The Body's Response to Stressors: The ANS and the HPA-Axis
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:
Sympathetic Branch of the ANS
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.
Parasympathetic Branch of the ANS
The parasympathetic branch of the autonomic nervous system restores the body to a relaxed state after the threating situation has passed
The HPA Axis
The Hypothalamic-Pituitary-Adrenal (HPA) Axis is a set of three organs that together are involved in controlling the body's response to stress through the release of hormones. The three organs are the hypothalamus, the pituitary gland and the adrenal glands.
The Hypothalamus
The hypothalamus is about the size of an almond and is located at the base of the brain, just above the brainstem and below the thalamus (hypo- means under). It is involved in controlling bodily functions such as hunger, thirst, temperature and circadian rhythms.
The Pituitary Gland
The pituitary gland is about the size of a pea and is located just below the hypothalamus, protected by a bony cavity. It secretes hormones involved in regulating functions such as growth, blood pressure, metabolic processes, breast milk production and function of the sex organs.
The Adrenal Glands
The adrenal glands are triangular structures that sit on top of the kidneys. They produce hormones that regulate the fight-or-flight response, such as adrenaline and noradrenaline. They also produce corticosteroids such as cortisol, which has a suppressant effect on the hypothalamus and pituitary gland.
The HPA Axis and Stress
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 to negatively feedback to reduce the production of CRF and ACTH in the hypothalamus and pituitary gland.
