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CHAPTER 16: THE ENDOCRINE SYSTEM

The endocrine system works directly with the nervous system to provide homeostasis. The NS works electrically and the endocrine system works chemically. They form an interlocking super system for regulation of homeostasis. The two systems connect at the hypothalamus (part of the CNS) and pituitary (an endocrine gland). TWO TYPES OF GLANDS IN THE HUMAN BODY Exocrine glands have ducts and carry their products to a surface via ducts. Endocrine gland lack ducts and release their products (hormones) directly into tissue spaces or capillaries. The endocrine system affects body activities by releasing hormones (chemical messengers) directly into the bloodstream or indirectly into the bloodstream via distribution in the tissue spaces. Hormones are part of a system of intercellular communication, which includes Direct communication by use of structures such as gap junctions. Limited to adjacent cells. Paracrine secretions into extracellular fluid. Paracrines are hormone-like chemicals that usually act locally not systematically. An example is the lipid group called the prostaglandins. Endocrine secretions are released throughout the circulatory system. They affect only target cells, which are cells that have appropriate receptors on membrane or internally. Synaptic communication releases neurotransmitters across synaptic clefts. Limited range requiring the assistance of neurotransmitters, such as ACh or serotonin. Target cells must have appropriate receptors. CHARACTERISTICS OF HORMONES Hormones can be divided into three groups based upon their chemical structure: (1) amino acid derivatives, (2) peptides and (3) lipid derivatives. CHEMISTRY OF HORMONES Some hormones are water-soluble, others are lipid soluble. MECHANISMS OF HORMONE ACTION There are two mechanisms (1) direct activation of genes and (2) second-messenger systems. ENDOCRINE GLANDS AND THEIR HORMONES (1) The Pituitary (Hypophysis) The pituitary lies in sella turcica of the sphenoid bone. The pituitary was called the "master gland" of endocrine system until the hormonal role of the hypothalamus was revealed. The pituitary has a direct connection with the CNS via the hypothalamus. The gland is attached to the hypothalamus by a stalk called the infundibulum. The communication between them is both electrical and chemical. The hypothalamus produces and releases chemicals called releasing factors or releasing hormones (a/k/a RF's or RH's) that stimulate the pituitary to make and release specific hormones as well as inhibiting factors. The pituitary can be described either as two lobes (anterior and posterior) or three regions: the pars distalis (which includes a subsection called the pars tuberalis), pars intermedia and pars nervosa. The pars distalis and pars tuberalis are regions of the anterior pituitary, the pars intermedia wraps around the infundibulum and the pars nervosa is the posterior lobe of the pituitary.

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Anterior Lobe (Adenohypophysis) This lobe connects to posterior lobe via pars intermedia. All except GH, MSH and PRL are tropic a/k/a trophic, meaning they stimulate other endocrine glands. The anterior lobe produces: growth hormone, prolactin, thyroid stimulating hormone, follicle-stimulating hormone, lutenizing hormone, adrenocorticotropic hormone and melanocyte stimulating hormone. Posterior Lobe (Neurohypophysis/Pars nervosa) This lobe consists of cells called pituicytes, which are similar to neuroglia. Not really an endocrine gland because it does not synthesize hormones. The hormones associated with the posterior pituitary are actually produced in the neurosecretory (glial) cells of the hypothalamus and then passed into the special pituicytes of the posterior pituitary. Hormones released from the posterior pituitary include oxytocin and antidiuretic hormone. (2) The Thyroid The thyroid is located just below larynx. It consists of right and left lateral lobes, which lie on either side of the trachea. The lobes are connected by a mass of tissue called the isthmus that lies in front of the trachea and below the cricoid cartilage. The thyroid contains follicular cells that reach the surface of the lumen of the follicle and parafollicular cells (C-cells) that are tucked in-between the follicular cells and the basement membranes of the follicular cells. The follicular cells manufacture thyroxine (T4) and triiodothyronine (T3). These hormones are referred to as thyroid hormones (which are the regulators of metabolic rate). Thyroid hormones are derivatives of the amino acid tyrosine. Parafollicular cells produce calcitonin (CT), which lowers calcium levels in blood. (3) The Parathyroids The parathyroids are embedded on posterior surfaces of lateral lobes of the thyroid. They are small, round masses. There are two attached to each thyroid lobe. There are two kinds of cells in parathyroid: (1) chief (principal) cells, which are the major producers of parathyroid hormone (PTH) and (2) oxyphil cells, which may synthesize a reserve capacity of PTH. (4) The Adrenal Glands There are two adrenal glands. One adrenal gland is superior to each kidney. Each gland has a cortex and medulla. The cortical regions are: (1) zona glomerulosa (mineralocorticoid production), (2) zona fasciculata (glucocorticoid production) and (3) zona reticularis (gonadocorticoid production). The medulla contains hormone-producing cells called chromaffin cells, which surround large blood-filled sinuses. They produce epinephrine and norepinephrine (both are catecholamines and are based on the amino acid tyrosine). These particular catecholamines are the "fight or flight" hormones. (5) The Pancreas The pancreas is a mixed gland: both exocrine and endocrine. The endocrine portion of the gland consists of the pancreatic islets (islets of Langerhans). The pancreatic islets consist of several kinds of hormone-producing cells including: alpha, beta, delta and F-cells. (6) The Gonads (Ovaries and Testes) Ovaries and testes are gonads. Gonads are glands that produce both hormones and gametes.

