Hypothalamus-Pituitary-Adrenal Axis (HPA)

Fig.1 HPA Axis.

The hypothalamus-pituitary-adrenal axis (HPA axis or HTPA axis) is a complex set of direct influence and feedback interactions, consisting of three parts: hypothalamus, pituitary and adrenal (also called "adrenal") glands. These organs and their interactions constitute the HPA axis.

HPA axis, hypothalamus-pituitary-gonad axis (HPG), hypothalamus-pituitary-thyroid axis (HPT) and hypothalamus-neuro pituitary system are the four main neuroendocrine systems for the direct neuroendocrine function of the hypothalamus and pituitary. Among them, the HPA axis is a major neuroendocrine system, which controls stress response and regulates many physical processes, including digestion, immune system, mood and emotion, sexual behavior, and energy storage and consumption.

Functions

Fig.2 HPA Axis Signaling.
(CRH, corticotropin-releasing hormone;
ACTH, adrenocorticotropic hormone)

The HPA axis plays a central role in regulating many homeostatic systems in the body, including the metabolic system, cardiovascular system, immune system, reproductive system and central nervous system.

The release of corticotropin-releasing hormone (CRH) from the hypothalamus is affected by stress, physical activity, disease, cortisol blood concentration and sleep/wake cycle (circadian rhythm). Abnormally flat circadian cortisol circulation is associated with chronic fatigue syndrome, insomnia, and fatigue.

The anatomical connections between the amygdala, hippocampus, prefrontal cortex, and hypothalamus help to activate the HPA axis. In the hypothalamus, the impulse that sends out fear signals activates the sympathetic nervous system and the HPA axis regulation system.

The increase in cortisol production during stress leads to an increase in the availability of glucose to promote fight or flight. Cortisol can not only directly increase the utilization rate of glucose, but also inhibit the metabolic process of the immune system, thereby further increasing the utilization rate of glucose.

Glucocorticoids have many important functions, including regulating the stress response, but too much can damage health. Prolonged exposure to high concentrations of glucocorticoids can cause hippocampus atrophy in humans and animals. Hippocampal deficiency may reduce the memory resources available to help the body respond appropriately to stress.

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