The startle response is an extreme response to a highly novel or intense stimulus that carries potential major salience for the intact organism that abrupt and unexpected sound, causes in all mammals and this response consists of a rapid and involuntary extension and then flexion of a series of muscles. Although the startle response is a reflex, it can be modulated by many different stimuli. Startle reactions tend to be higher in the presence of threats, fear and pain and are conversely decreased by anxiolytics. The pharmacology and circuitry of this reflex is virtually identical in most of rodents and humans. Startle response is measured in rodents by using automated startle chambers or stabilimeter chambers, to detect whole body reactions recording.
In humans, the movements of oculomotor muscles i. e; eye blink reflex assessed using electromyography recording of orbicularis oculi muscle are most typically used. The acoustic startle reflex in rats and cats is mediated primarily by a small cluster of giant neurons in the ventrocaudal part of the nucleus Reticularis Pontis Caudalis (RPC) of the reticular formation. Activation of these RPC neurons occurs 3-8 ms after the acoustic stimulus reaches the ear.
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