Endocrine System

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Skeletal Muscles

9.2     Muscles attached to bones are called skeletal muscles. They are also called “voluntary” muscles because a person can choose whether to move them. In addition to muscle fibres, skeletal muscles contain connective tissue, nerves, and blood vessels. Each muscle is surrounded by a fibrous membrane. At one end of the muscle, the fibres of this membrane connect with a tough band of elastic tissue called a tendon. Tendons connect most skeletal muscles to bones.

 Smooth and Cardiac Muscles

9.3     Smooth muscles are found in internal organs, such as the stomach, intestines, urinary bladder, uterus, and blood vessels. Unlike skeletal muscles, which can be contracted voluntarily, smooth muscles are involuntary. Smooth muscles are responsible for the contractions of the oesophagus and the intestines called peristalsis. Smooth muscles are also responsible for the contractions of labour during childbirth. Cardiac muscle fibres are strong like skeletal muscles, but the heart is an involuntary muscle. The contraction and relaxation of cardiac muscle fibres pumps blood through the body automatically and rhythmically.

 Muscle Action

9.4     Muscle fibres respond to nerve impulses by contracting. When enough nerve impulses are received, the entire muscle contracts and pulls any attached tissue along with it, resulting in movement of that tissue. Skeletal muscles often work in pairs to bend and straighten joints. The biceps and triceps muscles move the arms at the elbow. Contracting the biceps muscle pulls upward on one of the bones of the lower arm, causing the arm to bend. When the biceps relaxes and the triceps contracts, the arm is pulled in the opposite direction. Even the simplest body movements involve the coordination of many muscles. For example, it takes 13 muscles to smile and 34 muscles to frown.

 Common Muscle Problems

9.5     Considering that muscles are used every time you move, it is not surprising that muscle injuries are common. Although regular exercise preceded by a proper warm-up reduces the risk of muscle injury, it can still occur – especially when a person ignores his or her limitations. Charley horses and pulled hamstrings are common jogging-related injuries. A charley horse is a painful contusion, or bruising, of the muscle and is frequently accompanied by a tear in muscle fibres. A pulled hamstring is an overextension or strain of one of the muscles in the back or the thigh. Inflammation of the flexor muscles of the front portion of the lower leg causes another common condition, known as shin splints. A muscular cramp is a prolonged and painful involuntary muscular contraction. A spasm is a brief, involuntary contraction. The exact cause of muscle cramps and spasms is unknown. Experts suggest that they may result from several causes: insufficient oxygen, calcium, or magnesium; fatigue; or various drugs. Tendinitis is the inflammation of the connective tissue surrounding a tendon.