The cerebrum

Anxiety is an emotion characterized by an unpleasant state of inner turmoil; often accompanied by nervous behavior such as pacing back and forth, somatic complaints, and rumination. It is the subjectively unpleasant feelings of dread over anticipated events, such as the feeling of imminent death. Anxiety is not the same as fear, which is a response to a real or perceived immediate threat, whereas anxiety involves the expectation of a future threat.
Anxiety is a feeling of uneasiness and worry; as an overreaction to a situation that is only subjectively as menacing. It is often accompanied by muscular tension, restlessness, fatigue, and problems in concentration. Anxiety can be appropriate, but when experienced regularly the individual may suffer from an anxiety disorder.

The human brain

The cerebrum is the largest part of the human brain. It is into two cerebral hemispheres. The cerebral cortex is an outer layer of grey matter, covering the core of white matter. The cortex is split into the neocortex and the much smaller allocortex. The neocortex is up of six neuronal layers, while the allocortex has three or four. Each hemisphere is conventionally into four lobes the frontal, temporal, parietal, and occipital lobes.
Damage to parts of the orbitofrontal cortex (OFC); a region within the prefrontal cortex, heightens defensive responses to threat. The study proposes a critical role for subregions of this brain area in different anxiety disorders. A network of brain regions including the OFC has implicated in human anxiety disorders. For example, previous research has linked arachnophobia and generalized anxiety disorder to decreased activity in lateral and medial OFC, respectively.

Elisabeth Murray and colleagues from the National Institute of Mental Health and Icahn School of Medicine at Mount Sinai investigated the role of these OFC subregions in male rhesus macaques trained to retrieve a fruit snack reward in the presence of one of two fake; rubber predators a grayish green snake or a black; hairy spider or neutral objects.

Damage to a subregion of OFC

After producing selective lesions in the lateral or medial OFC; the researchers observed that these animals took longer to reach for the fruit snack in the presence of the predator stimuli compared to 12 animals with intact OFC. The researchers also found that this effect could attributed to the heightened defensive and reduced approach behaviors that followed damage to either subregion of OFC.
Both experimental groups also failed to reduce their defensive responses to the snake and spider over time. Finally, monkeys with medial OFC lesions showed a greater tendency to express defensive behaviors even in the absence of threat. Overall, these findings suggest that specific symptoms of various anxiety disorders may arise from dysfunction in distinct subregions of OFC.