In this chapter, you learned about the muscular system. Specifically, you learned that:

• The muscular system consists of all the muscles of the body. There are three types of muscle: skeletal muscle (which is attached to bones by tendons and enables voluntary body movements), cardiac muscle (which makes up the walls of the heart and makes it beat) and smooth muscle (which is found in the walls of internal organs and other internal structures and controls their movements).
• Muscles are organs composed mainly of muscle cells, which may also be called muscle fibres or myocytes. Muscle cells are specialized for the function of contracting, which occurs when protein filaments inside the cells slide over one another using energy from ATP. Muscle tissue is the only type of tissue that has cells with the ability to contract.
• Muscles can grow larger, or hypertrophy. This generally occurs through increased use, although hormonal or other influences can also play a role. Muscles can also grow smaller, or atrophy. This may occur through lack of use, starvation, certain diseases, or aging. In both hypertrophy and atrophy, the size — but not the number — of muscle fibres changes. The size of muscles is the main determinant of muscle strength.
• Skeletal muscles need the stimulus of motor neurons to contract, and to move the body, they need the skeletal system to act upon.
• Skeletal muscle is the most common type of muscle tissue in the human body. To move bones in opposite directions, skeletal muscles often consist of pairs of muscles that work in opposition to one another to move bones in different directions at joints.
• Skeletal muscle fibres are bundled together in units called muscle fascicles, which are bundled together to form individual skeletal muscles. Skeletal muscles also have connective tissue supporting and protecting the muscle tissue.

• • Each skeletal muscle fibre consists of a bundle of myofibrils, which are bundles of protein filaments. The filaments are arranged in repeating units called sarcomeres, which are the basic functional units of skeletal muscles. Skeletal muscle tissue is striated, because of the pattern of sarcomeres in its fibres.
  • Skeletal muscle fibres can be divided into two types, called slow-twitch and fast-twitch fibres. Slow-twitch fibres are used mainly in aerobic endurance activities (such as long-distance running). Fast-twitch fibres are used mainly for non-aerobic, strenuous activities (such as sprinting). Proportions of the two types of fibres vary from muscle to muscle and person to person.
• Smooth muscle tissue is found in the walls of internal organs and vessels. When smooth muscles contract, they help the organs and vessels carry out their functions. The pattern of smooth muscle contraction to move substances through body tubes is called peristalsis.  Contractions of smooth muscles are involuntary and controlled by the autonomic nervous system, hormones, and other substances.

• • Cells of smooth muscle tissue are not striated because they lack sarcomeres, but the cells contract in the same basic way as striated muscle cells. Unlike striated muscle, smooth muscle can sustain very long-term contractions and maintain its contractile function, even when stretched.
• Cardiac muscle tissue is found only in the wall of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Contractions of cardiac muscle are involuntary, like those of smooth muscles. They are controlled by electrical impulses from specialized cardiac cells.

• • Like skeletal muscle, cardiac muscle is striated because its filaments are arranged in sarcomeres. The exact arrangement, however, differs, making cardiac and skeletal muscle tissues look different from one another.
  • The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime. Its cells contain a great many mitochondria to produce ATP for energy and to help the heart resist fatigue.
• A muscle contraction is an increase in the tension or a decrease in the length of a muscle. A muscle contraction is isometric if muscle tension changes, but muscle length remains the same. It is isotonic if muscle length changes, but muscle tension remains the same.

• • A skeletal muscle contraction begins with electrochemical stimulation of a muscle fibre by a motor neuron. This occurs at a chemical synapse called a neuromuscular junction. The neurotransmitter acetylcholine diffuses across the synaptic cleft and binds to receptors on the muscle fibre. This initiates a muscle contraction.
  • Once stimulated, the protein filaments within the skeletal muscle fibre slide past each other to produce a contraction. The sliding filament theory is the most widely accepted explanation for how this occurs. According to this theory, thick myosin filaments repeatedly attach to and pull on thin actin filaments, thus shortening sarcomeres.
  • Crossbridge cycling is a cycle of molecular events that underlies the sliding filament theory. Using energy in ATP, myosin heads repeatedly bind with and pull on actin filaments. This moves the actin filaments toward the center of a sarcomere, shortening the sarcomere and causing a muscle contraction.
  • The ATP needed for a muscle contraction comes first from ATP already available in the cell, and more is generated from creatine phosphate. These sources are quickly used up. Glucose and glycogen can be broken down to form ATP and pyruvate. Pyruvate can then be used to produce ATP in aerobic respiration if oxygen is available, or it can be used in anaerobic respiration if oxygen is not available.
• Physical exercise is defined as any bodily activity that enhances or maintains physical fitness and overall health. Activities such as household chores may even count as physical exercise! Current recommendations for adults are 30 minutes of moderate exercise a day.
• Aerobic exercise is any physical activity that uses muscles at less than their maximum contraction strength, but for long periods of time. This type of exercise uses a relatively high percentage of slow-twitch muscle fibres that consume large amounts of oxygen. Aerobic exercises increase cardiovascular endurance, and include cycling and brisk walking.
• Anaerobic exercise is any physical activity that uses muscles at close to their maximum contraction strength, but for short periods of time. This type of exercise uses a relatively high percentage of fast-twitch muscle fibres that consume small amounts of oxygen. Anaerobic exercises increase muscle and bone mass and strength, and they include push-ups and sprinting.
• Flexibility exercise is any physical activity that stretches and lengthens muscles, thereby improving range of motion and reducing risk of injury. Examples include stretching and yoga.
• Many studies have shown that physical exercise is positively correlated with a diversity of physical, mental, and emotional health benefits. Physical exercise also increases quality of life and life expectancy.

