CONCEPTS IN BIOLOGY

PART VI. PHYSIOLOGICAL PROCESSES

 

26. The Body's Control Mechanisms and Immunity

 

26.6. Output Coordination

The nervous system and endocrine system cause changes in several ways. Both systems can stimulate muscles to contract and glands to secrete. The endocrine system is also able to change the metabolism of cells and regulate the growth of tissues. The nervous system acts on two kinds of organs: muscles and glands. The actions of muscles and glands are simple and direct: Muscles contract and glands secrete.

Muscular Contraction

The ability to move is one of the fundamental characteristics of animals. Through the coordinated contraction of many muscles, the intricate, precise movements of a dancer, basketball player, or writer are accomplished. Muscles can do work only when they pull while contracting. When muscles relax, they do not lengthen unless there is some force available to stretch a muscle after it has stopped contracting and relaxes. Therefore, the muscles that control the movements of the skeleton are present in antagonistic sets—for every muscle’s action there is another muscle with the opposite action. For example, the biceps muscle causes the arm to flex (bend) as the muscle shortens. The contraction of its antagonist, the triceps muscle, causes the arm to extend (straighten) and simultaneously stretches the relaxed biceps muscle (figure 26.18).

FIGURE 26.18. Antagonistic Muscles

Because muscles cannot actively lengthen, sets of muscles oppose one another. The contraction and shortening of one muscle causes the stretching of its relaxed antagonistic partner.

What we recognize is that a muscle is composed of many muscle cells, which in turn are made up of threadlike fibers, myofibrils, composed of two kinds of myofilaments arranged in a regular pattern. Thin myofilaments composed of the proteins actin, tropomyosin, and troponin alternate with thick myofilaments composed primarily of the protein myosin (figure 26.19). The mechanism by which muscle contracts involves the movement of protein filaments past one another as adenosine triphosphate (ATP) is used.

FIGURE 26.19. The Microanatomy of a Muscle

(a-c) Muscles are made of cells that contain bundles known as myofibrils. The myofibrils are composed of two kinds of myofilaments: thick myofilaments composed of myosin, and thin myofilaments containing actin, troponin, and tropomyosin. (d) The actin- and myosin-containing myofilaments are arranged in a regular fashion into units called sarcomeres. Each sarcomere consists of two sets of actin-containing myofilaments inserted into either end of bundles of myosin-containing myofilaments. The actin-containing myofilaments slide past the myosin-containing myofilaments, shortening the sarcomere.

Myosin molecules are shaped like a golf club. The head of the club-shaped molecule sticks out from the thick myofilament and can combine with the actin of the thin myofilament. However, the troponin and tropomyosin proteins associated with the actin (i.e., a troponin-tropomyosin-actin complex), cover actin in such a way that myosin cannot bind with it. When actin is uncovered, myosin can bind to it, and muscle contraction occurs when ATP is used.

The process of muscle-cell contraction involves several steps. The arrival of a nerve impulse at a muscle cell causes the muscle cell to depolarize. When muscle cells depolarize, calcium ions (Ca2+) contained within membranes are released among the actin and myosin myofilaments. The calcium ions (Ca2+) combine with the troponin molecules, causing the troponin-tropomyosin complex to expose actin, so that it can bind with myosin. While the actin and myosin molecules are attached, the head of the myosin molecule can flex as ATP is used and the actin molecule is pulled past the myosin molecule. Thus, a tiny section of the muscle cell shortens (figure 26.20). When one muscle contracts, thousands of such interactions take place within a tiny portion of a muscle cell, and many cells within a muscle contract at the same time.

FIGURE 26.20. Interaction Between Actin and Myosin

When calcium ions (Ca2+) enter the region of the muscle cell containing actin and myosin, they allow the actin and myosin to bind to each other. ATP is broken down to ADP and P with the release of energy. This energy allows the club-shaped head of the myosin to flex and move the actin along, causing the two molecules to slide past each other.

The Types of Muscle

There are three major types of muscle: skeletal, smooth, and cardiac. These differ from one another in several ways.

Skeletal muscle is voluntary muscle; it is under the control of the nervous system. The brain or spinal cord sends a message to skeletal muscles, and they contract to move the legs, fingers, and other parts of the body. This does not mean that we must make a conscious decision every time we want to move a muscle. Many of the movements we make are learned initially but become automatic as a result of practice. For example, walking, swimming, and riding a bicycle required a great amount of practice originally but become automatic for many people. They are, however, still considered voluntary actions.

Skeletal muscles are constantly bombarded with nerve impulses, which result in repeated contractions of differing strength. Many neurons end in each muscle, and each one stimulates a specific set of muscle cells, called a motor unit (figure 26.21). A motor unit is a single neuron and all the muscle fibers to which it connects. Because each muscle consists of many motor units, it is possible to have a wide variety of intensities of contraction within one muscle organ. This allows a single set of muscles to have a wide variety of functions. For example, the same muscles of the arms and shoulders that are used to play a piano can be used in other combinations to grip and throw a baseball.

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FIGURE 26.21. Motor Unit

This photo shows a motor unit—the muscle fibers stimulated by the endings of one axon.

