Muscle Functions and Tissue Types

• Muscles facilitate movement, stabilize posture, produce heat, aid communication, constrict organs, and enable heart contraction.

Functional Properties

• Excitability means muscles respond to stimuli.
• Conductivity refers to muscles transmitting electrical signals.
• Contractility enables muscles to shorten for movement.
• Extensibility means muscles can stretch without damage.
• Elasticity allows muscles to return to their original shape.

Muscle Tissue Types

• Skeletal muscle is striated, multinucleated, and under voluntary control; it facilitates locomotion, posture, respiration, and facial expression, attaching to bones via tendons or aponeuroses.
• Cardiac muscle is striated, single-nucleated, and involuntary; it is found in the heart and contracts rhythmically, connected by intercalated discs to allow electrical communication.
• Smooth muscle is non-striated, single-nucleated, and involuntary; it is in the walls of hollow organs like the digestive tract and blood vessels, moving substances via peristalsis.

Skeletal Muscle Structure

• Epimysium surrounds the entire muscle.
• Perimysium surrounds fascicles, or muscle bundles.
• Endomysium surrounds individual muscle fibers.

Components

• Sarcolemma is the muscle cell membrane.
• Sarcoplasm is cytoplasm containing myoglobin and glycogen.
• Sarcoplasmic Reticulum (SR) stores and releases calcium.
• T-tubules conduct electrical impulses deep into the muscle.
• Myofibrils contain thick (myosin) and thin (actin) filaments.
• Sarcomere is the functional unit of muscle contraction.

Sarcomere Structure

• Z-line marks the boundary of a sarcomere.
• A-band contains thick (myosin) and overlapping thin filaments.
• I-band contains only thin filaments (actin).
• H-zone is the center of the A-band with no thin filaments.
• M-line sits in the middle of the sarcomere.

Muscle Contraction

• Motor neurons stimulate muscle fibers at the Neuromuscular Junction (NMJ).
• Acetylcholine (ACh) release binds to ligand-gated ion channels which triggers an action potential.
• An action Potential travels along the sarcolemma and T-tubules, which releases Ca²⁺ from the SR.

Sliding Filament Model

• Myosin heads attach to actin, forming cross-bridges.
• ATP is required for myosin to detach and reset.
• Actin filaments slide toward the M-line, shortening the sarcomere.
• I-band and H-zone shorten, while A-band remains unchanged.

Cross-Bridge Cycle

• Cross-bridge formation occurs when myosin binds to actin.
• Power stroke involves myosin pulling actin toward the M-line.
• Detachment requires ATP to bind to myosin, releasing actin.
• Myosin resets when ATP is hydrolyzed to reposition myosin.

Muscle Relaxation

• ACh release stops, leading to no more action potentials.
• Ca²⁺ is pumped back into the SR.
• Without calcium, myosin cannot bind to actin, stopping contraction.
• ATP is required for both contraction and relaxation.

Types of Muscle Contractions

• Isometric contractions involve muscle contracting without changing length.
• Isotonic contractions involve muscle contracting and changing length.
• Concentric contractions cause muscle shortening.
• Eccentric contractions cause muscle lengthening.

Motor Units & Muscle Twitch

• Motor Unit: One motor neuron plus all muscle fibers it innervates.
• Muscle Twitch: Single contraction from a motor neuron stimulus.
• Latent Phase: Action potential triggers Ca²⁺ release.
• Contraction Phase: Cross-bridge formation, muscle shortens.
• Relaxation Phase: Ca²⁺ reabsorbed, muscle relaxes.

Summation & Recruitment

• Summation involves increasing force in individual fibers.
• Recruitment involves activating more motor units for a stronger contraction.

Muscle Fiber Types

• Slow-Twitch (Type I) fibers have high myoglobin content, are fatigue-resistant, and are used for endurance.
• Fast-Twitch (Type II) fibers have low myoglobin content, fatigue quickly, and are used for quick, powerful movements.

Aerobic vs. Anaerobic Metabolism

• Aerobic metabolism (with oxygen) allows efficient ATP production via oxidative phosphorylation.
• Anaerobic metabolism (without oxygen) facilitates short-term ATP production via glycolysis, producing lactic acid and leading to fatigue.

Muscle Fatigue Causes

• ATP depletion
• Lactic acid buildup (lowers pH)
• Oxidative stress (free radical damage)
• Inflammatory reactions

Exercise & Muscle Adaptation

• Short-term effects of exercise include fatigue and soreness due to enzyme release.
• Long-term effects of exercise include muscle hypertrophy, which is increased muscle size and strength.

Muscle Attachments

• Origin is the fixed (proximal) attachment of a muscle.
• Insertion is the movable (distal) attachment of a muscle.
• Belly is the central part of the muscle.

Muscle Actions

• Agonist (Prime Mover) is the main muscle responsible for movement.
• Antagonist opposes the agonist’s action.
• Synergist helps the agonist by stabilizing movement.

Muscle Shapes

• Circular muscles surround openings.
• Convergent muscles have a broad origin and converge to a single point.
• Parallel muscles have fibers that run parallel.
• Pennate muscles have a feather-like structure which can be: Unipennate (fibers on one side), Bipennate (fibers on both sides), or Multipennate (many fascicle attachments).
• Fusiform muscles are spindle-shaped.