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Adrenaline is normally produced by the adrenal glands and by a small number of neurons in the medulla oblongata. [14] It plays an essential role in the fight-or-flight response by increasing blood flow to muscles, heart output by acting on the SA node, [15] pupil dilation response, and blood sugar level.
The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. [1] It is anterior and partially inferior to the cerebellum . It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions, ranging from vomiting to sneezing . [ 2 ]
If the amygdala perceives a match to the stimulus, i.e., if the record of experiences in the hippocampus tells the amygdala that it is a fight, flight or freeze situation, then the amygdala triggers the HPA (hypothalamic–pituitary–adrenal) axis and "hijacks" or overtakes rational brain function. [5]
The fight-or-flight or the fight-flight-freeze-or-fawn [1] (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. [2] It was first described by Walter Bradford Cannon in 1915.
Chromaffin cells contained in the adrenal medulla act as postganglionic nerve fibers that release this chemical response into the blood as a circulating messenger. The sympathoadrenal system can activate and discharge chemical messengers as a single unit to activate an organism's “fight or flight” response.
Autonomic nervous system, showing splanchnic nerves in middle, and the vagus nerve as "X" in blue. The heart and organs below in list to right are regarded as viscera. The autonomic nervous system has been classically divided into the sympathetic nervous system and parasympathetic nervous system only (i.e., exclusively motor).
The clusters of the amygdala are activated when an individual expresses feelings of fear or aggression. This occurs because the amygdala is the primary structure of the brain responsible for fight or flight response. Anxiety and panic attacks can occur when the amygdala senses environmental stressors that stimulate fight or flight response.
The adrenal medulla is at the centre of each adrenal gland, and is surrounded by the adrenal cortex. The chromaffin cells of the medulla are the body's main source of the catecholamines, such as adrenaline and noradrenaline, released by the medulla. Approximately 20% noradrenaline (norepinephrine) and 80% adrenaline (epinephrine) are secreted here.