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

Area of Science:

• Neuroscience
• Human Physiology

Background:

• Human sympathetic neurons (muscle vasoconstrictor, cutaneous vasoconstrictor, and sudomotor) typically fire once per sympathetic burst.
• The short duration of sympathetic bursts may limit neuronal firing frequency.

Purpose of the Study:

• To investigate whether the firing pattern of human sympathetic neurons is constrained by burst duration.
• To use the human skeletomotor system as a model for the sympathetic nervous system.

Main Methods:

• Single-unit recordings were obtained from motor units in the tibialis anterior muscle of awake human subjects.
• Subjects generated brief electromyogram bursts with durations similar to muscle sympathetic bursts.

Main Results:

• Eight motor units were recorded, and most fired only once per burst.
• This firing pattern was identical to that observed in sympathetic neurons.

Conclusions:

• The firing pattern of individual sympathetic neurons appears to be constrained by the short duration of sympathetic bursts.
• If not for this constraint, sympathetic neurons would likely fire in long trains, similar to alpha motor neurons.