in cardiac and pulmonary branches of the cat and dog.

The location, within the brain stem, of vagal efferent neurones with axons in the cardiac and pulmonary branches of the cat, and the cardiac branch of the dog, have been defined. Horseradish peroxidase was applied to the cut central ends of these vagal branches and, after an appropriate time, sections of the brain stem were examined for labelled cell bodies. Labelled cell bodies were found ipsilaterally in the dorsal motor vagal nucleus, the nucleus ambiguus and in the reticular formation between these two nuclei. We suggest that the dual location for cell bodies of vagal axons may represent a separation of somata with myelinated and non-myelinated vagal axons to the nucleus ambiguus and the dorsal motor vagal nucleus respectively.

Cardiac and non-cardiac preganglionic neurones of the thoracic vagus nerve: an HRP study in the cat.

Using the HRP method, we studied cardiac and non-cardiac preganglionic neurones of the thoracic vagus nerve in the cat. The inferior cardiac branches of the right thoracic vagus nerve were differentiated from the neighboring non-cardiac branches by positive cardiac slowing following electrical stimulation. A small amount of crystalline HRP was placed at the central cut end of the cardiac or non-cardiac branches. After a survival period of 2 days, the medulla oblongata, the stellate ganglion (SG), the superior cervical (SCG) and the nodose ganglion (NG) were serially sectioned and processed for histochemical demonstration of HRP. In the medulla oblongata, HRP-labeled cells were mainly located in 2 regions: (1) within and around the ambiguous complex (site A); (2) within the dorsal vagus nucleus (site X). After applying HRP to the cardiac branches, 968 HRP-labeled cells were observed in 7 animals bilaterally with the ipsilateral dominance (94%); 81% of them were found in site A. After applying HRP to the non-cardiac branches, 447 HRP-labeled cells were seen in 2 animals bilaterally with the ipsilateral dominance (92%); 79% of them were found in site X. Many HRP-labeled cells were found in the SG and NG, indicating that the cardiac branches are composed of sensory afferent and sympathetic efferent fibres as well as cardiac vagus efferent fibres.

An ultrastructural study of the cardia ganglia in the bulbar plexus of the developing chick heart.

The development of the chick bulbar cardiac ganglia has been studied ultrastructurally from the 3rd day of incubation to hatching. For descriptive purposes, their development has been divided into three phases. The first phase includes migration and aggregation of undifferentiated neuroblasts, stages 21-26 (day 3.5-5). Cardiac branches of the vagus first grow toward the heart at 3.5 days of incubation. The nerves are accompanied by migrating neural crest neuroblasts. The neuroblasts begin to aggregate at 4.5 days around the aortic arch arteries and truncus arteriosus. Some of these cells sprout axonal processes by day 5. A sparse population of developing supporting cells can be found near some neuroblasts and nerves. During the second phase of development, stages 27-36 (days 5-10), the neuroblasts assume a globular shape and have eccentric, indented nuclei. Supporting cells become more numerous as this phase of development progresses. Axodendritic synapses appear first at stage 34 in the ganglia. The neurons and supporting cells in the ganglia undergo maturation during the final phase from stage 37 to hatching (11-21 days). The immature/mature ganglion cells are large ovoid to pyriform neurons which become rounder as the cytoplasm accumulates. Axosomatic synapses are seen first at stage 38. Small supporting cells become more numerous. Clusters of catecholamine-containing cells near the cardiac ganglia are present during the final phase of development.

Distribution of vagal cardioinhibitory neurons in the medulla of the cat.

Experiments were done in cats anesthetized with chloralose, paralyzed and artificially ventilated cats to obtain electrophysiological evidence on the medullary site of origin of vagal cardioinhibitory fibers. The regions of the nucleus ambiguus (AMB), dorsal motor nucleus of the vagus (DMV), nucleus tractus solitarius (NTS), and external cuneate nucleus (ECN) were systematically explored for units responding both to antidromic stimulation of the cardiac branches of the vagus (CBV) and to orthodromic stimulation of the carotid sinus and aortic depressor nerves. Eighty-six single units conforming to these criteria were found in the medulla: 30 in the AMB, 26 in the DMV, 12 in the NTS, 8 in the NTS-DMV border region, and 10 in the ECN. Antidromically evoked spikes had durations of 0.5--2.5 ms and followed stimulation frequencies of 20--500 Hz. The axons of these units conducted at velocities of 3.3--20.8 m/s. The specificity of activation of medullary units by cardioinhibitory fibers was tested in 11 units, which were found to respond consistently with an antidromic spike to stimulation of CBV but not to stimulation of the thoracic vagus. In eight spinal animals low threshold (less than 15 microA) sites eliciting vagal bradycardia were found in the same medullary nuclei where cardioinhibitory units had been located. These results indicate that vagal cardioinhibitory axons, originate in at least three medullary nuclei, the AMB, DMV, and NTS. Unit activity from the ECN may have been recorded from carioinhibitory fibers because of the short duration of the spike potentials.

Localization of vagal cardioinhibitory preganglionic neurons within rat brain stem

In chloralose-urethane anesthetized spinal rats, electrical stimulation of systematically chosen points over the entire caudal brain stem area was carried out to explore the site(s) responsible for vagally mediated bradycardia. A dorsomedial locus including the nucleus dorsalis and the adjacent structures, the nucleus tractus solitarius, the nucleus commissuralis and the area postrema, and a ventrolateral locus around the nucleus ambiguus were found to elicit bradycardia with low threshold and high responsiveness. In another series of experiments, horseradish peroxidase (HRP) was iontophoretically administered through a glass capillary microelectrode into the identified cardiac branch of the vagus nerve of rats in order to localize more precisely the cells of origin of vagal cardioinhibitory fibers within the brain stem. Distribution of the HRP-labeled cells was not confined to one area, but these cells were found within the nucleus dorsalis, the reticular formation surrounding the nucleus ambiguus and an intermediary zone between the two nuclei. Such a pattern of distribution of vagal cardioinhibitory preganglionic cells is discussed in relation to phylogenetic and ontogenetic development of the vagal motor nuclei.