"Rapid" rhythmic discharges of sympathetic nerves: Sources, mechanisms of generation, and physiological relevance

Like virtually all other physiological control systems, the sympathetic ner vous system controlling cardiovascular function is characterized by the pre sence of rhythmic activity. These include slow rhythms with frequencies at or below that of the respiration and rapid rhythms with frequencies at or a bove that of the heart beat. The rapid rhythms are the subject of this revi ew. The specific questions entertained are as follows: (1) Are the rapid ca rdiac-related and IO-Hz rhythms inherent to central sympathetic networks, o r are they imposed on sympathetic nerve discharge (SND) by extrinsic period ic inputs? (2) Does basal SND arise from an anatomically circumscribed "vas omotor center" composed of pacemaker neurons in the rostral ventrolateral m edulla or from an anatomically distributed network oscillator composed of d ifferent types of brainstem neurons, none of which necessarily have intrins ic pacemaker properties? (3) Are the rapid rhythms generated by single circ uits or by systems of coupled oscillators, each with a separate target? (4) Are the rapid rhythms in SND simply by-products of the sympathetic generat ing mechanisms, or do they subserve selective and special functions, such a s the formulation of differential patterns of spinal sympathetic outflow th at 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.