The fear response is a complex cascade of chemical and electrical signaling triggered by the autonomic nervous system, the involuntary part of the central nervous system. Involved in fear is the fight or flight response, which extends the emotion of fear to physical manifestations, such as increased heart rate, increased blood pressure, the stress response and increased muscle contractility. A number of regions in the brain are involved in sensing and responding to stimuli that result in the fear response.
The main portion of the brain involved in the fear response is thought to be the amygdala. This small region of the brain is part of the limbic system, processing emotions and unconsciously perceiving fearful stimuli. A 2006 study from Columbia University Medical Center also found that the amygdala is activated more in anxious individuals when the fear response is unconscious. The region sends out signals indicating cognitive and emotional conflict when fearful stimuli are present, which allows the brain to compensate for the unconscious reaction and maintain the ability to respond. The rostral anterior cingulate cortex in the frontal lobe dampens the amygdala’s emotional response to allow the individual to evaluate whether fight or flight is appropriate.
The hippocampus is another part of the limbic system and involved in memory. Memory plays a role in fear because it gives the stimulus context, and the hippocampus is involved in fear learning and conditioning. Lesions in this region of the brain can result in inappropriate fearful responses.
The sensory cortex plays a role in fear based on emotional responses to visual, acoustic and olfactory stimuli. When the receptors process a stimulus that evokes a fearful response, the secondary sensory cortices interact with subregions of the amygdala that trigger the fear response based on memories stored and created by the hippocampus. This has been shown in studies of fear conditioning.
The thalamus is a portion of the brain under the cerebral cortex. This region receives and integrates signals from several areas of the brain. It is involved in funneling the stimuli to appropriate brain regions for recognition. The two pathways by which a stimulus can proceed, starting with the thalamus, are shown in this graphic from Discovery.
The hypothalamus is involved in the fight or flight response. Central projections from the amygdala interact with the hypothalamus, as well as the brain stem, to stimulate the behavioral and physiological reactions of fear, including those of the cardiovascular system. The interaction appears to be mediated by oxytocin. When activated, the hypothalamus triggers the sympathetic nervous system and adrenal-cortical axis, which includes the pituitary gland.
The brain and fear
Though many aspects of the physiological reactions to emotion are not clear, the fear response is known to proceed through the amygdala, hippocampus, thalamus, hypothalamus and sensory cortex. These regions of the brain are known to interact to respond to stimuli, but studies continue to elucidate the chemical signals and pathways involved in regulating this complex neurological response.