Introduction to endocrinology Dr. Shafali Singh Coordinated functions Control system Nervous system Endocrinal system Control systems vary in their speed & specificity – Specificity • Nervous control very specific • Hormonal control more general – Nature of signal • Nervous system uses both electrical and chemical • Endocrine system uses only chemicals – Speed • Nervous system much faster, up to 120 m/sec • Endocrine system much slower – Duration of action • Nervous system shorter, very brief • Endocrine system lasts longer Chemical messenger system • Coordinating multiple activities of the cell, tissues and organs of the body. • Maintain homeostasis • Regulating almost all body functions. hormone "to set in motion," Classic definition- hormones are secretory product of the ductless glands, which are released in catalytic amount into blood stream and transported to specific target cells(or organs),where they elicit physiological, morphological and biochemical responses. What is the Endocrine System? • Mutiple hormone system controls numerous body processes • growth and development • metabolism • reproduction • behavior • Water and electrolyte balance • homeostasis Regulating all most all body functions •Numerous organs and tissues located throughout the body • Works in conjunction with the nervous system Classification of hormones • Proteins and polypeptides, • Steroids • Derivatives of the amino acid tyrosine, Regulation of hormone secretion 1. Feed back control 2. Neural control 3. Chronotrophic control 1.Feedback control (i)Negative feedback control Gland - • (ii) Positive feedback control (+) (-) Hormone Gland Hormone Target Target Product Product Regulation of hormone secretion Depending upon the product involved ,the feedback mechanism may be Hormone hormone feed back control Substrate hormone feed back control Hormone Hormone feedback control Substrate Hormone feedback control • A rise in blood glucose level promotes the secretion of insulin, while a fall in blood glucose promotes the secretion of glucagon 2. Neural control • Act to evoke or suppress hormone secretion in response to both external and internal stimuli. • External stimulivisual, auditory, olfactory, gustatory and tactile • Internal stimulipain, emotion, stress, fright, change in blood volume 3. Chronotropic control • Oscillating and pulsatile release of certain hormones • Diurnal variation in hormonal levels • Menstural rhythm • Seasonal rhythm • Developmental rhythm A circadian pacemaker directs numerous endocrine and body functions, each with its own daily profile. The nighttime rise in plasma melatonin may mediate certain other circadian patterns Hypothalamus and Pituitary Objectives • • • • • After the end of this lecture a student should be able to Name the hypothalamic hormones that regulate anterior pituitary function. Explain hypothalamo – hypophyseal portal system Name the hormones that are released from anterior pituitary Name the hormones that are released from posterior pituitary Explain the functions of pituitary hormones Hypothalamus and Pituitary • The hypothalamus-pituitary unit is the most dominant portion of the entire endocrine system. • The output of the hypothalamus-pituitary unit regulates the function of the thyroid, adrenal and reproductive glands and also controls somatic growth, lactation, milk secretion and water metabolism. • Pituitary function depends on the hypothalamus and the anatomical organization of the hypothalamus-pituitary unit reflects this relationship. neocortex Reituclar activating substance Sleep/ wake Thalamus Limbic system pain Emotion, fright, rage, smell Heat regulation (temperature) Water balance (blood volume, intake--thirst, output—urine volume) Energy regulation (hunger, BMI) Optical system vision Autonomic regulation (blood pressure etc) Regulation of Hypothalamus Metabolic rate, stress response, growth, reproduction, lactation) posterior pituitary hormones Anterior pituitary hormones Releasing Hormones • secreted by hypothalamic neurons and transported to the anterior pituitary by the hypothalamic-hypophyseal portal system • function as trophic hormones to either stimulate or inhibit release of anterior pituitary hormones Hypothalamic releasing hormones Hypothalamic releasing hormone Effect on pituitary Corticotropin releasing hormone (CRH) Stimulates ACTH secretion Thyrotropin releasing hormone (TRH) Stimulates TSH and Prolactin secretion Stimulates GH secretion Growth hormone releasing hormone (GHRH) Somatostatin Gonadotropin releasing hormone (GnRH) a.k.a LHRH Prolactin inhibiting hormone (dopamine) Inhibits GH (and other hormone) secretion Stimulates LH and FSH secretion Inhibits PRL secretion endocrine axes have the following important features 1. The activity of a specific axis is normally maintained at a set point, which varies from individual to individual, usually within a normal range. 