10.2 Skeletal Muscle

Skeletal muscles generate the force needed for movement, posture, and stability. They're built from layers of connective tissue and specialized fibers that work together in a precise sequence. Understanding their structure from the macro level down to the sarcomere is essential for making sense of how contraction actually happens.

Skeletal Muscle Structure and Function

Connective Tissue Layers of Muscle

Skeletal muscle is wrapped in several layers of connective tissue, each serving a different organizational role. Think of it like packaging: each fiber is individually wrapped, then grouped into bundles, then the whole muscle is encased.

• Endomysium envelops each individual muscle fiber. It's a thin layer of reticular fibers (collagen type III) that supports the fiber and allows nutrient and waste exchange between the fiber and surrounding capillaries.
• Perimysium surrounds bundles of muscle fibers called fascicles. It's made of thicker collagen fibers (collagen type I) that provide structural strength and help coordinate groups of fibers within each bundle.
• Epimysium is a dense irregular connective tissue layer that encases the entire muscle. It protects the muscle from friction against other muscles and bones, and it's continuous with the perimysium internally and with tendons or aponeuroses externally.
• Fascia surrounds groups of muscles, allowing them to slide smoothly against each other during movement. Deep fascia compartmentalizes muscle groups (important clinically for things like compartment syndrome), while superficial fascia lies just beneath the skin.

Muscle-Tendon Interaction for Movement

Muscles on their own just generate tension. Tendons are what translate that tension into movement at a joint.

• Tendons are composed of dense regular connective tissue, rich in parallel collagen fibers. They're continuous with the epimysium, creating a direct mechanical link from muscle fiber to bone.
• The sites where tendons attach to bone are called entheses. These come in two types: fibrous entheses (where tendon attaches directly to bone) and fibrocartilaginous entheses (where a layer of cartilage cushions the transition).
• When a muscle contracts, tension travels through the connective tissue layers into the tendon and then pulls on the bone, producing movement at the joint.

Tendons aren't just passive ropes. They have some elasticity, which lets them store and release elastic energy during activities like running or jumping. This makes movement more efficient. Tendon stiffness also affects how quickly force is transmitted, influencing the speed and power of a movement.

Together, the muscle and tendon form a muscle-tendon unit. The muscle provides active tension (from contraction), and the tendon provides passive tension (from its elastic properties). Different muscle-tendon units are adapted to different demands: some prioritize force production, others prioritize speed or range of motion.

Key Components of Muscle Fibers

Each skeletal muscle fiber is a single, multinucleated cell packed with specialized structures for contraction and energy supply.

• Sarcolemma is the plasma membrane of the muscle fiber. It has deep infoldings called transverse tubules (T-tubules) that plunge into the interior of the fiber. T-tubules are continuous with the extracellular space and carry action potentials deep into the cell so the entire fiber contracts at once. The sarcolemma also contains structural proteins like dystrophin and integrins that anchor the internal cytoskeleton to the extracellular matrix.
• Sarcoplasm is the cytoplasm of the muscle fiber. It contains glycogen (stored glucose for energy), mitochondria (for ATP production via oxidative phosphorylation), and myoglobin (an oxygen-binding protein that gives muscle its red color). Key sarcoplasmic enzymes like creatine kinase and glycolytic enzymes support the energy demands of contraction.
• Myofibrils are the contractile elements, running the length of the fiber in parallel bundles. Each myofibril is made of repeating units called sarcomeres, composed of organized myofilaments (actin and myosin). The parallel arrangement of these myofibrils is what gives skeletal muscle its characteristic striated (striped) appearance. Structural proteins like titin (provides elasticity and keeps myosin centered) and nebulin (regulates actin filament length) maintain sarcomere integrity.
• Sarcoplasmic reticulum (SR) is a specialized smooth endoplasmic reticulum that wraps around each myofibril. Its job is calcium handling. During relaxation, SERCA pumps (calcium ATPases) actively transport $Ca^{2+}$ back into the SR lumen. During contraction, ryanodine receptors (calcium release channels) on the SR open in response to T-tubule signals, flooding the sarcoplasm with $Ca^{2+}$.
• Sarcomere is the basic functional unit of contraction. It's bounded on each end by Z-lines (also called Z-discs), which anchor the thin (actin) filaments and transmit force between adjacent sarcomeres. The thick (myosin) filaments sit in the center. During contraction, the thin filaments slide over the thick filaments, pulling the Z-lines closer together and shortening the sarcomere. This is the sliding filament theory.

Excitation-Contraction Coupling Process

This is the step-by-step sequence that converts a nerve signal into muscle contraction. Every step matters, and exam questions love to test the order.

1. Nerve signal arrives. A motor neuron action potential reaches the neuromuscular junction (the synapse between the motor neuron and the muscle fiber). This triggers the release of acetylcholine (ACh) into the synaptic cleft.

2. End-plate potential generated. ACh binds to nicotinic acetylcholine receptors on the sarcolemma, causing a localized depolarization called an end-plate potential.

3. Action potential propagates. The end-plate potential triggers voltage-gated sodium channels to open, generating a full action potential. This action potential spreads along the sarcolemma and dives into the T-tubules, reaching deep into the fiber.

4. Calcium is released. T-tubule depolarization activates dihydropyridine receptors (voltage-gated calcium channels), which are mechanically coupled to ryanodine receptors on the SR. The ryanodine receptors open, and $Ca^{2+}$ floods into the sarcoplasm. (Note: in skeletal muscle, this coupling is mechanical, not true calcium-induced calcium release as seen in cardiac muscle.)

5. Troponin-tropomyosin shift. The released $Ca^{2+}$ binds to troponin C on the thin filaments. This causes a conformational change that shifts tropomyosin away from the myosin-binding sites on actin, exposing them.

6. Cross-bridge cycling begins. Myosin heads bind to the exposed actin sites (cross-bridge formation), then perform a power stroke, pulling the thin filaments toward the center of the sarcomere. ATP hydrolysis by myosin ATPase provides the energy for each cycle of attachment, power stroke, detachment, and re-cocking. This cycling repeats as long as $Ca^{2+}$ and ATP are available.

7. Relaxation. When the nerve signal stops, SERCA pumps actively transport $Ca^{2+}$ back into the SR. As $Ca^{2+}$ dissociates from troponin C, tropomyosin slides back over the myosin-binding sites. Cross-bridges can no longer form, and the muscle fiber returns to its resting length.

Muscle Fiber Types and Motor Units

Not all skeletal muscle fibers are the same. They vary in how fast they contract and how they produce ATP, which determines what kinds of activities they're best suited for.

Motor units consist of a single motor neuron and all the muscle fibers it innervates. A motor unit is the smallest controllable unit of force.

• Motor units are recruited according to the size principle: smaller motor units (fewer fibers, often Type I) are recruited first for low-force tasks. As more force is needed, larger motor units (more fibers, often Type II) are progressively recruited.
• Fine-control muscles like those in the eye have small motor units (few fibers per neuron). Powerful muscles like the quadriceps have large motor units (many fibers per neuron).
• The combination of which motor units are active and how frequently they fire determines the total force output and fatigue resistance of the muscle.

Muscle metabolism differs by fiber type and activity demand:

• Aerobic metabolism (oxidative phosphorylation) dominates in Type I fibers. It's efficient and sustainable but requires oxygen, which is why these fibers are rich in mitochondria, myoglobin, and capillaries.
• Anaerobic metabolism (glycolysis and the phosphocreatine system) is more prominent in Type II fibers. It produces ATP rapidly but in limited quantities, leading to quicker fatigue and lactic acid accumulation.