Biology 218 – Human Anatomy RIDDELL
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Biology 218 – Human Anatomy RIDDELL Chapter 23 Adapted form Tortora 10th ed. LECTURE OUTLINE A. Comparison of Nervous and Endocrine Systems (see Table 23.1): (p. 704) 1. The nervous and endocrine systems together coordinate functions of all body systems. 2. The nervous system controls homeostasis via nerve impulses that trigger release of neurotransmitter molecules which act on other neurons, muscle cells, or gland cells. 3. The endocrine system controls homeostasis by secreting hormones, i.e., messenger molecules, into the bloodstream for delivery to virtually all body cells. 4. The nervous and endocrine systems act as a coordinated supersystem called the neuroendocrine system: some neurons stimulate or inhibit the release of hormones and some hormones promote or inhibit the initiation of nerve impulses. 5. The nervous system causes muscles to contract and glands to secrete either more or less of their product; the endocrine system affects virtually all body tissues by altering metabolic activities, regulating growth and development, and influencing reproductive processes. 6. Nerve impulses typically produce their effects within several milliseconds and the effects are relatively brief in duration; hormones may take seconds to hours to produce their effects and these effects are generally longer in duration. 7. Endocrinology is the science concerned with the structure and functions of the endocrine glands and the diagnosis and treatment of disorders of the endocrine system. B. Endocrine Glands Defined (p. 705) 1. The body contains two types of glands: i. exocrine glands (e.g., sudoriferous, sebaceous, and mucous glands) secrete their products into ducts which deliver the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body ii. endocrine glands secrete their products, called hormones, into the surrounding interstitial fluid from which they diffuse into capillaries to be carried away by the blood 2. The endocrine glands (e.g., pituitary, thyroid, and adrenal glands) collectively form the endocrine system; there are several organs of the body (e.g., hypothalamus, thymus, pancreas, stomach, skin, heart, etc.) that contain endocrine tissue but are not endocrine glands exclusively. C. Hormones (p. 705) 1. Although hormones are delivered by the bloodstream throughout the body, hormones affect only certain target cells. 2. Target cells for a particular hormone are those cells that have the appropriate receptor molecules (proteins or glycoproteins) that can bind to the hormone. 3. The number of receptor molecules in a target cell may increase or decrease in order to increase or decrease, respectively, the sensitivity of that target cell to a particular hormone. 4. The amount of hormone secreted by endocrine cells, usually in short bursts, is regulated to maintain homeostasis and prevent overproduction or underproduction of that hormone. D. Hypothalamus and Pituitary Gland (p. 707) 1. The hypothalamus is the major integrating link between the nervous and endocrine systems. Page 1 of 6 Biology 218_Lecture Outline_23 Endocrine System Biology 218 – Human Anatomy RIDDELL 2. The hypothalamus receives input from numerous sources and, in addition to many other functions, exerts control over the pituitary gland and is itself a crucial endocrine gland; the hypothalamus and pituitary gland together play important roles in regulation of virtually all aspects of growth, development, metabolism, and homeostasis. 3. The pituitary gland or hypophysis: i. is about 1-1.5 cm in diameter and lies in the sella turcica of the sphenoid bone ii. is attached to the hypothalamus by the infundibulum iii. has two anatomically and functionally separate portions: a. anterior pituitary gland (anterior lobe) accounts for about 75% of the total weight of the gland and contains many glandular epithelial cells that form the glandular part of the pituitary gland; in the adult, it consists of the pars distalis and the pars tuberalis b. posterior pituitary gland (posterior lobe), consisting of the pars nervosa and the infundibulum, contains axons and axon terminals of neurons whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus - the axon terminals in the posterior pituitary gland are associated with specialized neuroglia called pituicytes iv. has a third region called the pars intermedia that atrophies during fetal development 4. Anterior Pituitary Gland (Anterior Lobe) or Adenohypophysis: i. Secretion of anterior pituitary hormones is regulated by releasing hormones and inhibiting hormones that are delivered from the neurosecretory cells of the hypothalamus by a hypophyseal portal system of blood vessels: a. superior hypophyseal arteries that form the primary