Chapter 9: Muscular System: Skeletal Muscle & Organization

Structurally, each skeletal muscle is enveloped by three connective tissue layers—the epimysium, the perimysium which separates fibers into bundles called fascicles, and the inner endomysium housing regenerative myosatellite cells—all of which merge to form tough tendons or broad aponeuroses. The specialized cells, or muscle fibers, are uniquely large and multinucleate, featuring invaginations of the cell membrane known as T tubules that rapidly convey electrical impulses (action potentials) into the cell interior. These impulses are triggered by the release of the neurotransmitter acetylcholine (ACh) from a motor neuron at the neuromuscular synapse, where the enzyme acetylcholinesterase (AChE) promptly limits the duration of stimulation. Internally, muscle fibers are packed with myofibrils, which are built from repeating functional units called sarcomeres. Sarcomere shortening, the essence of contraction, is explained by the sliding filament theory, relying on the interaction between thick filaments (myosin) and thin filaments (actin). Contraction is regulated by the proteins troponin and tropomyosin, which normally cover active sites on actin until calcium ions (Ca2+), released from the sarcoplasmic reticulum (SR) following the T-tubule impulse, bind to troponin and expose the sites. Tension production is modulated by the nervous system through motor units (a neuron and all the fibers it controls); precise movements use small motor units, whereas powerful movements use large units. Muscles increase force via recruitment (multiple motor unit summation) and maintain continuous resting tension through muscle tone. The chapter also differentiates between three metabolic fiber types: rapidly fatiguing fast fibers (relying on anaerobic glycolysis), fatigue-resistant slow fibers (rich in myoglobin and highly efficient aerobic metabolism), and intermediate fibers. The overall power and range of motion are influenced by fiber arrangement, categorized as parallel, convergent, tension-generating pennate, or circular (sphincter) muscles. Movement analysis employs muscle terminology, classifying actions by identifying the relatively fixed origin and movable insertion, and grouping muscles functionally as agonists (prime movers), antagonists (opposers), synergists (helpers), or fixators (stabilizers). Skeletal mechanics rely on the principle of levers (bones) acting around a fulcrum (joint), with the third-class lever being the most common arrangement, often utilizing bony structures like the patella or lateral malleolus as anatomical pulleys to redirect force. Finally, the aging process results in muscular decline, marked by decreasing fiber size, reduced elasticity due to the buildup of fibrous connective tissue (fibrosis), and a lessened capacity for muscle repair.