CONCEPT

35.1

The Endocrine and Nervous Systems Play Distinct, Interacting Roles

• Nerve and endocrine cells control and coordinate the functions of the body by releasing chemical signals that travel to another cell, called the target cell. Review Figure 35.1
• Neuronal signals are fast and addressed, whereas endocrine signals are slow and broadcast. The nervous system controls predominantly the fine, rapid movements of discrete skeletal muscles. The endocrine system typically controls more widespread, prolonged activities such as developmental or metabolic changes.
• Animals use chemical signaling over a very broad range of spatial scales. Autocrines and paracrines act on, respectively, the cells producing the signals and on their immediate neighbors. Neurotransmitters and hormones work at intermediate distances. Pheromones are released into the environment and can affect targets hundreds of meters away. Review Figure 35.2

CONCEPT

35.2

Hormones Are Chemical Messengers Distributed by the Blood

• Animals have two types of secretory glands: exocrine glands, which have ducts to carry away their secretions, and endocrine glands, which do not have ducts. Endocrine cells secrete chemical signals into the blood.
• A hormone is a chemical substance that is secreted into the blood by endocrine cells and that regulates the function of other cells that it reaches by blood circulation.
• Some endocrine cells are neurosecretory cells; they propagate action potentials and secrete hormones into the blood from their axon terminals. Other endocrine cells are nonneural endocrine cells; they are non-excitable cells that are typically stimulated to secrete their hormones by other hormones. Review Figure 35.3
• Most hormones are peptide hormones (polypeptides or proteins), steroid hormones, or amine hormones. Peptide hormones and some amine hormones are water-soluble; steroids and some amine hormones are lipid-soluble. Review Figure 35.4
• Receptors for water-soluble hormones are located on the cell surface of a target cell. Receptors for most lipid-soluble hormones are inside the cell.
• Hormones cause different responses in different target cells, depending on each target cell’s type of receptor and the processes activated inside the cell by binding of hormone to that receptor.
• Each hormone has a characteristic half-life, the time required for half of a group of simultaneously secreted hormone molecules to be removed from the blood. Typical half-lives range from minutes to as long as a week.

CONCEPT

35.3

The Vertebrate Hypothalamus and Pituitary Gland Link the Nervous and Endocrine Systems

• The pituitary gland has two parts—the anterior pituitary and posterior pituitary—which have different developmental origins and function in different ways. Both parts of the pituitary have close functional links with the brain.
• The posterior pituitary is a neurohemal organ where hormones produced by hypothalamic neurosecretory cells are released into the blood. In mammals it secretes two peptide hormones: antidiuretic hormone (ADH) and oxytocin. Review Figure 35.6
• The anterior pituitary is a nonneural endocrine gland that secretes four tropic hormones as well as growth hormone (GH), prolactin, and a few other hormones. It is controlled by releasing hormones (RHs) and inhibiting hormones (IHs; also called release-inhibiting hormones) produced by neurosecretory cells in the hypothalamus. Review Figure 35.7
• Endocrine cells often act on each other in sequence, a system known as an axis. For example, the hypothalamus–pituitary–adrenal cortex (HPA) axis controls the adrenal secretion of glucocorticoids. Review ANIMATED TUTORIAL 35.1
• Hormone release often is controlled by negative-feedback loops. Review Figure 35.8
• Releasing hormones and inhibiting hormones are often released from the hypothalamus in pulses. Pulsed release is hypothesized to prevent loss of sensitivity in the target cells in the pituitary. Review Figure 35.9

CONCEPT

35.4

Hormones Regulate Mammalian Physiological Systems

• The thyroid gland is controlled by thyroid-stimulating hormone (TSH) and secretes the thyroid hormones thyroxine (T₄) and triiodothyronine (T₃), which control cellular metabolism. Iodine deficiency impairs thyroid hormone production and can lead to impaired mental development in children and to goiter in adults. Review Figure 35.10, Figure 35.11 and Figure 35.12 and ACTIVITY 35.1
• Sex steroids (androgens in males, estrogens and progesterone in females) are produced by the gonads under control of tropic hormones, called gonadotropins, secreted by the anterior pituitary gland. Sex steroids control prenatal sexual development, puberty, and adult reproductive functions. Review Figure 35.13

CONCEPT

35.5

The Insect Endocrine System Is Crucial for Development

• Two hormones, prothoracicotropic hormone (PTTH) and ecdysone, control molting in insects. A third hormone, juvenile hormone (JH), prevents maturation. When an insect stops producing juvenile hormone, it molts into an adult. Review Figures 35.14 and 35.15 and ANIMATED TUTORIAL 35.2

See ACTIVITY 35.2 for a concept review of this chapter.