Spinal segmental preganglionic outflow to cervical sympathetic trunk and postganglionic cardiac sympathetic nerves

The aim of this study was to verify in which spinal cord segments the preganglionic neurones projecting to the cervical sympathetic trunk or converging onto the somata of the postganglionic cardiac sympathetic neurones are located in cats. The thoracic white rami T1 to T5 were electrically stimulated and the evoked responses were recorded in the cervical sympathetic trunks and postganglionic cardiac nerves. The responses were mostly evoked by electrical stimulation of group B preganglionic fibres. The maximum amplitude of evoked responses in the cervical sympathetic trunk was obtained when the T2 white ramus was stimulated and decreased gradually when followed by the stimulation of T1, T3, T4 and T5 white rami. In most cases the maximum amplitude of evoked responses in the left inferior cardiac nerve, right inferior cardiac nerve and left middle cardiac nerve was obtained when the T3 white ramus was stimulated. The size of the responses decreased when more cranial and caudal white rami were stimulated. It was found that the somata of the postganglionic neurones of the right and left inferior cardiac nerves were placed in the right and left stellate ganglion, respectively. Somata of the postganglionic neurones with axons in the left middle cardiac nerve were mainly located in the left middle cervical ganglion and some in the left stellate ganglion.     

Spinal segmental sympathetic outflow to cervical sympathetic trunk, vertebral nerve, inferior cardiac nerve and sympathetic fibres in the thoracic vagus

The spinal segmental localization of preganglionic neurons which convey activity to the sympathetic nerves, i.e. vertebral nerve, right inferior cardiac nerve, sympathetic fibres in the thoracic vagus and cervical sympathetic trunk, was determined on the right side in chloralose anaesthetized cats. For that purpose the upper thoracic white rami were electrically stimulated with a single pulse, suprathreshold for B and C fibres, and the evoked responses were recorded in the sympathetic nerves. The relative preganglionic input from each segment of the spinal cord to the four sympathetic nerves was determined from the size of the evoked responses. It was found that each sympathetic nerve receives a maximum preganglionic input from one segment of the spinal cord (dominant segment) and that the preganglionic input gradually decreased from neighbouring segments. The spinal segmental preganglionic outflow to the cervical sympathetic trunk, thoracic vagus, right inferior cardiac nerve and vertebral nerve gradually shifted from the most rostral to the most caudal spinal cord segments. In some cases, a marked postganglionic component was found in the cervical sympathetic trunk. It was evoked by preganglionic input from the same spinal cord segments which transmitted activity to the vertebral nerve. These results indicate that there is a fixed relation between the spinal segmental localization of preganglionic neurons and the branch of the stellate ganglion receiving the input from these neurons.     

Vagal stimulation during muscarinic and beta-adrenergic blockade increases atrial contractility and heart rate.

We determined the effects of continuous cardiac vagal nerve stimulation on atrial contractility and on heart rate in mongrel dogs in which we blocked the muscarinic and beta-adrenergic receptors. Each dog received atropine, 0.5 mg/kg and propranolol, 0.5-1 mg/kg. We stimulated the cardiac vagus nerves in each dog for three separate 5-min periods at frequencies of 0 (control), 20, and 40 Hz (5 ms, 15 V) and measured the changes in atrial contractility and heart rate. Vagal nerve stimulation increased right atrial contractility from the control value by 27% at 20 Hz and by 19% during stimulation at 40 Hz (P < 0.001). Vagal nerve stimulation also increased the heart rate from 114 +/- 5 beats/min during the control period to 146 +/- 10 beats/min (P < 0.01) during stimulation at a frequency of 20 Hz and to 140 +/- 11 beats/min (P < 0.05) during stimulation at 40 Hz. Injection of the vasoactive intestinal peptide (VIP) antagonist, [4Cl-D-Phe6,Leu17]VIP, directly into the dog right coronary artery in concentrations of 0 (control), 2, and 4 micrograms/kg did not influence spontaneous atrial contractility or the heart rate. However, 4 micrograms/kg of the VIP antagonist significantly reduced the augmentation in right atrial contractility and the increase in heart rate during vagal nerve stimulation. Our experiments suggest that cardiac vagal nerve stimulation, during muscarinic and beta-adrenergic receptor blockade, releases VIP or a 'VIP-like substance', that significantly augments atrial contractility and increases heart rate.     

