Sympathetic nerve and cardiovascular responses to auditory startle and prepulse inhibition.

While sudden (startling) sensory stimuli are generally thought of as inducing sympathetic excitation, in humans there is a short-lasting inhibition of limb muscle sympathetic nerve activity (MSNA). This study is the first to examine and contrast the effects of acoustic startle and the prepulse inhibition of startle (PPI) on MSNA, blood pressure, heart rate, and eye blinks. Startle elicited a two-component withdrawal of MSNA: an early inhibition of one sympathetic burst followed by a second inhibition. PPI abolished the early, but not the late MSNA inhibition. Prepulse stimuli alone had no early inhibitory effects on MSNA. Early MSNA inhibition, which may occur at latencies of approximately 100 ms, appears to be part of a CNS-generated startle reflex which subserves automatic defensive responses to potential threats. The late MSNA inhibition coincided with the stimulus-induced blood pressure increase and is probably an inhibitory reflex response.