2. Hypothalamic hypophysiotropic neurons are often secreted in a pulsatile manner and are entrained to daily and seasonal rhythms through CNS input 3. Abnormally low or high levels of a peripheral hormone (e.g., thyroid hormone) may be due to a defect at the level of the peripheral endocrine gland (e.g., thyroid), the pituitary gland, or the hypothalamus. Such lesions are referred to as primary, secondary, and tertiary endocrine disorders, respectively. Structure of the Pituitary Gland Two distinct lobes: • anterior pituitary (adenohypophysis; pars distalis) - true endocrine tissue - secretes classic hormones • posterior pituitary (neurohypophysis; pars nervosa) - neural tissue - secretes neurohormones Anterior pituitary cells and hormones Cell type Pituitary Product population Corticotroph 15-20% Thyrotroph 3-5% Target Adrenal gland ACTH b-lipotropin Adipocytes Melanocytes TSH Thyroid gland Gonadotroph 10-15% LH, FSH Gonads Somatotroph 40-50% GH Lactotroph 10-15% PRL All tissues, liver Breasts Adrenocorticotropic hormone (ACTH; also called corticotropin) The Lactotrope • The lactotrope is not part of an endocrine axis. This means that PRL acts directly on nonendocrine cells (primarily of the breast) to induce physiological changes. • Production and secretion of PRL are predominantly under inhibitory control by the hypothalamus. • Thus, disruption of the pituitary stalk and the hypothalamohypophysial portal vessels (e.g., secondary to surgery or physical trauma) results in an increase in PRL levels but a decrease in ACTH, TSH, FSH, LH, and GH. Summary of Hypothalamic-anterior-pituitary system Hypothalamus GnRH FSH and LH Gonads Germ cell development Hormones-estrogen, progesterone and testosterone GHRH SS TRH Anterior Pituitary GROWTH TSH HORMONE Liver Other organs IGF-1 Growth Metabolism Thyroid Thyroxine T3 PIH PROLACTIN Breasts Development Milk CRH ACTH Adrena l cortex Cortisol Action of Growth Hormone 1. stimulates growth of bone, cartilage & connective tissue through somatomedins & other growth factors. 2. Growth Hormone Promotes Protein deposition in Tissues. 3. Growth Hormone Enhances Fat Utilization for Energy. 4. Growth Hormone Decreases Carbohydrate Utilization Hypothalamic Control of Posterior Pituitary • Hypothalamic neuron cell bodies produce: ADH: supraoptic nuclei. Oxytocin: para ventricular nuclei. • Transported along hypothalamo-hypophyseal tract. • Stored in posterior pituitary. • Release controlled by neuroendocrine reflexes. ACTIONS OF ADH 1.ACTION ON KIDNEY Maintenance of ECF volume & Osmolarity Acts on DCT and CD of kidney Reabsorbs water Maintenance of volume more important that maintenance of osmolarity 2.VASOCONSTRICTOR EFFECT 3.ACTION ON ANTERIOR PITUITARY-cause increased ACTH secretion from the corticotroph Actions of oxytocin In females: Milk ejection Contraction of uterus during labor In females: during coitus causes uterine contraction and transport of sperms In Males: Increases at time of ejaculation May cause increase contraction of smooth muscle of vas deferens - propelling sperm through urethra IN THE CLINIC-- hypophysectomy (pituitary removal) • Because posterior pituitary hormones are synthesized in the hypothalamus rather than the pituitary, hypophysectomy (pituitary removal) does not necessarily permanently disrupt synthesis and secretion of these hormones. • Immediately after hypophysectomy, secretion of the hormones decreases. • However, over a period of weeks, the severed proximal end of the tract will show histological modification and pituicytes will form around the neuron terminals. Secretory vacuoles are seen, and secretion of hormone resumes from this proximal end. Secretion of hormone can even potentially return to normal levels. • In contrast, a lesion higher up on the pituitary stalk can lead to loss of neuronal cell bodies in the PVN and SON.