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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).

In children

The Moro reflex, also known as the startle reflex, is one of the infantile reflexes. It may be observed in incomplete form in premature birth after the 28th week of gestation, and is usually present in complete form by week 34 (third trimester). It is normally lost by the 6th month of life postpartum. It was discovered and first described by Austrian pediatrician Ernst Moro (1874-1951).

This reflex is a response to unexpected loud noise or when the infant feels like it is falling. It is believed to be the only unlearned fear in human newborns. The little Albert study used the startle reflex in the famous classical conditioning experiment to make him fear white fuzzy things.

The primary significance of this reflex is in evaluating integration of the central nervous system (CNS), since the reflex involves 4 distinct components:

  1. Startle
  2. spreading out the arms (abduction)
  3. unspreading the arms (adduction)
  4. Crying (usually)

Absence of any component (except crying) or any asymmetry in the movements is abnormal, as is persistence of the reflex in older infants, children and adults.

The origin of this reflex can be found in that fact that primate infants of our ancestors clung to their mother's fur soon after birth, giving their mother the advantage of having her hands free to forage for food and such. If human babies are falling backward with their head their innate reflex will be to stretch out the arms to grab and cling to their mother.

Acoustic Startle Reflex

File:Acoustic startle reflex.JPG

Acoustic startle reflex test for mice

The pathway for this response was largely elucidated in rats in the 1980s [1]. 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[2].

See also


  1. Davis, M., Gendelman, D. S., Tischler, M. D., & Gendelman, P. M. (1982). A primary acoustic startle circuit: Lesion and stimulation studies. Journal of Neuroscience, 6, 791-805. [1]
  2. Castellote, J.M., Kumru, H., Queralt, A. and Valls-Solé, J. (2007) A startle speeds up the execution of externally guided saccades. Exp. Brain Res. 177:129-136 [2]

External links

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