Neural organization of the viscero-cardiac reflexes

The reflex responses evoked in the postganglionic nerves to the heart were tested in chloralose-anaesthetized cats. Electrical stimulation of the A delta afferent fibres from the left inferior cardiac nerve evoked spinal and supraspinal reflex responses with the onset latencies of 36 ms and 77 ms respectively. The most effective stimulus was a train of 3-4 electrical pulses with the intratrain frequency of 200-300 Hz. Electrical stimulation of the high threshold afferent fibres (C-fibres) from the left inferior cardiac nerve evoked the reflex response with the onset latency of 200 ms. The C-reflex was present in intact animals and disappeared after spinalization. The most effective stimulus to evoke this reflex was a train of electrical pulses delivered at a frequency of 1-2 Hz with an intratrain frequency of 20-30 Hz. The most prominent property of the C-reflex was its marked increase after prolonged repeated electrical stimulation. We conclude that: (1) viscero-cardiac sympathetic reflexes may be organized at the spinal and supraspinal level; (2) viscero-cardiac sympathetic reflexes evoked by stimulation of the A delta and C afferent fibres from the left inferior cardiac nerve have different central organization.     

The effect of selective electrical stimulation of non-myelinated vagal fibres on heart rate in the rabbit

The bradycardia evoked by electrical stimulation of the peripheral cut end of the rabbit vagus nerve is mediated by both myelinated and non-myelinated fibres. The purpose of this study was to assess the effects of non-myelinated fibres on heart rate in the rabbit using selective electrical stimulation techniques. In 8 rabbits selective activation of non-myelinated fibres using reversed polarity triangular shaped pulses (10 Hz, 20 s), resulted in a slowly developing fall in heart rate of 24.1 +/- 1.1 beats/min which outlasted the period of stimulation by 58.4 +/- 4.2 s. In 4 rabbits stimulation of myelinated fibres at 10 Hz for 20 s resulted in a fall in heart rate of 24.5 +/- 2.6 beats/min. On stimulation of both myelinated and non-myelinated fibres heart rate fell by 39.9 +/- 3.2 beats/min. Heart rate returned rapidly to control value following stimulation of myelinated fibres (5.6 +/- 0.5 s) but only slowly after stimulation of both myelinated and non-myelinated fibres (56.7 +/- 4.9 s). Atropine (5 mg/kg, i.v.) abolished all effects of vagal stimulation on heart rate. Hexamethonium (15 mg/kg, i.v.) abolished the effect of myelinated fibres on heart rate but did not affect the fall in heart rate produced by non-myelinated fibres. We suggest that the prolonged effects on stimulation of non-myelinated fibres may reflect the persistent action of a non-cholinergic excitatory transmitter at the cardiac parasympathetic ganglia.     

Pathway-specific connections between peptide-containing preganglionic and postganglionic neurons in the vagus nerve of the toad (Bufo marinus)

We have examined immunohistochemically the distribution of postganglionic nerve cell bodies and their preganglionic inputs in the vagus nerve of the toad, Bufo marinus. Nerve cell bodies containing immunoreactivity (IR) to somatostatin (SOM) were found at the origin of the oesophago-gastric ramus; these neurons projected to the lung. Cell bodies with SOM-IR also occurred in the intracardiac branches of the vagus, but were absent from the distal segments of the pulmonary and oesophageal rami of the vagus. Cell bodies with IR to vasoactive intestinal peptide (VIP) also occurred at the origin of the oesophago-gastric ramus, but most of these neurons projected to the oesophagus. Most neurons in the distal pulmonary and oesophageal rami were VIP-IR. Some nerve cell bodies in the vagosympathetic trunk and in the intracardiac rami contained both SOM-IR and VIP-IR. Vagal preganglionic nerve fibres with IR both to a somatostatin-like peptide and to substance P were associated exclusively with those postganglionic VIP-IR neurons that projected to the oesophagus. These results provide evidence for highly specific connections between immunohistochemically defined populations of preganglionic and postganglionic neurons in the vagus nerve.     

