startle reflex /stär″təl/ [ME, stertlen, to rush; Gk, syn, together, dromos, course] , a reflex response to a sudden unexpected stimulus that may be accompanied by physiological effects such as increased heartbeat and respiration, closing of the eyes, and flexion of trunk muscles. The reaction is rapid, pervasive, and uncontrollable, regardless of the unexpected stimulus, which may be as simple as a touch. It is a normal reflex in neonates. However, premature and immature infants may not show the reaction. Also called startle reaction, startle syndrome. See also Moro reflex.
