The descending control of respiratory-related motoneurones in the thoracic spinal cord remains the subject of some debate. In this study, direct connections from expiratory bulbospinal neurones to identified motoneurones were investigated using spike-triggered averaging and the strengths of connection revealed were related to the presence and size of central respiratory drive potentials in the same motoneurones. Intracellular recordings were made from motoneurones in segments T5-T9 of the spinal cord of anaesthetized cats. Spike-triggered averaging from expiratory bulbospinal neurones in the caudal medulla revealed monosynaptic EPSPs in all groups of motoneurones, with the strongest connections to expiratory motoneurones with axons in the internal intercostal nerve. In the latter, connection strength was similar irrespective of the target muscle (e.g. external abdominal oblique or internal intercostal) and the EPSP amplitude was positively correlated with the amplitude of the central respiratory drive potential of the motoneurone. For this group, EPSPs were found in 45/83 bulbospinal neurone/motoneurone pairs, with a mean amplitude of 40.5 μV. The overall strength of the connection supports previous measurements made by cross-correlation, but is about 10 times stronger than that reported in the only previous similar survey to use spike-triggered averaging. Calculations are presented to suggest that this input alone is sufficient to account for all the expiratory depolarization seen in the recorded motoneurones. However, extra sources of input, or amplification of this one, are likely to be necessary to produce a useful motoneurone output.

Propriospinal interneurons in the thoracic spinal cord have vital roles not only in controlling respiratory and trunk muscles, but also in providing possible substrates for recovery from spinal cord injury. Intracellular recordings were made from such interneurons in anesthetized cats under neuromuscular blockade and with the respiratory drive stimulated by inhaled CO(2). The majority of the interneurons were shown by antidromic activation to have axons descending for at least two to four segments, mostly contralateral to the soma. In all, 81% of the neurons showed postsynaptic potentials (PSPs) to stimulation of intercostal or dorsal ramus nerves of the same segment for low-threshold (≤ 5T) afferents. A monosynaptic component was present for the majority of the peripherally evoked excitatory PSPs. A central respiratory drive potential was present in most of the recordings, usually of small amplitude. Neurons depolarized in either inspiration or expiration, sometimes variably. The morphology of 17 of the interneurons and/or of their axons was studied following intracellular injection of Neurobiotin; 14 axons were descending, 6 with an additional ascending branch, and 3 were ascending (perhaps actually representing ascending tract cells); 15 axons were crossed, 2 ipsilateral, none bilateral. Collaterals were identified for 13 axons, showing exclusively unilateral projections. The collaterals were widely spaced and their terminations showed a variety of restricted locations in the ventral horn or intermediate area. Despite heterogeneity in detail, both physiological and morphological, which suggests heterogeneity of function, the projections mostly fitted a consistent general pattern: crossed axons, with locally weak, but widely distributed terminations.

Activity in vagal preganglionic motoneurones running to the heart provokes a generalized cardioinhibition producing a decrease not only in heart rate (chronotropy), but also in the rate of atrioventricular (AV) conduction (dromotropy), and in the force of myocardial contraction (inotropy). These effects are secondary to, and are contingent upon, excitation of ganglion cells located in clusters on the epicardium of the dorsum of the atria, closely apposed to the sites of entry of the major veins. There is evidence that in many of the larger mammalian species, the ganglion cells are clustered into discrete zones or 'fat pads' which project to different regions of the heart and selectively control cardiac function (Ardell & Randall, 1986; Randall et al. 1986a,b;Gatti et al. 1995Gatti et al. , 1997. Several recent studies have described the anatomy of the cardiac vagal ganglia in the rat (Pardini et al. 1987;Abrahamian et al. 1991;Klimaschewski et al. 1992;Burkholder et al. 1992;deSouza et al. 1996;Cheng et al. 1999;Cheng & Powley, 2000).Although the results of some of these studies suggest the existence of a functional topography (e.g. Pardini et al. 1987), there is little clear physiological or pharmacological evidence of this in the rat. In the only previous study of a possible functional organization of rat cardiac ganglia (Burkholder et al. 1992), a brief description is given of a partial selectivity revealed by differing responses to vagal stimulation following local infiltration of hexamethonium into identified fat pads.Data from preliminary experiments in this laboratory suggest that glutamate excitation of cardiac vagal preganglionic motoneurones from different regions of the rat nucleus ambiguus results in differential chronotropic and dromotropic effects (Sampaio et al. 2000). That this may result from a different termination pattern of preganglionic axons onto ganglion cells from different ganglionic clusters is supported by the finding of restricted ganglionic innervation from motoneurones labelled with anterograde Vagal cardioinhibition is exerted through a reduction not only in the heart rate but also in the rate of propagation of the cardiac action potential and in myocardial contractility. In several species, such effects can be produced independently by selective activation of ganglia in identified 'fat pads'. In this study we investigate differential control of heart rate and atrioventricular conduction by two ganglionic clusters in the rat, a species increasingly important in studies of cardiovascular control. Epicardial sites producing low-threshold changes in P-P and P-R interval of the ECG in an arterially perfused preparation were explored with concentric bipolar stimulating electrodes. Stimulation sites centred on two principal ganglia, the sinoatrial (SA) ganglion at the junction of the right superior vena cava and right atrium, and the atrioventricular (AV) ganglion at the junction of the inferior pulmonary veins and left atrium. Stimulation of the SA ganglion decreased heart rate in all ...

Background-Temporary, antegrade amnesia is one of the core desirable endpoints of general anesthesia. Multiple lines of evidence support a role for the hippocampal θ-rhythm, a synchronized rhythmic oscillation of field potentials at 4-12 Hz, in memory formation. Previous studies have revealed a disruption of the θ-rhythm at surgical levels of anesthesia. We hypothesized that modulation of θ-rhythm would also occur at subhypnotic but amnestic concentrations. Therefore we examined the effect of three inhaled agents on properties of the θ-rhythm that are considered to be critical for the formation of hippocampus-dependent memories.

It has been widely accepted that the rat aortic depressor nerve contains only baroreceptors. However, the experiments which have provided these negative data have employed whole aortic nerve recording. In the present study, the technical difficulties associated with recording single fibres in vivo, from the rat aortic nerve (diameter 25‐50 μm), have been surmounted using a small tip, glass suction electrode technique. Upon switching from normocapnic hyperoxia to hypercapnic hypoxia, irregularly firing units (n= 13) appeared and these were significantly excited by intravenous injections of sodium cyanide (20 μg) but not by rises in arterial blood pressure induced by methoxamine (an α1‐adrenoreceptor agonist; 10 μg). Inhalation of 100 % oxygen rapidly and reversibly silenced, or profoundly reduced, ongoing activity. Intravenous injection of phenylbiguanide (PBG; a 5‐HT3 receptor agonist; 8 μg) strongly stimulated the chemoreceptors and was followed by a period of chemodepression (3‐21 s). In contrast none of the single fibre baroreceptors recorded (n= 15) were excited by PBG but all significantly increased their discharge in response to the increases in arterial blood pressure associated with methoxamine and cyanide. Both the excitatory and inhibitory effects of PBG on the chemoreceptor fibres were abolished by ondansetron (a 5‐HT3 receptor antagonist: 1 mg kg−1 i.v.; n= 5 animals) whilst the chemoexcitatory action of cyanide was preserved. It is concluded that there are chemoreceptor afferents contained in the aortic nerve of the Sprague‐Dawley rat. The 5‐HT3 receptor appears not to be a pre‐requisite for aortic body chemoexcitation.