Chapter 11 BIO203

Excitability (responsiveness): Muscle tissue responds to chemical signals, stretch, and electrical changes across the plasma membrane.
Conductivity: Local electrical changes trigger a wave of excitation traveling along the muscle fiber.
Contractility: Muscle fibers shorten when stimulated, generating force.
Extensibility: Muscle tissue can be stretched between contractions.
Elasticity: After stretching, muscle fibers return to their original length.

Characteristics:
- Voluntary: Conscious control over contraction.
- Striated appearance: Alternating light and dark transverse bands due to internal contractile protein arrangement.
- Muscle fiber (myofiber): Can be as long as 30 cm.

Endomysium: Connective tissue enveloping individual muscle fibers.
Perimysium: Connective tissue surrounding muscle fascicles.
Epimysium: Connective tissue encasing the entire muscle.
Tendons: Connect muscle to bone; continuous with collagen fibers of bones.
- Collagen properties: Somewhat extensible and elastic, resists excessive stretching, returns to resting length, and contributes to muscle efficiency.

Components:
- Sarcoplasma: Cytoplasm containing myofibrils, glycogen, and myoglobin.
- Sarcoplasmic reticulum (SR): Smooth ER forming networks around myofibrils, acting as a calcium reservoir.
- Transverse (T) tubules: Extensions of the sarcolemma that penetrate the cell and facilitate excitation contraction coupling.

Sarcolemma: Plasma membrane of muscle fibers.
Nuclei: Comprised of flattened nuclei pressed against the inner sarcolemma.
- Myoblasts: Stem cells that fuse into muscle fibers.
- Satellite cells: Unspecialized myoblasts aiding in muscle regeneration.
Mitochondria: Packed between myofibrils, providing energy for contraction.

Thick Filaments: Composed mainly of myosin molecules; head and tail arrangement allows interaction with actin.
- Heads are arranged in a helical pattern, creating a bare zone in the center.
Thin Filaments: Composed of actin, tropomyosin, and troponin proteins, facilitating contraction by binding with myosin heads.
Elastic Filaments (Titin): Stabilize thick filaments and prevent overstretching; play a role in muscle elasticity.

Mechanism: Sliding Filament Theory states that during contraction, thick and thin filaments slide past each other.
Cross-bridge cycling: Myosin heads hydrolyze ATP, bind to actin, pivot, and release, powered by ATP.
Relaxation: Calcium ions are removed from troponin, tropomyosin reblocks active sites on actin, muscle fiber returns to resting length.

The amount of tension generated by a muscle depends on its initial length before stimulation.
- Optimal length maximizes cross-bridge formation between actin and myosin.

Action Potential: Voltage change that results from sodium influx and triggers contraction.
Resting Membrane Potential: Approximately −90 mV, primarily maintained by the sodium-potassium pump.

Structure: Synapse between a motor neuron and muscle fiber, facilitating signal transmission.
Components: Contains synaptic knobs that store acetylcholine (ACh) for muscle fiber activation.

Energy Sources:
- Immediate Energy: Uses creatine phosphate and ADP for short bursts of activity.
- Short-Term Energy: Relies on anaerobic respiration and lactic acid formation during high-intensity activities.
- Long-Term Energy: Aerobic respiration dominates after several minutes, utilizing glucose and fatty acids.

Fast-Twitch Fibers: Adapted for quick, powerful contractions; lower fatigue resistance.
Slow-Twitch Fibers: Well-suited for endurance; higher fatigue resistance due to aerobic metabolism.
Physiological Classes: Muscles contain mixed fiber types, influencing their functional abilities.