The<unk> , 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 present in all infants/newborns up to 4 or 5 months of age, and its absence indicates a profound disorder of the motor system. An absent or inadequate<unk>  response on one side is found in infants with hemiplegia, brachial plexus palsy, or a fractured clavicle. Persistence of the<unk>  response beyond 4 or 5 months of age is noted only in infants with severe neurological defects.  It was discovered and first described by Austrian pediatrician Ernst<unk>  (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:
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. However, in individuals with cerebral palsy, persistence and exacerbation of this reflex is common.
In adults, the startle reflex is modulating by expressions of observers.
Moro reflex
Optokinetic reflex
Baroreflex
Biceps reflex
Bezold-Jarisch reflex
Ankle reflex
Dirac delta function
Eyelid twitch