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(7) Pineal Gland The pineal gland is attached to roof of third ventricle. It is made of masses of neuroglial cells and secretory cells called pinealocytes. The pineal gland produces melatonin. Helps regulate the biological clock mechanism. The biological clock mechanism is associated with a pair of nuclei called the suprachiasmatic nuclei (SCN). These nuclei are located in the hypothalamus, just behind and below the eyes. Melatonin is produced from the neurotransmitter serotonin. It may play a role in inducing sleep. The pineal gland possibly produces other hormones - still uncertain. (8) The Thymus Bilobed lymphatic structure located in superior mediastinum. It is posterior to the sternum and between the lungs. The thymus produces T-cells and hormones including thymosins, thymic humoral factor (THF) and thymopoietin. (9) Body structures that act as endocrine producers include: GI tract: stomach gastrin, enteric (intestinal) gastrin, secretin and cholecystokinin (CCK). Placenta: human chorionic gonadotropin, estrogens, progesterone and relaxin. Heart: atrial natriuretic peptide (ANP; formerly ANF: atrial natriuretic factor), which helps with sodium balance. Produced by cells located in the right atrium. Kidneys: renal erythropoietic factor, Vitamin D and renin. Skin: cholecalciferol, an inactive form of vitamin D.

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DISORDERS/DISEASES Acromegaly Addison's Disease Cretinism Cushing's Disease Goiter Grave's disease Hashimoto's thyroiditis Hyperglycemia Excessive GH production after epiphyseal growth plates close off. Hyposecretion of mineralocorticoids and glucocorticoids. Resulting typically from adrenal neoplasms. A/k/a childhood hypothyroidism. Tumor of pituitary (usually) causes excess of cortisone production. Causes persistent high blood sugar (hyperglycemia). Enlargement of the thyroid due to problems with hormone production. Hyperthyroidism. Hypothyroidism ­ has an autoimmune basis. A/k/a diabetes. Two forms predominate:

Insipidus: deficiency of ADH from posterior pituitary. Mellitus: insulin-related problems cause high glucose levels in the blood. Two types: Type I Juvenile onset.

Type II Hyperparathyroidism Hyperthyroidism Hypoglycemia Hypoparathyroidism Hypothyroidism Adult onset.

Rare, usually caused by tumors. Bones deform due to lack of calcium and their mineral salts are replaced by fibrous CT. Grave's disease. Autoimmune antibodies mimic TSH and continuously stimulate release of thyroid hormone. Excessive insulin secretion. Overexcites neurons causing tetany. If untreated will lead to death.

Myxedema Pituitary dwarfism

Myxedema in adults and cretinism in youngsters. Mental sluggishness and low metabolic rate. When caused by a lack of iodine it is called colloidal goiter. A/k/a hypothyroidism.

Dwarfism that results from GH insufficiency from the anterior pituitary.

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