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.

Fear vs Anxiety - are they the same?

Fear n. an emotional state evoked by the threat of danger and usually characterized by unpleasant subjective experiences as well as physiological and behavioural changes. Fear is often distinguished from anxiety in having a specific object. Associated physiological changes include increases in heart rate, blood pressure, sweating, etc. (Oxford Medical Dictionary)
Fear n. an unpleasant often strong emotion caused by anticipation or awareness of danger and accompanied by increased autonomic activity (Medline Plus)

Anxiety n. a generalized pervasive fear. (Oxford Medical Dictionary)
Anxiety n. 1: a painful or apprehensive uneasiness of mind usually over an impending or anticipated ill 
2: an abnormal and overwhelming sense of apprehension and fear often marked by physiological signs (as sweating, tension, and increased pulse), by doubt concerning the reality and nature of the threat, and by self-doubt about one's capacity to cope with it. (Medline Plus)

From the above definitions fear and anxiety have much in common and in common language fear and anxiety may be used synonymously. Fear is an emotional response whilst anxiety is a psychological and physiological state and both are unpleasant, yet despite this unpleasantness both can be considered natural responses with evolutionary roles. Both have roles in protecting ourselves or others from negative consequences by eliciting appropriate reactions in difficult situations.

Fear is more specific and usually driven by more external stimuli. The impeding threat may be well defined as may the consequences. An example may be an oncoming car speeding towards us. The natural reaction is quick one of escape and avoidance, to get out of the way of the oncoming car. An evolutionary example would be of that of encountering a lion or snake inducing fear producing a survival driven response to flee the situation.

Fear conditioning is the learned recognition and appropriate response of an aversive stimulus and fears are largely conditioned human behaviour. The recognition of a stimulus as aversive may be learned from personal experience of bad outcomes such as a child learning of the hazards of water by near-drowning, or through observing the outcomes of others. Many fears are learned through experience in early childhood because babies naturally fear few things. Loud noises are one example of unconditioned fears related to sensory overload that produce an unconditioned response (usually crying) in babies. In Watson's Little Albert experiment, Albert wasn't frightened of the white rat. He was however upset by loud noise, but wasn't able to predict when loud noise might occur. The rat was a neutral stimulus, he had no fear conditioned or unconditioned towards the rat, in fact he appeared to like the white rat. The white rat (neutral stimulus) was then paired with the loud sound (unconditioned stimulus) producing the unconditioned response of crying. Over time the introduction of the white rat would make Albert cry, thus the rat had become a conditioned stimulus and the anticipatory fear it produced was the conditioned response, even if no loud sound occurred. Fear generalization also occurred where Little Albert associated anything fluffy and white as being a condtioned fear stimulus associated with the noise outcome.

Anxiety can be perceived as being different to fear in that its stimulus is less specific and often less externalized and may be related to scenarios that are less controllable or avoidable. Unlike fear which is a conditioned negative emotional response, anxiety may not have an identifiable trigger as its stimulus and produces a state of mind with generalized fear and negative emotion, underlying heightened arousal and perception of sensory inputs. Anxiety can also be seen as a future-oriented state of mind versus fear which may be a subjective emotion related to a stimulus being perceived at the present. Anxiety as a normal physiological state may serve to prepare us or keep us in a heightened state of arousal to interpret and cope with new or impending threats and to learn and adapt new and suitable responsive behaviours. If however the level of anxiety is disproportionate or occurs non-objectively to stimuli or situations not normally deemed as threatening or dangerous it may be considered unhelpful and possibly pathological if it results it changes to the ability to function normally or starts to induce abnormal learned behaviours such as avoidance or ongoing generalized perpetual fear.