Vagal action on atrioventricular conduction and its inhibition by sympathetic stimulation and neuropeptide Y in anaesthetised dogs

The observed change in atrioventricular conduction time (PR interval) in response to vagal stimulation is the result of two opposing effects; PR interval increases in response to the direct action of the vagus on atrioventricular nodal cells (direct effect), and the accompanying slowing of heart rate acts to decrease PR interval (indirect effect). The relationships between these opposing effects were studied in anaesthetised dogs. This study has shown that the increase in PR interval in response to vagal stimulation is well correlated with vagal stimulation frequency and can be regarded as linear. This is so for unpaced and paced hearts. We have also shown there is an increase in the sensitivity of the relationship between increase in PR interval and vagal stimulation frequency during pacing. This increase in sensitivity is attributable to the elimination of the indirect effect of the slowing of heart rate. During atrial pacing, the relationship between pulse interval and PR interval resembles a hyperbola. At low-pulse intervals (i.e. fast heart rates) the PR interval increases. This is in agreement with previous qualitative findings and is related to the functional refractory period of the atrioventricular cells. The action of sympathetic stimulation and injection of neuropeptide Y has not been studied previously. The vagally induced increase in atrioventricular conduction time is attenuated for many minutes following stimulation of the cardiac sympathetic nerve at 16 Hz for 2 min or by intravenous injection of neuropeptide Y (25-50 micrograms/kg). Stimulation of the right cardiac sympathetic nerve evokes a significantly stronger inhibition of the vagally induced prolongation of pulse interval than stimulation of the left sympathetic nerve. On the other hand, stimulation of the left or right sympathetic nerves cause similar inhibition of vagal action on atrioventricular conduction time.     

Reciprocal and non-reciprocal action of the vagal and sympathetic nerves innervating the heart

Simultaneous recordings were made from vagal and sympathetic fibers innervating the heart in dogs anesthetized with chloralose. Reciprocal relationship between the two autonomic nerves was clearly seen in the baroreceptor reflex. Stimulation of chemoreceptors, however, evoked non-reciprocal responses of the two nerves; at the onset of the chemoreceptor reflex cardiac vagal and sympathetic discharges both increased, then, as baroreceptors became excited due to a pressor response, sympathetic nerve activity suddenly decreased while vagal discharges remained high, indicating the appearance of the reciprocal action typifying the baroreceptor reflex. Decrease in ventilatory volume and a slight increase in end-expired CO2 level augmented greatly both vagal and sympathetic discharges. As the phrenic-locked activity of the two nerves (i.e. the activity in vagus nerve occurs only in the absence of phrenic bursts while sympathetic discharges increase with phrenic bursts) increased, the alternate discharges between the two nerves became more conspicuous and the heart rate fluctuated with the respiratory (phrenic) rhythm. Thus, strong reciprocity between vagus and sympathetic can result in an oscillatory heart rate. When ventilatory volume was increased, both nerve activities decreased below control level. Mild hypoxia had similar effects to hypercapnia though changes in nerve activity were greater. When coactivation of vagal and sympathetic nerve was produced in reflex action, changes in vagal discharges occurred earlier and faster than in the sympathetic fibers. The magnitude of change in vagus activity was also far greater. The elimination of afferents in the vagi, the aortic and sinus nerves reduced cardiac vagal activity greatly. However, discharges were still present and occurred between phrenic bursts, indicating that the vagal "tone" is maintained centrally as well as peripherally by input from receptors in the cardiovascular system. The physiological significance of reciprocal and non-reciprocal control of vagal and sympathetic nerves innervating the heart was discussed.     

Sites at which neuropeptide Y modulates parasympathetic control of heart rate in guinea pigs and rats.

