Hormones are chemical signals secreted by cells of the endocrine system. Circulating hormones diffuse into the blood and can activate target cells Paracrine signals affect only target cells near the site of release. Autocrine signals affect the cells that released the signal. Endocrine glands – aggregations of cells that secrete hormones into in extracelllar fluid Exocrine glands have ducts that carry products to surface of skin or body cavity (like sweat glands, salivary glands)

Neurons communicate with other cells by chemical signals—neurotransmitters. Some neurotransmitters diffuse into the blood—called neurohormones. Pheromones are chemical signals released into the environment.

Chemical communication was critical for evolution of multicellular organisms. Plants, sponges, protists all use chemical signals. Signaling molecules are highly conserved, but their functions differ.

In arthropods, hormones control molting and metamorphosis The rigid exoskeleton is shed during molts to allow growth. Growth stages between molts are called instars. Two hormones regulate molting: PTTH (prothoracicotropic hormone) from cells in the brain stimulates the prothoracic gland to secrete ecdysone. Ecdysone diffuses to target tissues and stimulates molting. The nervous system and hormonal system are linked. Nervous system (brain) controls the endocrine gland (prothoracic gland), which produces the ecdysone that orchestrates the physiological response.

Hormones are in three chemical groups: Peptides or proteins—water-soluble, transported in blood with receptors on cell surfaces Example: insulin Steroid hormones—synthesized from cholesterol, lipid-soluble; must be bound to carrier proteins to be carried in blood Example: testosterone Amine hormones—derivatives of tyrosine Example: thyroxine Hormone receptors can be membrane-bound or intracellular. Lipid soluble hormones—receptors are inside the cell Water-soluble hormones cannot readily pass cell membrane— receptors are on the outside

One hormone can trigger different responses in different types of cells. Example: Epinephrine (amine), fight-or- flight response. Epinephrine binds to receptors in the heart, blood vessels, liver, and fat cells. Increases heart rate, vasodilation increases blood blow to muscles, break down of glycogen for energy supply

The pituitary gland is attached to the hypothalamus of the brain. Hypothalamus is involved in physiological regulatory systems The pituitary connects the nervous and endocrine systems. The hypothalamus (part of the brain) secretes two neurohormones into the posterior pituitary (endocrine gland): Oxytocin and antidiuretic hormone (ADH) via neurons ADH serves to increase the water retained by the kidneys when necessary. Oxytocin stimulates contractions, milk flow, promotes bonding. Hypothalumus secretes neurohormones into anterior pituitary via blood vessels Anterior pituitary secretes 4 tropic hormones that have effects on other endocrine glands

Main hypothalamic neurohormones include: Thyrotropin-releasing hormone (TRH) Gonadotropin-releasing hormone (GnRH) Prolactin-releasing and release-inhibiting hormones Growth hormone-releasing hormone Corticotropin-releasing hormone All control anterior pituitary function.

The anterior pituitary secretes four hormones that act as tropic hormones (control other endocrine glands): Thyrotropin Luteinizing hormone Follicle-stimulating hormone Corticotropin The anterior pituitary also secretes growth hormone, prolactin, melanocyte-stimulating hormone, enkephalins, and endorphins. Growth hormone (GH) promotes growth by stimulating cells to take up amino acids. GH stimulates the liver to produce somatomedins or insulin- like growth factors (IGFs).

Endorphins and enkephalins are the body’s natural painkillers, acting as neurotransmitters in the brain. They are the by-product of two other anterior pituitary hormones.

Negative feedback loops control hormone secretion from the anterior pituitary. Corticotropin is released by pituitary— adrenal produces cortisol in response. Circulating cortisol in bloodstream reaches pituitary and inhibits production. Hypothalamus slows release of corticotropin-releasing hormone.

The thyroid gland contains two cell types that each produce a hormone. Follicles formed by epithelial cells produce and store thyroxine. becomes T 3 (more active) or T 4 with the addition of iodine. Thyroxine regulates cell metabolism by acting as a transcription factor for many genes. Anterior pituitary secretes thyrotropin or thyroid-stimulating hormone (TSH), which activates the follicles to produce thyroxine.

Hyperthyroidism (thyroxine excess) is often caused by an autoimmune disease. Antibody-binding activates TSH receptors on follicle cells and increases thyroxine. Thyroid remains stimulated and grows bigger (goiter) Hypothyroidism (thyroxine deficiency) is the result of low circulating thyroxine. The most common cause is iodine deficiency— follicle cells can’t produce thyroxine. Thyroglobulin is produced but not converted efficiently to T 3 or T 4, and thyroid enlarges.

The pancreas is a gland located below the stomach that produces insulin. Insulin facilitates glucose transporters on target cells and allows uptake of glucose. When insulin binds to its receptor on the cell membrane, the cell becomes more permeable to glucose. In the absence of insulin or its function, glucose builds up in the urine. Water moves from cells into blood osmotically, increasing blood volume and thus urine production. High glucose in kidneys pulls more water into the urine. Diabetes Lack of insulin: Type I or juvenile-onset diabetes. autoimmune Insulin replacement therapy allows those with Type I diabetes to live normally. Lack of insulin responsiveness on target cells: Type II or adult-onset diabetes.

The adrenal gland is a gland within a gland. The core adrenal medulla produces epinephrine and norepinephrine Epinephrine and norepinephrine are both water-soluble and bind to the same set of receptors on the cell surface. Target cell receptors a-adrenergic receptors respond more strongly to norepinephrine. b-adrenergic receptors respond equally to both, but one type responds to circulating epinephrine—the target of b-blockers. The cortex of the adrenal produces steroid hormones

The outer adrenal cortex produces corticosteroids from cholesterol. Glucocorticoids influence blood glucose concentration Cortisol, the main glucocorticoid, mediates metabolic stress response Mineralocorticoids influence salt and water balance Aldosterone, the main mineralocorticoid, stimulates kidneys to conserve sodium and excrete potassium. Sex steroids are important in sexual development, behavior, and anabolism Only produced by adrenal in small amounts, main producer are the gonads

Gonads produce sex steroids. Androgens—male, testosterone Estrogens and progesterone—female, estradiol In development, sex hormones determine whether fetus will become male or female. Sex hormones exert their effects by the seventh week of human development. Androgens, produced in the presence of a Y chromosome, are required for male reproductive structures to develop. Without androgen, female reproductive structures develop.

At puberty, production of sex hormones increases. Controlled by tropic hormones called gonadotropins from the anterior pituitary: Luteinizing hormone (LH) Follicle-stimulating hormone (FSH) Increase in gonadotropins leads to increase in sex steroids and development of secondary sex characteristics.

The pineal gland produces melatonin, an amine hormone, from tryptophan. Melatonin is released in the dark and marks the length of the night—light inhibits release. Involved in photoperiodicity—seasonal changes in day length trigger physiological changes.