The Muscular System: An Overview

• The muscular system is the engine of the human body.
• Its functions include maintaining posture, locomotion, controlling the circulatory system, and facilitating movement with the skeletal system.
• Muscle tissues are responsible for these functions, contracting or shortening to enable body movement.

Key Terms

• Myofibril: A fiber of myofilaments that generates tension in a myocyte (muscle cell).
• Myofilament: A filament of myosin or actin proteins that generate tension by sliding over each other.
• Myosin: A motor protein that forms myofilaments, interacting with actin filaments to produce tension.
• Actin: A protein that forms myofilaments, interacting with myosin filaments to generate tension.
• Striated: The striped appearance of muscle types due to aligned myofibrils, producing directional tension.
• Voluntary: Muscle movement under conscious control.
• Involuntary: Muscle movement not under conscious control.
• Myocyte: A muscle cell.

Muscle Fiber Composition

• Myofibrils are composed of numerous sarcomeres.
• Sarcomeres are the basic contractile units and are composed of myofilaments.
• Myofilaments consist of thick filaments (myosin) and thin filaments (actin, tropomyosin, troponin).
• A myocyte contains thousands of myofibrils.
• Sarcoplasm is the cytoplasm of a muscle cell.
• Sarcoplasmic reticulum is the smooth endoplasmic reticulum (SER) in muscle cells.
• Sarcolemma is the cell membrane of a muscle cell.
• The nucleus controls the cell, and mitochondria provide energy.

Control of Motor Movement

• Motor neuron output is controlled by input from afferent neurons, the primary motor cortex, and the brain stem.

Muscle Tissue Classification

• Muscles can be divided according to their function as voluntary (skeletal) or involuntary (smooth and cardiac).
• Muscles can also be divided according to morphology as striated (skeletal and cardiac) or non-striated (smooth).

Muscle Types

• Striated muscles have transverse bands and myosin and actin organized into sarcomeres.
• Cardiac muscle is striated with irregular, branched cells with 1-2 nuclei near the center.
• Skeletal muscle is striated with long, cylindrical cells and several nuclei around the periphery.
• Unstriated (smooth) muscles have myosin and actin but lack sarcomeres, they are fusiform with one nucleus.

Skeletal Muscle

• Skeletal muscle is attached directly or indirectly to bone and produces voluntary movement, and has a striped appearance.
• Fascia are layers of fibrous connective tissue that wrap the muscle.
• Tendons are formed from fascia, tapering at each end.
• Motor neurons stimulate the muscles.
• The myoneural conjunction/neuromuscular junction is the point where the motor nerve contacts a muscle fiber.
• Each muscle fiber contains hundreds of parallel, cylindrical myofibrils.
• Myofibrils are 1 to 2μm in diameter and consist of bundles of interconnected protein filaments in striated muscles.
• They produce contraction and relaxation.
• Bands of myofibrils include dark, thick A bands; lighter I bands; narrow, dense Z discs or Z lines; and sarcomeres.
• A sarcomere is the portion of a myofibril between two Z discs, and one myofibril consists of a longitudinal series of repeating sarcomeres.

Myofibril vs Sarcomere

• A myofibril is a cylindrical structure of repeating sarcomere units to give the muscle structure.
• A sarcomere is the basic contractile unit of a myofibril, containing actin (thin) and myosin (thick) filaments between two Z-discs to allow muscle contraction.

Actin and Myosin

• Actin is a globular protein and an actin filament is associated with myosin and responsible for cell movements.
• Myosin is a motor protein that converts ATP chemical energy into mechanical energy for movement and force generation.

Muscle Contraction

• Muscle contracts when fibers get shorter, which begins with a nervous system signal, the action potential.
• Requires ATP.

Regulatory Proteins

• Tropomyosin blocks muscle contraction.
• Troponin promotes muscle contraction.

Excitation-Contraction Coupling

• This is the physiological process linking nervous system excitation with mechanical muscle contraction.

Smooth Muscle

• Found in the skin (arrector pili muscles), tracts of the reproductive, respiratory, and urinary systems, hollow organs (intestines, bladder, uterus, stomach), vessels (to constrict blood vessels), and eyes (iris, lens).
• Classification includes single-unit smooth muscle found in hollow organs, and multiunit smooth muscle is found in skin, eyes, and blood vessels.
• Calcium availability controls smooth muscle contraction by myosin-linked regulation.

Cardiac Muscle

• Cardiac muscle is found in the walls of the heart, also known as the myocardium.
• It typically has uninucleate cells.
• The functional properties include having intercalated discs like gap junctions and localized mechanical adhesions to transmit the force to ensure coordinated pumping.
• They can generate endogenous action potentials at periodic intervals, and have action potentials of vertebrate cardiac fibers.
• Action potentials have very long durations, typically 100 to 500 ms long, which prevent contractions from summing and ensures coordinated blood pumping.
• Cardiac conducting cells are specialized cells that initiate and propagate contractions throughout the heart, forming the heart's conducting system.

Contribution of Muscle Physiology

• Movement and Mobility: Muscles enable movement.
• Maintenance of Posture: They maintain posture against gravity.
• Heat Production and Thermoregulation: Muscle contraction produces heat to maintain body temperature.
• Metabolism and Energy Expenditure: Muscles use metabolic pathways for contraction energy.
• Role in Respiratory System: Muscles like the diaphragm are crucial for breathing.
• Contribution to Cardiovascular Health: Aerobic exercises improve cardiovascular health.
• Endocrine System Interaction: Muscle activity influences hormone release.