The anatomy of startle

Abstract

The involuntary startle is part of the first rapid fear reactions an organism experiences in response to a sudden threatening stimulus. It is adaptive in the sense that it allows the organism to immediately withdraw from an object that might possibly be dangerous, while the higher centres of the brain are still busy processing whether the object is in fact dangerous. The involuntary startle reaction is colloquially described as “to jump with fright”. This hair-trigger system is fine-tuned to protect organisms from danger and tends to err on the side of caution. Therefore, everyone has probably jumped with fright in response to an entirely harmless stimulus, for example mistaking a twig for a snake. This paper aims to explain why and how we jump with fright, even though we realise only milliseconds later that it was completely unnecessary to get a fright at all. The fear centres of the brain are the amygdala, two small nuclei in the limbic system. Visual, auditory and olfactory input from the sensory organs is relayed to the amygdala via two different pathways. Like most types of sensory inputs to the brain the information that activates the fear response via the indirect route is routed via the thalamus to cortical areas where it is analysed in terms of previous experience quality and context. From here the analysed information reaches the amygdala – the cerebral structures generally associated with fear. Impulses from the amygdala will then stimulate the typical fear reactions. In contrast to the indirect route from thalamus to amygdala just described, a direct route is taken in the case of the startle reaction. Impulses from the thalamus are relayed directly to the amygdala and the person experiences the fear reaction and response before the information has been analysed by the cortical structures. This short-cut to the amygdala is a direct, fast and crude pathway from the senses through the relevant modalities to the thalamus to the central nucleus of the amygdala. In general its purpose is to prime the amygdala for detailed incoming information, but in conditions of sudden danger it provides for a rapid response to a potentially aversive situation. The advantage of the direct pathway is that it allows for a quicker reaction, almost half the time that it takes for the cortical input pathway to the amygdala. The startle reaction is essential when speed is more important than accuracy, for example, when a life may be at stake. The cortical pathway is indirect, slower and refined, which allows for cortical processing and thus a much more accurate presentation of the stimulus. This route can also inhibit an inappropriate fear response initiated through the direct route. In this paper the neuroanatomy, specifically explaining the direct and indirect (cortical) route by which perceptual information reaches the amygdala, is reviewed first. Then the physiology of the fear reaction is alluded to, after which we conclude with an integrating figure and state a few interesting implications.