plexus at the base of the hypothalamus b. hypophyseal portal veins that pass down outside of the infundibulum to form a secondary plexus in the anterior pituitary; anterior pituitary hormones are carried away by the anterior hypophyseal veins for distribution to target tissues throughout the body ii. Five types of anterior pituitary cells secrete seven major hormones: a. somatotrophs secrete human growth hormone (hGH) or somatotropin which stimulates several tissues to secrete insulinlike growth factors, hormones that stimulate general body growth and regulates aspects of metabolism b. thyrotrophs secrete thyroid-stimulating hormone (TSH) or thyrotropin which regulates activities of the thyroid gland c. gonadotrophs secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH) which affect activities of the ovaries and testes d. lactotrophs secrete prolactin (PRL) which stimulates milk production e. corticotrophs secrete adrenocorticotropic hormone (ACTH) or corticotropin which stimulates the adrenal cortex to secrete glucocorticoids; some corticotrophs also secrete melanocyte-stimulating hormone (MSH) which is thought to affect brain activity and skin pigmentation f. the 5 different types of secretory cells can be classified according to their staining reactions into basophils, acidophils and chromophobes. iii. Some of the anterior pituitary hormones are tropic hormones or tropins, i.e., hormones that influence another endocrine gland. iv. Table 23.2 provides a summary of anterior pituitary gland hormones and their actions. 5. Posterior Pituitary Gland (Posterior Lobe) or Neurohypophysis: Page 2 of 6 Biology 218_Lecture Outline_23 Endocrine System Biology 218 – Human Anatomy RIDDELL i. The posterior pituitary does not synthesize hormones but it does store and release two hormones synthesized in the hypothalamus. ii. The posterior pituitary consists of: a. pituicytes b. axon terminals of hypothalamic neurosecretory cells whose cell bodies are located in the paraventricular and supraoptic nuclei of the hypothalamus - axons of these cells form the hypothalamohypophyseal tract which extends from the hypothalamus to the posterior pituitary - the cell bodies of these cells produce: 1. oxytocin (OT) which enhances uterine contractions during labor and stimulates milk ejection 2. antidiuretic hormone (ADH), also called vasopressin, which decreases urine volume and increases blood pressure - these hormones are delivered in secretory vesicles to and stored in the axon terminals until nerve impulses that are delivered to the axon terminals trigger exocytosis to release the hormones into nearby capillaries iii. Blood is supplied to the posterior gland by inferior hypophyseal arteries that drain into the capillary plexus of the infundibular process; from this plexus, hormones pass into the posterior hypophyseal veins for delivery to target cells in other tissues. iv. Table 23.3 provides a summary of posterior pituitary gland hormones and their actions. E. Thyroid Gland (p. 711) 1. The butterfly-shaped thyroid gland is located just below the larynx. 2. It has right and left lateral lobes, located on either side of the trachea, that are connected by an isthmus that lies in front of the upper end of the trachea; a small pyramidal-shaped lobe sometimes extends upward from the isthmus. 3. It contains thyroid follicles; the wall of each follicle consists of two types of cells: i. numerous follicular cells which secrete: a. thyroxine or tetraiodothyronine or T4 b. triiodothyronine or T3 - these two hormones are collectively called the thyroid hormones - these hormones regulate the rate of metabolism, growth, and development ii. less numerous parafollicular cells or C cells which secrete calcitonin - calcitonin influences calcium homeostasis by decreasing blood calcium concentration 4. The thyroid gland receives its blood supply from the superior and inferior thyroid arteries; blood is drained by the superior, middle, and inferior thyroid veins. 5. The thyroid gland is innervated by postganglionic fibers from the superior and middle cervical sympathetic ganglia; the preganglionic fibers of these ganglia emerge from the second through seventh thoracic segments of the spinal cord. 6. Table 23.4 provides a summary of thyroid gland hormones and their actions. F. Parathyroid Glands (p. 713) 1. There is usually one superior and one inferior parathyroid gland attached to the posterior surface of each lateral lobe of the thyroid gland, i.e., usually 4 in total. 2. The parathyroid glands contain two types of cells: i. numerous principal cells which secrete parathyroid hormone (PTH) or parathormone Page 3 of 6 Biology 218_Lecture Outline_23 Endocrine System Biology 218 – Human Anatomy RIDDELL - this hormone increases blood calcium and magnesium