Muscle Tissue

Types of Muscle Tissue

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

Skeletal Muscle Characteristics

• Skeletal muscle cells are long, multinucleated fibers with diameters up to 100 μm.
• Each fiber has a sarcolemma surrounded by an external lamina and thin connective tissue called endomysium, which contains capillaries.

Organization of Skeletal Muscle Fibers

• Groups of fibers are called fascicles, which are surrounded by perimysium.
• All fascicles are enclosed within a dense connective tissue called epimysium.
• Each muscle fiber is filled with myofibrils, which are composed of thousands of thick myosin filaments and thin actin filaments.
• Myofibrils are highly organized into contractile units called sarcomeres.

Sarcomere Structure

• Within sarcomeres, thick and thin filaments interdigitate.
• Globular myosin heads project from the thick filaments toward the F-actin filaments.
• F-actin filaments are associated with tropomyosin and troponin.
• Sarcomeres are separated by Z discs that bisect the light-staining I bands.
• I bands contain mainly thin filaments attached to α-actinin in the Z disc.

Sarcomere Structure

• Dark-staining A band contains thick myosin filaments
• Alternating light and dark bands create microscopic striations along fibers

Sarcoplasmic Reticulum

• Located between parallel myofibrils in sarcoplasm
• Composed of mitochondria and cisternae of smooth ER
• Specialized for Ca2+ sequestration and release

Transverse Tubule System

• Deep invagination of sarcolemma, called a transverse or T tubule
• Two terminal cisterns of SR contact T tubule at each sarcomere, forming a triad
• Triad triggers Ca2+ release when sarcolemma is depolarized

Mechanism of Contraction

• Ca2+ binding to troponin causes a shape change in tropomyosin, allowing myosin heads to bind to actin subunits and form crossbridges between thick and thin filaments.
• Myosin heads pivot with ATP hydrolysis, pulling thin filaments along thick filaments and shortening the sarcomere.
• The contraction cycle involves repeated attachment, pivoting, detachment, and return of myosin heads, causing filaments to slide past each other and shorten the sarcomere.
• When membrane depolarization ends, Ca2+ is sequestered, ending contraction and allowing sarcomeres to lengthen again as the muscle relaxes.

Motor Unit and Neuromuscular Junction

• Synapses of motor axons with skeletal muscle are called motor end plates (MEPs), neuromuscular junctions (NMJs), or myoneural junctions.
• The neurotransmitter at the neuromuscular junction is acetylcholine.
• A motor axon may form many terminal branches, each ending on an MEP of a muscle fiber.
• All fibers innervated by branches of a single axon comprise a motor unit.

Muscle Spindles & Tendon Organs

• Muscle spindles and tendon organs are sensory proprioceptors that consist of sensory axons wrapped around intrafusal fibers in small specialized fascicles or around myotendinous collagen bundles, respectively.

Muscle Fiber Types

• Skeletal muscles contain three main types of fibers:
  • Slow, oxidative (type I) fibers
  • Fast, intermediate oxidative-glycolytic (type IIa) fibers
  • Fast, glycolytic (type IIb) fibers

Cardiac Muscle

• Cardiac muscle fibers are striated, cylindrical cells with one or two central nuclei.
• Each cardiac muscle fiber is linked by adherent and gap junctions at prominent intercalated discs.
• Sarcomeres of cardiac muscle are organized and function similarly to those of skeletal muscle.
• Contraction of cardiac muscle is intrinsic at nodes of impulse-generating pacemaker muscle fibers.
• Autonomic nerves regulate the rate of cardiac muscle contraction.

Smooth Muscle Characteristics

• Smooth muscle fibers are individual, small, fusiform cells connected by numerous gap junctions.
• Thin and thick filaments in smooth muscle fibers do not form sarcomeres, and no striations are present.
• Thin actin filaments attach to α-actinin in dense bodies throughout the sarcoplasm and near the sarcolemma.
• Contraction of smooth muscle cells causes individual shortening.

Muscle Fiber Structure

• Sarcoplasmic reticulum is less well-organized in smooth muscle fibers.
• There is no transverse tubule system in smooth muscle fibers.

Muscle Contraction Regulation

• Troponin is absent in smooth muscle.
• Myosin light-chain kinase (MLCK) and calmodulin regulate the sliding filaments in smooth muscle.

Muscle Tissue Regeneration

• Skeletal muscle has a population of reserve muscle satellite cells, enabling regeneration through proliferation, fusion, and new muscle fiber formation.
• Cardiac muscle lacks satellite cells and has limited regenerative capacity.
• Smooth muscle regeneration occurs rapidly due to its relatively small, less differentiated cells, which can resume mitotic activity after injury.