Chapter 6 The Muscular System 195 Copyright Goodheart-Willcox Co., Inc. Infants and Children Since muscles do not develop tension unless stimu- lated by a nerve impulse, an infant’s ability to make purposeful movements depends on the development of the nervous system, as well as the development of muscular strength. As previously mentioned, due to this relationship between the nervous and muscular systems, they are often referred to collectively as the neuromuscular system. Neuromuscular development in the infant and toddler proceeds in a superior to inferior direction through the body. Babies can raise their heads before they can sit and can sit before beginning to crawl. The ability to stand and walk requires the ability to balance in an upright position, as well as sufficient leg strength and motor control. During childhood, neuromuscular control con- tinues to develop. Generally, gross motor skills, (involving large limb movements), are acquired before fine motor skills, (involving eye-hand coordination). A task such as tying a shoe is challenging for a young child but is readily mastered once the neuromuscular system has reached a sufficient level of maturity. The strength of boys and girls is essentially equal during preadolescence, increasing as muscle mass increases. O ne of the exciting moments in a pregnancy occurs when the mother can first feel the developing fetus begin to move. This can only occur, of course, after the fetus has developed skeletal muscle tissues and a nervous system to stim- ulate them. This section describes the development and growth of muscular system, as well as the chal- lenges to aging skeletal muscles. Development and Growth of Muscles Human muscle tissue begins to form from dividing stem cells in the embryo in a process known as myogenesis. The embryonic stem cells destined to become muscle cells are called myoblasts. The myoblasts fuse into myotubes that contain multiple nuclei. These myotubes mature into muscle cells, or myocytes. The myocytes then further specialize into skeletal, cardiac, or smooth muscle cells, which in turn organize into tissues and organs. By the 16th week of pregnancy, the muscles are sufficiently developed that fetal movements can be felt. The skeletal muscle fibers grow in length and diameter from birth to adulthood. linea alba a longitudinal band of connective tissue that separates the rectus abdominis muscles rectus sheath connective tissue that encases the rectus abdominis muscles rotator cuff the four muscles that attach the humerus to the scapula and their tendons Review Questions 1. List the major muscles of the face. 2. What are the major functions of the trunk muscles? 3. Explain the purpose of the diaphragm. 4. Using the drawings and tables in this lesson, identify the agonist/antagonist pairs for abduction/adduction of the hip and plantar flexion/dorsiflexion of the ankle. 5. What type of motor units do you think the forearm and hands have? Why? 6. Compare and contrast inversion and eversion of the foot and supination and pronation of the hand. 7. Try writing the answers to one of the questions at the left without using your thumb. Why is opposition important? 8. What position are you in if all of your joints that can perform flexion do so at the same time? 9. In a push-up, what movements are happening at the shoulder and elbow when you are moving up? Which muscles are performing these movements? What movements occur when you move down? Which muscles cause these movements? SECTION 6.4 Development of Muscle Objectives • Explain the process by which muscle cells develop both before birth and throughout a person’s lifetime. • Discuss the causes of sarcopenia and the interven- tions that can slow it. Key Terms myoblasts myocytes myogenesis myotubes sarcopenia