Startle

Startle refers to a sudden, involuntary reaction to an unexpected stimulus, typically involving a brief body jerk and a rapid orienting response, often accompanied by a reflexive blink. The term covers both the general phenomenon of surprise or alarm and the specific reflex known as the startle reflex, most commonly elicited by loud or abrupt sounds.

Neurophysiologically, the acoustic startle reflex is mediated by a relatively short neural circuit. Auditory input travels from the cochlear nerve to the cochlear root nucleus, then to the caudal pontine reticular nucleus (PnC), which activates motor pathways that produce eye blinking and bodily reflexes. The amygdala and other limbic structures modulate the response, influencing its magnitude in emotional or fearful contexts. The reflex can be measured in humans by recording facial muscle activity, particularly the orbicularis oculi, with response latencies typically in the tens of milliseconds range.

The startle response can be modulated in several ways. Prepulse inhibition (PPI) occurs when a weak, nonstartling stimulus precedes the startling one, reducing the reflex and reflecting sensorimotor gating. PPI is widely used in research on schizophrenia and other neuropsychiatric conditions. Startle magnitude also varies with attention, arousal, fear, and habituation to repeated stimuli, with responses often diminishing over time.

Clinically, exaggerated startle responses occur in hyperekplexia (startle disease), frequently due to mutations affecting glycine signaling. Heightened startle responsiveness can also be observed in trauma-related disorders and certain anxiety conditions. In research and clinical settings, measuring startle and PPI provides insight into fear processing, attention, and the integrity of sensorimotor pathways.