Definitions

alarm-reaction

Startle reaction

The startle reaction, also called the startle response, startle reflex or alarm reaction, is the response of mind and body to a sudden unexpected stimulus, such as a flash of light, a loud noise (acoustic startle reflex), or a quick movement near the face. In human beings, the reaction includes physical movement away from the stimulus, a contraction of the muscles of the arms and legs, and often blinking. It also includes blood pressure, respiration, and breathing changes. The muscle reactions generally resolve themselves in a matter of seconds. The other responses take somewhat longer. An exaggerated startle reaction is called hyperexplexia (also hyperekplexia) The exaggerated startle response is often seen in patients with Posttraumatic stress disorder (PTSD).

Acoustic Startle Reflex

The pathway for this response was largely elucidated in rats in the 1980s . In summary the basic pathway follows the audiory pathway from the ear up to the Nucleus of the Lateral Lemniscus (LLN) from where it the 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.

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