Muscle Tissue Types and Functions

Muscle Tissue Types and Characteristics

• Three types of muscle tissue exist: skeletal, cardiac, and smooth.
• Skeletal muscle is attached to bone.
• Cardiac muscle is found in the heart.
• Smooth muscle lines hollow organs.
• All muscle tissues share excitability (respond to stimuli), contractility (generate force), extensibility (ability to stretch), and elasticity (ability to return to original shape).

Skeletal Muscle Organization and Function

• Skeletal muscle functions include producing movement, maintaining posture, supporting soft tissues, guarding entrances/exits, and maintaining body temperature.
• Connective tissues organize skeletal muscle: epimysium (surrounds muscle), perimysium (sheathes bundles of fibers), and endomysium (covers individual fibers). Tendons or aponeuroses attach muscle to bone or other muscles.

Skeletal Muscle Histology and Contraction

• Skeletal muscle fibers contain a sarcolemma (cell membrane), sarcoplasm (cytoplasm), sarcoplasmic reticulum (stores Ca2+), T-tubules (invaginations of sarcolemma), and sarcomeres (functional units of muscle contraction).
• Myofibrils are composed of thick (myosin) and thin (actin, tropomyosin, troponin) filaments.
• The sliding filament theory describes muscle contraction: action potential triggers ACh release at the neuromuscular junction, leading to Ca2+ release from the sarcoplasmic reticulum. Ca2+ binds to troponin, shifting tropomyosin to expose actin's myosin-binding sites. Myosin heads bind to actin, forming cross-bridges and pulling actin filaments towards the center of the sarcomere, shortening it. ATP is required for cross-bridge cycling and detachment.

Tension Production and Contraction Types

• The all-or-none principle applies to muscle fiber contraction; a single fiber contracts completely or not at all. Tension depends on the number of cross-bridges formed.
• A twitch is a single contraction-relaxation cycle. Treppe is a series of twitches with increasing tension. Summation involves repeated stimulation before relaxation, resulting in wave summation, incomplete tetanus, or complete tetanus.
• Motor units consist of a motor neuron and all the muscle fibers it innervates. Precise movement control depends on motor unit size and number, with progressive recruitment increasing tension.
• Isometric contractions generate tension without changing muscle length (no joint angle change). Isotonic contractions generate tension with muscle length change (joint angle change), including concentric (muscle shortening) and eccentric (muscle lengthening) contractions.

Muscle Energy Production

• Muscle contraction requires significant energy.
• Creatine phosphate (CP) provides immediate energy to convert ADP to ATP.
• Aerobic metabolism is the primary ATP source during sustained activity.
• Anaerobic glycolysis generates ATP during peak activity.
• Fatigue results from lactic acid buildup and energy resource depletion. The recovery period involves oxygen debt (EPOC) to restore normal conditions.

Muscle Fiber Types and Performance

• Slow fibers (Type I, red) are fatigue-resistant, smaller, have abundant mitochondria and myoglobin, and contract slowly.
• Fast fibers (Type II, white) are larger, powerful, fatigue quickly, have fewer mitochondria and high glycogen reserves, and contract rapidly.
• Intermediate fibers combine characteristics of slow and fast fibers.
• Muscle fiber type distribution varies between muscles and individuals. Training can lead to hypertrophy (increased muscle size).

Physical Conditioning and Muscular Changes

• Anaerobic endurance refers to the duration muscle contractions are sustained by glycolysis and ATP/CP reserves.
• Aerobic endurance is about the time muscle contraction can continue with mitochondrial activity.
• Muscular atrophy is a decrease in myofibrils and muscle size from disuse or denervation.
• Muscular hypertrophy is an increase in muscle size due to increased myofibril number, not cell number.