From Encyclopedia of Sex and Sexuality
The chemical messengers of the body, called hormones, are secreted by the endocrine glands. The endocrine glands include the pancreas, which secretes insulin regulating sugar use by the cells of the body; the gonads, which secrete the sex hormones regulating reproduction; and the thyroid, which secretes thyroxine to control metabolism. Regulation of body growth, reproduction, and many metabolic functions, such as energy use, is managed by hormone secretion. The hormones are released by the endocrine system at a slow and steady rate and reach their targets through the blood. This is quite different from the other regulatory system, the nervous system, which relays signals from cell to cell electronically in fractions of seconds. The chemical messages take from several minutes to hours to be received and acted upon, and their effects are usually long-lasting.
The endocrine glands themselves are regulated by a complex mechanism called “feedback.” When the activity of the responding tissue reaches a critical level, the information is relayed back to the regulating endocrine gland, reducing the activity of the gland. Endocrine glands can respond to increases in activity of the target tissue by decreasing their own activity and can respond to decreases in target activity with subsequent increases.
There are many classes of chemical structures that are defined as hormones. The most common hormones are either small peptides, steroid molecules, or complex proteins. In evolutionary terms, steroids are possibly the oldest of the hormones used by the body. They are constant throughout the vertebrate world, functioning in fish as they do in mammals. The steroid structure is simple and easily recognized by the cell that will react to the hormone. Hormones interact with cells of the body through cellular structures called receptors. Receptors are shaped so that only a specific hormone will bind on a specific receptor, like a lock and key. If a cell does not have the specific hormone receptor, it will not respond to the hormone. Receptors are found both in the cell plasma membrane and within the cell. Steroids pass through the plasma membrane, binding to receptors inside the cell. Receptors in the plasma membrane bind hormones that do not enter the cell. Once the hormone has bound to the receptor, it changes the shape of the receptor, thereby activating it. The activated receptor transfers the stimulus from outside the cell wall to chemical mechanisms within the cell and causes a response. Often, the response will be an increase in protein production or the production of a particular metabolite. The response can also affect the amount of enzymes found in a particular cellular reaction, increasing the rate of reaction. The end product may be a protein that will increase or decrease metabolism, or it could even be another hormone.
Hormonal control is found at many levels of reproductive function. Hormones stimulate gonadal development, sperm and oocyte (egg) production, and secondary sexual characteristics. They also play a role in regulating sexual behavior and libido.