Immunohistological evidence indicates that neuropeptide Y (NPY) is present in the cardiac innervation of numerous species. The present experiments determined if NPY influences in vivo parasympathetic control of heart rate in guinea pigs and rats by either pre- or postganglionic mechanisms or by an interaction at muscarinic receptors at the sino-atrial node. Urethane-anesthetized animals were prepared with arterial and venous catheters, and ECG leads. The cervical vagi were sectioned and propranolol was administered to minimize reflex changes in heart rate. Methacholine injection, carbachol injection, or electrical stimulation of the peripheral end of the vagus nerve was performed to activate the neuroeffector site, intracardiac ganglion cells, or preganglionic neurons, respectively. All three trials were performed before, during, and after NPY infusion. No differences in methacholine- or carbachol-induced bradycardia were observed between control and NPY groups in either species. NPY infusion inhibited vagal-mediated bradycardia in guinea pigs and in rats. However, NPY inhibited vagal-mediated bradycardia at a lower dose in guinea pigs (1 microgram/kg/min) than in rats (4 micrograms/kg/min). These data indicate that NPY modulates cardiac vagal preganglionic, but not postganglionic nerve function or neuroeffector sites at the sino-atrial node, in guinea pigs and rats. Furthermore, due to the different effective dosages, NPY may play a greater modulatory role in guinea pigs than in rats.     

Selective vagal postganglionic innervation of the sinoatrial and atrioventricular nodes in the non-human primate

The distribution of parasympathetic postganglionic nerves to the atrioventricular (AVN) and sinoatrial nodal (SAN) regions was investigated in the non-human primate heart. Eight male monkeys (Macaca fascicularis) weighing 5.5-7.0 kg. were anesthetized (alpha-chloralose, 50 mg/kg and urethane, 500 mg/kg) and instrumented to measure arterial pressure, electrocardiogram, atrial and ventricular electrograms. The cervical vagi were electrically stimulated (20 Hz, 4 V, 2 ms) before and after selective denervation (D) of the AVN and/or SAN. Vagal stimulation was repeated during atrial pacing to assess parasympathetic modulation of AVN conduction. Ablation of parasympathetic pathways to the AVN, accomplished by the disruption of the epicardial fat and surface muscle layer at the junction of the inferior vena cava and inferior left atrium eliminated (P less than 0.01) the dromotropic effects of vagal stimulation without affecting the heart rate response (right vagus, before D, paced: atrial rate 218.0 +/- 6.3, ventricular rate 67.1 +/- 23.7; after D: atrial rate 210.3 +/- 6.4, ventricular rate 210.3 +/- 6.4 beats/min, means +/- S.D.). In sharp contrast, surgical dissection of the fat pad overlying the right pulmonary vein-superior vena cava junction significantly (P greater than 0.01) attenuated negative chronotropic effects of vagal stimulation (left vagus, before D the R-R interval increased by 832.7 +/- 146.4 ms, 209.5% increase; after D 37.4 +/- 18.0 ms, 8.8% increase). These data demonstrate discrete vagal efferent pathways innervate both the SAN and AVN regions of the non-human primate heart.     

Sympathetic-parasympathetic interactions at the heart in the anaesthetised rat

This study observed the effects of stimulation of the cardiac sympathetic nerve on vagal slowing of the heart in rats, and compared these with any actions of exogenous neuropeptide Y (NPY) and galanin (GAL). In rats anaesthetised with pentobarbitone, stimulation of the cardiac sympathetic nerve for 2 min at 20 Hz in the rat evoked an attenuation of subsequent cardiac vagal action, which could be mimicked by exogenous NPY, but not GAL. The galanin antagonist, GAL1-13/NPY24-36, known to block the inhibitory action of galanin on the cardiac vagus in cats, did not alter the effect of sympathetic stimulation on cardiac vagal activity. We suggest on the basis of results here that in the rat, NPY released during stimulation of the cardiac sympathetic nerve, causes inhibition of acetylcholine release from the vagus nerve.     

Effect of neuropeptide-Y and galanin on autonomic control of heart rate in the toad, Bufo marinus.

The effects of neuropeptide-Y (NPY) and galanin (GAL) on the autonomic control of heart rate were investigated in the anaesthetised toad, Bufo marinus. Both vagosympathetic trunks were sectioned to prevent reflex changes in heart rate, and the cardiac responses to electrical stimulation of either the vagal or sympathetic fibres to the heart assessed. Intravenous, bolus doses of 10 or 20 micrograms (2 or 4 nmol) NPY and 5 or 10 micrograms (1.5 or 3 nmol) GAL caused pronounced pressor responses but small direct changes in heart rate. Pulse intervals measured after peptide administration were within 5% of control values. All doses of both peptides caused inhibition of action of the cardiac vagus nerves, the maximum inhibition observed in response to 20 micrograms NPY: mean 49.5 +/- 14% (SEM). No significant changes in cardiac sympathetic nerve action were observed. It is concluded that NPY and GAL have similar, important cardiovascular actions in the toad. Similarities between the responses of toads and mammals to NPY suggest a phylogenetic conservation of function for this peptide.     

