Startle reaction

In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect. Usually the onset of the startle response is a startle reflex reaction. The startle reflex is a brainstem reflectory reaction (reflex) that serves to protect vulnerable parts, such as the back of the neck (whole-body startle) and the eyes (eyeblink) and facilitates escape from sudden stimuli. It is found across the lifespan of many species. An individual's emotional state may lead to a variety of responses.

A startle reflex can occur in the body through a combination of actions. A reflex from hearing a sudden loud noise will happen in the primary acoustic startle reflex pathway consisting of three main central synapses, or signals that travel through the brain.

First, there is a synapse from the auditory nerve fibers in the ear to the cochlear root neurons (CRN). These are the first acoustic neurons of the central nervous system. Studies have shown a direct correlation to the amount of decrease of the startle to the number of CRNs that were killed. Second, there is a synapse from the CRN axons to the cells in the nucleus reticularis pontis caudalis (PnC) of the brain. These are neurons that are located in the pons of the brainstem. A study done to disrupt this portion of the pathway by the injection of PnC inhibitory chemicals has shown a dramatic decrease in the amount of startle by about 80 to 90 percent. Third, a synapse occurs from the PnC axons to the motor neurons in the facial motor nucleus or the spinal cord that will directly or indirectly control the movement of muscles. The activation of the facial motor nucleus causes a jerk of the head while an activation in the spinal cord causes the whole body to startle.

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