Age-related loss of hysteresis in the spontaneous vascular sympathetic baroreflex is associated with altered neural rather than mechanical control.

The vascular sympathetic baroreflex is critical for stabilising blood pressure. A notable feature of this reflex is hysteresis, a directional asymmetry in responsiveness during rising versus falling pressure that is evident in young adults but attenuated in older age. Whether age-related reduction reflects altered mechanical transduction at baroreceptors or changes in neural processing remains unclear. We compared the mechanical and neural components of the sympathetic baroreflex, with specific focus on hysteresis, in healthy young (21-30 years old, n = 20) and middle-aged (50-63 years, n = 14) males. Continuous recordings of arterial pressure (photoplethysmography), muscle sympathetic nerve activity (MSNA; microneurography) and common carotid artery diameter (ultrasound) were obtained during 6 min of supine rest. Spontaneous baroreflex gain was quantified for mechanical (diastolic pressure into carotid diameter) and neural (carotid diameter into MSNA) components, and overall (diastolic pressure into MSNA), with rising and falling pressure sequences analysed separately. Young men exhibited clear hysteresis in the overall baroreflex loop, whereas this was absent in middle-aged men. Mechanical hysteresis was not observed in either age group. In contrast, young men displayed pronounced neural hysteresis (Falling: -0.302 ± 0.157 vs. Rising: -0.128 ± 0.88% bursts µm-1, P < 0.001; g = 1.37), whereas neural hysteresis was absent in middle-aged men (Falling: -0.345 ± 0.400 vs. Rising: -0.396 ± 0.368% burst µm-1, P = 0.348, g = 0.13). These findings suggest that age-related loss of spontaneous sympathetic baroreflex hysteresis is more apparent within the neural rather than the mechanical component.