Enkephalins and functionally specific vagal preganglionic neurons to the heart: ultrastructural studies in the cat

In cat, distinct populations of vagal preganglionic and postganglionic neurons selectively modulate heart rate, atrioventricular conduction and left ventricular contractility, respectively. Vagal preganglionic neurons to the heart originate in the ventrolateral part of nucleus ambiguus and project to postganglionic neurons in intracardiac ganglia, including the sinoatrial (SA), atrioventricular (AV) and cranioventricular (CV) ganglia, which selectively modulate heart rate, AV conduction and left ventricular contractility, respectively. These ganglia receive projections from separate populations of vagal preganglionic neurons. The neurochemical anatomy and synaptic interactions of afferent neurons which mediate central control of these preganglionic neurons is incompletely understood. Enkephalins cause bradycardia when microinjected into nucleus ambiguus. It is not known if this effect is mediated by direct synapses of enkephalinergic terminals upon vagal preganglionic neurons to the heart. The effects of opioids in nucleus ambiguus upon AV conduction and cardiac contractility have also not been studied. We have tested the hypothesis that enkephalinergic nerve terminals synapse upon vagal preganglionic neurons projecting to the SA, AV and CV ganglia. Electron microscopy was used combining retrograde labeling from the SA, AV or CV ganglion with immunocytochemistry for enkephalins in ventrolateral nucleus ambiguus. Eight percent of axodendritic synapses upon negative chronotropic, and 12% of axodendritic synapses upon negative dromotropic vagal preganglionic neurons were enkephalinergic. Enkephalinergic axodendritic synapses were also present upon negative inotropic vagal preganglionic neurons. Thus enkephalinergic terminals in ventrolateral nucleus ambiguus can modulate not only heart rate but also atrioventricular conduction and left ventricular contractility by directly synapsing upon cardioinhibitory vagal preganglionic neurons.     

Cells of origin of sympathetic pre- and postganglionic cardioacceleratory fibers in the pigeon

The pre- and postganglionic cardioacceleratory innervation is described in the pigeon. The peripheral course of the postganglionic cardiac nerves has been determined using microdissection and electrical stimulation. Using these techniques and retrograde degeneration methods, the distribution within the sympathetic ganglia of the cells of origin of these fibers has been localized to the three right caudal cervical ganglia (12, 13 and 14). It has also been shown on the basis of electrical stimulation combined with selective ablation of the right sympathetic chain that cardioaccelerator preganglionic fibers probably arise from the most caudal cervical segment of the spinal cord (14), always arise from the upper two thoracic segments (15 and 16), and occasionally arise from a mid-thoracic segment (17). The left sympathetic chain was shown to have an inconsistent influence on heart rate. On the basis of retrograde degeneration, the cells of origin of sympathetic preganglionic fibers have been localized to a welldefined cell column dorsal to the central canal (column of Terni).     

Conduction pathways of the lumbar ganglia of the paravertebral sympathetic trunk in the rabbit

The conducting pathways of the lumbar (L3-L5) sympathetic ganglia were studied in rabbits by recording action potentials evoked in the nerves of the ganglia by stimuli applied to their other nerves and intracellular recording. It is established that some presynaptic fibres enter the sympathetic chain via grey rami and then pass upward and downward making synaptic contacts with ganglionic neurons. Other fibres enter the sympathetic chain through white communicating rami and pass in descending direction giving collaterals to the neurons of the ganglia. Descending preganglionic fibres with different conduction velocity converge on ganglionic neurons.     

