Partition of the Carotid Sinus Baroreceptor Response in Dogs between the Mechanical Properties of the Wall and the Receptor Elements

Abstract

The purpose of this investigation was to learn what part of the carotid sinus baroreceptor response is attributable to the gross mechanical properties of the wall and what part to the receptor elements. Static pressure forcings were applied to an isolated dog carotid sinus preparation while baroreceptor nerve activity was recorded; carotid sinus deformation was measured from still photographs taken during the experiment. Pressure-nerve activity data were obtained from four dogs and pressure-deformation data from another five dogs. The average electrical power in the nerve signal was used as the measure of nerve activity, and strain-energy density, a scalar quantity, was selected as the best indicator of the mechanical state of the sinus wall. Strain-energy density was calculated by measuring the circumferential and the longitudinal strains and by estimating the corresponding stresses in accordance with a thin-walled, axially symmetric model. The pressure-nerve activity data followed an S-shaped pattern, but the pressure-strain-energy density data were linear over the pressure range of 50 to 250 mm Hg. A curve of strain-energy density vs. nerve activity constructed from these two graphs, with pressure as the parametric variable, showed a linear relationship between nerve activity and strain-energy density over the pressure range of 75 to 175 mm Hg, but the slope of the curve rapidly went to zero with increasing pressure. We concluded that the nonlinearity in the pressure-nerve activity relationship was primarily due to the inability of the receptor elements to respond to increasing wall strains.