“Rapid” Rhythmic Discharges of Sympathetic Nerves: Sources, Mechanisms of Generation, and Physiological Relevance

Abstract

Like virtually all other physiological control systems, the sympathetic nervous system controlling cardiovascular function is characterized by the presence of rhythmic activity. These include slow rhythms with frequencies at or below that of the respiration and rapid rhythms with frequencies at or above that of the heart beat. The rapid rhythms are the subject of this review. The specific questions entertained are as follows: (1) Are the rapid cardiac-related and 10-Hz rhythms inherent to central sympathetic networks, or are they imposed on sympathetic nerve discharge (SND) by extrinsic periodic inputs? (2) Does basal SND arise from an anatomically circumscribed "vasomotor center" composed of pacemaker neurons in the rostral ventrolateral medulla or from an anatomically distributed network oscillator composed of different types of brainstem neurons, none of which necessarily have intrinsic pacemaker properties? (3) Are the rapid rhythms generated by single circuits or by systems of coupled oscillators, each with a separate target? (4) Are the rapid rhythms in SND simply by-products of the sympathetic generating mechanisms, or do they subserve selective and special functions, such as the formulation of differential patterns of spinal sympathetic outflow that support particular behaviors? The controversial aspects of these issues and the state-of-the-art analytical methods used to study them are stressed in this review.

Rhythmic Properties of Neurons in the Rostral Ventrolateral Medulla of the Rat In Vitro: Effects of Clonidine

“…These rhythms are pertinent as 1) markers of sympathetic discharge and 2) features that could aid the study of mechanisms producing sympathetic tone. In studies of the rat in vivo, the presence of rhythms locked to the cardiac cycle (presumably via baroreceptor afferents) in brain stem unit discharges has served to identify those units as being involved in sympathetic activity (Granata and Kitai 1992;Sun et al 1988a;Zagon and Spyer 1996), but there have been no reports of other types of fast rhythm, such as the "10-Hz" rhythm found in the cat (Barman and Gebber 2000).…”

“…A well-known hypothesis is that sympathetic activity is generated by neuronal networks in the medulla (Barman and Gebber 2000). Another hypothesis is that sympathetic activity is generated by pacemaker-like neurons in the RVLM (Sun et al 1988a).…”

“…In the study of sympathetic neural discharge, the existence of rhythms in population and unit discharges, at both peripheral and central levels, has merited the attention of researchers (Barman and Gebber 2000). These rhythms are pertinent as 1) markers of sympathetic discharge and 2) features that could aid the study of mechanisms producing sympathetic tone.…”

Burst activity and heart rhythm modulation in the sympathetic outflow to the heart

“…Nonetheless, they are accompanied by faster rhythms that can not be put in relation with cardiovascular variability by means of classical methods in time and frequency domain, simply because they are found at a frequency equal or higher then the cardiac frequency (CF) itself [1].…”

“…Rhythms synchronous to cardiovascular variability and autonomic control oscillations have been described at various levels of the central nervous sy stem and autonomic nervous system; i.e., the Mayer's low frequency (LF) waves and the respiratory high frequency (HF) ones [1,2,3]. Nonetheless, they are accompanied by faster rhythms that can not be put in relation with cardiovascular variability by means of classical methods in time and frequency domain, simply because they are found at a frequency equal or higher then the cardiac frequency (CF) itself [1].…”