N-desmethylclozapine an M1 receptor agonist enhances nitric oxide's cardiac vagal facilitation in the isolated innervated rat right atrium

We have previously determined that neuronal nitric oxide (NO) may partly mediate its established cholinergic effect via activation of muscarinic type 1 (M1) receptors located at the preganglionic/postganglionic synapse. In this series of experiments we set out to confirm this finding using an M1 agonist. Experiments were carried out on the isolated vagally innervated right atrium in the presence of atenolol (4 microM). The right vagus was stimulated at 4, 8, 16, 32 Hz; pulse duration 1 ms at 20 V for 20 s and the effect on cardiac interval (ms) assessed. N-desmethylclozapine (100 nM), a potent M1 agonist, enhanced the vagally induced increase in cardiac interval, a lower concentration of 50 nM had no significant effect on cardiac interval. This effect was prevented by pre-treatment of the atria with the neuronal NO synthase inhibitor 1 (2-trifluoromethylphenyl)imidazole (TRIM) at 0.14 mM. The vagal stimulation protocol was repeated in order to rule out a reduction in vagal effectiveness which may have been due to the experimental stimulation protocol used in this study. TRIM (0.14 mM) alone causes a small but significant attenuation of the vagally induced increase in cardiac interval. These results show that agonism of M1 receptors on cardiac vagal preganglionic fibres enhances vagal cardiac effects which can be prevented by a neuronal NO inhibitor.     

Motor innervation of the striated oesophagus muscle: Part 1. Intramural distribution of the right and left vagus nerve in the rat oesophagus as revealed by the glycogen depletion technique

The muscularis propria of the rat oesophagus is entirely made up of striated muscle fibres. All fibres are of the same histochemical type, which is characterized by high activity of actomyosin ATPase, medium activity of oxidative enzymes and relatively strong reaction for phosphorylase. Prolonged stimulation (10 Hz, 30 min) of the vagus nerves causes depletion of the glycogen content of the oesophageal muscle fibres. This stimulation effect can be visualized by means of the PAS technique as well as by the histochemical reaction for phosphorylase.In 8 animals the right and in 8 animals the left vagus nerve were stimulated repetitively and the stimulated muscle fibres were identified in transverse sections of the oesophagi, stained for phosphorylase. The muscle fibres supplied by one vagus nerve are distributed all over the circumference of the oesophagus. In the upper third of the oesophagus stimulation of either vagus nerve depletes slightly less than 50% of the muscle fibres, whereas in the lower two-thirds the right vagus nerve seems to predominate to a certain degree.In 3 animals both vagus nerves were stimulated simultaneously. Bilateral stimulation produced a very extensive depletion. Only a few muscle fibres remained unaffected. Functional implications of the results, the question of polyneuronal innervation and the role of the myenteric plexus are discussed.     

Responses of cardiac vagus and sympathetic nerves to excitation of somatic and visceral nerves

Somato-vagal and somato-sympathetic reflex responses were studied by recording simultaneously the activity of cardiac vagal and sympathetic efferents following excitation of various somatic (and 1 visceral) nerves in chloralose-anesthetized dogs.Stimulation of pure cutaneous (infraorbital, superficial radial, sural nerves), muscle (gastrocnemius, hamstring nerves) and mixed nerves (sciatic, brachial, intercostal, spinal) with short trains of pulses inhibited the activity of cardiac vagus nerve and excited that of cardiac sympathetic nerve after a latency of approximately 40–60 ms, depending on the nerve stimulated. These responses were followed by the opposite response, i.e. excitation of vagus and long-lasting inhibition (`silent period') of sympathetic nerve activity. These biphasic reflex responses recorded from both autonomic nerves had similar latencies so that a clear reciprocal relationship was observed. In addition to the above reflex responses which were observed in most instances, two peaks of excitation of short duration were recorded from the vagus nerve, in some instances, and an ‘early (spinal) reflex’ in sympathetic nerve was also observed. Both excitatory and inhibitory responses described above in either nerve were readily evoked by excitation of Group II (Aβ), but not Group I (Aα), afferent fibers and increased in magnitude when Group III (Aδ) afferents were also excited. Group IV (C) afferent contributed insignificantly to the somato-vagal reflex. The vagus nerve discharge evoked by sinus nerve stimulation was inhibited during reflex inhibition produced by somatic nerve stimulation. The latency of such inhibition was less than 20 ms and lasted for 100 ms after sural nerve stimulation. We conclude that, as in case of the baroreceptor reflex and autonomic component of the ‘defense reaction’, the somato-vagal and somato-sympathetic reflex responses are reciprocal in nature.     

