The Endocrine System

Includes all cells and endocrine tissues that produce hormones or paracrine factors Endocrine system

Nervous system performs short term crisis management Endocrine system regulates long term ongoing metabolic Endocrine communication is carried out by endocrine cells releasing hormones –Alter metabolic activities of tissues and organs –Target cells Paracrine communication involves chemical messengers between cells within one tissue Endocrine vs Nervous System

Endocrine reflexes are the counterparts of neural reflexes Hypothalamus regulates the activity of the nervous and endocrine systems –Secreting regulatory hormones that control the anterior pituitary gland –Releasing hormones at the posterior pituitary gland –Exerts direct neural control over the endocrine cells of the adrenal medullae Control of Endocrine Activity

Figure 18.5 Three Methods of Hypothalamic Control over the Endocrine System

Hypothalamic Control of Adenohypophysis Hypothalamus regulates secretion of hormones Secretes releasing factors to release hormones Secretes inhibiting hormones to turn off secretion of hormones

The Hypothalamus The hypothalamus sends a chemical stimulus to the anterior pituitary to releasing hormones stimulate the synthesis and release of hormones –TRH - Thyrotropin releasing hormone (TRH) >> release of TSH –CRH - Corticotropin releasing hormone (CRH) >> release of ACTH –GnRH - Gonadotropin releasing hormone (GNRH) >> release of FSH and LH –GHRH – Growth hormone releasing hormone >> release of GH

Hypothalamic Stimulation–from CNS

The Pituitary Gland - Hypophysis Figure 25.3a–c Attached to the hypothalamus by the infundibulum Two basic divisions of the pituitary gland –Anterior pituitary or Adenohypophysis –Posterior pituitary or Neurohypophysis

Releases nine important peptide hormones All nine bind to membrane receptors and use cyclic AMP as a second messenger The Pituitary Gland - Hypophysis

Anterior Pituitary Pars distalis - largest division of the adenohypophysis –Contains five different types of endocrine cells Somatotropic cells -secrete growth hormone (GH) Mammotropic cells - secrete prolactin (PRL) Thyrotropic cells - secrete thyroid-stimulating hormone (TSH) Corticotropic cells - secrete adrenocorticotropic hormone (ACTH) and melanocyte-stimulating hormone (MSH) Gonadotropic cells - secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH) –Tropic hormones TSH, ACTH, FSH, and LH Regulate the secretion of hormones by other endocrine glands

Thyrotropin releasing hormone promotes the release of TSH Thyroid stimulating hormone (TSH) triggers the release of thyroid hormones Corticotropin releasing hormone causes the secretion of ACTH Adrenocorticotropic hormone (ACTH) stimulates the release of glucocorticoids by the adrenal gland Hormones of the Adenohypophysis

Follicle stimulating hormone (FSH) - stimulates follicle development and estrogen secretion in females and sperm production in males Luteinizing hormone (LH) causes ovulation and progestin production in females and androgen production in males Gonadotropin releasing hormone (GNRH) promotes the secretion of FSH and LH Hormones of the Adenohypophysis

Prolactin (PH) –stimulates the development of mammary glands and milk production Growth hormone (GH or somatotropin) - stimulates cell growth and replication Melanocyte stimulating hormone (MSH) -stimulates melanocytes to produce melanin Hormones of the Adenohypophysis

Posterior Pituitary Structurally part of the brain Contains axons of hypothalamic nerves Manufactures antidiuretic hormone (ADH) –Decreases the amount of water lost at the kidneys –Elevates blood pressure Manufactures oxytocin –Stimulates contractile cells in mammary glands –Stimulates smooth muscle cells in uterus Figure 25.5

Table 25.1

Figure 18.8a Feedback Control of Endocrine Secretion

Negative Feedback Controls: Long & Short Loop Reflexes Figure 7-14: Negative feedback loops in the hypothalamicanterior pituitary pathway

Negative Feedback Controls: Long & Short Loop Reflexes Figure 7-15: Control pathway for cortisol secretion

Lies near the thyroid cartilage of the larynx Thyroid

Thyroid gland contains numerous follicles –Release several hormones such as thyroxine (T4) and triiodothyronine (T3) that regulate metabolism increases protein synthesis promotes glycolysis, gluconeogenesis, glucose uptake –C cells produce calcitonin - helps regulate calcium concentration in body fluids Thyroid Follicles and Thyroid Hormones

The Thyroid Follicles Figure 18.12b

Held in storage Bound to mitochondria, thereby increasing ATP production Bound to receptors activating genes that control energy utilization Exert a calorigenic effect C cells produce calcitonin - helps regulate calcium concentration in body fluids Thyroid hormones

Embedded in the posterior surface of the thyroid gland Chief cells produce parathyroid hormone (PTH) in response to lower than normal calcium concentrations Parathyroid hormones are regulators of calcium levels in healthy adults Four parathyroid glands

Figure Homeostatic Regulation of Calcium Ion Concentrations

Adrenal Cortex Synthesizes and releases steroid hormones called corticosteroids Different corticosteroids are produced in each of the three layers –Zona glomerulosa – mineralocorticoids (chiefly aldosterone) –Zona fasciculata – glucocorticoids (chiefly cortisol) –Zona reticularis – gonadocorticoids (chiefly androgens)

Adrenal Cortex Secretes over 30 different steroid hormones (corticosteroids) –Mineralocorticoids Aldosterone: maintains electrolyte balance –Glucocorticoids Cortisol: –Stimulates gluconeogenesis –Mobilization of free fatty acids –Glucose sparing –Anti-inflammatory agent –Gonadocorticoids testosterone, estrogen, progesterone

Mineralocorticoids Regulate the electrolyte concentrations of extracellular fluids Aldosterone – most important mineralocorticoid –Maintains Na+ balance by reducing excretion of sodium from the body –Stimulates reabsorption of Na+ by the kidneys Aldosterone secretion is stimulated by: –Rising blood levels of K+ –Low blood Na+ –Decreasing blood volume or pressure

Glucocorticoids (Cortisol) Help the body resist stress by: –Keeping blood sugar levels relatively constant –Maintaining blood volume and preventing water shift into tissue Cortisol provokes: –Gluconeogenesis (formation of glucose from non- carbohydrates) –Rises in blood glucose, fatty acids, and amino acids

Gonadocorticoids (Sex Hormones) Most gonadocorticoids secreted are androgens (male sex hormones), and the most important one is testosterone Androgens contribute to: –The onset of puberty –The appearance of secondary sex characteristics –Sex drive in females Androgens can be converted into estrogens after menopause

Clusters of endocrine cells within the pancreas called Islets of Langerhans or pancreatic islets –Alpha cells secrete glucagons –Beta cells secrete insulin –Delta cells secrete GH-IH Pancreatic Islets

Insulin lowers blood glucose by increasing the rate of glucose uptake and utilization Glucagon raises blood glucose by increasing the rates of glycogen breakdown and glucose manufacture by the liver Insulin and glucagon

Figure Regulation of Blood Glucose Concentrations