THE LIVING WORLD
Unit Six. Animal Life
30. Chemical Signaling Within the Animal Body
In vertebrates and most other animals, the central nervous system coordinates and regulates the diverse activities of the body by using chemical signals called hormones to effect changes in physiological activities. Many hormones—adrenaline (epinephrine), estrogen, testosterone, insulin, thyroid hormone—are probably familiar to you. Some of these hormones have very different roles in other animals, however. For example, thyroid hormone is needed in amphibians for the metamorphosis of larvae into adults. If the thyroid gland is removed from a tadpole, it will not change into a frog. Conversely, if an immature tadpole is fed pieces of a thyroid gland, it will undergo premature metamorphosis and become a miniature frog! Similarly, melanocyte-stimulating hormone, a peptide hormone, is present in mammals, but we don’t know what it does. In reptiles and amphibians, this hormone stimulates color changes. The green anole (Anolis carolinensis) shown here in the upper photo has changed to a tan color in the lower photo, in response to an environmental or physiological cue. This color change is the result of dispersal of pigment-containing granules from the center of the anole’s skin cells into extensions of the cells, darkening the skin. Triggered by melanocyte-stimulating hormone, the reversible color change takes 5 to 10 minutes. In this chapter you will encounter many other hormones, some familiar, others less so, but all used by vertebrates to regulate their body condition.
A hormone is a chemical signal produced in one part of the body that is stable enough to be transported in active form far from where it is produced and that typically acts at a distant site. There are three big advantages to using chemical hormones as messengers rather than speedy electrical signals (like those used in nerves) to control body organs. First, chemical molecules can spread to all tissues via the blood (imagine trying to wire every cell with its own nerve!) and are usually required in only small amounts. Second, chemical signals can persist much longer than electrical ones, a great advantage for hormones controlling slow processes like growth and development. Third, many different kinds of chemicals can act as hormones, so different hormone molecules can be targeted at different tissues. For all these reasons, hormones are excellent messengers for signaling widespread, slow-onset, long duration responses.
Hormones, in general, are produced by glands, most of which are controlled by the central nervous system. Because these glands are completely enclosed in tissue rather than having ducts that empty to the outside, they are called endocrine glands (from the Greek, endon, within). Hormones are secreted from them directly into the bloodstream (this is in contrast to exocrine glands, like sweat glands, that have ducts). Your body has a dozen principal endocrine glands, shown in figure 30.1, that together make up the endocrine system.
The endocrine system and the motor nervous system are the two main routes the central nervous system (CNS) uses to issue commands to the organs of the body. The two are so closely linked that they are often considered a single system— the neuroendocrine system. The hypothalamus can be considered the main switchboard of the neuroendocrine system. The hypothalamus is continually checking conditions inside the body to maintain a constant internal environment, a condition known as homeostasis. Is the body too hot or too cold? Is it running out of fuel? Is the blood pressure too high? If homeostasis is no longer maintained, the hypothalamus has several ways to set things right again. For example, if the hypothalamus needs to speed up the heart rate, it can send a nerve signal to the medulla oblongata, or it can use a chemical command, causing the adrenal gland to produce the hormone adrenaline, which also speeds up the heart rate. Which command the hypothalamus uses depends on the desired duration of the effect. A chemical message is typically far longer lasting than a nerve signal.
Figure 30.1. Major glands of the human endocrine system.
Hormone-secreting cells are clustered in endocrine glands. The pituitary and adrenal glands are each composed of two glands.
The Chain of Command
The hypothalamus issues commands to a nearby gland, the pituitary, which in turn sends chemical signals to the various hormone-producing glands of the body. The pituitary is suspended from the hypothalamus by a short stalk, across which chemical messages pass from the hypothalamus to the pituitary. The first of these chemical messages to be discovered was a short peptide called thyrotropin-releasing hormone (TRH), which was isolated in 1969. The release of TRH from the hypothalamus triggers the pituitary to release a hormone called thyrotropin, or thyroid-stimulating hormone (TSH), which travels to the thyroid and causes the thyroid gland to release thyroid hormones.
Several other hypothalamic hormones have since been isolated, which together govern the pituitary. Thus, the CNS regulates the body’s hormones through a chain of command. The “releasing” hormones made by the hypothalamus cause the pituitary to synthesize a corresponding pituitary hormone, which travels to a distant endocrine gland and causes that gland to begin producing its particular endocrine hormone. The hypothalamus also secretes inhibiting hormones that keep the pituitary from secreting specific pituitary hormones.
The key reason why hormones are effective messengers within the body is because a particular hormone can influence a specific target cell. How does the target cell recognize that hormone, ignoring all others? Embedded in the plasma membrane or within the target cell are receptor proteins that match the shape of the potential signal hormone like a hand fits a glove. As you recall from chapter 28, nerve cells have highly specific receptors within their synapses, each receptor shaped to “respond” to a different neurotransmitter molecule. Similarly, cells that the body has targeted to respond to a particular hormone have receptor proteins shaped to fit that hormone and no other. Thus, chemical communication within the body involves two elements: a molecular signal (the hormone) and a protein receptor on or in target cells. The system is highly specific because each protein receptor has a shape that only a particular hormone fits.
Hormones secreted by endocrine glands belong to four different chemical categories:
1. Polypeptides are composed of chains of amino acids that are shorter than about 100 amino acids. Some important examples include insulin and antidiuretic hormone (ADH).
2. Glycoproteins are composed of polypeptides significantly longer than 100 amino acids to which are attached carbohydrates. Examples include follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
3. Amines, derived from the amino acids tyrosine and tryptophan, include hormones secreted by the adrenal medulla, thyroid, and pineal glands.
4. Steroids are lipids derived from cholesterol, and include the hormones testosterone, estrogen, progesterone, aldosterone, and cortisol.
The path of communication taken by a hormonal signal can be visualized as the series of simple steps shown in the Key Biological Process illustration above:
1. Issuing the command. The hypothalamus of the CNS controls the release of many hormones. Some hormones produced in cells in the hypothalamus are stored in the posterior pituitary and are released into the bloodstream in response to a signal from the brain.
2. Transporting the signal. While hormones can act on an adjacent cell, most are transported throughout the body by the bloodstream.
3. Hitting the target. When a hormone encounters a cell with a matching receptor, called a target cell, the hormone binds to that receptor.
4. Having an effect. When the hormone binds to the receptor protein, the protein responds by changing shape, which triggers a change in cell activity.
Key Learning Outcome 30.1. Hormones are effective because they are recognized by specific receptors. Thus, only cells possessing the appropriate receptor will respond to a particular hormone.