Muscle Tissue: Types, Properties, and Functions

• There are three types of muscle tissue: skeletal, cardiac, and smooth.

Muscle Tissue Types

• Skeletal muscle is voluntary, striated, multinucleated, and attached to bones for movement.
• Cardiac muscle is involuntary, striated, found in the heart, and contains intercalated discs for synchronized contraction.
• Smooth muscle is involuntary, non-striated, found in the walls of hollow organs, and responsible for slow, sustained contractions.

Muscle Properties

• Excitability is the ability of muscle tissue to respond to stimuli.
• Contractility is the ability of muscle cells to shorten and generate force.
• Extensibility is the ability of muscle tissue to be stretched without damage.
• Elasticity is the ability of muscle to return to its original shape after stretching or contracting.

Functions of Muscle

• Muscles allow for body movement and internal movement.
• Skeletal muscles maintain posture by stabilizing joints and keeping the body upright.
• Muscle contraction generates heat, which helps regulate body temperature.
• Muscles help reinforce joints and prevent unwanted movement.

Nerve and Blood Supply in Muscles

• Muscles are controlled by motor neurons that transmit electrical signals.
• Muscles require a rich blood supply to deliver oxygen and nutrients, and to remove waste products.

Connective Tissue Components of Muscle

• Epimysium surrounds the entire muscle.
• Perimysium surrounds muscle fascicles.
• Endomysium surrounds individual muscle fibers.
• Connective tissues provide support and protect muscle fibers while allowing for force transmission.

Muscle Attachments

• The origin is the fixed, immovable attachment of a muscle to a bone.
• The insertion is the movable attachment of a muscle that moves during contraction.

Muscle Organization

• The organization of muscle from whole muscle to myofilament goes: whole muscle, fascicle, muscle fiber, myofibril, sarcomere, and myofilaments.
• A whole muscle is an organ composed of skeletal muscle tissue, connective tissue, blood vessels, and nerves, surrounded by the epimysium.
• Within the whole muscle, muscle fibers are bundled into fascicles, which are surrounded by the perimysium.
• Individual muscle fibers are the muscle cells themselves, bundled together to form fascicles, and surrounded by the endomysium.
• Within each muscle fiber are myofibrils that are responsible for the muscle's contractile properties.
• Each myofibril is composed of sarcomeres, the basic functional unit of muscle contraction which gives skeletal muscle its striated appearance.
• Within each sarcomere are myofilaments, the contractile proteins composed of thick myofilaments (myosin) and thin myofilaments (actin, troponin, and tropomyosin).

Striations in Skeletal Muscle

• The A band is a dark band containing thick filaments (myosin) and overlapping thin filaments.
• The I band is a light band containing only thin filaments (actin).
• The H zone is the center of the A band, where there is no overlap of actin and myosin.
• The Z disc separates one sarcomere from another.
• The M line is the middle of the sarcomere, where thick filaments are anchored.

Thick and Thin Filaments

• The thick filament is composed of myosin, which has heads that form cross-bridges with actin during contraction.
• The thin filament is composed of actin, along with regulatory proteins troponin and tropomyosin.
• Troponin binds calcium to initiate contraction.
• Tropomyosin blocks myosin-binding sites on actin until contraction is needed.

Muscle Cell Structures

• The sarcolemma is the plasma membrane of a muscle fiber.
• T-tubules are extensions of the sarcolemma that transmit electrical signals into the muscle fiber.
• The sarcoplasmic reticulum (SR) stores and releases calcium for muscle contraction.

Sarcomere and Sliding Filament Theory

• The sarcomere is the basic functional unit of a muscle fiber.
• The sliding filament theory describes how muscle contraction occurs: Myosin heads attach to actin, pulling thin filaments toward the M line and shortening the sarcomere.

Key Molecules for Contraction

• Key molecules for contraction include ATP, calcium, actin, myosin, troponin, and tropomyosin.

Neuromuscular Junction & ACh Role

• The neuromuscular junction is where motor neurons release acetylcholine (ACh) to stimulate muscle contraction.
• ACh binds to receptors on the muscle fiber, triggering an action potential.

Motor Units and Recruitment

• A motor unit is a motor neuron and all the muscle fibers it controls.
• Recruitment is the process of activating more motor units to generate more force.

Types of Muscle Contractions

• Isometric contractions occur when the muscle contracts but does not shorten.
• Concentric contractions occur when the muscle shortens while generating force.
• Eccentric contractions occur when the muscle lengthens while maintaining force.

ATP Production in Muscle Contraction

• Direct phosphorylation is the fastest method, using creatine phosphate to regenerate ATP.
• The anaerobic pathway produces ATP without oxygen, resulting in lactic acid build-up.
• The aerobic pathway produces ATP using oxygen and is the most efficient but slowest method.

Length-Tension Relationship

• The optimal length of a sarcomere allows maximum overlap of actin and myosin for the greatest force.

Muscle Fiber Types

• Slow-twitch (Type I) fibers are fatigue-resistant, have high endurance, and use aerobic metabolism.
• Fast-twitch (Type IIa/IIb) fibers fatigue quickly, produce powerful contractions, and use anaerobic metabolism.

Energy Use

• Oxidative fibers use oxygen for ATP (aerobic).
• Glycolytic fibers use glucose without oxygen (anaerobic).

Differences Between Skeletal, Cardiac, and Smooth Muscle

• Skeletal muscles have myofibrils and T-tubules, are voluntary, regulated by the sarcoplasmic reticulum, contract fast, and use both aerobic and anaerobic energy.
• Cardiac muscles have myofibrils and T-tubules, are involuntary, use SR and extracellular calcium, contract at a moderate speed, and use aerobic energy.
• Smooth muscles lack myofibrils and T-tubules, are involuntary, use extracellular calcium, contract slowly, and use aerobic energy.

Muscular Dystrophy

• Muscular dystrophy is a genetic disorder causing progressive muscle weakness due to a defect in dystrophin.

Muscular System & Other Body Systems

• The integumentary system generates heat to regulate body temperature.
• The endocrine system uses hormones like growth hormone to regulate muscle growth.
• The digestive system uses smooth muscle to move food through the digestive tract.