H_P Axis 2024.pdf

Hypothalamic-Pituitary Axis (HP Axis) VP 2024 Clara Camargo, DVM 1. List the endocrine glands regulated by the Hypothalamic-Pituitary Axis 2. Explain the specific structures and characteristics of the anterior and the posterior pituitary 3. List and understand the functions of the hormones produced...

Hypothalamic-Pituitary Axis (HP Axis) VP 2024 Clara Camargo, DVM 1. List the endocrine glands regulated by the Hypothalamic-Pituitary Axis 2. Explain the specific structures and characteristics of the anterior and the posterior pituitary 3. List and understand the functions of the hormones produced by the hypothalamus and pituitary gland HYPOTHALAMUS The HYPOTHALAMUS is a place of neurological and endocrine connection Composed of more than 12 nuclei (groups of neurons) and areas that regulate:  AUTONOMIC SYSTEM (sympathetic and parasympathetic)  ENDOCRINE SYSTEM  LIMBIC SYSTEM (Instintic and emotions: fear, aggression, memory, reflex memories related to smell, learning, feeding behavior, pleasure, sex, care for offspring, dominance) Diencephalon HYPOPHYSIS The HYPOPHYSIS is a small gland located in the sella turcica (bony cavity at the base of the brain – pituitary fossa)  Connected to the hypothalamus by the pituitary stalk (infundibulum)  Divided into 2 distinct parts: 1. Anterior Pituitary (ADENOHYPOPHYSIS) 2. Posterior Pituitary (NEUROHYPOPHYSIS) ANTERIOR PITUITARY (ADENOHYPOPHYSIS) The anterior hypophysis originates from the Rathke’s pouch (embryonic invagination of the pharyngeal epithelium) o Contains 5 major types of cells with epithelioid nature Each cell type produces a different type of hormone ANTERIOR PITUITARY (ADENOHYPOPHYSIS) Adenohypophysis 5 major types of cells: 1. SOMATOTROPES – produce growth hormone (GH) – about 50% of the total adenohypophysis gland 2. CORTICOTROPES – produce adrenocorticotropin (ACTH) – 15 to 20% 3. LACTOTROPES – produce prolactin - 15 to 20% 4. THYROTROPES – produce thyroid-stimulating hormone (TSH) - 5% 5. GONADOTROPES – gonadotropic hormones (LH and FSH) – 10 to 15% Luteinizing hormone LH and Follicle-stimulating hormone FSH HYPOTHALAMIC - PITUITARY AXIS Cerebral hemisphere When regulating the endocrine system, the Diencephalon hypothalamus works in close connection with the  Pituitary gland (Hypophysis) HYPOTHALAMIC-PITUITARY AXIS H P Medulla oblongata Cerebellum https://www.researchgate.net/figure/A-schematicrepresentation-of-the-hypothalamic-nuclei-and-thedistribution-of-tanycytes_fig1_278794183 HYPOTHALAMIC-PITUITARY AXIS Hypothalamus and pituitary gland function in a coordinated way to affect the endocrine system regulating: THYROID GLANDS GROWTH ADRENAL GLANDS MILK PRODUCTION/EJECTION REPRODUCTIVE GLANDS OSMOREGULATION HYPOTHALAMIC - PITUITARY AXIS The HYPOTHALAMUS controls the anterior pituitary (adenohypophysis) secretion via:  Releasing hormones  Inhibiting hormones 1. Thyrotropin releasing hormone (TRH) 2. Corticotropin-releasing hormone (CRH) 3. Gonadotropin-releasing hormone (GnRH) 4. Growth hormone-releasing hormone (GHRH) 5. Growth hormone inhibiting hormone (GHIH /Somatostatin) 6. Prolactin inhibiting hormone (PIH/Dopamine) HYPOTHALAMIC - PITUITARY AXIS Special neurons in the hypothalamus synthesize and secrete the  Hypothalamic releasing and inhibiting hormones  Neurons originate in various parts of the hypothalamus (nuclei) The endings of these fibers are different from most nerve endings in the CNS  They secrete hormones into the ECF (extra cellular fluid)  Hormones are immediately absorbed into the: neurosecretory neurons Hypothalamic - Hypophysial Portal System → carry hormones directly to the sinuses