SUMMARY1. Intracellular recordings from sympathetic neurones in the isolated coeliac ganglion of guinea-pigs have been used to define the synaptic input to three subtypes of neurone, classified on the basis of their discharge during maintained depolarizing current as phasic neurones, neurones with prolonged after-hyperpolarizations (LAH), and tonic neurones.2. The three classes of neurone were distributed characteristically in different parts of the ganglion.3. Passive membrane properties differed between the three neurone types. Mean input resistance was highest in phasic neurones and was inversely related to the size of the prolonged calcium-activated potassium conductance in LAH neurones. Mean input time constant was highest in tonic neurones, because of significantly higher cell capacitance.4. Phasic and LAH neurones usually received one suprathreshold ('strong') as well as several subthreshold excitatory synaptic potentials (ESPs) from the ipsilateral splanchnic nerve. In general, the amplitude and number of splanchnic inputs were greater, and the occurrence of two strong inputs more common, in phasic than in LAH neurones. The input to tonic neurones was small and usually subthreshold, even with supramaximal splanchnic stimulation. In a few (mostly tonic) neurones lying close to the midline, small ESPs were evoked by contralateral splanchnic stimulation.5. Antidromic action potentials were evoked in more than half of all neurones by high voltage coeliac nerve stimulation. In addition, multiple small subthreshold ESPs were recorded in virtually all tonic neurones (99 %) on coeliac nerve stimulation. In contrast, coeliac stimulation rarely evoked a few very small ESPs in LAH neurones (9%), but no synaptic response in phasic neurones.6. In about half of the tonic neurones tested (but no phasic or LAH neurones), small ESPs were evoked by stimulation of the intermesenteric nerve.7. Slow depolarization elicited by repetitive activation of splanchnic and coeliac nerve trunks, at voltages supramaximal for the fast cholinergic responses, were * Present address: Department of Physiology and Pharmacology, University of Queensland, St Lucia, Queensland 4067, Australia. E. M. McLACHLAN AND R. L. MECKLERrecorded from about half of both phasic and tonic neurones, but only one of twentyfour LAH neurones. These responses commonly faded during subsequent trials, so that it was difficult to characterize them.8. The data indicate that the three broad groups of coeliac neurone, classified on the basis of their voltage-and calcium-dependent potassium conductances, receive different patterns of synaptic input. The differences may be related to the three major functions of vasoconstriction, motility and mucosal secretion in the small intestine.

1. Sympathetic neurons in superior mesenteric ganglion and inferior mesenteric ganglion (IMG) isolated from guinea pigs were classified as tonic, phasic, or long after hyperpolarizing (LAH) on the basis of their discharge characteristics and the different types of potassium currents recorded from them with the soma under single-microelectrode voltage clamp. 2. Passive electrical properties showed a progressive increase in input resistance across the prevertebral ganglia in the rostrocaudal direction when compared with those previously reported for the same classes of neurons in celiac ganglia (CG). 3. The proportions of tonic, phasic, and LAH neurons changed markedly in a rostrocaudal progression from 37, 14, and 49%, respectively, in the CG to 80, 18, and 2%, respectively, overall in the IMG. 4. Three populations of neurons distinguished immunohistochemically by their content of somatostatin (SOM), neuropeptide Y (NPY), or neither neuropeptide were present in different proportions in each prevertebral ganglion. The proportions of SOM, NPY, and no peptide neurons changed from 27, 34, and 39%, respectively, in the CG to 45, 20, and 35%, respectively, in the IMG. There was no significant difference in these distributions between the sexes. 5. Individual electrophysiologically characterized neurons were filled with biocytin and later examined for SOM immunoreactivity. All SOM-positive neurons (9/9) in the CG but only 7/10 in the IMG were tonic, whereas SOM-negative neurons were classified in all electrophysiological classes. 6. Other than this one group of sympathetic neurons (constituting nearly 30% of neurons in both CG and IMG), the three electrophysiological classes do not correlate directly with the three neurochemical types so far identified. This is consistent with the existence of more than three functional groups of sympathetic neurons in the prevertebral ganglia. The findings also suggest that a major inhibitory effect on mucosal secretion, as well as on motility, is mediated by peripheral reflex pathways along much of the length of the gastrointestinal tract.

Afferent neurons contained within cardiac sympathetic nerves may have important influences on the circulation when activated during myocardial ischemia. Although such activation is known to reflexly excite upper thoracic sympathetic efferent neurons, effects on other components of sympathetic outflow are unknown. Therefore, cardiac sympathetic afferent nerves were stimulated by occlusion of coronary arteries to investigate their reflex influences on renal sympathetic nerve activity and systemic arterial blood pressure. Responses were observed in anesthetized cats in which sympathetic and/or vagal cardiac afferent nerves remained intact and arterial baroreceptors remained intact or had been denervated. Stimulating sympathetic afferent neurons caused excitation of renal nerve activity, which was accompanied by variable changes in arterial pressure. Stimulation of vagal afferents by coronary occlusion consistently produced inhibition of renal nerve activity and marked depressor responses. When both components of cardiac innervation remained intact, increases or decreases in renal nerve activity and blood pressure were elicited by coronary artery occlusion in the presence or absence of arterial baroreceptors. These results illustrate that cardiac sympathetic afferent nerves can contribute significantly to cardiovascular control during myocardial ischemia.

SUMMARY1. Electrical discharge of thirty-nine single splenic and renal postganglionic nerve fibres was recorded in artificially respired, chloralose-anaesthetized cats.2. Ongoing discharge rates, averaged over 10 s periods, did not differ between renal and splenic fibres. All neurones of both groups had irregular discharge frequencies.3. Half of the splenic population and all renal fibres had cardiac-related discharge patterns. Of those tested for respiratory-related firing, 30 % of the splenic fibres and 69% of the renal fibres exhibited this pattern.4. Firing of splenic fibres was less inhibited than that of renal fibres by stimulation of pressoreceptors with phenylephrine-induced increases in blood pressure. Firing of splenic fibres also was less excited than that of renal fibres by unloading pressoreceptors with depressor doses of sodium nitroprusside.5. Chemical stimulation of splenic afferent nerves with bradykinin consistently elicited greater increases in splenic than renal nerve discharge by causing large increases in firing of all splenic fibres and smaller excitatory responses in 75 % of the renal fibres.6. Application of bradykinin to the intestinal serosa produced greater increases in renal than splenic nerve discharge by consistently causing increased firing of renal fibres and by causing excitation, inhibition, or no change in splenic fibre discharge.7. Responses of splenic and renal fibres to stimulation of splenic and intestinal afferent nerves after spinal cord transection were similar to those responses elicited when the neuraxis was intact.8. In conclusion, the differerntial reflex responses of splenic and renal neuronal populations can be due to the heterogeneity or to the intensity of responses within a neuronal population.

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