Histology (Lecture)

• Muscle tissue


  Differentiated cells that posses contractile filaments
• Muscle tissue classification

  According to morphology and function
• Muscle contraction


  May be voluntary or involuntary
• General Functional Characteristics of Muscle

  • Posture maintenance
  • Movement
  • Joint stabilization
  • Heat generation
• Special Functional Characteristics of Muscle

  • Contractility - Shorten and generate pulling force
  • Excitability - Respond to stimuli
  • Extensibility - Stretch with contraction of opposing muscle
  • Elasticity - Recoil passively after stretch
• Types of muscle tissue

  • Skeletal
  • Cardiac
  • Smooth
• Skeletal muscle

  • Attach to and move skeleton
  • 40% of body weight
  • Fibers are multinucleated
  • Visible striations
  • Voluntary
• Cardiac muscle


  • Found only in heart wall
  • Myocardium
  • Striated
  • Involuntary
• Smooth muscle

  • Found in hollow visceral organs
  • No visible striations
  • Involuntary
• Skeletal muscle structure

  • Myofibril
  • Muscle fiber
  • Muscle fascicle
  • Skeletal muscle (organ)
• Skeletal muscle

  • Multinucleated
  • Cytoplasm is referred to as sarcoplasm
  • Plasma membrane is sarcolemma
  • Sarcolemma forms deep tubular invaginations T-tubules
  • Consists largely of myofibrils
  • Sarcoplasmic reticulum is a modified Smooth ER
  • T-tubules and sarcoplasmic reticulum form a triad
• Skeletal muscle fiber

  • Composed of contractile proteins (myofibrils)
  • Myofibrils are divided into segments called sarcomeres (contractile / functional units of a muscle)
  • Composed of alternating thick (myosin) filaments and thin (F-Actin) filaments (Collectively known as Myofilaments)
  • Z line/disc marks sarcomere boundary
• Muscle proteins


  • Contractile proteins (e.g. Actin and Myosin) - Generate force during contraction
  • Regulatory proteins (e.g. Troponin and Tropomyosin) - Switch the contraction process on and off
  • Structural proteins (e.g. Titin and Dystrophin) - Align the thick and thin filaments properly, Provide elasticity and extensibility, Link the myofibrils to the sarcolemma
• Sarcomere


  • During contraction, thick and thin filaments slide past each other
  • I-band shortens
  • A-band length remains constant
  • One Z-disc comes close to another Z-disc
• Action potential at the NMJ

  1. Transmitted along the T tubules to terminal cisterna of SR

  2. Release of calcium
• Resting muscle


  • Myosin heads can't bind actin because the binding sites are blocked by the troponin-tropomyosin complex on actin
• Muscle contraction


  1. Calcium binds to troponin, myosin heads bind to actin, creating cross bridges

  2. Cross bridges pull on thin filaments, sarcomere shortens

  3. Calcium goes back into sarcoplasmic reticulum, contraction stops
• Skeletal muscle cells


  • Elongated cells that do not branch with larger diameter
  • Nucleus: appears multinucleated, flattened, and peripherally located
  • Myoblasts - embryonic cells that fuse to develop muscle fibers
• Skeletal muscle fibers


  • Myofibrils are striated with distinct myofilaments (actin & myosin)
  • Alternating light and dark bands
  • Dark bands (A bands) - Anisotropic / birefringent
  • Light bands (I bands) - isotropic / do not alter polarized light
• Skeletal muscle distribution


  • Widely distributed, Attached to entire skeletal system of the body, External urethral & External anal sphincter, Tongue, Upper 1/3 of the esophagus
• Types of skeletal muscle

  • Red/Slow (Type I), Red/Fast (Type IIa), White/Fast (Type IIb)
• Red/Slow (Type I) skeletal muscle

  • Color: Red Contraction time: Slow Oxidative capacity: High Mitochondrial density: High Resistance to fatigue: High Major storage fuel: Fatty acids Metabolic pathway: Aerobic
  • Force production: Low Typical use: Posture; Low-level contraction
• Red/Fast (Type IIa) skeletal muscle

