Search

1. In anaesthetized rats, using a focal recording technique, activity was recorded from single sympathetic postganglionic neurones innervating the caudal ventral artery of the tail. The following hypotheses were tested: (i) that the frequency of the dominant rhythmic discharge of the neurones can be different from the frequency of the central respiratory rhythm (as indicated by rhythmic phrenic discharge); and (ii) that the dominant sympathetic rhythm is not reliant on afferent feedback carried in aortic, sinus and vagus nerves. 2. Four types of preparation were used: spontaneously breathing (group 1), artificially ventilated (group 2), artificially ventilated with vagi cut (group 3), and artificially ventilated with vagus and sino-aortic denervation (group 4). 3. The frequencies of the dominant sympathetic rhythm under control conditions were: group 1, 0.91 +/- 0.12 Hz (mean +/- S.E.M., n = 5); group 2, 0.81 +/- 0.04 Hz (n = 18); group 3, 0.83 +/- 0.03 Hz (n = 17); group 4, 0.95 +/- 0.06 Hz (n = 11). The frequency of the dominant sympathetic rhythm was different from that of the phrenic rhythm in thirty-five out of fifty-one cases. 4. The mean frequency of the dominant sympathetic rhythm was not influenced significantly by hypocapnic apnoea. 5. Hyperthermia increased the frequency of the phrenic rhythm whilst decreasing that of the dominant sympathetic rhythm. 6. In all cases the frequency of the dominant sympathetic rhythm was different from that of the artificial ventilation cycle. 7. It is concluded that the frequency of the dominant sympathetic rhythm can be different from that of central respiratory drive and that it is not "driven' by afferent feedback relayed via sinus, aortic and vagus nerves. 8. It is proposed that the dominant sympathetic rhythm is unlikely to be generated by a central respiratory oscillator.

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.

The central respiratory-drive-related inputs to antidromically identified cervical sympathetic preganglionic neurones have been investigated, in the rat, using extracellular recording techniques, the ionophoretic application of an excitatory amino acid (glutamate) to increase their excitability, and phrenic nerve discharge as an indicator of central respiratory drive. Three distinct firing patterns of sympathetic preganglionic neurones are described: maximal discharge during phrenic nerve activity, maximal discharge during phrenic silence, and a firing pattern unrelated to phrenic nerve discharge. Both spontaneously active and glutamate-activated silent cervical sympathetic preganglionic neurones had similar, if not identical, firing patterns. The application of glutamate, using ionophoretic currents of up to 100 nA, did not disrupt central respiratory-drive-related discharge patterns indicating that these inputs are an important contribution in the regulation of the firing pattern of a proportion of sympathetic preganglionic neurones. On the basis of these observations it is proposed that some sympathetic preganglionic neurones may receive central respiratory drive potentials similar to those received by respiratory motoneurones.

In halothane-anaesthetized, vagotomized, SA-denervated rats, the activity of various sympathetic nerves has been analyzed with respect to phrenic nerve discharge (an indicator of central respiratory drive (CRD)). The cervical and lumbar sympathetic nerves had maximal activity following, and were least active during phrenic nerve discharge. In contrast, the splanchnic, cardiac, renal and adrenal nerves exhibited their activity peak during phrenic nerve discharge (i.e. inspiration). Similar activity profiles were observed after ganglion blockade in the mixed pre- and postganglionic fibre preparations. These observations indicate that it is the subpopulations of preganglionic neurones and the proportional contribution of each to whole-nerve activity which give rise to the differences in CRD-related activity profiles between nerves.