Multiple Baroreceptors Operate in A Single Sensory Unit

Arterial baroreceptors play a vital role in the regulation of the cardiopulmonary system. However, what is known about how these sensors operate at the subcellular level is limited. Until recently, one afferent axon was considered to be connected to a single baroreceptor (one‐sensor theory). However, a single airway mechanosensory unit is now known to house many sensors (multiple‐sensor theory). The sensory unit is not merely a transducer, but also a processor that integrates information. In the current study, we tested the hypothesis that multiple‐sensor theory also operates in barosensory units with both morphological and electrophysiological approaches. The rabbit aortic arch (in whole mount) was labeled with Na+/K+‐ATPase, as well as myelin basic protein antibodies, and examined microscopically. Sensory structures were located near the bifurcation or origin of the common carotid and subclavian arteries. These sensors presented in compact clusters, similar to bunches of grapes. Sensory terminals, like those in the airways, formed leaf‐like or knob‐like expansions, that is, a single myelinated axon connected with multiple sensors forming a network. We also recorded single‐unit activities from aortic baroreceptors in the depressor nerve in anesthetized rabbits and examined the unit response to a bolus intravenous injection of phenylephrine (10mM, 0.2ml). Unit activity increased progressively as blood pressure (BP) increased. Five of eleven units abruptly changed their discharge pattern to a lower activity level after BP attained a plateau for a minute or two (when BP was maintained at the high level). These findings clearly show that the high discharge baroreceptor deactivates after over‐excitation and unit activity falls to a low discharge sensor. In conclusion, our morphological and physiological data support the hypothesis that multiple‐sensory theory can be applied to barosensory units.