Tonic Activity of Sympathetic Nerves
"Tonic" refers to a state of continuous activity that exists in both the sympathetic and parasympathetic divisions of the autonomic nervous systems under a wide variety of conditions. Strictly defined, "autonomic" or "autonomous" signifies independence or freedom from control by external forces. In actuality, this definition should not be applied to pre- and postganglionic autonomic neurons because their tonic discharges are synaptically driven. In the case of the sympathetic division, many of the inputs that are now referred to as "presympathetic" originate in the brainstem.
This essay deals exclusively with the tonic activity of sympathetic nerves with cardiovascular targets.
Recordings from pre- and postganglionic sympathetic nerve bundles with cardiovascular targets reveal a complex and variable mixture of rhythmic and aperiodic components. Depending upon experimental conditions, rhythmic