The startle response is a brainstem reflex 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. The startle response is distinctively different from the Moro reflex which is a specific response to a loss of support and is said the change around 3-6 months.[1][2]
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The pathway for this response was largely elucidated in rats in the 1980s.[3]
In summary 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.[4]
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