Skeletal Muscle

• Muscle tissue is composed of cells optimized for contractility.
• Actin microfilaments and associated proteins generate forces for muscle contraction.
• Muscle cells are primarily of mesodermal origin.
• Differentiation involves cell lengthening and synthesis of myofibrillar proteins.
• Sliding interaction of thick myosin filaments along thin actin filaments causes contraction.
• Forces for sliding are generated by proteins affecting actin-myosin bridge interactions.
• Muscle cell cytoplasm is called sarcoplasm.
• Smooth ER in muscle cells is the sarcoplasmic reticulum.
• The muscle cell membrane is the sarcolemma.
• Variation in muscle fiber diameter depends on muscle, age, gender, nutrition, and training.
• Exercise enlarges skeletal muscles by stimulating new myofibrils and fiber diameter growth.
• Hypertrophy, increase in cell volume, characterizes this muscle growth.
• Hyperplasia, tissue growth by increased cell number, occurs readily in smooth muscle.

Skeletal Muscle

• Skeletal muscle consists of long, cylindrical, multinucleated fibers (10-100 μm diameter).
• Myoblasts fuse to form myotubes during development.
• Myotubes differentiate into striated muscle fibers.
• Nuclei are located peripherally under the sarcolemma.
• Muscle satellite cells act as reserve progenitor cells.
• Connective tissue organizes contractile fibers.
• The epimysium is an external sheath of dense irregular connective tissue around the entire muscle.
• Septa extend inward, carrying nerves and vessels.
• The perimysium is a thin layer surrounding muscle fiber bundles (fascicles).
• Each fascicle makes up a functional unit.
• The endomysium is a thin layer of reticular fibers surrounding individual muscle fibers within fascicles.
• Capillaries form a network in the endomysium.
• Collagen transmits mechanical forces generated by contraction.
• Muscle fibers rarely extend the entire muscle length.
• Epimysium, perimysium, and endomysium connect to tendons at myotendinous junctions.
• Collagen fibers from tendons insert among muscle fibers.
• Sectioned fibers show striations of light and dark bands.
• Sarcoplasm contains myofibrils (filament bundles) running parallel to the long axis.
• Dark bands are A bands.
• Light bands are I bands.
• Each I band is bisected by a Z disc.
• The sarcomere is the functional subunit, extending from Z disc to Z disc (2.5 μm long in resting muscle).
• Mitochondria and sarcoplasmic reticulum are found between myofibrils.
• Sarcomeres in adjacent myofibrils align laterally, causing striations.
• A and I banding patterns are due to thick (myosin) and thin (F-actin) myofilaments.
• Thick myosin filaments are 1.6 μm long and 15 nm wide, occupying the A band.
• Myosin has two heavy chains and two pairs of light chains.
• Myosin heavy chains are rodlike motor proteins twisted together as myosin tails.
• Globular projections with light chains form a head at one end of each heavy chain.
• Myosin heads bind actin and ATP, catalyzing energy release (ATPase activity).
• Several hundred myosin molecules are within each thick filament with overlapping tails and heads directed toward either end.
• Thin, helical actin filaments are 1.0 μm long and 8 nm wide, running between thick filaments.
• Each G-actin monomer contains a binding site for myosin.
• Thin filaments have two regulatory proteins: tropomyosin and troponin
• Tropomyosin is a 40 nm long coil of two polypeptide chains located in the groove between the two twisted actin strands.
• Troponin is complex of three subunits: TnT attaches to tropomyosin, TnC binds Ca2+, and TnI regulates actin-myosin.
• Troponin complexes attach at specific sites regularly spaced along each tropomyosin molecule.
• I bands are portions of thin filaments that do not overlap thick filaments and stain lightly.
• Actin filaments are anchored perpendicularly on the Z disc by α-actinin and exhibit opposite polarity.
• Titin (3700 kDa) supports thick myofilaments and connects them to the Z disc.
• Nebulin binds thin myofilaments laterally and anchors them to α-actinin.
• A bands contain thick and overlapping thin filaments.
• The H zone is a lighter zone in the A band center with only myosin tails.
• The M line bisects the H zone, containing myomesin that holds thick filaments in place.
• Creatine kinase catalyzes phosphate transfer from phosphocreatine to ADP.
• Myosin and actin make up over half of striated muscle protein.
• The arrangement of thin and thick filaments in sarcomeres creates hexagonal patterns in TEM cross-sections.
• Sarcoplasmic reticulum (smooth ER) contains Ca2+ pumps and surrounds myofibrils.
• Ca2+ release is triggered by membrane depolarization from a motor nerve.
• Transverse or T-tubules are sarcolemma infoldings that encircle myofibrils at A- and I-band boundaries.
• Terminal cisternae of sarcoplasmic reticulum are adjacent to T-tubules.
• A triad is a T-tubule with two terminal cisternae, allowing depolarization to trigger Ca2+ release.
• During contraction, thick and thin filaments slide past one another without changing length.
• An action potential arrives at the neuromuscular junction (NMJ) and is transmitted along T-tubules.
• Ca2+ ions released upon neural stimulation bind troponin, moving tropomyosin and exposing myosin-binding sites.
• Myosin heads bind actin, producing a pivot that pulls thin filaments toward the Z disc.
• Energy for the pivot is provided by ATP hydrolysis.
• Myosin binds another ATP and detaches from actin.
• Attach-pivot-detach events occur repeatedly in the presence of Ca2+ and ATP.
• When the neural impulse stops, Ca2+ levels diminish, tropomyosin covers myosin-binding sites, and sarcomeres relax.
• Rigor mortis occurs due to stable actin-myosin crossbridges in the absence of ATP.
• Myelinated motor nerves branch within the perimysium, forming synapses (neuromuscular junctions) with muscle fibers.
• Schwann cells enclose axon branches.
• Axon terminals contain acetylcholine-filled vesicles.
• Synaptic cleft separates axon and muscle.
• Sarcolemma has junctional folds with acetylcholine receptors.
• Acetylcholine is liberated, diffuses across the cleft, and binds its receptors.
• The acetylcholine receptor contains a cation channel that depolarizes the sarcolemma.
• Acetylcholinesterase removes free neurotransmitter.
• The muscle action potential moves along the sarcolemma and T-tubules.
• Depolarization signal triggers Ca2+ release at triads.
• A single motor neuron can form MEPs with one or many muscle fibers.
• Innervation of single fibers provides precise control.
• Larger muscles have motor axons that innervate 100+ fibers.
• A motor unit consists of a single axon and all its muscle fibers.
• Striated muscle fibers contract either all the way or not at all.
• The firing of a single motor axon will generate tension proportional to the number of muscle fibers it innervates, used to vary the force of contraction.
• The number of motor units and their variable size control the intensity and precision of a muscle contraction.
• Myasthenia gravis is an autoimmune disorder involving antibodies against acetylcholine receptors.