Intracellular recordings from "recurrent neurons" in the rat superior cervical ganglion

Intracellular recordings in the isolated superior cervical ganglion of the rat showed that electrical stimulation of the cervical sympathetic trunk elicited in a cluster of neurons localized in the caudal part of the ganglion synaptically driven action potentials, and propagated potentials having the features of typical antidromic spikes. The results demonstrate that these neurons, besides synapsing with common preganglionic fibres, project their axons to the cervical sympathetic trunk. The recurrent neurons showed a very low threshold to direct intracellular stimulation and a high input resistance, suggesting that they have a small size. Almost all recurrent neurons were activated synaptically also by stimulating the postganglionic trunks, indicating that they are innervated by collaterals of preganglionic through-fibres which are known to sustain a direct pathway between pre- and postganglionic nerves. Moreover, some recurrent neurons could also be activated antidromically following stimulation of the external carotid nerve, indicating that their axons divide into collaterals which project not only to the preganglionic trunk but also to a postganglionic nerve. The presence of recurrent neurons in the superior cervical ganglion of the rat provides further evidence for the concept that sympathetic ganglia consist of discrete cell subpopulations which are segregated in different regions and probably subserve different functions.     

Cardiotopic organization of the nucleus ambiguus? An anatomical and physiological analysis of neurons regulating atrioventricular conduction

Previous data indicate that there are anatomically segregated and physiologically independent parasympathetic postganglionic vagal motoneurons on the surface of the heart which are capable of selective control of sinoatrial rate, atrioventricular conduction and atrial contractility. We have injected a retrograde tracer into the cardiac ganglion which selectively regulates atrioventricular conduction (the AV ganglion). Medullary tissues were processed for the histochemical detection of retrogradely labeled neurons by light and electron microscopic methods. Negative dromotropic retrogradely labeled cells were found in a long column in the ventrolateral nucleus ambiguus (NA-VL), which enlarged somewhat at the level of the area postrema, but reached its largest size rostral to the area postrema in an area termed the rostral ventrolateral nucleus ambiguus (rNA-VL). Three times as many cells were observed in the left rNA-VL as compared to the right (P < 0.025). Retrogradely labeled cells were also consistantly observed in the dorsal motor nucleus of the vagus (DMV). The DMV contained one third as many cells as the NA-VL. The right DMV contained twice as many cells as the left (P < 0.05). These data are consistent with physiological evidence that suggests that the left vagus nerve is dominant in the regulation of AV conduction, but that the right vagus nerve is also influential. While recording the electrocardiogram in paced and non-paced hearts,l-glutamate (GLU) was microinjected into the rNA-VL. Microinjections of GLU caused a 76% decrease in the rate of atrioventricular (AV) conduction (P < 0.05) and occasional second degree heart block, without changing heart rate. The effects of GLU were abolished by ipsilateral cervical vagotomy. These physiological data therefore support the anatomical inference that CNS neurons that are retrogradely labeled from the AV ganglion selectively exhibit negative dromotropic properties. Retrogradely labeled negative dromotropic neurons displayed a round nucleus with ample cytoplasm, abundant rough endoplasmic reticulum and the presence of distinctive somatic and dendritic spines. These neurons received synapses from afferent terminals containing small pleomorphic vesicles and large dense core vesicles. These terminals made both asymmetric and symmetric contacts with negative dromotropic dendrites and perikarya, respectively. In conclusion, the data presented indicate that there is a cardiotopic organization of ultrastructurally distinctive negative dromotropic neurons in the NA-VL. This central organization of parasympathetic preganglionic vagal motoneurons mirrors the functional organization of cardioinhibitory postganglionic neurons of the peripheral vagus nerve. These data are further discussed in comparison to a recent report on the light microscopic distribution and ultrastructural characteristics of negative chronotropic neurons in the NA-VL⁴². The data support the hypothesis that anatomically separated and functionally selective parasympathetic preganglionic vagal motoneurons in the NA may independently control AV conduction and cardiac rate.