42 Hormones and Their Actions In multicellular animals, nerve impulses provide electric signals; hormones provide chemical signals. Hormones are secreted by cells, diffuse into the extracellular fluid, and often are distributed by the circulatory system. Hormones work much more slowly than nerve impulse transmission and are not useful for controlling rapid actions. Hormones coordinate longer-term developmental processes such as reproductive cycles.

42 Hormones and Their Actions Hormone-secreting cells are called endocrine cells. Cells receiving the hormonal message are called target cells and must have appropriate receptors.

42 Hormones and Their Actions Hormones can be classified into three main groups:  Peptides or proteins. They are water soluble and transported by vesicles out of the cell that made them.  Steroid hormones are lipid-soluble and can diffuse out of the cell that made them but in the blood they must be bound to carrier proteins.  Amine hormones are derivatives of the amino acid tyrosine. Some are water-soluble and some are lipid-soluble.

42 Hormones and Their Actions Some hormones act locally. Autocrine hormones act on the secreting cell itself. Paracrine hormones act on cells near the site of release. Most hormones travel in blood to different tissues and cells.

42 Hormones and Their Actions Endocrine refers to cells or glands that do not have ducts leading to the outside of the body; they secrete their products directly into the extracellular fluid. In vertebrates, nine major endocrine glands make up the endocrine system.

Figure 42.2 The Endocrine System of Humans

42 Vertebrate Endocrine Systems The pituitary gland of mammals is a link between the nervous system and many endocrine glands and plays a crucial role in the endocrine system. The pituitary gland sits in a depression at the bottom of the skull and is attached to the hypothalamus. The pituitary is made of two parts: anterior and posterior.

42 Add anatomy slides in here Make a handout of the major hormones and actions.

Figure 42.8 Multiple Feedback Loops Control Hormone Secretion

Figure The Adrenal Gland Has an Outer and an Inner Portion

42 Vertebrate Endocrine Systems Until the seventh week of an embryo’s development, either sex may develop. In mammals, the Y chromosome causes the gonads to start producing androgens in the seven-week-old embryo, and the male reproductive system develops. If androgens are not released, the female reproductive system develops. In birds, the opposite rules apply: male features are produced unless estrogens are present to trigger female development.

Figure The Development of Human Sex Organs

42 Vertebrate Endocrine Systems Sex steroid production increases rapidly at puberty, or sexual maturation, in humans. Control of sex steroids (both male and female) is under the anterior pituitary tropic hormones called luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These gonadotropins are controlled by the hypothalamic gonadotropin-releasing hormone (GnRH). Before puberty, the hypothalamus produces low levels of GnRH.

42 Vertebrate Endocrine Systems Puberty starts when the hypothalamus becomes less sensitive to negative feedback by the sex steroids. GnRH release increases, stimulating gonadotropin production and, hence, sex steroid production. In females, increased LH and FSH at puberty induce the ovaries to begin female sex hormone production to initiate sexually mature body traits. In males, increased LH stimulates cells in the testes to make androgens which induce changes associated with adolescence.

42 Vertebrate Endocrine Systems Melatonin hormone is produced by the pineal gland, located between the cerebral hemispheres of the brain. Melatonin release occurs in the dark, marking the length of night. Exposure to light inhibits melatonin release. Melatonin is involved in biological rhythms, including photoperiodicity. In many animals, increasing day length signals the onset of reproductive behavior. Humans are not photoperiodic, but melatonin may be involved in daily rhythms of the body (light/dark cycles).

42 Hormone Actions: The Role of Signal Transduction Pathways Hormones are released in very small amounts, yet they cause large and very specific responses in target organs and tissues. Strength of hormone action results from signal transduction cascades that amplify the original signal. Selective action is keyed to appropriate receptors of cells responding to hormones. Specific receptors can also be linked to different response mechanisms, as is the case with receptors for epinephrine and norepinephrine.

42 Hormone Actions: The Role of Signal Transduction Pathways The abundance of hormone receptors can be under feedback control. Continuous high levels of a hormone can decrease the number of its receptors, a process called downregulation. High levels of insulin in type II diabetes mellitus result in a loss of insulin receptors. Upregulation of receptors is a positive feedback mechanism and is less common than downregulation.

42 Hormone Actions: The Role of Signal Transduction Pathways The concentration of hormones and receptors can be determined by a technique called immunoassay. A saturating concentration of a labeled hormone is mixed with an antibody until all the binding sites of the antibody are used. A sample of unlabeled hormone is then added to the mixture. The unlabeled hormone displaces some of the labeled hormone from the antibody. The ratio of labeled to unlabeled antibody is a measure of the amount of unlabeled hormone.