S2 L2 Physio

Posterior Pituitary Hormones

• Posterior pituitary hormones include vasopressin and oxytocin
• These are polypeptides containing 9 amino acids
• They share similar structures and functions
• Vasopressin (also known as antidiuretic hormone or ADH) is crucial in maintaining blood pressure and serum osmolality
• Oxytocin plays a role in uterine contractions during labor and milk ejection
• Oxytocin and vasopressin are produced in the hypothalamus and stored in the posterior pituitary
• They're transported to the posterior pituitary via the hypothalamo-hypophysial tract
• A carrier protein called neurophysin is involved in their transport
• Oxytocin is primarily produced by paraventricular nuclei, and ADH by supraoptic nuclei of the hypothalamus

Pituitary Gland

• The pituitary gland, also called the hypophysis, is approximately 1 cm in diameter and weighs 0.5 to 1 gram
• It's located in the sella turcica, a bony cavity at the base of the brain
• It's connected to the hypothalamus via the hypophysial stalk
• The pituitary is divided into two parts:
  • Anterior pituitary (adenohypophysis)
  • Posterior pituitary (neurohypophysis)

Embryological Origin

• The anterior pituitary originates from pharyngeal epithelium
• The posterior pituitary originates from neural tissue

Anterior Pituitary Hormones

• The anterior pituitary secretes 6 hormones: growth hormone (GH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL)

Vasopressin Receptors

• Vasopressin binds to different tissues using various vasopressin receptors (V1A, V1B, and V2)
• These receptors trigger cellular responses, including vasoconstriction and water retention
• V1A receptors are primarily found in vascular smooth muscle cells, causing vasoconstriction and raising blood pressure
• V2 receptors are mainly located in the renal collecting ducts, promoting water reabsorption

Renal Water Reabsorption

• Vasopressin (ADH) regulates water reabsorption in the kidneys
• It increases water reabsorption in the collecting ducts, leading to more concentrated urine
• V2 receptors play a key role in this process

Other Functions

• Vasopressin plays a role in the release of adrenocorticotropic hormone (ACTH)
• It also influences platelet aggregation
• It can help in maintaining blood pressure
• Controlling the release of vasopressin is crucial for regulating blood volume and blood pressure

Regulation of Vasopressin

• The release of vasopressin is regulated by factors like effective circulating volume, serum osmolality, and other factors
• It is inhibited, in part ,by cortisol and atrial natriuretic peptide (ANP)

Oxytocin

• It's a water-soluble peptide produced in the hypothalamus
• It travels in circulation unbound
• It interacts with oxytocin receptors in target tissues (breast and brain)
• This triggers a signaling cascade leading to altered gene expression
• It plays a key role in uterine contractions during childbirth

Actions of Oxytocin

• Oxytocin's actions include uterine contractions during labor, and milk ejection from the breasts
• Oxytocin production is stimulated by sensory nerve impulses
• It mediates the contraction of myoepithelial cells in the milk-producing glands, inducing milk ejection.
• During pregnancy, oxytocin receptor expression increases in the uterus to enhance responsiveness to the hormone

Role of Oxytocin After Delivery

• Oxytocin release continues after delivery, causing uterine contractions
• This is crucial for the uterus to return to its pre-pregnancy size

Actions in the Central Nervous System

• Overall, oxytocin's effect is to reduce the stress response
• It influences various regions of the brain
• It plays a role in feelings of love, anticipation, interpersonal interactions, and rewards

Other Behavioral Effects

• Oxytocin has effects on maternal-fetal bonding, motivation, and group decision-making

Clinical Correlation

• Synthetic oxytocin, known as pitocin, is used clinically to induce or strengthen uterine contractions during labor
• It's also a treatment option for postpartum hemorrhage

Case Studies

• Case studies are presented demonstrating the importance of understanding hormone function. Physiological aspects and clinical implications are discussed.