Startle reaction

Startle Reaction

The startle reaction, also known as the startle response, startle reflex, or alarm reaction, is a rapid, involuntary response to a sudden or unexpected stimulus such as a loud noise, quick movement, or sudden change in light intensity. This reaction is a universal, protective response that prepares the body for a potential threat.

Physiology

The startle reaction is mediated by a neural circuit in the brainstem. The primary sensory input is through the auditory system, but other sensory systems such as the visual system and somatosensory system can also trigger the reaction. The reaction involves a complex sequence of muscle contractions that typically begin with a sudden jerk of the head, followed by movements of the limbs and trunk.

The startle reaction is characterized by a rapid contraction of the skeletal muscles in response to a stimulus. This is followed by a slower relaxation phase. The reaction is thought to be mediated by the reticular formation, a network of neurons in the brainstem that plays a key role in controlling arousal and attention.

Clinical significance

The startle reaction can be exaggerated in certain neurological conditions such as Hyperekplexia, a rare genetic disorder characterized by an exaggerated startle response. It can also be diminished or absent in other conditions such as Parkinson's disease.

In psychiatry, an exaggerated startle reaction is one of the symptoms of post-traumatic stress disorder (PTSD). It is also seen in other anxiety disorders and can be induced by certain drugs.

Measurement

The startle reaction can be measured in a laboratory setting using a technique called the startle response test. This involves presenting a sudden, loud noise and measuring the subject's physical reaction, typically a blink response.

See also

• Reflex
• Fight-or-flight response
• Acoustic startle reflex
• Moro reflex