Muscle Spindles and Tendon Organs

• Striated muscles contain proprioceptors, providing CNS data from the musculoskeletal system.
• Muscle spindles are stretch detectors encapsulated by modified perimysium.
• Intrafusal fibers are thin muscle fibers within the spindle, filled with nuclei.
• Sensory nerve axons wrap around intrafusal fibers.
• Changes in length are detected by the muscle spindles and sensory nerves relay this information to the spinal cord.
• Golgi tendon organs are encapsulated structures enclosing sensory axons at the myotendinous junction.
• Tendon organs detect changes in tension produced by muscle contraction.
• Because they detect increases in tension, helps regulate the amount of effort required to perform movements that call for variable amounts of muscular force.

Skeletal Muscle Fiber Types

• Fiber types are identified based on contraction rate (fast/slow) and ATP synthesis (oxidative/glycolysis).
• Fast vs slow contraction rates depend on myosin isoforms.
• Histochemical staining identifies fibers with differing ATPases.
• Metabolic differences include capillary density, mitochondria number, and myoglobin levels.
• Slow oxidative fibers are adapted for long contractions without fatigue.
• Features include many mitochondria, capillaries, and myoglobin, resulting in a dark or red color.
• Fast glycolytic fibers are specialized for rapid, short-term contraction.
• Fast oxidative-glycolytic fibers are intermediate.
• All fibers of a motor unit are similar.
• Most muscles contain a mixture of fiber types.
• Useful for diagnosing myopathies and other causes of atrophy.
• Dystrophin is an actin-binding protein involved in myofibril organization.

Cardiac Muscle

• Mesenchymal cells align into chainlike arrays during development.
• Cardiac muscle cells form complex junctions between interdigitating processes.
• Cells often branch and join with adjacent fibers.
• Bundles of cells are interwoven in spiraling layers for contraction.
• Mature cells are 15-30 μm diameter and 85-120 μm long, with striations.
• Each cell usually has one centrally located nucleus.
• Endomysium surrounds the muscle cells.