of the Anterior Pituitary gland The hypothalamic-hypophyseal portal system Is a microcirculation system of blood vessels at the base of the brain, connecting Its main function is to quickly the hypothalamus with the anterior pituitary transport and exchange hormones between the hypothalamus and anterior pituitary gland The capillaries in the portal system are fenestrated (small pores with high vascular permeability) which allows a rapid exchange between the hypothalamus and the pituitary HYPOTHALAMIC-HYPOPHYSIAL PORTAL SYSTEM HYPOTHALAMUS Pituitary Gland ADENOHYPOPHYSIS NEUROHYPOPHYSIS (anterior PITUITARY) (posterior PITUITARY) ANTERIOR PITUITARY (ADENOHYPOPHYSIS) HORMONES ANTERIOR PITUITARY (ADENOHYPOPHYSIS) HORMONES The peptide hormones of the anterior pituitary play a major role in the control of metabolic functions throughout the body  GROWTH HORMONES (GH) – promote growth of the entire body by affecting protein formation, cell multiplication and cell differentiation  CORTICOTROPIN (adrenocorticotropic hormone ACTH) – controls secretion of some of the adrenocortical hormones, which affect metabolism of glucose, proteins and fats ANTERIOR PITUITARY (ADENOHYPOPHYSIS) HORMONES  THYROTROPIN (thyroid stimulating hormone TSH) controls the secretion rate of thyroxine (T4) and triiodothyronine (T3) by the thyroid gland, and these hormones control the rates of most intracellular chemical reactions in the body  PROLACTIN (PRL) – promotes mammary gland development and milk production  GONADOTROPINS (FSH and LH) – control growth of the ovaries and testes, as well as their hormonal and reproductive activities POSTERIOR PITUITARY (NEUROHYPOPHYSIS) The NEUROHYPOPHYSIS originates from a neural tissue outgrowth from the hypothalamus  Composed by glial-like cells and axons of large neurons called MAGNOCELLULAR NEURONS  Their cell bodies are located in the hypothalamus paraventricular and supraoptic nucleus  The hormones are produced in the hypothalamus and transported in the axoplasm to the posterior pituitary  The nerve endings contain many secretory granules and lie on the surface of capillaries where they secrete 2 hormones POSTERIOR PITUITARY (NEUROHYPOPHYSIS) 2 important peptide hormones are secreted by the neurohypophysis 1. ANTIDIURETIC HORMONE (ADH) or VASOPRESSIN o Controls the rate of water excretion into the urine, helping to control the concentration of water in the body fluids 2. OXYTOCIN o Released during copulation – mating behavior, parenting behavior/monogamy o Released during pregnancy and nursing. Effects on smooth muscles:  Helps with the delivery of the baby at the end of gestation  Helps express milk from the mammary gland to the nipples during suckling POSTERIOR PITUITARY ADH PRODUCTION AND RELEASE ADH IS PRODUCED IN THE NEURON’S CELL BODY AND TRANSPORTED THROUGH THE AXON USING A SPECIFIC PROTEIN CALLED NEUROPHYSIN PROCESSING OF THE PRECURSOR TO THE MATURE PEPTIDE TAKES PLACE AS THE SECRETORY GRANULES MOVE DOWN THE AXON ADH PRODUCTION & RELEASE VASOPRESSINERGIC NEURONS IN THE HYPOTHALAMUS RECEIVE INPUT FROM SEVERAL SOURCES IN RESPONSE, THE NEURON SYNTHESIZES THE PRECURSOR HORMONE WHICH IS PACKED FOR TRANSPORTATION TO THE NERVE ENDING THE AXON TERMINATES ON THE CAPILLARY WALLS WHERE HORMONE WILL BE RELEASED IN A CALCIUM DEPENDENT MANNER HORMONE AND ITS ASSOCIATED NEUROPHYSIN DIFFUSE THROUGH FENESTRATIONS IN THE CAPILLARY WALL INTO THE CAPILLARIES HYPOTHALAMUS + RELEASING HORMONE (and inhibiting hormone) - SHORT-LOOP HYPOPHYSIS POSITIVE FEEDBACK + TROPIC HORMONE TARGET GLAND TARGET HORMONE NEGATIVE FEEDBACK LONG-LOOP - [HORMONE]  CARRIED TO TARGET TISSUES BLOOD CIRCULATION