The startle response or startle reaction is a response to sudden, startling stimuli, such as sudden noise or sharp movement. Usually the onset of the startle response is reflectory. The startle reflex is a brainstem reflectory reaction that serves to protect the back of the neck (whole-body startle), or the eye (eyeblink), and also facilitates escape from sudden stimuli. It is found across the lifespan and in many species. An individual's emotional state may lead to a variety of different responses.
During neuromotor examination of newborns, it is noted that for a number of techniques the patterns of the startle reaction and the Moro reflex may significantly overlap, the notable distinction being the absence of arm abduction (spreading) during startle response.
The pathway for this response was largely elucidated in rats in the 1980s. The basic pathway follows the auditory pathway from the ear up to the Nucleus of the Lateral Lemniscus (LLN) from where it then activates a motor centre in the reticular formation. This centre sends descending projections to lower motor neurones of the limbs. In slightly more detail this corresponds to: Ear (cochlea)->Cranial Nerve VIII (auditory) -> Cochlear Nucleus (ventral/inferior) -> LLN -> Caudal pontine reticular nucleus (PnC). The whole process has a less than 10ms latency. There is no involvement of the superior/rostral or inferior/caudal colliculus in the reaction that "twitches" the hindlimbs, but these may be important for adjustment of pinnae, gaze towards the direction of the sound or the associated blink.
Normal people are not startled by sounds at 180 decibels, even though sounds above 90 decibels cause hearing damage. People become desensitized since infancy. People suffering from hyperacusis are easily startled and may suffer from a fear of loud sounds.
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