Hypothalamic-pituitary-adrenal axis

Overview of the hypothalamic-pituitary-adrenal axis

Hypothalamic-pituitary-adrenal axis

200px
Diagram of the hypothalamic-pituitary-adrenal axis
System	Endocrine system

The hypothalamic-pituitary-adrenal axis (HPA axis) is a complex set of direct influences and feedback interactions among three endocrine glands: the hypothalamus, the pituitary gland (a pea-shaped structure located below the hypothalamus), and the adrenal glands (small, conical organs on top of the kidneys). The HPA axis is a major part of the neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure.

Structure

The HPA axis involves three key components:

• Hypothalamus: The hypothalamus is a region of the brain that controls an immense number of bodily functions. It releases corticotropin-releasing hormone (CRH) in response to stress.

• Pituitary gland: The pituitary gland, often termed the "master gland," releases adrenocorticotropic hormone (ACTH) into the bloodstream in response to CRH.

• Adrenal glands: The adrenal glands produce cortisol, a steroid hormone, in response to ACTH. Cortisol helps the body respond to stress and has many effects on different body systems.

Function

The HPA axis is activated in response to stress. When a stressor is perceived, the hypothalamus releases CRH, which travels to the pituitary gland and stimulates the release of ACTH. ACTH then travels through the bloodstream to the adrenal glands, prompting them to release cortisol. Cortisol helps mobilize energy, suppresses the immune system, and aids in the metabolism of fat, protein, and carbohydrates.

Cortisol also provides negative feedback to the hypothalamus and pituitary gland to reduce the release of CRH and ACTH, thus regulating the stress response.

Clinical significance

Dysregulation of the HPA axis is implicated in numerous disorders, including:

• Cushing's syndrome: Characterized by excessive cortisol production.
• Addison's disease: Characterized by insufficient production of cortisol.
• Depression: Altered HPA axis function is often observed in patients with depression.
• Anxiety disorders: Dysregulation of the HPA axis can contribute to anxiety.

Research

Research into the HPA axis continues to uncover its role in various physiological and psychological conditions. Studies are exploring how chronic stress affects the HPA axis and contributes to disease.

Also see

• Endocrine system
• Corticotropin-releasing hormone
• Adrenocorticotropic hormone
• Cortisol
• Stress (biology)