Figure 1. Functional structure of arterial baroreflex system.

A, theoretical considerations of the coupling of baroreflex neural and peripheral arcs. Although baroreflex is a negative feedback control system that senses AP by baroreceptors and regulates AP, we opened the loop by changing baroreceptor pressure independent of AP. By measuring SNA, we divided the baroreflex system into the neural arc (from baroreceptor pressure input to efferent SNA via central nervous system) and the peripheral arc (from SNA input to AP via cardiovascular organs system). B, block diagram of open-loop baroreflex system. Because of vascular isolation of carotid-sinus regions, CSP is independent of systemic AP. Noise is introduced to the neural and/or peripheral arcs. C, block diagram of closed-loop-spontaneous baroreflex system, where CSP equals AP. Noise is introduced to the neural and/or peripheral arcs. Because of the closed-loop nature, changes in AP (and thus, in CSP) control SNA via neural arc transfer function (H_n), which in turn modulate AP via peripheral arc transfer function (H_p). CSP, carotid sinus pressure; SNA, sympathetic nerve activity; AP, arterial pressure; NN, unknown noise in the neural arc; PN, unknown noise in the peripheral arc.