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.

CARDIOVASCULAR baroreceptors are connected to the central nervous system by either myelinated or unmyelinated axons. 1 The reflexes elicited by electrical stimulation of the two groups, which will be referred to as MBs and UBs, differ quantitatively and qualitatively. Electrical excitation of unmyelinated afferents in the aortic nerve produces more profound depressor reflexes than excitation of myelinated afferents (Fig. 1 in paper by Douglas and Ritchie2). Moreover, electrical excitation of myelinated cardiac vagal afferents may produce tachycardia, whereas excitation of both myelinated and unmyelinated cardiac vagal afferents elicits marked depressor effects. 3 These results emphasize the differences in the central connections of MBs and UBs but do not address the question of the functional differences between the two groups of receptors. Recently, Thoren and Thoren et al. have compared the static characteristics of aortic MBs and UBs and found that the threshold is higher and the sensitivity to suprathreshold pressures is lower in UBs. The dynamic characteristics have not been compared, however. A comparison would clearly be important since cardiovascular pressures are normally phasic and the discharge of MBs and UBs has frequency-dependent components.-In addition, the dynamic characteristics of baroreceptors are largely unknown except for a few receptors which have been studied over limited frequency and