Muscle

• Muscle tissue


  • Characterized by the ability to contract or move upon stimulation
  • Composed of cells that optimize the universal cell property of contractility
  • Muscle cells are of mesodermal origin and differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin
• Muscle cell organelles

  • Cytoplasm of muscle cells – sarcoplasm
  • Smooth Endoplasmic Reticulum of Muscle – sarcoplasmic reticulum
  • Cell membrane and external lamina – sarcolemma
• Types of muscle tissue

  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle
• Skeletal muscle


  • Multi nucleated cells with cross striations
  • Quick, Forceful, usually voluntary contractions
• Cardiac muscle


  • Contains cross striations
  • Branched cells bound to one another at structures [transverse line] called intercalated discs [unique to the cardiac muscle]
  • Contraction is involuntary, vigorous, and rhythmic
• Smooth muscle


  • Fusiform cells which lack striations
  • Slow, Involuntary Contractions
• Skeletal muscle


  • Striated muscle, Voluntary muscle
  • Responsible for the movement of the skeleton as well as organs such as the globe of the eye and the tongue
  • Consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of 10-100 μm
• Development of skeletal muscle

  1. Mesenchymal myoblasts fuse, forming myotubes with many nuclei

  2. Myotubes then further differentiate to form striated muscle fibers
• Satellite cells proliferate and produce new muscle fibers following muscle injury
• Interstitial connective tissue in skeletal muscle

  • Epimysium
  • Perimysium
  • Endomysium
  • Deep Fascia
• Myotendinous junctions


  Join the muscle to bone, skin, or another muscle
• Striations in skeletal muscle

  • Dark bands are called A Bands [Anisotropic Bands]
  • Light bands are called I Bands [Isotropic Bands]
• Sarcoplasm


  Highly organized, containing primarily long cylindrical filament bundles called myofibrils
• Sarcomere


  The repetitive functional subunit of the contractile apparatus, extending from Z disc to Z disc
• Components of sarcomere

  • Thick myofilament – Myosin; occupy the A bands
  • Thin filament – actin, tropomyosin, F-actin, and troponin; occupy the I band
• 1. actin


  Contains the binding site for the thick filaments [myosin]
• Troponin subunits

  • TnT – attaches to the tropomyosin
  • TnI – regulates actin myosin interaction
  • TnC – binds calcium
• Myosin heads


  Bind both actin, forming transient cross-bridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity)
• I bands


  Consist of the portions of the thin filaments which do not overlap the thick filaments in the A bands
• Alpha Actinin


  Supports and connects Z discs to the thin filaments
• Titin


  Binds Z discs to the thick filament; largest protein in our body, with some elastic properties
• A bands


  Contain both the thick filaments and the overlapping portions of thin filaments
• H zone


  Lighter zone in the center of A bands, corresponding to a region with only the rodlike portions of the myosin molecule and no thin filaments
• M line


  Contains myomesin [myosin binding protein that holds the thick filaments in place]; contains creatine kinase [catalyze (pinapabilis) the formation of phosphocreatine (storage for high energy phosphate)]
• ATP


  Soul for muscle contraction
• Sarcoplasmic Reticulum


  Membranous smooth ER in skeletal muscle fibers; Function for calcium-ion-sequestration during muscle contraction
• Transverse or T-tubules


  Long fingerlike invaginations of the cell membrane encircling each myofibril near the aligned A- and I-band boundaries of sarcomeres
• Terminal cisternae


  Expanded structures adjacent to each T-Tubule
• Mechanism of contraction

  1. Nerve impulse triggers release of Acetyl Choline [neurotransmitter] from the synaptic knob into the synaptic cleft

  2. ACh binds to ACh receptors in the motor end plate of the neuromuscular junction, initiating a muscle impulse in the sarcolemma of the muscle fiber

  3. As the muscle impulse spreads quickly from the sarcolemma along T tubules, calcium ions are released from terminal cisternae into the sarcoplasm

  4. Calcium ions bind to troponin

  5. Troponin changes shape, moving tropomyosin on the actin to expose active sites on actin molecules of thin filaments

  6. Myosin heads of thick filaments attach to exposed active sites to form crossbridges

  7. Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP binds myosin heads and is broken down into ADP and P

  8. Myosin heads detach from thin filaments and return to their prepivot position

  9. The sarcomere shortens and the muscle contracts

  10. When the impulse stops, calcium ions are actively transported into the sarcoplasmic reticulum
• Types of skeletal muscle fibers

  • Type I [Slow Oxidative Fibers]
  • Type II A [Fast Oxidative Glycolytic Fibers]
  • Type II B [Fast Glycolytic Fibers]
• Type I [Slow Oxidative Fibers]


  • Small fibers, appear red in fresh specimens, contain many mitochondria and large amounts of myoglobin and cytochrome complexes
  • Slow-twitch fatigue-resistant motor units
  • Muscles with mainly type I fibres are often postural muscles, for example, in the neck and spine
• Type II A [Fast Oxidative Glycolytic Fibers]


  • Intermediate fibers seen in fresh tissue
  • Medium size w/ many mitochondria and a high myoglobin content
  • Fast-twitch fatigue-resistant motor units that generate high peak muscle tension
  • They possess characteristics that are intermediate between fast fibers and slow fibers
  • Use both aerobic and anaerobic metabolism
  • Primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting
• Type II B [Fast Glycolytic Fibers]


  • Large fibers, w/c appear light pink in fresh specimens, contain less myoglobin & fewer mitochondria than type I & IIa fibers
  • Low levels of oxidative enzymes but exhibit high anaerobic enzyme activity and store a considerable amount of glycogen
  • Fast-twitch fatigue-prone motor units & generate high peak muscle tension
  • Used to produce rapid, forceful contractions to make quick, powerful movements
  • These fibers fatigue quickly, permitting them to only be used for short periods
  • This type of fibre can be turned into type IIa fibres by resistance training
  • Found in large quantities in the muscles of the arms
• Cardiac muscle


  • Myocardium
  • Exhibits many structural and functional characteristics intermediate between those of skeletal and visceral muscle
  • Are essentially long, cylindrical cells with one or at most two nuclei which are centrally located within the cell
  • Contains transverse lines that cross the fibers at irregular intervals where the myocardial cells join
  • Intercalated discs – represent the interfaces between adjacent cells and consist of many junctional complexes
• Components of intercalated disk

  • Fascia adherens (adhering junction)
  • Maculae adherens (desmosomes)
  • Gap junctions (communicating junctions)
• Smooth muscle


  • Visceral muscle
  • Major component of blood vessels and of the digestive, respiratory, urinary, and reproductive tracts and their associated organ
  • Specialized for slow, steady contraction under the influence of autonomic nerves and various hormones
  • Generally occurs as bundles or sheets of elongated fusiform cells w/ finely tapered ends
  • Cells possess a contractile apparatus of thin and thick filaments and a cytoskeleton of desmin and vimentin intermediate filaments
  • Thin and thick filaments in smooth muscle fibers do not form sarcomeres, and no striations are present
  • Troponin is lacking in smooth muscle
  • Proteins controlling the sliding filaments here include myosin light-chain kinase (MLCK) and the Ca2+-binding protein calmodulin
• Functional aspects of smooth muscle

  • Specialized for slow, prolonged contraction
  • Nerve terminals in smooth muscles are observed only in the connective tissue adjacent to muscle cells
  • Smooth muscles also secrete connective tissue matrix