Human Cycle W 9 2024-2025 Posterior Pituitary PDF

Human Cycle W 9 2024 – 2025 The Posterior Pituitary Prof Mohamed Fathelbab Physiology Department Al-Ryan College of Medicine  Posterior pituitary hormones  How the Posterior Pituitary Functions as a Neurohypophysis 1.Hormone Synthesis in the Hypothalamus: 1.The hypothalamus cont...

Human Cycle W 9 2024 – 2025 The Posterior Pituitary Prof Mohamed Fathelbab Physiology Department Al-Ryan College of Medicine  Posterior pituitary hormones  How the Posterior Pituitary Functions as a Neurohypophysis 1.Hormone Synthesis in the Hypothalamus: 1.The hypothalamus contains specialized neurons in two key nuclei: 1.Supraoptic nucleus: Produces antidiuretic hormone (ADH), also known as vasopressin. 2.Paraventricular nucleus: Produces oxytocin. 2.These hormones are neuropeptides, meaning they are synthesized in the cell bodies of neurons in the hypothalamus.  How the Posterior Pituitary Functions as a Neurohypophysis Transport to the Posterior Pituitary: – Once synthesized, ADH and oxytocin are transported down the axons of these neurons via the hypothalamo-hypophyseal tract. This is a bundle of nerve fibers that connects the hypothalamus to the posterior pituitary. Storage in the Posterior Pituitary: – The posterior pituitary contains the axon terminals of these hypothalamic neurons, where ADH and oxytocin are stored in vesicles until they are needed.  How the Posterior Pituitary Functions as a Neurohypophysis Release of Hormones: 1.When the body signals the need for ADH or oxytocin, electrical impulses (action potentials) from the hypothalamus travel down the axons to the posterior pituitary. 2.These action potentials trigger the release of the stored hormones from the axon terminals into the bloodstream. 1.ADH is released in response to increased blood osmolarity or decreased blood volume, promoting water reabsorption in the kidneys. 2.Oxytocin is released in response to stimuli such as uterine contractions during labor or suckling during breastfeeding, promoting uterine contractions and milk ejection.  THE ADH RECEPTORS (V1 AND V2) ADH (antidiuretic hormone), also known as vasopressin, exerts its physiological effects primarily through two types of receptors: V1 and V2. These receptors are distributed in different tissues and mediate distinct physiological functions. 1. V1 Receptors (V1a and V1b) Location: V1a receptors: Found mainly in vascular smooth muscle, liver, platelets, and certain areas of the brain. V1b receptors: Located in the anterior pituitary and are involved in regulating ACTH release. Physiological Effects: Vascular Smooth Muscle Contraction (V1a): ADH binding to V1a receptors in vascular smooth muscle cells activates a signaling pathway that causes vasoconstriction, leading to increased blood pressure. This effect is particularly important in response to significant drops in blood pressure or blood volume (e.g., during hemorrhage). ACTH Release (V1b): ADH stimulates V1b receptors in the anterior pituitary, promoting the release of adrenocorticotropic hormone (ACTH), which subsequently stimulates cortisol production by the adrenal glands. This response is more associated with stress and metabolic regulation. Mechanism of Action: V1 receptors stimulates calcium release from intracellular stores (endoplasmic reticulum), increasing cytosolic calcium levels. The rise in calcium triggers smooth muscle contraction in the blood vessels, causing vasoconstriction and an increase in blood pressure.  THE ADH RECEPTORS (V1 AND V2) 2. V2 Receptors Location: Primarily located in the renal collecting ducts (principal cells) of the kidneys. Physiological Effects: Renal Water Reabsorption: ADH binding to V2 receptors promotes the insertion of aquaporin-2 water channels into the apical membrane of the collecting duct cells. water to be reabsorbed from the filtrate back into the bloodstream. Mechanism of Action: V2 increases cyclic AMP (cAMP) production. cAMP leading to phosphorylation aquaporin-2 (AQP2) channels. Phosphorylated AQP2 channels are then inserted into the apical membrane of the collecting ducts, allowing water to pass from the urine back into the kidney cells, and eventually into the blood. This water reabsorption helps concentrate the urine and reduce water loss, particularly in response to dehydration or elevated plasma osmolality.  The physiological mechanism and functions of Oxytocin receptor locations and their roles in parturition and lactation. Oxytocin is a hormone produced by the hypothalamus and released by the posterior pituitary gland. Its primary physiological roles are in parturition (childbirth) and lactation (milk ejection), mediated by oxytocin receptors. Oxytocin Receptors: Locations and Functions Oxytocin receptors are G-protein-coupled receptors found in several tissues, primarily the uterus and mammary glands, but also in other areas like the brain. Upon binding to oxytocin, these receptors trigger specific physiological processes important for childbirth and breastfeeding. 