Tissues

• Overview of 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
• Excitability

  -Respond to stimuli
• Contractility

  -Shorten and generate pulling force
• Extensibility

  -Stretch w/ contraction of opposing muscle
• Elasticity

  -Recoil passively after stretch
• Muscle contraction

  
• Skeletal

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

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

  • Found in hollow visceral organs
  • No visible striations
  • Involuntary
• Skeletal: General concepts and structures

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

  • Muscle fiber is 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
• Myofibrils are built from three kinds of 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
• Skeletal: Skeletal muscle ultrastructure

  • 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 → transmitted along the T tubules to terminal cisterna of SR → release of calcium
• release of calcium to ICF
• In resting muscle, myosin heads can’t bind actin because the binding sites are blocked by the troponin-tropomyosin complex on actin
• Binding of calcium to Troponin displaces tropomyosin → exposure of myosin binding sites
• Energy provided by hydrolysis of ATP
• formation of crossbridges
• Skeletal: Molecular basis of contraction

  • 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
• Myoblasts

  -embryonic cells that fuse to develop muscle fibers
• Skeletal: Microscopic appearance

  • Cells

  elongated cells that do not branch

  w/ larger diameter

  Nucleus: appears multinucleated , flattened , and peripherally located
• Skeletal: Microscopic appearance

  • 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 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

  
• Layers of the Heart

  Pericardium - is the outermost layer. It consists of 2 thin, fibrous protective layer that contains fluid to protect them from friction

  Myocardium - is the middle layer that contains the heart muscle

  Endocardium - is the innermost layer that lines the heart
• Cardiac: General concepts and structure

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

  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: 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 one SR
• Cardiac

  Distribution

  -Muscle layer of the heart (myocardium)

  -Walls of the aorta, vena cava and pulmonary vessels

  Functions

  -to provide the contractile activity of the heart
• Conduction System of the Heart

  • Responsible for generating and conducting electrical impulses for the heart
  • Cause heart to contract and pump blood throughout the body
• Made up of 5 elements:

  • Sino-atrial (SA)
  • Atrio-ventricular (AV) node
  • Bundle of His
  • Left and right bundle branches
  • Purkinje fiber node
• Sinoatrial Node (Pacemaker, Node of Keith & Flack)

  -Located below the epicardium at the junction of superior vena cava & right atrium

  Function: Regulates heart rate and rhythm
• Atrioventricular Node (Node of Tawara)

  -Located below the endocardium, on the lower part of the interatrial septum

  Functions: Delays cardiac impulses from sinoatrial node to allow atria to contract and empty the contents first

  Relays cardiac impulses to the atrioventricular bundle
• Atrioventricular Bundle of His

  -bundle of fibers located within the septum of the heart

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