Introduction to the endocrine system

The endocrine system refers to the collection of glands that produce hormones that regulate bodily functions. The neuroendocrine system is how neural tissues (hypothalamus) communicates with the endocrine system to regulate homeostasis.

Hormones are chemical products released in small quantities into the blood stream to exert effects on target organs. It is a form of extrinsic homeostatic control. Different organs and glands are responsible for releasing specific hormones which serve a specific function (Figure 1). Hormone release is regulated by neural mechanisms (hypothalamus, autonomic nervous system), releasing hormones (i.e hormones released by the pituitary gland that trigger release of hormones in a distant gland), direct or indirect regulation by nutrient or ion ( e.g glucose triggering insulin production, plasma Ca2+, PO4- and parathyroid hormone).

Figure 1
The hypothalamus is a small region in the centre of the brain below the thalamus and near the pituitary gland. The main role of the hypothalamus is to maintain homeostasis and to achieve this it regulates the autonomic nervous system and modulates the endocrine system. The hypothalamus regulates:
  • Vegetative control
    • Autonomic nervous system: Sympathetic/parasympathetic nerves to all internal organs, sympathetic nerves to blood vessels, sympathetic (cholinergic) nerves to sweat glands, sympathetic nerves to adrenal glands releasing adrenaline and norarenaline into blood stream, GIC motility and secretions (enteric NS)
    • Cardiovascular regulation
    • Body temperature regulation
    • Body water regulation (ADH and posterior pituitary)
    • Uterine contractibility an milk ejection
    • GIT secretions
  • Neuroendocrine: Controls release of hormones from pituitary gland (anterior and posterior pituitary gland)
  • Behavioural
    • Body water regulation (thirst)
    • Feeding regulation (Hunger and satiety)
    • Affective nature of sensations (reward, punishment, satisfaction, aversion)
    • Rage and anger
    • Anxiety
    • Sex drives
The pituitary gland is split into the anterior and posterior pituitary which serve different functions. 
  • The posterior pituitary is responsible for the secretion of the hormones Vasopressin (ADH=Antidiuretic hormone) and oxytocin (Figure 2). These hormone are produced in the hypothalamus in paraventricular and supraoptic nuclei (nuclei are a cluster of neurons) by neurosecretory cell bodies and travel down neuron axons in the hypothalamic-posterior pituitary stalk to be stored in the neuronal terminals within the posterior pituitary). Vasopressin is critical for body water homeostasis and arterial BP regulation. It results in water reabsorption in times of hypovolemia. Osmolarity and volume status are the main stimulators of ADH release through baroreceptors and stimulation by the vagus nerve. Oxytocin stimulus uterine contractions in pregnancy and milk ejection from the breast.
Figure 2
  • Anterior pituitary gland (Figure 3) is key in regulating the endocrine system by releasing tropic hormones (hormones that has distant endocrine tissue as a target). These act on target hormones to trigger an effect (usually the release of another hormone). Anterior pituitary hormone release is controlled by the hypothalamus median eminance releasing hypophysiotropic neuropeptites  (e.g TRH, CRH, GnRH) as opposed to the posterior pituitary which is stimulated by direct neural stimulation. The hormones produced by the anterior pituitary have feedback systems to regulate the production of the hypothalamic neuropeptides.
    • Thyroid stimulating hormone (TSH) acts on the thyroid gland to stimulate the production of thyroid hormone resulting in growth and differentiation and energy balance
    • Lutenising hormone (LSH) acts on the gonads to stimulate sex hormone production (estrogen, progesterone, testosterone) affecting reproductive function and behaviour
    • Follicle stimulating hormone (FSH) acts on the gonads to stimulate growth of the reproductive system
    • Adrenocorticotropic hormone (ACTH) acts on the adrenal gland to stimulate secretion of cortisol, aldosterone (both important for BP and the stress response) and androgens
    • Prolactin (Prl) actors on the ovaries, mammary glands, testes, prostate to stimulate secretion of estrogens/progesterone, milk production, spermatogenesis, prostate hyperplasia
    • Growth hormone (GH) acts on the liver and adipose tissue to promote growth through IGF-1 production, lipid and carbohydrate metabolism. Acts on skeletal muscle and bone to stimulate growth.
Figure 3






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