Muscle Contraction

Muscle Function and Interaction

• Muscle function relies on the interaction of protein filaments.

• Muscle activity is initiated by input from the nervous system.

• Muscle action is characterized by contraction; extension occurs passively.

Vertebrate Skeletal Muscle

• Skeletal muscle in vertebrates facilitates movement of bones and the body.

• It is structured hierarchically into smaller and smaller units.

• Composed of bundles of long fibers, each fiber is a single cell that runs parallel to the muscle’s length.

Muscle Fiber Structure

• Each muscle fiber is made up of smaller myofibrils arranged longitudinally.

• Myofibrils consist of two types of myofilaments:

  • Thin filaments: Composed of actin strands and regulatory proteins.

  • Thick filaments: Consist of staggered arrays of myosin molecules.

Striated Muscle Characteristics

• Skeletal muscle is also termed striated owing to the regular arrangement of myofilaments.

• The functional unit of muscle, known as a sarcomere, is defined by Z lines.

The Sliding-Filament Model of Muscle Contraction

• The sliding-filament model proposes that thin and thick filaments slide past one another.

• This results in increased overlap between thin and thick filaments, facilitating contraction.

Mechanism of Muscle Contraction

• Contraction occurs through the interaction of actin and myosin:

  • The myosin head binds to actin, forming a cross-bridge that pulls the thin filament toward the sarcomere’s center.

  • Muscle contraction relies on repeated cycles of binding and release between actin and myosin.

Energy for Muscle Contraction

• Two processes, glycolysis and aerobic respiration, produce the ATP needed to sustain muscle contraction.

Role of Calcium and Regulatory Proteins

• During rest, tropomyosin and the troponin complex prevent interaction between actin and myosin.

• When a muscle fiber is activated, calcium shifts regulatory proteins on actin strands, allowing muscle contraction.

Nervous Control of Muscle Tension

• Muscle contraction is graded, implying the extent and strength can be altered voluntarily.

• Mechanisms include varying both the number of contracting fibers and the rate of stimulation of fibers.

Motor Units

• Each motor neuron can synapse with multiple muscle fibers; however, each muscle fiber is activated by a single motor neuron.

• A motor unit comprises one motor neuron and the muscle fibers it controls.

• Recruitment of motor neurons enhances contraction strength.

Muscle Twitch and Tetanus

• A twitch is a contraction resulting from a single action potential in a motor neuron.

• Rapidly delivered action potentials lead to graded contractions through summation.

• Tetanus involves a sustained contraction achieved via frequent action potentials.

Types of Muscle Tissue

• Beyond skeletal muscle, vertebrates also possess cardiac and smooth muscle:

  • Cardiac Muscle: Found in the heart, striated cells are interconnected by intercalated disks, enabling autoregulation of contractions.

  • Smooth Muscle: Located mainly in hollow organs (e.g., circulatory, digestive); contractions are slower and can initiate independently or through autonomic nervous system stimulation.

• Smooth muscle lacks striations due to irregular arrangements of actin and myosin.