Skeletal muscle

• 1. Introduction

  ›

  Skeletal muscle makes up about 35% of the human body by weight, and there are more than 600 of these muscles working in a single person. A single biceps in a young adult male holds around 253,000 muscle fibers. One of those fibers, ten centimeters long, can carry as many as 3,000 nuclei inside a single cell. These are some of the largest cells in the body, roughly two to three centimeters long and 100 micrometers across. They are the reason you can lift, walk, hold your posture, and stay warm. Yet skeletal muscle is far stranger than a simple engine for movement. It is one of three muscle types in vertebrates, sitting alongside cardiac muscle and smooth muscle, and it is the one you command at will. This documentary asks what a muscle fiber actually is, how it converts a nerve signal into force, why some fibers sprint while others endure, and how a tissue built for motion quietly behaves like a gland that talks to the brain. The answers reach from a turtle's neck to a Canadian study of aging minds.

• 2. Anatomy Of A Fiber

  ›

  A muscle fiber begins as a fusion event. During myogenesis, developmental cells called myoblasts merge into long multinucleated cells, each myoblast donating a nucleus. This fusion depends on muscle-specific proteins called fusogens, named myomaker and myomerger. The many nuclei that result, called myonuclei, sit elongated against the inside of the cell membrane rather than floating in the center. Each myonucleus governs its own myonuclear domain, responsible for the cytoplasm in its stretch of the fiber.

  The cell membrane of a muscle fiber has its own vocabulary. It is called the sarcolemma, the cytoplasm is the sarcoplasm, and packed inside are myofibrils, long protein bundles about one micrometer in diameter. Pressed against the sarcolemma are the flattened myonuclei, and between the myofibrils sit the mitochondria that meet the cell's energy needs. Wrapping the myofibrils is the sarcoplasmic reticulum, which holds a reserve of calcium ions and ends in dilated sacs called terminal cisternae.

  The whole muscle is organized in nested layers of connective tissue called fascia. Deep fascia specializes to enclose each muscle fiber as endomysium, each bundle of fibers, or fascicle, as perimysium, and each whole muscle as epimysium. Together these are called the mysia. At each end of the contractile fibers lies a non-contractile tendon of dense fibrous tissue, and the length of a muscle includes those tendons.

  Not every nucleus in a muscle belongs to a muscle fiber. By count, myocyte nuclei may be only half of the nuclei present. The rest belong to smaller mononuclear cells. One survey by Cameron and colleagues identified nine cell types, with endothelial cells lining capillaries making up 45%, fibro-adipogenic progenitors 20%, and pericytes 14%. Among them sit muscle stem cells, the satellite cells, waiting beneath the basement membrane for the call to repair.

• 3. How A Signal Becomes Force

  ›

  Acetylcholine is the chemical that starts a contraction. Released by motor neurons at the neuromuscular junction, it depolarizes the muscle fiber. A motor neuron and all the fibers it touches form a motor unit, and a single muscle is driven by many such units. The signal travels across the cell surface and dives inward through a network of transverse tubules, the T tubules, which carry the action potential deep into the fiber.

  The coupling between electrical signal and physical pull happens at a structure called the triad. There, a transverse tubule sits between two regions of sarcoplasmic reticulum studded with ryanodine receptors, RYR1. On the tubule membrane are voltage-gated channels called dihydropyridine receptors. When the inner fiber depolarizes, these receptors physically interact with the ryanodine receptors through foot processes, prying them open. Calcium floods out of the sarcoplasmic reticulum, and the near-synchronous firing of thousands of calcium sparks lifts calcium across the whole cell.

  The contraction itself is a story of four proteins. Thick filaments of myosin and thin filaments of actin repeat in units called sarcomeres, the basic contractile units that give skeletal muscle its striped, striated look under a microscope. Guarding the actin are two regulatory proteins, troponin and tropomyosin, which normally block myosin from binding. When calcium binds troponin, troponin changes shape, tropomyosin shifts aside, and the myosin-binding sites on actin are exposed. Myosin heads then cycle through ATP-dependent cross-bridges, and the muscle shortens.

  Relaxation is just as active. A pump called SERCA drives calcium back into the sarcoplasmic reticulum, where a binding protein called calsequestrin gives the reservoir its large buffering capacity. As calcium falls to resting levels, force fades and the fiber lets go.

• 4. Sprinters And Marathoners

  ›

  Type I and Type II divide the muscle world. Type I fibers are slow, while Type II are fast, and Type II splits further into the oxidative IIA and the glycolytic IIX, giving three main fiber types. Slow oxidative Type I fibers appear dark red, rich in myoglobin, mitochondria, and capillaries, and they resist fatigue using aerobic respiration. The pale Type IIX fibers run on anaerobic glycolysis, contract fast, and tire quickly. Type IIA fibers sit between, fast but largely aerobic.

  Proportions vary muscle by muscle, and the numbers tell the story. In humans the quadriceps run about 52% Type I, the soleus about 80% Type I, and the orbicularis oculi of the eye only about 15% Type I. Sedentary men and women carry roughly 45% Type II and 55% Type I. Endurance athletes show higher Type I, sprinters need large numbers of Type IIX, and middle-distance athletes show roughly equal shares.

