This is the first of a two part series explaining howmuscles work. In this first article I describe the gross structure of skeletalmuscle; in the second article I describe skeletal muscle ultrastructure and howmuscles develop tension.
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Gross Structure of Skeletal Muscle
Humans possess three types of muscle—cardiac, smooth, and skeletal—each exhibiting distinct functional and anatomical differences, see below.
Cardiac muscle residesonly in the heart. It shares several common features with skeletal muscle asboth appear striated (striped) under low-magnification microscopic examinationand both shorten (contract) in a similar manner. Smooth muscle lacks a striatedappearance but shares cardiac muscle’s characteristic of nonconsciousregulation under autonomic nervous system control. Skeletal muscle operatesunder voluntary control, as incurling a 25-lb barbell.
Individuals can easilycontrol the velocity of movement in a barbell curl, the range of motion duringthe lifting movement, and the number of repetitions completed. In golf, theplayer controls all aspects of the coordinated and hopefully perfectly timedmovements of the arms, legs, and torso during the backswing and downswing. Adifferent situation exists for both cardiac and smooth muscle tissue becauseactivity of these tissues occurs involuntarily,although mediation from central centers can exert some influence. This means ageneral absence of conscious control as to how fast the heart beats, or howfast food moves through the digestive system, or how the miles of blood vesselscontract and expand throughout the day.
The figure below illustrates a cross section of skeletal muscle structures and arrangement of connective tissue wrappings, including the thousands of cylindrical wells called fibers.
Each of the body’sapproximately 600-plus skeletal muscles contain various wrappings of fibrousconnective tissue. These long, slender, multinucleated fibers lie parallel toeach other, with the force of action directed along the fiber’s long axis.Their number probably remains largely fixed by the second trimester of fetaldevelopment. Individual fiber length varies from a few millimeters in the eyemuscles to nearly 30 cm in the large antigravity leg muscles (with widthreaching 0.15 mm).
Skeletal Muscle Organization
The endomysium, a fine layer of connective tissue, wraps each muscle fiber and separates it from neighboring fibers. Another layer of connective tissue, the perimysium, surrounds a bundle of up to 150 fibers called a fasciculus. A fascia of fibrous connective tissue, the epimysium, surrounds the entire muscle. This protective sheath tapers at its distal and proximal ends as it blends into and joins the intramuscular tissue sheaths to form the tendon’s dense, strong connective tissue. Tendons connect both ends of the muscle to the periosteum, the bone’s outermost covering.
|FYI— Tendinitis, a condition of tendon inflammation, most commonly occurs from trauma at the patellar tendon of the knee (common in basketball and volleyball athletes) and other body regions. These include the Achilles region of the ankle (common in sports requiring high impact during lunging and jumping activities), or at the attachment of the rotator cuff muscles, a group of muscles and their tendons that act to stabilize the shoulder (common in sports that involve high-velocity baseball pitching, shotput, or discuss throwing). Tendinitis also can occur from overuse and putting limbs through extreme movements that exceed the joints’ normal range of motion. In less severe tendon trauma, common therapies include nonsteroidal anti-inflammatory medicines (NSAIDs), immobilization, ice, and rest, with gradual return to normal physical activities.|
The tissues of thetendon intermesh with the collagenous fibers within bone. This forms a powerfullink between muscle and bone that remains inseparable except during severestress when the tendon can sever or literally pull away from the bone. When thetendon attaches to the end of a long bone, the bone adapts by enlarging at thatend to create a more stable union.
The force of muscleaction transmits directly from the connective tissue harness to the tendons,which then pull on the bone at the point of attachment. The forces exerted onthe tendinous attachments under muscular exertion range from 20 to 50 N (197 to492 kg) per cm2 of cross-sectional area—forces often larger than themuscle fibers themselves can tolerate.
The muscle’s origin refers to the location where thetendon joins a relatively stable skeletal part, generally the proximal or fixedend of the lever system or that nearest the body’s midline; the point of distalmuscle attachment to the moving bone represents the insertion.
