CONCEPTS IN BIOLOGY

PART VI. PHYSIOLOGICAL PROCESSES

26. The Body's Control Mechanisms and Immunity

26.4. The Integration of Nervous and Endocrine Function

Although we still tend to think of the nervous and endocrine systems as separate and different, it is becoming clear that they are interconnected and cooperate to bring about appropriate responses to environmental challenges. The nervous system is particularly involved in activities that are of short duration, such as sensory input and muscle contractions. The endocrine system participates in some short-duration activities, such as the actions of epinephrine, but is more typically involved in medium- to long-term activities such as regulating glucose levels or modifying growth.

When endocrine and nervous systems interact, usually the pituitary gland is involved. The pituitary gland is located at the base of the brain and is divided into two parts. The posterior pituitary, which is directly connected to the brain, develops from nervous tissue during embryology. The other part, the anterior pituitary, develops from the lining of the roof of the mouth in early fetal development. Certain pituitary hormones are produced in the brain and transported down axons to the posterior pituitary, where they are stored before being released. The anterior pituitary also receives a continuous input of messenger molecules from the brain, but these are delivered by way of blood vessels, which pick up hormones produced by the hypothalamus and deliver them to the anterior pituitary.

The pituitary gland produces a variety of hormones that are responsible for causing other endocrine glands, such as the thyroid, ovaries and testes, and adrenals, to secrete their hormones. Pituitary hormones also influence milk production, skin pigmentation, body growth, mineral regulation, and blood glucose levels (figure 26.11).

FIGURE 26.11. Hormones of the Pituitary and Their Target Organs

The anterior pituitary gland produces several hormones that regulate growth and the secretions of target tissues. The posterior pituitary produces hormones that change the behavior of the kidney and uterus but do not influence the growth of these organs.

Because the pituitary is constantly receiving information from the hypothalamus, many kinds of sensory stimuli to the body can affect the functioning of the endocrine system. One example is the way in which the nervous system and endocrine system interact to influence the menstrual cycle. At least three hormones are involved in the cycle of changes that affects the ovary and the lining of the uterus (see chapter 27 for details). It is well documented that stress caused by tension or worry can interfere with the normal cycle of hormones and delay or stop menstrual cycles. In addition, young women living in groups, such as those in college dormitories, often find that their menstrual cycles become synchronized. Although the exact mechanism involved in this phenomenon is unknown, it is suspected that input from the nervous system causes this synchronization. (Odors and sympathetic feelings have been suggested as causes.)

In many animals, the changing length of the day causes hormonal changes related to reproduction. In the spring, birds respond to lengthening days and begin to produce hormones that gear up their reproductive systems for the summer breeding season. The pineal body, a portion of the brain, serves as the receiver of light stimuli and changes the amounts of hormones secreted by the pituitary, resulting in changes in the levels of reproductive hormones. These hormonal changes modify the birds’ behavior. Courtship, mating, and nest-building behaviors increase in intensity. Therefore, it appears that a change in hormone level is affecting the animals’ behavior; the endocrine system is influencing the nervous system (figure 26.12). It has been known for centuries that changes in the levels of sex hormones cause changes in animals’ behavior. The castration (the removal of the testes) of male domesticated animals, such as cattle, horses, and pigs, is sometimes done in part to reduce their aggressive behavior and make them easier to control. The use of anabolic steroids by humans to increase muscle mass is known to cause behavioral changes and “moodiness.”

FIGURE 26.12. Interaction Between the Nervous and Endocrine Systems

In birds and many other animals, reproduction is a seasonal activity triggered by changing length of day. The brain receives information about the changing length of day, which causes the pituitary to produce hormones that stimulate sex organs. The testes or ovaries grow and secrete their hormones in increased amounts. Increased levels of testosterone or estrogen result in changed behavior, including increased aggression, mating behavior, and nest-building activity.

As scientists learn more about the molecules produced in the brain, it is becoming clear that the brain produces many molecules that act as hormones. Some of these molecules affect adjacent parts of the brain, others affect the pituitary, and still others may have effects on more distant organs (How Science Works 26.2).

HOW SCIENCE WORKS 26.2

Endorphins: Natural Pain Killers

The pituitary gland and the brain produce a group of small molecules that act as pain suppressors, called endorphins. It is thought that these molecules are released when excessive pain or stress occurs in the body. They attach to the same receptor molecules of brain cells associated with the feeling of pain. Endorphins work on the brain in the same manner as morphine and opiate drugs. Once endorphins are attached to the brain cells, the feeling of pain goes away, and a euphoric feeling takes over. Long-distance runners and other athletes talk about feeling good as a "runner's high." These responses may be due to an increase in endorphin production. It is thought that endorphins are also released by mild electric stimulation and the use of acupuncture needles.

26.4. CONCEPT REVIEW

9. Describe two ways in which the function of the nervous system differs from that of the endocrine system.

10. Give an example of the interaction between the endocrine system and the nervous system.