  The split shapes how muscles sound and shake. Slow twitch fibers produce 10 to 30 contractions per second, fast twitch fibers 30 to 70. Clench a firm fist and you can feel the vibration; press that tensed muscle to your ear and hear a rumble. Fast twitch fibers can develop tension at two to three times the rate of slow twitch fibers.

  Fiber identity is flexible. Endurance events may push some Type IIX fibers toward Type IIA, though there is no consensus, and the shift may instead reflect more and larger mitochondria. Across species the variation widens. American lobsters carry slow-tonic fibers that hold long contractions, and chimpanzee muscle is 67% fast-twitch, with maximum dynamic force and power about 1.35 times that of human muscle of similar size.

• 5. Measuring Strength

  ›

  Calling any one muscle the strongest is misleading, because three factors overlap at once. Physiological strength depends on size and cross-sectional area, neurological strength on the signal that tells the muscle to contract, and mechanical strength on leverage, moment arm, and joint geometry. Muscles never work alone, which is why naming a single champion is a trap.

  By the everyday meaning of force on an external object, the masseter, the jaw muscle, wins. The 1992 Guinness Book of Records logged a bite strength of 4337 newtons held for 2 seconds. Its edge is leverage, working against a much shorter lever arm than other muscles, not anything unusual in the muscle tissue itself. Each individual fiber exerts force on the order of 0.3 micronewtons, and that figure barely varies from fiber to fiber.

  By force at the point of insertion, the largest cross-sectional area wins, usually named as the quadriceps femoris or the gluteus maximus. By weight, the myometrial layer of the uterus may be the strongest in the female body. At delivery the whole uterus weighs about 1.1 kilograms, and during childbirth it exerts 100 to 400 newtons of downward force with each contraction. Popular claims about the tongue collapse on inspection, since the tongue is eight muscles, not one.

  Clinicians grade strength on a scale from 0 to 5, from no contraction at all to normal strength. Grade 3 marks movement against gravity but not against added resistance, the boundary where weakness starts to signal disease. Significant drops can point to an underlying myopathy or neuropathy.

• 6. The Engine Room

  ›

  Skeletal muscle burns more calories than other organs. At rest it consumes 54.4 kilojoules per kilogram per day, far above adipose tissue at 18.8 and bone at 9.6. That appetite has a payoff and a cost, because human muscle is not an efficient machine. Measured in rowing and cycling, its efficiency runs only 18% to 26%, the ratio of mechanical work out to total metabolic cost.

  The fuel comes in tiers. A short-term store sits in creatine phosphate, which can regenerate ATP through creatine kinase when demand spikes. A larger store is glycogen, converted rapidly to glucose for sustained, powerful contractions. Glycolysis breaks a glucose molecule down anaerobically into two ATP and two lactic acid molecules, while aerobic systems take longer, run through many more steps, and yield far more ATP from the same fuel.

  Muscle is also a furnace. Muscle contraction produces 85% of the body's heat, mostly as a wasted by-product of activity. When the body is dangerously cold, muscles are signaled into the contractions of shivering, generating heat on demand as a homeostatic response.

  Lactic acid is both waste and resource. High intracellular concentrations inhibit ATP generation and can even stop it, but endurance training raises capillarization and myoglobin to clear it faster. Once removed, lactic acid feeds other muscles and tissues, or travels to the liver to be converted back to pyruvate. Delayed onset muscle soreness, felt one to three days after exercise, is now attributed not to lactic acid but to tiny tears from eccentric contraction.

• 7. Muscle As A Gland

  ›

  After it contracts, a muscle starts to speak. Skeletal muscle acts as an endocrine organ, secreting myokines, a range of cytokines and peptides that travel through the bloodstream as signaling molecules. Interleukin 6 is the most studied myokine, with others including BDNF, FGF21, and SPARC. Under different conditions, subsets of 654 different proteins, along with lipids, amino acids, metabolites, and small RNAs, appear in the muscle secretome.

  Exercise rewires which of those messages get sent. Williams and colleagues took thigh biopsies from eight 23-year-old originally sedentary men before and after a six-week endurance program of one hour of stationary cycling, five days a week. Of 13,108 genes with detected expression, 641 were upregulated and 176 downregulated, and 531 of the altered genes were identified as part of the secretome. The pathways most affected related to cardiac, cognitive, kidney, and platelet function, suggesting the effect of exercise is more endocrine than metabolic.

  The link to the mind shows up in aging. A study in Canada measured the arm and leg muscle mass of 8,279 people over 65, then tested them again three years later. The 1,605 participants with low muscle mass at baseline declined in executive mental function far more sharply than those with more muscle. Memory and psychomotor speed, by contrast, showed no such correlation.

  The simple act of walking carries its own weight. Paluch and colleagues pooled 15 studies covering 47,471 adults over 7 years. Among those over 60, with the lowest quartile averaging 3,553 steps a day set at a mortality risk of 1.0, the second, third, and fourth quartiles fell to 0.56, 0.45, and 0.35. The fourth quartile averaged 10,901 steps a day, and the briskness of those steps made no difference once volume was accounted for.