Normal startle response
The normal human startle response consists of a brief flexion response, most marked in the face and upper half of the body, elicited by an unexpected auditory, or sometimes somaesthetic, visual or vestibular stimulus. Activity is most prominent in orbicularis oculi and sternocleidomastoid. Habituation of the normal generalized startle response is rapid, although the blink reflex tends to persist.
Studies in animals suggest that the startle response originates in the lower brainstem. The short latency startle response to sound persists after decerebration (Forbes & Sherrington, 1914; Szabo & Hazafi, 1965). Lesioning experiments in the rat have implicated the medial bulbopontine reticular formation, particularly the nucleus reticularis pontis caudalis, as the primary centre subserving the acoustic startle reflex (Szabo & Hazafi, 1965; Hammand, 1973; Leitner et al., 1980; Davis et al., 1982). Thus electrical stimulation of the nucleus reticularis pontis caudalis elicits short latency startle-like responses (Davis et al., 1982).
If the medial pontomedullary reticular formation is the primary centre subserving the acoustic startle reflex, the efferent limb of the reflex may be provided by the bulbobulbar and reticulospinal pathways originating in this area. In particular, Shimamura and Livingston have identified a spino-bulbo-spinal reflex system relayed in the medial medullary reticular formation in cats, the efferent limb of which is formed by the moderately slowly conducting component of the medial reticulospinal pathway, running in the ventrolateral funiculus (Shimamura & Livingston, 1963; Shimamura & Kogure, 1979; Shimamura et al., 1964). This efferent pathway may form the basis of the audio-spinal reflex response, believed to underlie the auditory startle reflex in animals (Wright & Barnes, 1972).