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