FROM MOVEMENT to meat production, the muscular

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1 Structures and Functions of the Muscular System FROM MOVEMENT to meat production, the muscular system provides benefits to animals and producers. Like other body systems, the muscular system is a fantastic example of small units working together to produce major results. An effective use of the muscular system is critical for an animal to live and grow. Objective: Describe the muscular system and the way in which muscles contract to create movement. Key Terms: actin cardiac muscle muscular system myosin sarcomere skeletal muscle smooth muscle tendons The Muscular System When we see a big, juicy steak on a plate, most of us do not immediately think of which muscle of the steer this steak came from. In fact, many people may not realize that meat is muscle tissue. Similarly, most of us know that cats can jump well and dogs can run fast, but we do not understand the types of muscles working in their bodies to give them these abilities. Page 1

2 Knowledge of the muscular system will improve our production in meat livestock and make us better pet owners in general. The muscular system can be defined as all forms of tissue in the body that can contract to perform movement or similar functions. Some types of muscle create movements that are easily seen, such as moving legs or blinking eyes. Other types of muscle are somewhat more difficult to observe, but these muscles are still important. Gizzards in poultry and heart muscle in all animals are two examples of critical muscle movement that we normally do not see, but we know the muscles are moving. The muscular system is a vast network that covers almost the entire body of the animal. This system, however, is composed of three types of muscle tissue. Each type of muscle tissue has a distinct structure and function within the body. SKELETAL MUSCLE FIGURE 1. Draft Horses exhibit excellent muscling. Skeletal muscle, or striated voluntary muscle, is attached to the skeleton and contracts to produce body movement. Skeletal muscle is voluntary muscle, so it can be moved whenever the animal desires and will not contract without effort. Most of the edible meat portions of livestock are composed of skeletal muscle, which is sometimes prepared still attached to the bone, as in the case of chicken drumsticks. FURTHER EXPLORATION ONLINE CONNECTION: Muscle Animations Muscles may seem to be a confusing mixture of proteins and filaments, and it may be difficult to visualize muscles working together. Use the following Web site to explore a number of anatomy- and physiology-related animations that can help explain the action of muscle contraction and the structure of muscle tissues. Using these animations, find your favorite that explains some part of the muscular system. Share it with the class. Page 2

3 Skeletal muscle is attached to bones directly or by the use of tendons. Tendons are connective tissue that anchor muscle to bone. Tendons are tough to chew and are not very appetizing. As a result, tendons are not normally consumed. Sometimes with extreme use or extension of a bone, tendons can be torn, producing a painful and slow-healing injury. Muscles do not extend to produce movement. They can only contract. Movement in opposite directions, such as a leg moving forward and then backward, is caused by two sets of muscle, each contracting in an order to produce the desired movement. The entire muscular system can be thought of as a series of rubber bands that contract to produce movement and then relax while another muscle contracts to produce a different movement. Trapezius Thoracalis Trapezius Cervicalis Latissimus Dorsi External Intercoastal Obliquus Abdominis Externus Tensor Fasciae Latae Brachiocephalicus Semitendinosis Sterno-Cephalicus Deltoid Long Head of Triceps Serratus Thoracis Posterior Deep Pectoral Lateral Head of Triceps FIGURE 2. The muscular system of a hog. Movement of a muscle is possible because the muscle contracts (shortens). This shortening of a muscle is due to the shortening of the cells that compose the muscle tissue. The basic unit of contraction within the muscle is known as a sarcomere. When all the sarcomeres in a muscle tissue contract, the net result is a total shortening of the muscle. Sarcomeres are found within myofibers, which resemble tiny strands like the wires in a wound cable. These myofibers are made of myofibrils, or even smaller strands, which are composed of two types of protein filaments: actin and myosin. Myosin, a thick filament, and actin, a thin filament, work together to produce the contractive force that causes muscles to shorten. During a contraction, the myosin actually pulls itself along the actin filament. Page 3

4 SMOOTH MUSCLE Smooth muscle is commonly found in the reproductive, digestive, and urinary organs of the body. Smooth muscle is an involuntary muscle, which means the animal has no control over the action of this muscle. The contraction of the intestines and the movement of wastes in the ureters are examples of smooth muscle in action. The oviducts, iris of the eye, and most blood vessels are also examples of smooth muscle. Smooth muscle is composed of the same actin and myosin protein filaments as skeletal muscle, but in different proportions, and will contract differently than voluntary skeletal muscles. Esophagus Rumen Small intestine Reticulum Omasum Abomasum FIGURE 3. The digestive system of a cow. CARDIAC MUSCLE Cardiac muscle is found only in the hearts of animals and is an involuntary muscle. Cardiac muscle is specialized to perform under continuous operation without fatigue, which is what keeps animals and humans alive. In humans, the heart begins to beat at 21 days after conception and must continue to beat regularly for possibly one hundred years. In an average lifespan, the human heart will beat nearly 2.5 billion times. The main purpose of cardiac muscle is Page 4

5 to pump blood throughout the body as part of the circulatory system. Without this blood flow to the lungs and throughout the body, the tissues and brain would quickly begin to die. Cardiac muscle differs from the other two types of muscle tissue in that it does not require outside stimuli for contraction. Special cells in the heart are able to contract at a regular interval on their own, which causes the rest of the heart tissue cells to contract. The autonomic nervous system regulates this rhythmic contracting of cardiac tissue. Cardiac tissue can go into fibrillation, or an uncontrolled contraction, when the timing of the contractions is somehow disrupted. Pericardium Fibrous pericardium Serous pericardium Myocardium Endocardium Pericardial cavity Epicardium FIGURE 4. Major parts of a heart (longitudinal section). Summary: Skeletal muscle, or striated voluntary muscle, is attached to the skeleton and contracts to produce body movement. Muscles do not extend to produce movement; they can only contract. The basic unit of contraction within the muscle is known as a sarcomere. Myosin (a thick filament) and actin (a thin filament) work together to produce the contractive force that causes muscles to shorten. Smooth muscle is commonly found in the reproductive, digestive, and urinary organs of the body. Cardiac muscle is found only in the hearts of animals and is an involuntary muscle. The main purpose of cardiac muscle is to pump blood throughout the body as part of the circulatory system. The autonomic nervous system regulates this rhythmic contracting of cardiac tissue. Without the use of all three types of muscle tissue, our movement, internal functions, and even blood circulation would not be possible. Page 5

6 Checking Your Knowledge: 1. List the three types of muscle fiber found in the body. 2. Using the terms sarcomere, actin, and myosin, explain how a muscle contracts. 3. List three organs that use smooth muscle for operation. 4. How does cardiac muscle differ from smooth or skeletal muscle? 5. What is the purpose of tendons? Expanding Your Knowledge: Visit a local meat locker to view the three different types of muscle tissue in a recently butchered livestock animal. If a visit is not possible, ask your instructor to obtain samples of the tissue. Web Links: Interactive Feline Dissection Images Interactive Muscular System of the Human Body Muscle Tissues, Common Muscles, and Microscopic Muscle Images Agricultural Career Profiles Page 6