Baroreceptor dynamics and their relationship to afferent fiber type and hypertension.

Static characteristics of baroreceptors differ depending on whether receptors are connected to myelinated or unmyelinated axons (MBs and UBs) and whether they come from normotensive or spontaneously hypertensive rats (NTRs and SHRs). Dynamic characteristics are incompletely known and were examined using an in vitro rat aortic arch-aortic nerve preparation in which static characteristics are similar to those present in vivo. Small amplitude sinusoidal pressures were applied at frequencies varying from 0.1 to 20 Hz over the range of linear responses to pressure. MBs from SHRs and NTRs show peaking in curves relating gain to frequency, gain being the ratio of sinusoidal discharge-rate amplitude and sine wave pressure amplitude. Similar curves from UBs of NTRs and SHRs are overdamped and show no such peaking. In MBs, the response phase leads initially, starts to lag around resonant frequencies, and falls behind progressively thereafter. In UBs, the phase also leads initially, then lags monotonically to about —60°. The discharge rectifies at high frequencies probably as the result of nonlinear threshold properties of the spike-initiating zone. The dynamic response curves of SHR and NTR aortas also were determined and found to be flat to much higher frequencies than those of the baroreceptors. Thus aortic wall dynamics do not limit the frequency range of the baroreceptors. There are no differences between NTRs and SHRs in the dynamics of either their aortas or their baroreceptors. The differences in dynamics between MBs and UBs may be due to differences in mechanical coupling related to structural differences between their endings.