Synaptic events evoked by brief noxious cutaneous stimuli were recorded in sympathetic neurones in the superior cervical ganglion of anaesthetized rats. On‐going excitatory synaptic potentials (ESPs) and/or action potentials (APs) were recorded in 69% of neurones at mean frequencies that varied from 0.01 to 6.3 Hz in different cells. From histograms of ESP amplitude during membrane hyper polarization, it appears that most cells received one (52%), or two or more (36%), suprathreshold inputs and several subthreshold inputs with overlapping amplitudes. Pinching the skin for 1–3 s evoked either a brief burst of synaptic events (lasting about 300 ms) preceding a few seconds of inhibition (burst–inhibitory (BI) neurones), or simply an excitation (excitatory (E) neurones), or no response (O neurones). In 60% of BI neurones, a second burst occurred after the end of the pinch. BI neurones had a higher frequency of on‐going synaptic activity (2.9 ± 0.5 Hz, n= 15) than E neurones (0.2 ± 0.1 Hz, n= 5) or O (0.2 ± 0.1 Hz, n= 5) neurones. Most neurones with two or more suprathreshold inputs were BI neurones. In 20% of neurones (all BI with high rates of synaptic activity), several other inputs had ESPs with amplitudes close to threshold. Subthreshold and suprathreshold inputs responded in the same way in only 45% of neurones, but suprathreshold inputs were excited in 73% of BI and all E neurones. The order of recruitment of different inputs varied from trial to trial. If classification was based only on suprathreshold responses, there were 36% BI, 32% E and 32% O neurones. In the majority of neurones, postganglionic discharge was initiated exclusively by supra‐threshold inputs, even during reflex excitation. Qualitatively similar, but smaller, responses were evoked by a puff of air on the abdomen in 71% of cells tested. The data suggest that the natural discharge of SCG neurones is largely determined by the activity of one or two preganglionic inputs with high quantal contents. BI neurones may include vasoconstrictor neurones, whereas the other types include secretomotor, pilomotor and other neurones projecting to targets in the head.

The patterns of on‐going synaptic events recorded intracellularly in neurones of superior cervical ganglia (SCG)of anaesthetized female rats were analysed by constructing inter‐event interval histograms, autocorrelograms, ln‐survivor curves and histograms triggered by the arterial pulse wave and by the intercostal EMG. In 11/12 cells with on‐going frequencies > 0.5 Hz, one or two inputs were strong (i.e. always suprathreshold). In five cells, action potentials also arose from synaptic potentials with amplitudes close to threshold. Synaptic events in 5/11 neurones tested were phase‐related to the arterial pressure wave (i.e. had cardiac rhythmicity, CR). Synaptic events in 9/10 neurones tested (including all with CR) were phase‐related to the intercostal EMG and/or their autocorrelograms showed peaks at multiples of the respiratory interval (i.e. had respiratory rhythmicity, RR). The intervals between all synaptic events were exponentially distributed in 8/12 neurones although intervals between single strong events showed peaks related to the respiratory cycle. Bursts occurred only by chance. Event patterns could be simulated by combining events from several respiration‐modulated inputs with their timing distributed over nearly half the cycle. From the simulations, the mean number of active preganglionic inputs was estimated to be ≈6 with mean discharge frequency ≈0.4 Hz. We conclude that, in the spontaneously breathing anaesthetized rat, most preganglionic neurones to the SCG fire with relatively low probability in relation to the respiratory cycle. Rhythms in a postganglionic neurone reflect the activity of its suprathreshold preganglionic inputs.

The electrotonic behavior of three phenotypes of sympathetic postganglionic neuron has been analyzed to assess whether their distinct cell input capacitances simply reflect differences in morphology. Because the distribution of membrane properties over the soma and dendrites is unknown, compartmental models incorporating cell morphology were used to simulate hyperpolarizing responses to small current steps. Neurons were classified as phasic (Ph), tonic (T), or long-afterhyperpolarizing (LAH) by their discharge pattern to threshold depolarizing current steps and filled with biocytin to determine their morphology. Responses were simulated in models with the average morphology of each cell class using the program NEURON. Specific membrane resistivity, R(m), was derived in each model. Fits were acceptable when specific membrane capacitance, C(m), and specific resistivity of the axoplasm, R(i,) were varied within realistic limits and when underestimation of membrane area due to surface irregularities was accounted for. In all models with uniform R(m), solutions for R(m) that were the same for all classes could not be found unless C(m) or R(i) were different for each class, which seems unrealistic. Incorporation of a small somatic shunt conductance yielded values for R(m) for each class close to those derived assuming isopotentiality (R(m) approximately 40, 27, and 15 k omega cm(2) for T, Ph, and LAH neurons, respectively). It is concluded that R(m) is distinct between neuron classes. Because Ph and LAH neurons relay selected preganglionic inputs directly, R(m) generally affects function only in T neurons that integrate multiple subthreshold inputs and are modulated by peptidergic transmitters.

Reflexes from the lower urinary tract clearly reached sympathetic neurons located in remote segments. The response incidence in the population studied suggests that most sympathetic neurons involved in cardiovascular regulation participate in these reflexes. The different reflex patterns probably occur in neurons with different functional targets in the head and neck.