Hormones and Cell Communication

• Hormones facilitate cell communication and regulate various bodily functions from glucose and salt metabolism to mood and behavior.
• The hypothalamus controls the secretion of multiple hormones in conjunction with the pituitary gland, forming the hypothalamo/pituitary/endocrine organ axis.
• The pituitary gland has two parts: the posterior pituitary (neurohypophysis, which secretes ADH and oxytocin) and the anterior pituitary (adenohypophysis, which secretes growth hormone and trophic hormones).
• Pituitary gland dysfunction can lead to significant health issues due to excessive or deficient hormone secretion.

Cell Communication

• Hormones allow cells to communicate, affecting the function of other cells.
• Hormones released circulate and affect cells with appropriate receptors.
• Hormonal effects are slower but sustained, lasting until hormone concentration decreases due to metabolic breakdown.
• Hormonal control is important in homeostasis regulation over minutes to days.
• Other cell communication methods include nerve cells communicating using neurotransmitters.

Mechanisms of Action

• Hormones travel through the bloodstream to specific target cells.
• Hormones bind to membrane or intracellular receptors.
• Membrane receptor binding often uses second messenger systems, affecting ion channels and enzyme activity.
• Intracellular acting hormones may alter RNA formation or mitochondrial function.

Hypothalamic Control

• The hypothalamus is near the third ventricle in the forebrain and connects to the pituitary gland (hypophysis cerebri).
• The pituitary has two embryologically distinct components: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis).
• The anterior pituitary is a gland with secretory epithelial cells derived from Rathke's pouch.
• The anterior pituitary receives blood from hypophyseal portal vessels connecting it to the hypothalamus.
• The posterior pituitary develops from the brain and comprises adapted axons releasing hormones.
• Neuronal cell bodies are in the paraventricular and supraoptic nuclei of the hypothalamus.
• The anterior pituitary's endocrine activity is regulated by hypothalamic hormones via the hypophyseal portal blood.
• These factors are often oligopeptides, and selectively promote (releasing hormones) or inhibit (inhibiting hormones) the secretion of specific pituitary hormones.

Pituitary Hormones

• The secretion of pituitary regulating hormones by the hypothalamus is influenced by neurological inputs and feedback.
• Pituitary hormones and the products of the systems they control can regulate hypothalamic secretion through feedback inhibition.
• The posterior pituitary secretes two peptides: oxytocin and antidiuretic hormone (ADH or vasopressin).
• These are manufactured in the hypothalamic neuron cell bodies, packaged in vesicles, and transported to the posterior pituitary for storage.

Anterior Lobe Hormones

• 6 peptide hormones with defined functions are secreted by the anterior pituitary
• These regulate the endocrine functions of the adrenal cortex, thyroid, and gonads, as well as prolactin and growth hormone.
• Adrenocorticotrophic hormone (ACTH): Secreted due to corticotrophin releasing hormone (CRH) from the hypothalamus, to stimulate glucocorticoid release from the adrenal cortex.
• Thyroid stimulating hormone (TSH): Released in response to thyrotrophin releasing hormone (TRH) from the hypothalamus to stimulate thyroid secretion.
• Follicle stimulating hormone (FSH) and luteinizing hormone (LH): Secreted in response to gonadotrophin releasing hormone (GnRH) to stimulate the male and female gonads.
• Prolactin: Primarily controlled by prolactin inhibiting hormone (PIH), likely dopamine.
• Reduced PIH stimulates prolactin increases during pregnancy and breastfeeding.
• Human growth hormone (hGH): Secretion is controlled by antagonistic hypothalamic hormones with the growth hormone releasing hormone (GHRH) more important than the growth hormone inhibiting hormone (GHIH, or somatostatin).
• Growth hormone is anabolic, stimulating cell division and growth, is mediated by growth promoters (somatomedins) from the liver.
• Growth hormone favors protein synthesis and fat breakdown, has a carbohydrate-sparing effect, and increases glycogen stores and blood glucose levels.
• Low blood glucose triggers hypothalamic GHRH and inhibits somatostatin secretion; growth hormone secretion is also stimulated by trauma and stress.
• Oxytocin, produced in the paraventricular and supraoptic nuclei, is released from the posterior pituitary due to sensory inputs, particularly from breast mechanoreceptors during suckling.
• Oxytocin stimulates milk ejection and uterine contraction.
• Antidiuretic hormone (ADH) is also produced in the paraventricular and supraoptic nuclei.
• Sensory inputs stimulate ADH when extracellular fluid osmolarity rises, or if blood volume or pressure falls.
• ADH promotes water reabsorption in the kidney's collecting ducts, reducing osmolarity and expanding extracellular fluids, while also increasing peripheral resistance through arteriolar constriction.

Abnormalities of Pituitary Function

• Pituitary function abnormalities stem from deficient pituitary secretion or excess hormone production.

• Deficient secretion: Deficiencies from insults to the entire gland (panhypopituitarism) or a single hormone deficit due to pituitary/hypothalamic cell defects.

• Main problems resulting from deficient secretion include:

  • Corticosteroid deficiency
  • Thyroid deficiency
  • Failure of sexual function
  • Dwarfism
  • Diabetes insipidus

• Excess secretion: Conditions resulting from over-secretion of a single pituitary hormone include:

  • Corticosteroid excess
  • Impaired reproductive function (hyperprolactinaemia)
  • Abnormal growth, which may lead to giantism or acromegaly
  • Fluid retention and low plasma osmolarity (inappropriate ADH secretion)