Muscle Tissue Types and Properties

Muscle Tissue Types

• Three types of muscle tissue exist: skeletal, cardiac, and smooth.
• Skeletal muscle moves the skeleton.
• Cardiac muscle pumps blood.
• Smooth muscle controls involuntary movements like hair raising and digestion.

Muscle Properties

• Excitability: Muscle cells can change their electrical state (polarized to depolarized) and generate action potentials.
• Skeletal muscle relies entirely on nervous system signaling. Cardiac and smooth muscle can respond to hormones and other stimuli.
• Contractility: Muscles shorten with force when proteins actin and myosin interact.
• Extensibility: Muscles can stretch.
• Elasticity: Muscle returns to its original length after contraction.

Muscle Contraction Mechanism

• Actin is pulled by myosin.
• In striated muscle, calcium ions expose binding sites on actin.
• Troponin and tropomyosin proteins regulate actin binding sites.
• All muscles need ATP for contraction.
• Calcium removal and re-shielding of actin binding sites cause relaxation.

Differences in Muscle Types

• Skeletal muscle: Multinucleated fibers exhibiting striations (regular arrangement of actin and myosin).
• Cardiac muscle: Single or two nuclei; physically and electrically connected (syncytium) to ensure as one unit.
• Smooth muscle: Single nucleus; non-striated (actin and myosin not arranged regularly).

Skeletal Muscle Structure

• Mysia: Three layers of connective tissue enclosing and compartmentalizing skeletal muscle fibers.
• Epimysium: Outermost layer surrounding the entire muscle.
• Perimysium: Middle layer organizing muscle fibers into bundles called fascicles.
• Endomysium: Innermost layer enclosing individual muscle fibers. Provides nutrients and extracellular fluid.

Skeletal Muscle Function

• Movement production and cessation.
• Maintaining posture and balance.
• Maintaining joint stability.
• Controlling movement of substances (swallowing, urination).
• Protecting internal organs.
• Generating heat through ATP breakdown during contraction.

Skeletal Muscle Fibers

• Long cylindrical cells.
• Can be large, up to 100μm diameter and 30 cm long in some cases.
• Multinucleated (formed by fusion of myoblasts), enabling large protein production.

Sarcomere

• The functional unit of a muscle fiber.
• Highly organized arrangement of actin (thin filament) and myosin (thick filament) proteins.
• Z-discs mark the boundaries between sarcomeres.
• Striations are due to the parallel arrangement of actin and myosin.

Neuromuscular Junction (NMJ)

• The site where a motor neuron's terminal meets a muscle fiber.
• The only way to functionally contract skeletal muscle.

Excitation-Contraction Coupling

• Stimulation of the skeletal muscle membrane initiates an action potential.
• Action potential triggers calcium release from the sarcoplasmic reticulum (SR).
• Calcium exposes actin binding sites for myosin interaction.
• Myosin pulls actin filaments, causing muscle contraction.

Motor Units

• A motor neuron and all the muscle fibers it serves.
• Large motor units lead to large movements. Small motor units are for precise adjustments.