HPA axis
The hypothalamic-pituitary-adrenal axis is a major part of the neuroendocrine system that controls reactions to stress. Species from humans to the most ancient organisms share components of the HPA axis. It is the mechanism for a set of interactions among glands, hormones and parts of the mid-brain that mediate a general adaptation syndrome.
Anatomy
The HPA axis includes parts of the hypothalamus, the anterior lobe of the pituitary gland, the adrenal cortices, hormones, systems that transport hormones and feedback mechanisms that transport cortisol from adrenal glands back to the hypothalamus and to other parts of the brain.
The hypothalamus releases corticotropin-releasing factor (CRF) from an area along the median eminance. The hormone is transported to the anterior lobe of the pituitary through the portal blood vessel system of the hypophyseal stalk, which descends from the hypothalamus. In the anterior pituitary gland, CRF stimulates release of stored adrenocorticotropic hormone (ACTH). ACTH is transported by the blood to the adrenal cortex of the adrenal gland, where it rapidly stimulates biosynthesis of corticosteroids from cholesterol.
Function
Release of CRF from the hypothalamus is influenced by stress, by blood levels of cortisol and by the sleep/wake cycle.
Anatomical connections between amygdala, hippocampus, and hypothalalamus facilitate activation of the HPA axis. Sensory information arriving at the lateral aspect of the amygdala is processed and conveyed to the central nucleus, which hosts projections to several parts of the brain in involved responses to fear. At the hypothalamus, fear-signaling impulses activate both the sympathetic nervous system and the modulating systems of the HPA axis.
Increased production of the glococorticoid cortisol mediates alarm reactions to stress, facilitating an adaptive phase of a general adaptation syndrome in which alarm reactions are suppressed, allowing the body to attempt countermeasures.
Glococorticoids serve important functions including modulation of stress reactions but they can be damaging. Atrophy of the hippocampus in humans and animals exposed to severe stress is believed to be caused by the presence of excessive stress-induced glococorticoids. Deficiencies of the hippocampus are believed to reduce the memory resources available to help a body formulate appropriate reactions to stress.
The HPA axis is believed to be involved in the neurobiology of mood disorders, including anxiety disorder, bipolar disorder and post-traumatic stress disorder.
Related topics
Brain
List of regions in the human brain
Referenced By
Adrenal | Adrenal cortex | Adrenal gland | Cortisol | Hypothalamus
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