Basis for phase relations between baroreceptor and sympathetic nervous discharge

Area of Science:

• Cardiovascular Physiology
• Autonomic Nervous System Regulation
• Neuroscience

Background:

• The cardiac cycle and sympathetic nervous discharge (SND) exhibit complex phase relationships.
• Baroreceptor nerve activity influences sympathetic outflow, but its precise role in cardiac-related synchronization is unclear.

Purpose of the Study:

• To investigate the phase relations between the cardiac cycle and SND in anesthetized cats.
• To determine how heart rate modulation affects the synchronization of SND with cardiac events.
• To elucidate the underlying mechanisms of SND-cardiac cycle entrainment.

Main Methods:

• Utilized an average-response computer to analyze cardiac cycle and SND phase relations.
• Manipulated heart rate by stimulating the vagus nerve in anesthetized cats.
• Observed shifts in the timing of maximum SND relative to the cardiac cycle at varying heart rates.

Main Results:

• Maximum SND occurred during early diastole at higher heart rates (3-4 beats/s).
• Progressive heart rate reduction shifted maximum SND from early diastole to peak systole and then to the preceding cardiac cycle's late diastole.
• Disruption of the 1:1 SND-cardiac cycle relationship at low heart rates (<2 beats/s) revealed spinal and brainstem sympathoinhibition components.

Conclusions:

• SND synchronization with the cardiac cycle is not solely due to baroreceptor activity fluctuations.
• Entrainment of a centrally generated sympathetic rhythm by baroreceptor reflexes explains the 1:1 locking of SND bursts to the cardiac cycle.
• An inhibitory-phasing hypothesis is proposed for this entrainment mechanism.