The physiology of stress, including general adaptation syndrome, the hypothalamic pituitary-adrenal system, the sympathomedullary pathway and the role of cortisol
On the left is the chronic stress response, the pituitary adrenal system. On the right is the acute stress response, the sympathomedullary pathway.
The Autonomic Nervous System (ANS)
The Peripheral Nervous System is the nervous system that extends beyond the brain and spinal cord, it is divided into the Somatic Nervous System, which is concerned with voluntary movements and reception of external stimuli; and the Autonomic Nervous System, which is concerned with maintaining homeostasis, acting largely without conscious control. Examples of functions controlled by the ANS include heart rate, digestion, respiration rate, salivation, perspiration, dilation and contraction of the pupils, micturition (urination), and sexual arousal.
The Autonomic Nervous System has been divided into two branches, the sympathetic branch and the parasympathetic branch:[space=10]
The Sympathomedullary Pathway
In threatening situations, the sympathetic branch is activated, which prepares the body for flight-or-flight: the pupils dilate, the heart beats faster, saliva production slows down, digestion slows down bronchi in the lungs dilate, the liver converts glycogen to glucose, the bladder relaxes and the adrenal-medulla (the core of the adrenal glands) produces adrenaline and noradrenaline which increases the sympathetic response. Adrenaline also acts to increase the HPA-axis response by stimulating the hypothalamus to produce more CRF and the pituitary to produce more ACTH.
The Parasympathetic Branch
The parasympathetic branch of the autonomic nervous system restores the body to a relaxed state after the threating situation has passed.
The Pituitary Adrenal System (HPA)
When a person experiences psychological or physiological stress, the hypothalamus releases a hormone called corticotrophin-releasing factor (CRF). CRF binds to receptors on cells in the pituitary gland, which, in turn, releases adrenocorticotropic hormone (ACTH). ACTH is carried in the bloodstream to the adrenal-cortex, which release cortisol. Cortisol acts upon the liver to make it secrete glucose into the bloodstream for the muscles to use as an energy source. Cortisol also negatively feedsback to reduce the production of CRF and ACTH in the hypothalamus and pituitary gland (this prevents an ever increasing production of the hormone).