biology

Hypothalamic–pituitary–adrenal axis

Hypothalamic–pituitary–adrenal axis is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Hypothalamic–pituitary–adrenal axis rather than just read about it. In short: The hypothalamic–pituitary–adrenal axis (HPA axis) is a neuroendocrine axis that controls the secretion of corticosteroid stress hormones. The HPA axis has three components: the hypothalamus (a part of the brain located below the thalamus), the pituitary gland (a pea-shaped structure located below the hypothalamus), and the adrenal (also called "suprarenal") glands (small, conical organs on top of the kidneys).

Key takeaways

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Reference excerpt

The hypothalamic–pituitary–adrenal axis (HPA axis) is a neuroendocrine axis that controls the secretion of corticosteroid stress hormones. The HPA axis has three components: the hypothalamus (a part of the brain located below the thalamus), the pituitary gland (a pea-shaped structure located below the hypothalamus), and the adrenal (also called "suprarenal") glands (small, conical organs on top of the kidneys). These structures and their interactions constitute the HPA axis. The HPA axis is a major neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, immune responses, mood and emotions, sexual activity, and energy storage and expenditure. It is the common mechanism for interactions among glands, hormones, and parts of the midbrain that mediate the general adaptation syndrome (GAS). While steroid hormones are produced mainly in vertebrates, the physiological role of the HPA axis and corticosteroids in stress response is so fundamental that analogous systems can be found in invertebrates and monocellular organisms as well. The HPA axis, hypothalamic–pituitary–gonadal (HPG) axis, hypothalamic–pituitary–thyroid (HPT) axis, and the hypothalamic–neurohypophyseal system are the four major neuroendocrine systems through which the hypothalamus and pituitary direct neuroendocrine function.

Anatomy The key elements of the HPA axis are:

The paraventricular nucleus of the hypothalamus: It contains neuroendocrine neurons which synthesize and secrete vasopressin and corticotropin-releasing hormone (CRH). The anterior lobe of the pituitary gland: CRH and vasopressin stimulate the anterior lobe of pituitary gland to secrete adrenocorticotropic hormone (ACTH), once known as corticotropin. The adrenal cortex: It produces glucocorticoid hormones (mainly cortisol in humans) in response to stimulation by ACTH. Glucocorticoids in turn, act back on the hypothalamus and pituitary (to suppress CRH and ACTH production) in a negative feedback cycle. CRH and vasopressin are released from neurosecretory nerve terminals at the median eminence. CRH is transported to the anterior pituitary through the portal blood vessel system of the hypophyseal stalk and vasopressin is transported by axonal transport to the posterior pituitary gland. There, CRH and vasopressin act synergistically to stimulate the secretion of stored ACTH from corticotrope cells. ACTH is transported by the blood to the adrenal cortex of the adrenal gland, where it rapidly stimulates the biosynthesis of corticosteroids such as cortisol from cholesterol. Cortisol is a major stress hormone and has effects on many tissues in the body, including the brain. In the brain, cortisol acts on two types of receptors: mineralocorticoid receptors and glucocorticoid receptors, and these are expressed by many different types of neurons. One important target of glucocorticoids is the hypothalamus, which is a major controlling centre of the HPA axis. Vasopressin can be thought of as "water conservation hormone" and is also known as "antidiuretic hormone(ADH)". It is released when the body is dehydrated and has potent water-conserving effects on the kidney. It is also a potent vasoconstrictor. Important to the function of the HPA axis are some of the following feedback loops:

Cortisol produced in the adrenal cortex will negatively feedback to inhibit both the hypothalamus and the pituitary gland. This reduces the secretion of CRH and vasopressin, and also directly reduces the cleavage of proopiomelanocortin (POMC) into ACTH and β-endorphins. Epinephrine and norepinephrine (E/NE) are produced by the adrenal medulla through sympathetic stimulation and the local effects of cortisol (upregulation enzymes to make E/NE). E/NE will positively feedback to the pituitary and increase the breakdown of POMCs into ACTH and β-endorphins.

Function Release of corticotropin-releasing hormone (CRH) from the hypothalamus is influenced by stress, physical activity, illness, by blood levels of cortisol and by the sleep/wake cycle (circadian rhythm). In healthy individuals, cortisol rises rapidly after wakening, reaching a peak within 30–45 minutes. It then gradually falls over the day, reaching a trough during the middle of the night. This corresponds to the rest-activity cycle of the organism. An abnormally flattened circadian cortisol cycle has been linked with chronic fatigue syndrome, insomnia and burnout. The HPA axis has 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 HPA axis integrates physical and psychosocial influences in order to allow an organism to adapt effectively to its environment, use resources, and optimize survival. Anatomical connections between brain areas such as the amygdala, hippocampus, prefrontal cortex and hypothalamus facilitate activation of the HPA axis. Sensory information arriving at the lateral aspect of the amygdala is processed and conveyed to the amygdala's central nucleus, which then projects out to several parts of the brain involved in 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 cortisol during stress results in an increased availability of glucose in order to facilitate fighting or fleeing. As well as directly increasing glucose availability, cortisol also suppresses the highly demanding metabolic processes of the immune system, resulting in further availability of glucose. Glucocorticoids have many important functions, including modulation of stress reactions, but in excess they can be damaging. Atrophy of the hippocampus in humans and animals exposed to severe stress is believed to be caused by prolonged exposure to high concentrations of glucocorticoids. Deficiencies of the hippocampus may reduce the memory resources available to help a body formulate appropriate reactions to stress.

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Illustrations

Worked examples

Example 1 — a first encounter with Hypothalamic–pituitary–adrenal axis

Start with the simplest possible case. Write down what Hypothalamic–pituitary–adrenal axis claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Hypothalamic–pituitary–adrenal axis before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Hypothalamic–pituitary–adrenal axis ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Hypothalamic–pituitary–adrenal axis

In research: Hypothalamic–pituitary–adrenal axis appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Hypothalamic–pituitary–adrenal axis in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Hypothalamic–pituitary–adrenal axis is common in secondary-school and first-year university syllabi. It links to neighbouring topics Anxiety, Attention, Neuroendocrinology, so understanding it makes those chapters shorter.
In everyday life: Look for Hypothalamic–pituitary–adrenal axis outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Hypothalamic–pituitary–adrenal axis in simple terms?

Why does Hypothalamic–pituitary–adrenal axis matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Hypothalamic–pituitary–adrenal axis?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Hypothalamic–pituitary–adrenal axis.