Histology Muscle Tissue

• Muscle tissue


  • Differentiated cells that possess contractile filaments
  • Classified according to morphology and function
  • Contraction may be voluntary or involuntary
• General Functional Characteristics of Muscle

  • Movement
  • Posture maintenance
  • 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
  • Striated
  • Voluntary
• Cardiac muscle

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

  • Found in hollow visceral organs
  • Not Striated
  • Involuntary
• Skeletal muscle structure

  1. Myofibril

  2. Muscle fiber

  3. Fascicle
• Skeletal muscle general concepts and structures

  • 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
• Skeletal muscle proteins

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

  1. During contraction, thick and thin filaments slide past each other

  2. I-band shortens

  3. A-band length remains constant

  4. One Z-disc comes close to another Z-disc
• Skeletal muscle excitation-contraction coupling

  1. Action potential at the NMJ transmitted along the T tubules to terminal cisterna of SR

  2. Release of calcium
• Calcium release in skeletal muscle

  • Ca++ binds to troponin, myosin heads bind to actin, creating cross bridges
  • Cross bridges pull on thin filaments, sarcomere shortens
  • Ca++ goes back into sarcoplasmic reticulum, contraction stops
• Skeletal muscle microscopic appearance

  • 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 - although unattached to the skeletal system, it is classified as striated voluntary
  • Upper 1/3 of the esophagus (lower part is involuntary in nature, hence, smooth muscle)
• 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 High
  • 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! (muscle cells beat 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
• Muscle types

  • Skeletal
  • Cardiac
  • Smooth
• Cardiac muscle

  • Branched shape
  • Single central nucleus
  • Striated
  • Dyad at Z disc
  • Involuntary contraction
• Smooth muscle

  • Short spindle shape
  • Single central nucleus
  • No striations
  • Caveola and some ER
  • Gap junctions
  • No sarcomeres
  • Involuntary contraction
• Smooth muscle contraction is initiated by the calcium-activated phosphorylation of myosin rather than calcium binding to troponin
• Smooth muscle contraction

  1. Intracellular calcium binds with calmodulin

  2. Calmodulin-myosin light-chain kinase complex phosphorylates myosin

  3. Activates myosin ATPase
• Smooth muscle contraction involves the interaction of sliding actin and myosin filaments, similar to skeletal muscle
• Smooth muscle is very fatigue resistant
• Smooth muscle contraction is slow and sustained
• Smooth muscle is involuntary and cannot be consciously controlled
• Functions of smooth muscle

  • Alter activity of various body parts to meet their specific needs
  • 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
• Smooth muscle is distributed in the eye, respiratory tubes, urinary organs, reproductive organs, digestive tubes, walls of blood vessels, and skin
• Cardiac muscle is responsible for generating and conducting electrical impulses for the heart, causing the heart to contract and pump blood throughout the body
• Components of the cardiac conduction system

  • Sinoatrial (SA) node
  • Atrioventricular (AV) node
  • Bundle of His
  • Left and right bundle branches
  • Purkinje fibers
• Sinoatrial (SA) node


  Regulates heart rate and rhythm
• Atrioventricular (AV) node


  Delays cardiac impulses from SA node to allow atria to contract and empty first, then relays impulses to the atrioventricular bundle
• Atrioventricular bundle of His


  Carries cardiac impulses down the septum to the ventricles via the Purkinje fibers