The neurobiology of anxiety and fear

The amygdala is implicated in both anxiety and fear. The amygdala are a pair of almond shaped nuclei deep in the medial temporal lobes of the brain, considered part of the basal ganglia and along with the hippocampus, are considered the major components of the limbic system.
The amygdala function is the memory of emotional responses to past experiences. In memorising our emotional responses, when we encounter negative stimuli that we have come across in the past, the amygdala can elicit a more rapid, instinctive response as may be involved in fear. It is thought that our conditioned response to fear can take one of two routes. The long route involves the higher centers of the cerebral cortex and the hippocampus producing a more accurate response analysing whether the perceived threat is true or not. The short route is quick and 'dirty' as in it produces a rapid conditioned response to a perceived threat without analysis of whether the threat is real or not.

The long route - Slow and accurate
Sensory stimulus is processed and relayed by the nuclei of the thalamus. These include the lateral geniculate nuclei for visual stimulus, the medial geniculate nuclei for auditory stimulus and the ventral posterior nucleus involved in processing of proprioception, touch, temperature, pain, etc.


The normal response to sensory stimulus is the relay of information from the thalmic nuclei to the relevant cortical areas of the brain, such as the primary visual centre in the occipital lobe for vision and so on. These cortical centres appraise the stimulus for meaning and are involved in accurate targeting of the stimulus to the relevant brain structures. From the cortex, the information is relayed either directly to the amygdala or indirectly via the hippocampus.
The hippocampus is also part of the limbic system and is involved in explicit memory. It will contextualise the stimulus with past memories, for example where, when and with whom did the past experience occur. In response to new stimuli (as well as autobiographical memory in general) the hippocampus helps in memory formation of new events.
The amygdala integrates information about the appraised and contextualised stimulus as to whether the threat is real or not, before eliciting an emotion response via the autonomic nervous system.

Green arrow = Long route, Purple = Short route

The short route - Quick and dirty
The short route is thought to pass directly from the thalamic nuclei to the amygdala bypassing the higher centres and the hippocampus. The emotional response to stimulus is much faster and occurs as a fear conditioned response assuming the perceived stimulus is true. Higher centres are not involved so there is little discrimination of the threat as to whether it is true or not and only the characteristics of the threat are analysed. The longer route may occur simultaneously and be used to confirm whether the rapid response was correct or not. An example may be our initial response to a loud noise or bang to jump. The longer route will analyse whether the noise was really a threat or not and from what direction it came.