1. Role in Parturition (Childbirth) Location of Receptors: Oxytocin receptors are highly expressed in the myometrium (smooth muscle layer of the uterus) and endometrium (inner lining of the uterus). The expression of these receptors increases significantly during late pregnancy, preparing the uterus for labor.  The physiological mechanism and functions of Oxytocin Physiological Functions: Uterine Contractions: During labor, oxytocin binds to its receptors in the myometrium, initiating strong rhythmic contractions of the uterine muscles. These contractions help dilate the cervix and push the fetus through the birth canal. Positive Feedback Mechanism: Oxytocin secretion is regulated by a positive feedback loop during labor. As uterine contractions progress, stretch receptors in the cervix send signals to the hypothalamus, stimulating the release of more oxytocin. This amplification of oxytocin release enhances uterine contractions, driving labor forward until the baby is delivered. Mechanism of Action: Oxytocin receptors leads to the release of calcium from intracellular stores, which enhances the contractile activity of uterine smooth muscle cells.  The physiological mechanism and functions of Oxytocin 2. Role in Lactation (Milk Ejection) Location of Receptors: Oxytocin receptors are found in the myoepithelial cells surrounding the alveoli of the mammary glands. Physiological Functions: Milk Ejection Reflex (Let-down Reflex): After childbirth, oxytocin plays a crucial role in breastfeeding. During suckling, sensory nerve impulses from the nipples are sent to the hypothalamus, stimulating the release of oxytocin. Oxytocin then binds to its receptors on the myoepithelial cells surrounding the milk-producing alveoli. Contraction of Myoepithelial Cells: When oxytocin binds to receptors on these myoepithelial cells, it triggers their contraction, causing the milk stored in the alveoli to be ejected into the milk ducts and released through the nipple. This is the milk ejection reflex. Mechanism of Action: Like uterine contractions, oxytocin receptors in the mammary glands activate leading to increased intracellular calcium levels. This calcium influx causes the contraction of myoepithelial cells, which expels milk from the alveoli.  Other Roles of Oxytocin: Social Bonding and Emotional Regulation: Oxytocin receptors are also found in certain regions of the brain, particularly in areas involved in emotion and social behavior. It is sometimes called the "love hormone" because it promotes bonding between individuals, such as mother and infant during breastfeeding, and enhances trust and emotional bonding in social relationships. Summary of Oxytocin Functions: 1. Parturition (Childbirth): 1. Receptor Location: Uterus (myometrium). 2. Function: Stimulates uterine contractions during labor. 3. Mechanism: stretch receptor stimulation → calcium release → uterine muscle contraction. 4. Positive Feedback Loop: Cervical stretching increases oxytocin release, amplifying contractions..‫تضخيم ا'نقباضات‬ 2. Lactation (Milk Ejection): 1. Receptor Location: Mammary glands (myoepithelial cells). 2. Function: Triggers milk ejection during breastfeeding. 3. Mechanism: Suckling → Mechanoreceptors in the Nipple-Areolar Complex → sends nerve impulses via sensory neurons to the hypothalamus in the brain, leading to the release of Oxytocin → calcium release → myoepithelial cell contraction. 4. Milk let down (ejection)  The structural comparisons between the anterior and posterior pituitary glands Feature Anterior Pituitary (Adenohypophysis) Posterior Pituitary (Neurohypophysis) H o r m o n e Synthesizes and secretes hormones (e.g., Stores and releases hormones produced in the Production ACTH, TSH, GH, LH, FSH, PRL) hypothalamus (e.g., ADH, oxytocin) S t o r a g e a n d Does not store hormones; releases directly Stores hormones in nerve endings until released into Release into the bloodstream circulation Regulated by hypothalamic hormones via Regulated by direct neural signals from the Regulation portal circulation hypothalamus  The anterior and posterior pituitary gland Feature Anterior Pituitary (Adenohypophysis) Posterior Pituitary (Neurohypophysis) Connection to Linked via the hypophyseal portal system (vascular Connected via the hypothalamo-hypophyseal tract Hypothalamus connection) (neural connection) Control by Controlled by hypothalamic releasing and inhibiting Controlled by neural signals from hypothalamus Hypothalamus hormones Does not synthesize hormones; stores and releases Hormone Synthesis Synthesizes and releases its own hormones hypothalamic hormones - GH (Growth Hormone) - ACTH (Adrenocorticotropic Hormone) - TSH (Thyroid-Stimulating Hormone) - ADH (Antidiuretic Hormone/Vasopressin) Hormones Released - PRL (Prolactin) - Oxytocin - LH (Luteinizing Hormone) - FSH (Follicle-Stimulating Hormone) Regulates growth, metabolism, reproduction, and Regulates water balance (via ADH) and uterine Main Functions lactation contractions/milk ejection (via oxytocin) Storage of Hormones No hormone storage; releases immediately into blood Stores ADH and oxytocin until release is triggered TSH release is controlled by TRH from the ADH release is controlled by osmoreceptors detecting Example of Regulation hypothalamus blood osmolarity