If the nerves going to a muscle are destroyed, the muscle becomes paralyzed and begins to shrink. The regular nervous stimulation of skeletal muscle is necessary for it to maintain its size and strength. Any kind of prolonged inactivity leads to the degeneration of muscles, known as atrophy. Muscle maintenance is one of the primary functions of physical therapy and a benefit of regular exercise.

Skeletal muscles can contract quickly, but they cannot remain contracted for long periods. Even when we contract a muscle for a minute or so, the muscle is constantly shifting the individual motor units within it that are in a state of contraction. A single skeletal muscle cell cannot stay in a contracted state but by shifting the individual muscle cells that are contracted, the muscle organ can remain contracted for a short time.

Smooth muscles make up the walls of muscular internal organs, such as the gut, blood vessels, and reproductive organs. They contract as a response to being stretched. Because much of the digestive system is being stretched constantly, the responsive contractions contribute to the normal rhythmic movements associated with the digestive system. These are involuntary muscles; they can contract on their own without receiving direct messages from the nervous system. This can be demonstrated by removing portions of the gut or uterus from experimental animals. When these muscular organs are kept moist with special solutions, they go through cycles of contraction without any possible stimulation from neurons. However, they do receive nervous stimulation, which can modify the rate and strength of their contraction. This kind of muscle also has the ability to stay contracted for long periods without becoming fatigued. Many kinds of smooth muscle, such as the muscle of the uterus, also respond to the presence of hormones. Specifically, the hormone oxytocin, which is released from the posterior pituitary, causes strong contractions of the uterus during labor and childbirth. Similarly, several hormones produced by the duodenum influence certain muscles of the digestive system to either contract or relax.

Cardiac muscle makes up the heart. It can contract rapidly, like skeletal muscle, but does not require nervous stimulation to do so. Nervous stimulation can, however, cause the heart to speed or slow its rate of contraction. Hormones, such as epinephrine and norepinephrine, also influence the heart by increasing its rate and strength of contraction. Cardiac muscle also has the characteristic of being unable to stay contracted; it will contract quickly but must have a short period of relaxation before it is able to contract a second time. This makes sense in light of its continuous, rhythmic, pumping function. Table 26.1 summarizes the differences among skeletal, smooth, and cardiac muscles.

TABLE 26.1. Characteristics of the Three Kinds of Muscle

Kind of Muscle

Stimulus

Length of Contraction

Rapidity of Response

Skeletal

Nervous system

Short; tires quickly

Most rapid

Smooth

Self-stimulated; also responds to nervous and endocrine systems

Long; doesn’t tire quickly

Slow

Cardiac

Self-stimulated; also responds to nervous and endocrine systems

Short; cannot stay contracted

Rapid

The Activities of Glands

Recall that there are two types of glands: endocrine glands, such as the pituitary, thyroid, ovary, and testis; and exocrine glands, such as the salivary glands, intestinal mucous glands, and sweat glands. Some of these glands, such as salivary glands and sweat glands, are under nervous control. When stimulated by the nervous system, they secrete their contents.

Russian physiologist Ivan Petrovich Pavlov showed that salivary glands are under the control of the nervous system, when he trained dogs to salivate in response to hearing a bell. You may recall from chapter 18 that, initially, the animals were presented with food at the same time the bell was rung. Eventually, they would salivate when the bell was rung even if food was not present. This demonstrated that saliva release is under the control of the central nervous system.

Many other exocrine glands are under hormonal control. Many of the digestive enzymes of the stomach and intestine are secreted in response to local hormones produced in the gut. These are circulated through the blood to the digestive glands, which respond by secreting the appropriate digestive enzymes and other molecules.

Growth Responses

The hormones produced by the endocrine system can have a variety of effects. Hormones can stimulate smooth muscle to contract and can influence the contraction of cardiac muscle. In addition, the hormones released by one gland can cause another gland to secrete its own hormone. However, the endocrine system has one major effect that is not equaled by the nervous system: Hormones regulate growth. Several examples of the many kinds of long-term growth changes that are caused by the endocrine system were given earlier in the chapter. Growth-stimulating hormone (GSH) is produced over a period of years to bring about the increase in size of most of the structures of the body. A low level of this hormone results in a person with small body size. The amount of growth-stimulating hormone (GSH) present varies from time to time; it is present in fairly high amounts throughout childhood and results in steady growth. It also appears to be present at higher levels at certain times, resulting in growth spurts. Finally, as adulthood is reached, the level of this hormone falls, and growth stops.

Similarly, testosterone produced during adolescence influences the growth of bone and muscle to provide men with larger, more muscular bodies than those of women. In addition, there is growth of the penis, of the larynx, and of hair on the face and body. The primary female hormone, estrogen, causes the growth of reproductive organs and the development of breast tissue. It is also involved, along with other hormones, in the cyclic growth and sloughing of the wall of the uterus.

26.6. CONCEPT REVIEW

14. How do skeletal, cardiac, and smooth muscles differ in (1) speed of contraction, (2) ability to stay contracted, and (3) cause of contraction?

15. What is the role of each of the following in muscle contraction: actin, myosin, ATP, troponin, and tropomyosin?

16. Why must muscles be in antagonistic pairs?

17. What determines the timing and rate of growth of tissues?