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Fear is a chain reaction in the [|brain] that starts with a stressful stimulus and ends with the release of chemicals that cause a racing [|heart], fast breathing and energized [|muscles], among other things, also known as the fight-or-flight response

he fear response is almost entirely autonomic: We don't consciously trigger it or even know what's going on until it has run its course.

Because cells in the brain are constantly transferring information and triggering responses, there are dozens of areas of the brain at least peripherally involved in fear


 * Thalamus** - decides where to send incoming sensory data (from [|eyes], ears, mouth, skin)


 * Sensory cortex** - interprets sensory data


 * Hippocampus** - stores and retrieves conscious memories; processes sets of stimuli to establish context


 * Amygdala** - decodes emotions; determines possible threat; stores fear memories


 * Hypothalamus** - activates "fight or flight" response

To produce the fight-or-flight response, the hypothalamus activates two systems: the sympathetic nervous system and the adrenal-cortical system.

The sympathetic nervous system uses nerve pathways to initiate reactions in the body, and the adrenal-cortical system uses the bloodstream.

When the hypothalamus tells the sympathetic nervous system to kick into gear, the overall effect is that the body speeds up, tenses up and becomes generally very alert.

The sympathetic nervous system sends out impulses to glands and smooth muscles and tells the adrenal medulla to release epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream.

At the same time, the hypothalamus releases corticotropin-releasing factor (CRF) into the pituitary gland, activating the adrenal-cortical system.

The pituitary gland (a major [|endocrine gland]) secretes the hormone ACTH (adrenocorticotropic hormone). ACTH moves through the bloodstream and ultimately arrives at the adrenal cortex, where it activates the release of approximately 30 different hormones that get the body prepared to deal with a threat.

What the hormones cause in the body to prepare for fight or flight.
 * heart rate and blood pressure increase
 * [|pupils] dilate to take in as much light as possible
 * veins in skin constrict to send more blood to major muscle groups (responsible for the "chill" sometimes associated with fear -- less blood in the skin to keep it warm)
 * blood-glucose level increases
 * [|muscles] tense up, energized by adrenaline and glucose (responsible for goose bumps -- when tiny muscles attached to each hair on surface of skin tense up, the hairs are forced upright, pulling skin with them)
 * smooth muscle relaxes in order to allow more oxygen into the [|lungs]
 * nonessential systems (like digestion and [|immune system]) shut down to allow more energy for emergency functions
 * trouble focusing on small tasks (brain is directed to focus only on big picture in order to determine where threat is coming from)