Skeletal muscle
More than 600 skeletal muscles exist in the human body, making up around 40% of body weight in healthy young adults. These structures attach to bones via tendons and form bilaterally-placed pairs that serve both sides of the body. A single muscle such as the biceps in a young adult male contains around 253,000 individual fibers. Each fiber is multinucleated with nuclei called myonuclei located along the inside of the cell membrane. Muscle fibers are formed from the fusion of developmental myoblasts during a process known as myogenesis. The tissue appears striated due to the arrangement of sarcomeres which are the basic functional units necessary for contraction. Connective tissue layers surround each fiber as endomysium, each fascicle as perimysium, and each whole muscle as epimysium. Deep fascia separates groups of muscles into compartments like the four groups found in the arm or leg. Sensory receptors within these tissues include muscle spindles and Golgi tendon organs. Muscle spindles act as stretch receptors located in the belly while Golgi tendon organs inform about tension at the myotendinous junction.
Muscle contraction relies on the interaction between actin and myosin filaments within repeating sarcomere units. Calcium ions released from the sarcoplasmic reticulum bind to troponin causing tropomyosin to move and expose binding sites on actin. This allows myosin heads to attach and perform ATP-dependent cross-bridge cycling that shortens the muscle. Excitation-contraction coupling occurs when depolarization spreads across T-tubules to activate dihydropyridine receptors. These receptors physically interact with ryanodine receptors to open calcium channels in the terminal cisternae. The resulting calcium spark diffuses into the bulk cytoplasm to trigger force generation. Skeletal muscle cells produce adenosine triphosphate molecules to power the movement of myosin heads. About 95 percent of ATP required for resting or moderately active muscles comes from aerobic respiration in mitochondria. Creatine phosphate serves as a short-term energy store that regenerates ATP during the first few seconds of exertion. Glycogen stored in muscle fibers converts rapidly to glucose for sustained powerful contractions. Anaerobic glycolysis produces lactic acid which can inhibit ATP generation if intracellular concentration becomes too high.
Type I fibers appear red due to high levels of myoglobin and contain more mitochondria than other types. Fast oxidative type IIA fibers have fast contractions but fatigue more quickly than slow oxidative fibers. Fast glycolytic type IIX fibers rely on anaerobic metabolism and generate large amounts of lactic acid. Most skeletal muscles contain all three fiber types though proportions vary depending on the specific action. In humans the quadriceps muscles contain approximately 52% type I fibers while the soleus is about 80% type I. The orbicularis oculi muscle of the eye contains only around 15% type I fibers. Sedentary men and women typically possess 45% type II and 55% type I fibers. Endurance athletes show higher levels of type I fibers whereas sprinters require large numbers of type IIX fibers. Fiber typing methods include histochemical staining for myosin ATPase activity or immunohistochemical staining for myosin heavy chain. Early researchers believed humans expressed MHC IIb but later research showed this was actually IIx in human tissue. Type distribution varies considerably from person to person and muscle to muscle within the same individual. Environmental factors like diet and exercise play a pivotal role in determining these proportions.
During embryonic development between the tenth and eighteenth weeks of gestation all muscle cells have fast myosin heavy chains. Two myotube types become distinguished in the developing fetus with one expressing both fast and slow chains. Between 10 and 40 percent of fibers express the slow myosin chain during early stages. Myoblasts migrate out into the body following chemical signals to form all muscles except those associated with the vertebral column. Satellite cells found underneath the basal lamina are necessary for postnatal development of muscle cells. Muscle fibers grow when exercised and shrink when not in use due to changes in myofibril numbers. Well-exercised muscles develop more mitochondria, myoglobin, glycogen and higher capillary density. Natural hypertrophy normally stops at full growth in the late teens after puberty accelerates the process. Inactivity malnutrition disease and aging can increase breakdown leading to sarcopenia or cachexia. Human spaceflight involving prolonged immobilization results in loss of as much as 30% of mass in some muscles. Exercise training shifts fiber proportions toward slow twitch fibers while explosive powerlifting transitions them toward fast twitch.
Skeletal muscle is identified as an endocrine organ because it synthesizes and secretes multiple factors that exert beneficial effects on remote organs. Subsets of 654 different proteins occur in the secretome of skeletal muscles under various physiological conditions. Interleukin 6 is the most studied myokine among other contraction-induced peptides like BDNF and FGF21. A study in Canada measured skeletal muscle mass of 8,279 Canadians over age 65 to test effects on mental functions. Individuals with lower skeletal muscle mass declined in executive mental function considerably more sharply than those with higher mass. Walking affects mortality risk significantly especially among older individuals according to a meta-analysis of 15 studies evaluating 47,471 adults. The lowest quartile averaged 3,553 steps per day while the fourth quartile reached 10,901 steps daily. Relative risk of mortality for the highest walking group was 0.35 compared to the lowest group set at 1.0. Biopsies from eight sedentary males showed 641 genes upregulated after six weeks of endurance training. Many exercise-regulated genes are identified as secreted indicating much of the effect has an endocrine rather than metabolic function.
Diseases of skeletal muscle are termed myopathies while diseases of nerves are called neuropathies falling under neuromuscular disease. Symptoms may include weakness spasticity myoclonus and myalgia affecting both function and causing pain. Diagnostic procedures include testing creatine kinase levels in blood or performing electromyography to measure electrical activity. Muscle biopsy identifies specific myopathies alongside genetic testing for DNA abnormalities associated with dystrophies. Inactivity malnutrition disease and aging increase breakdown leading to sarcopenia which causes frailty and its consequences. Cancer AIDS and heart failure can cause cachexia resulting in significant muscle loss known as wasting. Human spaceflight involving prolonged periods of weightlessness results in weakening and atrophy of muscles. Research uses cell culture systems derived from healthy or diseased tissue biopsies to model these conditions. Electrical stimulation determines force and contraction speed at different frequencies related to fiber-type composition within groups. Surface EMG measures action potentials occurring from hyperpolarization of motor axons sent by nerve impulses. Non-invasive elastography techniques measuring muscle noise are undergoing experimentation to monitor neuromuscular disease progression.
Common questions
What percentage of body weight does skeletal muscle make up in healthy young adults?
Skeletal muscles make up around 40% of body weight in healthy young adults. More than 600 skeletal muscles exist in the human body to perform this function.
How do calcium ions trigger contraction in Skeletal muscle cells?
Calcium ions released from the sarcoplasmic reticulum bind to troponin causing tropomyosin to move and expose binding sites on actin. This allows myosin heads to attach and perform ATP-dependent cross-bridge cycling that shortens the muscle.
Which Skeletal muscle fiber type contains approximately 80 percent type I fibers in humans?
The soleus muscle is about 80% type I fibers while other muscles like the quadriceps contain approximately 52% type I fibers. Type I fibers appear red due to high levels of myoglobin and contain more mitochondria than other types.
When does natural hypertrophy stop during human development?
Natural hypertrophy normally stops at full growth in the late teens after puberty accelerates the process. Muscle fibers grow when exercised and shrink when not in use due to changes in myofibril numbers.
What relative risk of mortality applies to the highest walking group compared to the lowest group?
Relative risk of mortality for the highest walking group was 0.35 compared to the lowest group set at 1.0. The fourth quartile reached 10,901 steps daily while the lowest quartile averaged 3,553 steps per day.