Pituitary Gland Dr. Amel Eassawi

Objectives The student should be able to: Explain the hypothalamus as the major integrative site for the neuroendocrine system. Contrast the anterior and posterior pituitary lobes with respect to cell types, vascular supply, development, and innervations. Describe the general characteristics of hypothalamic releasing and inhibiting (hypophysiotropic) hormones and describe their route of transport from the hypothalamus to the anterior pituitary. Identify appropriate hypothalamic hormones that control the secretion of each of the anterior pituitary hormones. Diagram short-loop and long-loop negative feedback control of anterior pituitary hormone secretion. List the target tissues for oxytocin and describe its effects on each. Name the stimuli for oxytocin release during parturition or lactation. Describe the stimuli and mechanisms that control vasopressin secretion.

Endocrine Control: Three Levels of Integration Hypothalamus Pituitary stimulation–from hypothalamic trophic hormones Endocrine gland stimulation–from pituitary trophic hormones

Pituitary gland Hypophysis Regulating the activity of most the other endocrine glands(so called master gland). Consists of: Neurohypophysis – posterior lobe (neural tissue) Receives, stores, and releases hormones from the hypothalamus. Adenohypophysis – anterior lobe, made up of glandular tissue Synthesizes and secretes a number of hormones 5

Anterior pituitary gland Anterior Pituitary: 6 hormones Growth hormone (GH) Thyroid stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH) Follicle stimulating hormone (FSH) Luteinizing hormone (LH) Prolactin (PL)

Control of anterior pituitary secretion The Hypothalamic Control: Through the hypothalamic releasing and inhibitory factors secreted by hypothalamus and carried to the anterior pituitary through the hypothalamic hypophyseal portal circulation Feed Back Control: blood levels of adrenocortical hormones, thyroid hormones and gonad steroid influence the pituitary secretion of ACTH, TSH and gonadotropins respectively. The influence in mainly inhibitory in nature ( negative feedback mechanism ) 10

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Negative feedback loops in the hypothalamicanterior pituitary pathway Negative Feedback Controls: Long and Short Loop Reflexes Short long Negative feedback loops in the hypothalamicanterior pituitary pathway

Control pathway for cortisol secretion

Negative Feedback Loops Regulate amount of hormones in blood Hormone levels stay within range needed for appropriate responses Eliminates waste of hormones

Hypothalamic-hypophyseal Portal system

Posterior pituitary gland Release of hormones from both anterior and posterior pituitary is controlled by hypothalamus Posterior pituitary Along with hypothalamus forms neuroendocrine system Does not actually produce any hormones Stores and releases two small peptide hormones Vasopressin (ADH) Conserves water during urine formation Oxytocin Stimulates uterine contraction during childbirth and milk ejection during breast-feeding

Neurohormones: secreted into the Blood by Neurons

The hormone, vasopressin and oxytocin are synthesized in supraoptic and paraventricular nuclei in the hypothalamus. The hormone travels down the axon to be stored in the neuronal terminals within the posterior pituitary. On excitation of the neuron, the stored hormone is released from these terminals into the systemic blood for distribution throughout the body. Relationship of the Hypothalamus and Posterior Pituitary gland

Posterior pituitary hormones 1. Antidiuretic hormone (ADH, vasppressin) chemistry: 9 amino acid peptide, produced primarily by supraoptic nucleus and small amount by paraventricular nucleus actions: 1) water retention by the kidney  urine volume and  ECF 2) vasoconstriction (in large amounts) regulation/stimuli: 1) blood (or ECF) osmolality/osmoreceptors 2) blood volume 3) others: alcohol, nicotine, barbiturates, etc. abnormality: Diabetes Insipidus

Vasopressin Secretion

Diabetes Insipidus Diabetes insipidus is a disorder of the posterior lobe of the pituitary gland. ADH is deficient. Causes could be head trauma, brain tumor, or irradiation of the pituitary gland, infections of the CNS (meningitis, encephalitis, tuberculosis) or tumors (eg, metastatic disease, lymphoma of the breast or lung). Another cause is failure of the renal tubules to respond to ADH; this may be related to hypokalemia,hypercalcemia, and a variety of medications (eg, lithium,Declomycin).

Clinical Manifestations Enormous daily output of very dilute, water-like urine. Intense thirst: The patient drinks 2 to 20 liters of fluid daily. Fluid intake cannot be limited; otherwise the patient will experience crave for fluid and would develop hypernatremia and severe dehydration.

Assessment and Diagnostic Finding Fluid deprivation test for 8 to 12 hours. The patient is weighed frequently during the test. Perform plasma and urine osmolality studies before and after the test. Low specific gravity and weight loss, rising serum osmolality, and elevated serum sodium levels. Terminate the test if tachycardia, excessive weight loss, or hypotension develops. Plasma levels of ADH (vasopressin) and plasma and urine osmolality.

Medical Management Aims to: Replace ADH Ensure adequate fluid replacement Identify and correct the underlying cause

Posterior pituitary hormones 2. Oxytocin Chemistry: 9 amino acid peptide, produced primarily by paraventricular nucleus and small amount by supraoptic nucleus actions: 1) regulating breast milk release 2) contraction of pregnant uterus regulation/stimuli: suckling by a nursing infant crying sounds from a baby fear and stress inhibit release

Regulation of Secretion of Oxytocin through Positive Feedback Mechanism Uterine Contractions Force the Baby's Head into the Cervix Stretch Receptors in Cervix (Detector) + Nerve Impulses Hypothalamus and Posterior Pituitary (Control Center) Stimulation Release of Oxytocin Uterine Smooth Muscle Contraction (effector) Stronger Contractions Force the Baby's Head Further into the Cervix Inhibition Occurs After Delivery When is no Uterine Contractions

References Human physiology, Lauralee Sherwood, seventh edition. Text book physiology by Guyton &Hall,11th edition. Physiology by Berne and Levy, sixth edition.