The firing properties of single sympathetic neurones in humans in health and disease

While most microelectrode recordings of sympathetic nerve activity in awake human subjects have been based on analysis of multi-unit activity, in which differences in burst amplitude cannot be compared across individuals, single-unit recordings allow one to compare sympathetic outflow across subjects and groups because the discharge occurs in a quantal fashion: an action potential(s) is either present (1) or is absent (0). Since the first unitary recordings from type-identified muscle vasoconstrictor neurones were made, the study of the firing behaviour of these neurones has been extended to physiological and pathophysiological increases in sympathetic drive. In addition, the firing properties of single cutaneous vasoconstrictor and sudomotor neurones have been characterised. Curiously, regardless of their type as muscle vasoconstrictor, cutaneous vasoconstrictor or sudomotor, individual postganglionic neurones tend to fire only once per burst, although they can fire multiple times. They also discharge with a low firing probability (~30%) and generate low firing rates (~0.5 Hz); even during pathophysiological increases in drive, firing probabilities rarely exceed 55% and mean firing rates rarely exceed 1 Hz. A comparison of modelled and actual data supports the idea that a given postganglionic neurone receives excitatory drive from, on average, two preganglionic neurones. Importantly, we now have a schema for analyzing the firing properties of single sympathetic neurones, in terms of (i) the mean firing frequency, (ii) the firing probability (the percentage of cardiac intervals in which a unit fires) and (iii) the number of spikes a unit generates per cardiac interval, and studies in different patient groups have revealed subtle differences in how an increase in burst amplitude is brought about. Nevertheless, the dominant mechanism by which the sympathetic nervous system grades its output appears to be through the recruitment of previously silent neurones.