Why do human postganglionic neurones primarily only fire once during a sympathetic burst?

Abstract

Single-unit recordings from muscle vasoconstrictor, cutaneous vasoconstrictor and sudomotor neurones in awake human subjects have shown that they tend to fire only once per sympathetic burst. We review the firing properties of human sympathetic neurones and examine the idea that the short duration of a sympathetic burst may limit the number of times a neurone can fire, using the human skeletomotor system as a model for the sympathetic nervous system. It is known that human alpha motor neurones usually fire in long trains during voluntary contractions, but what of their pattern when constrained by a brief burst? We recorded from single motor units in the tibialis anterior muscle while subjects generated brief electromyogram bursts in the intervals between heart beats, with a duration similar to that of muscle sympathetic bursts. Eight motor units fired mostly one spike per burst, with a pattern identical to that of sympathetic neurones. These results suggests that were it not for the constraint of the bursting pattern, individual sympathetic neurones would--like alpha motor neurones--tend to fire in long trains.