Skeletal muscles are the powerhouses of movement in our bodies. They're made up of fibers bundled together, with each fiber containing tiny contractile units called sarcomeres. These structures work together to create the force needed for everything from lifting weights to blinking.
Understanding how muscles work is key to grasping exercise physiology. From the cellular level to the whole muscle, every part plays a role in generating movement. We'll explore how signals from nerves trigger muscle action and how different muscle types affect performance.
Skeletal Muscle Structure and Function
Muscle Fiber Organization
- Skeletal muscle comprises muscle fibers (cells) organized into fascicles bundled together to form the whole muscle
- Muscle fibers contain myofibrils composed of sarcomeres functioning as the basic units of muscle contraction
- Sarcomeres consist of thick filaments (myosin) and thin filaments (actin) arranged in a specific pattern allowing for muscle contraction
- Myosin filaments form the A-band of the sarcomere
- Actin filaments extend from the Z-lines and form the I-band
- Connective tissue components provide structural support and force transmission
- Endomysium surrounds individual muscle fibers
- Perimysium encases fascicles (bundles of muscle fibers)
- Epimysium covers the entire muscle
Cellular Components and Membranes
- Sarcolemma functions as the cell membrane of the muscle fiber containing specialized structures called T-tubules for transmitting electrical signals
- T-tubules form a network throughout the muscle fiber allowing rapid signal propagation
- Sarcoplasmic reticulum surrounds the myofibrils and stores calcium ions necessary for muscle contraction
- Terminal cisternae of the sarcoplasmic reticulum form triads with T-tubules
- Mitochondria provide energy for muscle contraction through aerobic metabolism (oxidative phosphorylation)
- Glycogen granules serve as a readily available energy source for anaerobic metabolism
Excitation-Contraction Coupling in Muscle
Neuromuscular Junction and Action Potential Generation
- Excitation-contraction coupling converts an electrical stimulus into a mechanical response in skeletal muscle
- Process begins with an action potential arriving at the neuromuscular junction triggering acetylcholine release
- Acetylcholine binds to nicotinic receptors on the sarcolemma causing depolarization and generating an action potential in the muscle fiber
- Sodium ions flow into the muscle fiber through voltage-gated sodium channels
- Potassium ions flow out of the muscle fiber to repolarize the membrane
- Action potential propagates along the sarcolemma and into the T-tubules activating voltage-gated calcium channels
- Dihydropyridine receptors (DHPRs) in T-tubules sense the change in membrane potential
Calcium Release and Muscle Contraction
- Activated DHPRs trigger the opening of ryanodine receptors (RyRs) in the sarcoplasmic reticulum
- Calcium release from the sarcoplasmic reticulum through RyRs triggers the sliding filament mechanism
- Calcium binds to troponin C on the thin filaments
- Tropomyosin shifts position exposing myosin binding sites on actin
- Myosin heads attach to actin filaments forming cross-bridges and generating force through the power stroke
- ATP hydrolysis provides energy for the power stroke and cross-bridge cycling
- Process concludes with active reuptake of calcium into the sarcoplasmic reticulum by calcium ATPase pumps leading to muscle relaxation
Motor Units and Muscle Contraction
Motor Unit Structure and Recruitment
- Motor unit consists of a single motor neuron and all the muscle fibers it innervates
- Motor units vary in size with smaller units controlling fine movements (eye muscles) and larger units responsible for gross movements (leg muscles)
- Size principle of motor unit recruitment states that smaller motor units activate first followed by progressively larger units as force demands increase
- Ensures smooth and efficient force production
- Motor unit recruitment and firing rate modulation serve as primary mechanisms for controlling muscle force production
- Recruitment adds more motor units to increase force
- Rate coding increases the firing frequency of active motor units
Motor Unit Types and Properties
- Asynchronous firing of motor units allows for smooth sustained muscle contractions and prevents fatigue
- Different types of motor units have distinct physiological and metabolic properties
- Slow-twitch (Type I) motor units: fatigue-resistant with high oxidative capacity (marathon runners)
- Fast-twitch oxidative (Type IIa) motor units: intermediate fatigue resistance and force production (middle-distance runners)
- Fast-twitch glycolytic (Type IIx) motor units: high force production but low fatigue resistance (sprinters)
- Motor unit type composition varies between muscles and can be altered through training
- Endurance training increases the proportion of slow-twitch fibers
- Resistance training can increase the size and force production of fast-twitch fibers
Muscle Fiber Arrangement vs Force Production
Muscle Architecture and Force Generation
- Muscle fiber arrangement (muscle architecture) refers to the orientation of muscle fibers relative to the line of force generation
- Parallel fiber arrangement allows for greater range of motion but produces less force compared to pennate arrangements (biceps brachii)
- Pennate muscle arrangements increase force production due to increased physiological cross-sectional area
- Unipennate: fibers arranged at an angle to one side of the tendon (gastrocnemius)
- Bipennate: fibers arranged at angles on both sides of the tendon (rectus femoris)
- Multipennate: fibers arranged at multiple angles (deltoid)
- Angle of pennation affects force transmitted to the tendon with larger angles resulting in reduced force transmission but increased muscle packing
Muscle Fiber Characteristics and Force-Velocity Relationships
- Muscle fiber length influences the velocity of contraction with longer fibers capable of greater shortening velocities
- Sartorius muscle has long fibers for rapid knee flexion and hip flexion/rotation
- Force-length relationship of muscle fibers affects overall muscle force production with optimal force generated at resting sarcomere lengths
- Descending limb: force decreases as muscle shortens beyond optimal length
- Ascending limb: force decreases as muscle lengthens beyond optimal length
- Muscle fiber type composition influences the force-velocity characteristics and fatigue resistance of the muscle
- Fast-twitch fibers have higher maximum shortening velocities but fatigue more quickly
- Slow-twitch fibers have lower maximum shortening velocities but greater fatigue resistance