• • Many of the benefits of exercise may come about because contracting muscles release hormones called myokines, which promote tissue repair and growth and have anti-inflammatory effects.
  • Physical exercise can reduce risk factors for cardiovascular disease, including hypertension and excess body weight. Physical exercise can also increase factors associated with cardiovascular health, such as mechanical efficiency of the heart.
  • Physical exercise has been shown to offer protection from dementia and other cognitive problems, perhaps because it increases blood flow or neurotransmitters in the brain, among other potential effects.
  • Numerous studies suggest that regular aerobic exercise works as well as pharmaceutical antidepressants in treating mild-to-moderate depression, possibly because it increases synthesis of natural euphoriants in the brain.
  • Research shows that physical exercise generally improves sleep for most people, and helps sleep disorders, such as insomnia. Other health benefits of physical exercise include better immune system function and reduced risk of type 2 diabetes and obesity.
• There is great variation in individual responses to exercise, partly due to genetic differences in proportions of slow-twitch and fast-twitch muscle fibres. People with more slow-twitch fibres may be able to develop greater endurance from aerobic exercise, whereas people with more fast-twitch fibres may be able to develop greater muscle size and strength from anaerobic exercise.
• Some adverse effects may occur if exercise is extremely intense and the body is not given proper rest between exercise sessions. Many people who overwork their muscles develop delayed onset muscle soreness (DOMS), which may be caused by tiny tears in muscle fibres.
• Musculoskeletal disorders are injuries that occur in muscles or associated tissues (such as tendons) because of biomechanical stresses. The disorders may be caused by sudden exertion, over-exertion, repetitive motions, and similar stresses.

• • A muscle strain is an injury in which muscle fibres tear as a result of overstretching. First aid for a muscle strain includes the five steps represented by the acronym PRICE (protection, rest, ice, compression, and elevation). Medications for inflammation and pain (such as NSAIDs) may also be used.
  • Tendinitis is inflammation of a tendon that occurs when it is over-extended or worked too hard without rest. Tendinitis may also be treated with PRICE and NSAIDs.
  • Carpal tunnel syndrome is a biomechanical problem that occurs in the wrist when the median nerve becomes compressed between carpal bones. It may occur with repetitive use, a tumor, or trauma to the wrist. It may cause pain, numbness, and eventually — if untreated — muscle wasting in the thumb and first two fingers of the hand.
• Neuromuscular disorders are systemic disorders that occur because of problems with the nervous control of muscle contractions, or with the muscle cells themselves.

• • Muscular dystrophy is a genetic disorder caused by defective proteins in muscle cells. It is characterized by progressive skeletal muscle weakness and death of muscle tissues.
  • Myasthenia gravis is a genetic neuromuscular disorder characterized by fluctuating muscle weakness and fatigue. More muscles are affected, and muscles become increasingly weakened as the disorder progresses. Myasthenia gravis most often occurs because immune system antibodies block acetylcholine receptors on muscle cells, and because of the actual loss of acetylcholine receptors.
  • Parkinson’s disease is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. It occurs because of the death of neurons in the midbrain. Characteristic signs of the disorder are muscle tremor, muscle rigidity, slowness of movement, and postural instability. Dementia and depression also often characterize advanced stages of the disease.

As you saw in this chapter, muscles need oxygen to provide enough ATP for most of their activities. In fact, all of the body’s systems require oxygen, and also need to remove waste products, such as carbon dioxide. In the next chapter, you will learn about how the respiratory system obtains and distributes oxygen throughout the body, as well as how it removes wastes, such as carbon dioxide.