  • Color: White, Contraction time: Fast, Oxidative capacity: High, Mitochondrial density: High, Resistance to fatigue: Medium, Major storage fuel: Glycogen, creatine phosphate, Metabolic pathway: Both, Force production: Medium, Typical use: Speed, strength, power
• White/Fast (Type IIb) skeletal muscle

  • Color: White, Contraction time: Very fast, Oxidative capacity: Low, Mitochondrial density: Low, Resistance to fatigue: Low, Major storage fuel: Glycogen, creatine phosphate, Metabolic pathway: Anaerobic, Force production: Very High, Typical use: Short, fast, bursts of power
• Cardiac muscle


  • Bundles form thick myocardium
  • Cardiac muscle cells are single cells (not called fibers)
  • Auto rhythmicity: each cell beats separately even without any stimulation
  • Involuntary activation (like smooth muscle)
  • Very fatigue resistant
• Cardiac muscle cells

  • Single, spherical, centrally located nucleus (Branches have no nucleus)
  • With intercalated discs of Eberth: serves as junction between cardiac cells
  • Elongated branches, w/ numerous areolar CT
  • Myofibrils striated w/ distinct actin & myosin
• Cardiac muscle ultrastructure

  • T-tubules are larger, located at Z-disk
  • Sarcoplasmic reticulum is poorly defined, contributes to dyads
  • Dyads consist of one T-tubule and two terminal cisternae of SR
• Skeletal muscle


  • Long cylindrical shape
  • Multinucleated with peripheral nuclei
  • Striated
  • Triad structure with T-tubules and sarcoplasmic reticulum
  • Voluntary contraction
• Cardiac muscle


  • Branched shape
  • Single centrally located nucleus
  • Striated
  • Dyad structure with T-tubules and sarcoplasmic reticulum
  • Involuntary contraction
  • Autorhythmic - each cell can beat separately
• Smooth muscle


  • Short spindle shape
  • Single centrally located nucleus
  • Non-striated
  • Caveolae and some endoplasmic reticulum
  • Linked by gap junctions
  • Involuntary contraction
  • Slow and sustained contraction
  • Fatigue resistant
• Locations of smooth muscle

  • Inside the eye
  • Respiratory tubes
  • Urinary organs
  • Reproductive organs
  • Digestive tubes
  • Walls of blood vessels
  • Skin
• Function of smooth muscle

  To alter the activity of various body parts to meet their specific needs
• Examples of smooth muscle function

  • Contraction of urinary bladder
  • Move food through intestines (peristalsis)
  • Peristaltic movement to move feces down digestive system
  • Contraction in trachea and bronchi to decrease airway size
  • Constriction and dilation of blood vessels to regulate blood pressure
  • Constriction, accommodation and dilation of pupil
  • Uterine contraction during birthing
  • Expulsion of glandular contents
  • Arrector pili muscle causes "goosebumps" for thermoregulation
• Molecular basis of smooth muscle contraction

  1. Interaction of sliding actin and myosin filaments similar to skeletal muscle

  2. Contractions initiated by calcium-activated phosphorylation of myosin rather than calcium binding to troponin

  3. Intracellular calcium binds with calmodulin, which then binds and activates myosin light-chain kinase

  4. Calcium-calmodulin-myosin light-chain kinase complex phosphorylates myosin initiating contraction and activating myosin ATPase
• Comparison of muscle tissue properties

  • Skeletal: Long cylindrical shape, multinucleated with peripheral nuclei, striated, triad structure, voluntary contraction
  • Cardiac: Branched shape, single centrally located nucleus, striated, dyad structure, involuntary contraction, autorhythmic
  • Smooth: Short spindle shape, single centrally located nucleus, non-striated, caveolae and some ER, linked by gap junctions, involuntary contraction, slow and sustained, fatigue resistant