9.3 - HPA: Hypothalamic-Pituitary Axis

Hypothalamic-pituitary axis (HPA)

The "command center" of the endocrine system; a critical part involving complex interactions between the hypothalamus and pituitary gland that orchestrates hormone release to maintain homeostasis; also coordinates messages between the endocrine and nervous systems; stimuli received by the nervous system often pass through the HPA to be translated into hormones

Hypothalamus (location)

A structure of the diencephalon of the brain located anterior and inferior to the thalamus; has both neural and endocrine functions; produces and secretes many hormones

Pituitary gland (hypophysis)

A bean-sized organ suspended from the hypothalamus by a stem called the infundibulum (pituitary stalk); cradled within the sella turcica of the sphenoid bone; consists of two main lobes: anterior pituitary (adenohypophysis) and posterior pituitary (neurohypophysis)

Posterior pituitary lobe (neurohypophysis)

Neural tissue; a physical extension of hypothalamic neurons; does NOT produce hormones — stores and secretes ADH and oxytocin, which are produced by the hypothalamus and transported down axons via the hypothalamic-hypophyseal tract

Anterior pituitary lobe (adenohypophysis)

Glandular tissue that develops from the embryonic digestive tract; secretes seven hormones in response to hypothalamic releasing and inhibiting hormones; includes the pars intermedia (intermediate zone)

Infundibulum (pituitary stalk)

The stem connecting the hypothalamus to the pituitary gland; contains the hypothalamic-hypophyseal tract (axons carrying ADH and oxytocin to the posterior pituitary) and the capillaries of the hypophyseal portal system

Neurosecretory cells

Specialized neurons within the hypothalamus that respond to neurotransmitters and produce hormones

Three functions of the hypothalamus (endocrine control)

1) Releases regulatory hormones to control anterior pituitary hormone secretion; 2) Produces ADH and oxytocin for the posterior pituitary to release; 3) Has autonomic centers that directly control the adrenal medulla via the sympathetic nervous system

Hypothalamic-hypophyseal tract

The pathway of axons running from hypothalamic neuron cell bodies through the infundibulum to axon terminals in the posterior pituitary; carries ADH and oxytocin for storage and release

ADH (antidiuretic hormone) / vasopressin

Produced by the hypothalamus, stored and released by the posterior pituitary; released in response to high blood ion concentration (e.g., hypernatremia) or decreased blood volume (hypovolemia); targets the kidneys to decrease urine formation by retaining water, increasing blood volume and blood pressure; also dilutes blood to decrease osmolality; regulated by negative feedback

Oxytocin (OXT)

Produced by the hypothalamus, stored and released by the posterior pituitary; released continually during childbirth via positive feedback (cervical pressure → more oxytocin → stronger uterine contractions → more cervical pressure); also stimulates milk let-down from mammary glands for nursing

Releasing hormones (-RH)

Hypothalamic hormones that stimulate hormone secretion from the anterior pituitary; include TRH, CRH, GnRH, GH-RH, and PRF

Inhibiting hormones (-IH)

Hypothalamic hormones that inhibit hormone secretion from the anterior pituitary; include GH-IH (somatostatin) and PIH

Hypophyseal portal system

A network of capillaries within the infundibulum that connects the hypothalamus directly to the anterior pituitary; allows hypothalamic hormones to reach the anterior pituitary without being diluted by systemic circulation, keeping their concentration high

Seven hormones of the anterior pituitary

TSH (thyroid-stimulating hormone), ACTH (adrenocorticotropic hormone), FSH (follicle-stimulating hormone), LH (luteinizing hormone), GH (growth hormone), PRL (prolactin), and MSH (melanocyte-stimulating hormone, from pars intermedia)

Tropic hormones

TSH, ACTH, FSH, and LH; anterior pituitary hormones that turn on or off the function of other endocrine glands; the target glands in turn produce hormones that regulate hypothalamic hormone secretion via negative feedback

Thyrotropin-Releasing Hormone (TRH)

Hypothalamic releasing hormone → stimulates anterior pituitary to secrete TSH → targets thyroid gland → releases thyroid hormones

Corticotropin-Releasing Hormone (CRH)

Hypothalamic releasing hormone → stimulates anterior pituitary to secrete ACTH → targets adrenal cortex → releases glucocorticoids

Gonadotropin-Releasing Hormone (GnRH)

Hypothalamic releasing hormone → stimulates anterior pituitary to secrete FSH and LH; FSH targets testes (→ inhibin) and ovaries (→ inhibin, estrogen); LH targets testes (→ androgens) and ovaries (→ estrogens, progestins)

Growth Hormone-Releasing Hormone (GH-RH)

Hypothalamic releasing hormone → stimulates anterior pituitary to secrete GH → targets the liver → releases somatomedins

Growth Hormone-Inhibiting Hormone (GH-IH) / somatostatin

Hypothalamic inhibiting hormone → inhibits GH release from the anterior pituitary

Prolactin-Releasing Factor (PRF)

Hypothalamic releasing factor → stimulates anterior pituitary to secrete prolactin (PRL) → targets mammary glands

Prolactin-Inhibiting Hormone (PIH)

Hypothalamic inhibiting hormone → inhibits prolactin (PRL) release from the anterior pituitary

Pars intermedia

The intermediate zone of the anterior pituitary; secretes melanocyte-stimulating hormone (MSH)

Melanocyte-stimulating hormone (MSH)

Secreted by the pars intermedia of the anterior pituitary; targets melanocytes in the skin to induce melanin production, capable of darkening the skin; local skin production of MSH occurs in response to UV light exposure; increased during pregnancy and, combined with estrogens, can cause darker pigmentation of the areolas and labia minora; significance in healthy adults is uncertain