Lecture 19-Hypothalamus and Pituitary Gland PDF

Hypothalamus and Pituitary Gland • Dr. Kelly C. S. Roballo • [email protected] • VCOM-Main Building Room 341 Learning Objectives 1. Describe the structure of the hypothalamus, including the major hypothalamic nuclei and areas. 2. Describe the major functions of the hypothalamus and its nuclei/a...

Hypothalamus and Pituitary Gland • Dr. Kelly C. S. Roballo • [email protected] • VCOM-Main Building Room 341 Learning Objectives 1. Describe the structure of the hypothalamus, including the major hypothalamic nuclei and areas. 2. Describe the major functions of the hypothalamus and its nuclei/areas. 3. Describe the role and mechanisms of the hypothalamus as it relates to thirst, hunger, temperature regulation, and the defense mechanism. 4. Identify the hormones that are secreted by the anterior pituitary and the cell types that secrete them 5. Determinate the actions of the posterior pituitary hormones, and their receptors and how this controls their actions. Hypothalamus • The hypothalamus serves as an integrator of somatic, visceral, endocrine and immune functions Blumenfeld, Neuroanatomy The Anatomical (Nuclear) Divisions of the Hypothalamus Are Usually Described as Functional Groupings Thieme, Head & Neuroanatomy • The hypothalamus exerts CNS and systemic influences by two mechanisms: 1. Neural (local circuits, afferent and efferent projections) 2. Neurohumoral (i.e., hormonal, vascular, neuroendocrine functions, neuroimmune influences) The Hypothalamus Has Three Sites of Functional Influence Hypothalamus Anterior Pituitary Stress “Fight (aggressive behavior) or Flight (fear)” Sexual behavior and reproduction Growth Metabolism Posterior Pituitary Body fluid balance Maternal behavior Non-Pituitary including other regions of the hypothalamus nTemperature regulation nImmune system regulation nCircadian rhythm influences on hormonal fluctuations, sleep, etc. nAppetite regulation nAutonomic Nervous System Afferent Connections to the Hypothalamus • The fornix conveys afferent fibers from the hippocampus, it is an important fiber tract of the limbic system • The medial forebrain bundle transmits afferent fibers from the olfactory areas to the preoptic nuclei • The stria terminalis conveys afferent fibers from the amygdala • The peduncle of the mamillary bodies transmits visceral afferent fibers Efferent Connections from the Hypothalamus • The dorsal longitudinal fasciculus reaches to parasympathetic nuclei • The mamillotegmental tract distributes efferent fibers to the tegmentum of the midbrain; these are then relayed to the reticular formation. The fibers of this tract mediate the exchange of autonomic information between the hypothalamus, cranial nerve nuclei, and spinal cord • The mamillothalamic fasciculus connects to cingulate gyrus • Hypothalamic - hypophyseal tract communicates with pituitary gland hyperthermia Heat loss Heat conservation (satiety center) Inhibition of appetite • Anterior preoptic region detects increased body temperature and activates mechanisms of heat dissipation. Lesions can cause hyperthermia. In contrast, the posterior region functions to conserve heat. Bilateral lesions of the posterior hypothalamus usually cause poikilothermia, in which the body temperature varies with the environment. • The lateral part of anterior nuclei is important in appetite, and lateral hypothalamic lesion cause a decrease in body weight. Conversely, the medial part of anterior nuclei appears to be important in inhibiting appetite, and medial hypothalamic lesions can cause obesity. *Clinical correlation The Pituitary Gland Adenohypophysis Neurohypophysis Pituitary Vasculature and Hypothalamic Hormone Delivery (Neurosecretion) Portal veins branch into another series of capillaries within the anterior pituitary Capillaries in anterior and posterior pituitary coalesce into veins that drain into systemic venous blood. n Medial Preoptic Nucleus: Is characterized by sexual dimorphism involving hormonal influences on neuronal maturation Contains high density of gonadotropin-releasing hormone producing neurons which synthesize large peptides that are directed at target cells in the anterior pituitary n Anterior preoptic region: maintains constant body temperature n *Clinical correlation Ø *Clinical correlation Suprachiasmatic nucleus: receives direct retinal input; circadian rhythm Circadian Rhythmicity Pineal (melatonin) Via spinal cord and Sup. Cervical Ganglion retina Suprachiasmatic Nucleus (SCN, neurons Retinohypothalamic pathway with cyclic activity in firing rates) Other Hypothalamic Regions Production of TRH, CRH, GHRH, sleepwake cycle *Clinical correlation Other external Influences (sounds, temperature changes) Ø Paraventricular and Supraoptic Nuclei: ØNeurosecretory cells whose axons project to posterior pituitary ØParaventricular nucleus secrete oxytocin, ØSupraoptic nucleus secrete vasopressin. Posterior pituitary (Neurohypophysis) • Paraventricular and supraoptic nuclei contain neurons that produce oxytocin or vasopressin • Hormones travel down axon by axonal transport and are stored and released at terminals in the posterior pituitary • Hormones released into systemic circulation upon excitation • Systemic feedback to hypothalamus via CircumVentricular Organs (OVLT) Hypothalamic Regulation of Body Fluid Homeostasis “Watered Down” Version CircumVentricular Organs + (Osmoreceptive neurons in anterior hypothalamus) + Paraventricular Nuc. (PVN) Supraoptic Nuc. (SON) HypothalamoHypophyseal Tract Plasma Osmolality (e.g., [NaCl], [K+], [urea], [glucose]) increases > 280-303 mosmoles/kg (i.e., hyperosmolality) Activated by neurons in OVLT Increased release of vasopressin (ADH) Renal receptors H2O and electrolyte reabsorption from urine (restoration of normal plasma osmolality) Ø Dorsomedial and Ventromedial Nuclei.: ØNeurons involved in feeding, reproductive, and parenting behavior, thermoregulation Ø Anterior nuclei: regulate appetite and food intake. Medial part lesion causes obesity, lateral part lesion causes anorexia Ø Ø Posterior Hypothalamic Region : Temperature regulation function has been assigned. Responds to temperature changes, such as sweating. Lesion causes hypothermia Ø Also, arousal, shift of attention; especially as a function of behavioral states mediated by hypothalamus Ø Mammillary body: involved in limbic forebrain and midbrain functional integration Neuroepithelial cell proliferation • Inflammatory response-associated chemicals (i.e., cytokines) which may influence thyroid function during severe infection (i.e., sepsis) through systemic feedback to hypothalamus Negative Feedback Controls: Long & Short Loop Reflexes Endocrine Control by Hypothalamus: Three Levels of Integration That is what was on the table but in a different way. Lesions In Certain Cortical Regions Can Be Associated with Severe Autonomic Dysfunctions n n Limbic system n Amygdala n Cingulate gyrus n Lateral septal nuclei n Insular cortex, others Cortical influences on hypothalamic output leading to altered behavioral states n Emotion, pain n “Drives”: hunger, thirst, sex n Fight or flight n Forms of anxiety Circumventricular organs mediate peripheral hormone release and “afferent” humoral feedback Subcommissural organ (glycoprotein secretion into CSF) Subfornical organ (chemoreceptive to Angiotensin II, regulates water balance in body) Pineal gland (regulates circadian rhythms via melatonin release) Organum Vaculosum Lamina Terminalis (OVLT) Neurohypophysis (secretion of vasopressin, oxytocin, etc. from pituitary) Median Eminence (regulates anterior pituitary) Area Postrema (chemosensitive, emetic) Systemic Regulatory Feedback to Hypothalamus Cells with receptors to circulating angiotensin II that influence thirst Hyperosmolality and ADH release Formation of Angiotensin II Several systemic actions Decreased plasma volume 1. Via breaches in the blood-brain barrier Kidney renin Systemic Regulatory Feedback to Hypothalamus dehydration, hemorrhage 5-10% peripheral detection IncreasesVasopressin release and thirst Paraventricular Supraoptic Nc. 2. 3. Decreased Blood Pressure, Blood Volume, Cardiac Output Noradrenergic pathways Baroreceptors in carotid and aortic arches, stretch receptors in left atrium, pulmonary veins, etc. impulse propagation via CN IX, X (GLOSSOPHARINGEAL AND VAGUS) Medulla and Mesencephalon Via peripheral pressure (baro-) receptors and central pathways (e.g., hypovolemia (decrease in blood volume) Via lipid-soluble gonadal and adrenal steroids Feedback Loops (Positive or Negative) and the Hypothalamus Pituitary hormone acting on releasing- or inhibiting factor from hypothalamus Pituitary hormone acting on its own secretion by an autocrine mechanism (i.e., cell has receptors for what it releases so it feeds back on itself) blood-borne (e.g., gonadal steroids, adrenal corticoids) STRESS Corticotropinreleasing hormones in PVN which project axons to med. eminence Many CNS regions Fear, anxiety, disease Long-loop negative feedback CRH stimulates corticotrophs in Anterior pituitary corticotrophs Adrenaline ACTH + Heart rate Respiration Flight and Fight glucocorticoids Glucocorticoid synth. and release Ø Anterior nuclei: regulate appetite and food intake. Medial part lesion causes obesity, lateral part lesion causes anorexia Neuroendocrine control of lactation Auditory and visual cues from baby (crying) Infant suckling spinal cord & Brainstem pathways Paraventricular Nucleus (Stress can inhibit oxytocin Secretion) Supraoptic Nucleus Mammilary Body Inferior Hypophyseal Artery Oxytocin contracts the myoepithelial cells lining the ducts of the breast causing milk “let down”. Also potentiates contraction of the uterine smooth muscle during labor. Superior Hypophyseal Artery Neurohypophysis Adenohypophysis NOTE: Prolactin from the anterior pituitary stimulates milk production and ductal outgrowth Neuroendocrine Control of Reproduction n Gonadotropin-releasing hormone (GnRH) regulates luteinizing hormone (LH) and folliclestimulating hormone (FSH) secretion by gonadotropes in anterior pituitary n GnRH-producing cells prominent in medial preoptic area n GnRH release is subject to feedback loops which in females has both negative and positive components n Breast-feeding and effect on GnRH/FSH-LH activity (ant. – post. pituitary interaction)