Beneath the endomysiumand surrounding each muscle fiber lies the sarcolemma,a thin, elastic membrane that encloses the fiber’s cellular contents. Itcontains a plasma membrane (plasmalemma) and a basement membrane. Between thebasement and plasma membranes lie myogenic stem cells known as satellite cells, these normallyquiescent myoblasts function in regenerative cellular growth provide possibleadaptations to exercise training and recovery from injury.
Incorporation of satellite cell nuclei into existing muscle fibers seems a likely explanation for exercise-induced muscle fiber hypertrophy. The fiber’s aqueous protoplasm or sarcoplasm contains enzymes, fat and glycogen particles, nuclei (approximately 250 per mm of fiber length) that contain the genes, mitochondria, and other specialized organelles. The sarcoplasmic reticulum, an extensive longitudinal latticelike network of tubular channels and vesicles provides structural integrity to the cell.
Muscles’ Chemical Composition
Waterconstitutes approximately 75% of skeletal muscle mass while protein composes20%. The remaining 5% contains salts and other substances, includinghigh-energy phosphates; urea; lactate; the minerals calcium, magnesium, andphosphorus; various enzymes; sodium, potassium, and chloride ions; and aminoacids, fats, and carbohydrates. The most abundant muscle proteins includetitin, the largest protein in the body consisting of 27,000 amino acids(accounts for about 10% of muscle mass), myosin (approximately 60% of muscleprotein), actin, and tropomyosin. Each 100 g of muscle tissue contains about700 mg of the oxygen-binding, conjugated protein myoglobin.
Arteries andveins that lie parallel to individual muscle fibers provide a richvascular supply. These vessels divide into numerous arterioles, capillaries,and venules to form a diffuse network in and around the endomysium.Extensive branching of blood vessels ensures each muscle fiber an adequateoxygenated blood supply from the arterial system and rapid removal of carbondioxide in the venous circulation. During vigorous physical activity for anelite endurance athlete, the muscle’s oxygen uptake increases nearly 70 timesto approximately 11 mL per 100 g per minute. The local vascular bed deliverslarge quantities of blood through active tissues to accommodate this oxygenrequirement. Blood flow distribution fluctuates in rhythmic running, swimming,cycling, and other similar activities. Flow decreases during the muscle’scontraction phase and increases during relaxation to provide an auxiliary“milking action” that moves blood through the muscles and propels it via thevenous system back to the heart. Between 200 and 500 capillaries deliver bloodto each square millimeter of active muscle cross section, with up to fourcapillaries directly contacting each fiber. In endurance athletes, five toseven capillaries surround each fiber; this positive adaptation ensures greaterlocal blood flow and adequate tissue oxygenation when needed.
Physicalactivities that require “straining” (i.e., exerting force against an immovableobject) present a somewhat different picture for muscle blood flow. When amuscle generates about 60% of its force-generating capacity for severalseconds, elevated intramuscular pressure occludes local blood flow during thecontraction. With a sustained high-force contraction, the intramuscularhigh-energy phosphates and glycolytic anaerobic reactions provide the mainenergy source for muscular effort.
Trained muscles’ increased capillary-to-muscle fiber ratio helps to explain improved exercise capacity with endurance training. An enhanced capillary microcirculation expedites removal of heat and metabolic byproducts from active tissues in addition to facilitating delivery of oxygen, nutrients, and hormones. Electron microscopy reveals the total number of capillaries per muscle and capillaries per mm2 of muscle tissue averages about 40% higher in endurance-trained athletes than untrained counterparts. Enhanced vascularization at the capillary level proves particularly beneficial during activities that require a high level of steady-rate aerobic metabolism. Vascular stretch and shear stress on the vessel walls from increased blood flow during exercise stimulate capillary development with intense aerobic training.
Source:McArdle WD,Katch FI, Katch VL. Exercise Physiology:Nutrition Energy, and Human Performance.
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Eighth Edition. Wolters KluwerPubl. 2015.