[Function and dysfunction of the startle reaction in humans].

INTRODUCTION AND DEVELOPMENT

The startle reaction is one of the fastest movements of human beings in response to a sensory stimulus. It is likely that the central nervous system uses the circuits of the SR for the execution of a ballistic movement. Evidence in favour of the participation of the startle reaction circuits in the execution of a ballistic movement has been gathered by combining the presentation of startle reaction-inducing stimuli with the imperative signal in a reaction time task experiment. In this situation, in which subjects are highly prepared for movement execution, the reaction becomes accelerated but the movement pattern is not modified. On top of that, acceleration is limited to the movement per se, and it does not involve the subjective perception of the movement. Impairment of the central control of the startle reaction may lead to excess or reduction of the response. One of the examples of excessive response is hyperekplexia, in which patients cannot avoid having a startle reaction to unexpected stimuli. Reduced startle reaction occurs in parkinsonism and, specially, in progressive supranuclear palsy.

CONCLUSIONS

One of the best known mechanisms of control of the startle reaction is prepulse inhibition. This phenomenon is likely the consequence of sensory signal processing at the brainstem level. The role of the prepulse inhibition is to avoid disruption of the sensory processing by the interference of another input. Prepulse inhibition is abnormally reduced in patients with parkinsonism, as an expression of the abnormal control of brainstem circuits by the basal ganglia. It has been also found abnormal in patients with Huntington's disease and in some patients with dystonia.