Chapter 24. Chapter 24: Hormones

Oxytocin causes humans to be more trusting of others, facilitates birth, makes milk available to nursing babies, and increases an individual’s tendency to form social attachments.

The nervous and endocrine systems both act as systems of communication and coordination within the body. The nervous system controls rapid movements and sensations. The endocrine system controls chemical signals, called hormones, that influence cells close to and far from the hormone-secreting cells. In contrast to the nervous system, the endocrine system is generally responsible for longer-term, slower regulation, such as growth and development, as well as a variety of secretions. Note, however, that some nervous system cells secrete hormones, so the systems overlap somewhat in function.

All hormones act by binding to receptors on or in target cells. The chemical structure of the hormone determines how it regulates activity in the target cell. Receptors for peptide and protein hormones are embedded within and extend through cell membranes. Once the hormone binds to the receptor on the outside of the cell, it alters the receptor and causes some type of change within the cell. Steroid hormones pass through cell membranes and bind to receptors in the cell, eventually affecting DNA transcription in the nucleus.

The hypothalamus, part of the underside of the brain, functions as a liaison between the nervous and endocrine systems. It receives input from neurons throughout the brain and the rest of the body.

In a stress-related negative feedback loop, an animal takes action in response to a stressful situation, and the source of stress is removed. The secretions of cortisol and adrenaline are first elevated, then reduced through negative feedback.

Peptide and protein hormones are water-soluble and cannot pass through cell membranes; they interact on the outside of the cell with receptors embedded in the membrane, and this interaction causes specific types of change within the cell. Lipid-soluble steroid hormones can diffuse through cell membranes and interact with receptors in the cytoplasm (then passing into the nucleus) or in the nucleus, causing changes in DNA transcription.

Signals from the hypothalamus directly influence the pituitary gland. The pituitary gland consists of two separate glands fused together. The posterior pituitary does not itself produce hormones; two important peptide hormones are produced by neurons within the hypothalamus and travel down axon tracts into the posterior pituitary, from which they are released. The anterior pituitary produces numerous hormones in response to commands by the hypothalamus. Many of the anterior pituitary hormones direct endocrine glands elsewhere to release other hormones.

Castration almost completely eliminates production of the hormone testosterone and may reduce the criminal activity of violent sex offenders.

Women with congenital adrenal hyperplasia, a genetic condition that causes them to have higher than typical levels of circulating testosterone, also (like men) have increased performance on tests of spatial ability.

Women taking oral contraceptives and women giving birth to higher numbers of children have a reduced lifetime exposure to estrogen and other hormones produced by developing follicles. This is associated with a decreased incidence of ovarian cancer.

When endocrine disruptors are taken up by organisms, these chemicals can mimic, block, or otherwise interfere with hormones, leading to harmful effects. These effects are a consequence of the close chemical similarity between endocrine disruptors and hormones, particularly estrogen. For animals that consume other animals, even small amounts of endocrine disruptors taken up from the environment can become concentrated as they pass through the food chain.

DDT disrupts the development and functioning of a bird’s reproductive tract and impairs its ability to produce normal eggshells—the shells crack under the weight of the parent incubating the eggs. During the years of DDT use, this caused serious reductions in bird populations.