Muscle Tissue (PART 1)

• Muscle tissue is responsible for movement of the body and its parts, and changes in the size and shape of internal organs

  
• Muscle tissue


  • Characterized by aggregates of specialized, elongated cells arranged in parallel arrays that have the primary role of contraction

  
• General properties of muscular tissues

  • Contractility
  • Excitability
  • Extensibility
  • Elasticity

  
• Contractility


  Ability of muscle cells to forcefully shorten
• Excitability


  Ability to respond to a stimulus, which may be delivered from a motor neuron or a hormone
• Extensibility


  Ability of a muscle to be stretched or extended
• Elasticity


  Ability of a muscle to return to its original length when relaxed

  
• General functions of muscular tissues

  • Movement
  • Maintenance of posture
  • Respiration
  • Heat generation
  • Communication
  • Constriction of organs and blood vessels
  • Pumping of blood
  • Protecting organs

  
• Myofilament interaction


  Responsible for muscle cell contraction
• Thin filaments


  • Composed primarily of the protein actin, each thin filament of fibrous actin is a polymer formed from globular actin molecules
• Thick filaments


  • Composed of the protein myosin II, each thick filament consists of 200-300 myosin II molecules, the long, rod-shaped tail portion of each molecule aggregates in a regular parallel but staggered array, whereas the head portions project out in a regular helical pattern
• Sarcoplasm


  The cytoplasm of muscle cells, where the two types of myofilaments occupy the bulk

  
• Muscle classification


  Striated muscle - cells exhibit cross-striations at the light microscope, Smooth muscle - cells do not exhibit cross-striations

  
• Skeletal muscle


  • Each skeletal muscle functions as an organ composed of several interconnected tissues including skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue

  
• Skeletal muscle functions


  Maintains posture and enables movement

  
• Sarcolemma


  • The plasma membrane of muscle fibers, it is the site of action potential conduction, which triggers muscle contraction
• Terminal cisternae


  • Extension of sarcolemma that stores calcium, it surrounds the transverse tubules in muscle cells
• Transverse tubules


  • Tube-like inward folds (channels) of the sarcolemma, they carry electrical impulses into the center of the muscle fiber so that every contractile unit of the muscle fiber contracts in unison

  
• Sarcoplasm


  The cytoplasm of muscle fibers

  
• Sarcoplasmic reticulum


  • A highly specialized smooth endoplasmic reticulum in skeletal muscle fibers that stores high levels of Ca2+, release of Ca2+ from the sarcoplasmic reticulum is a "switch" for muscle contraction

  
• Myofilaments


  • Protein filaments that form the building blocks of muscles, they work together to produce muscle contractions, allowing us to move our bodies

  
• Sarcomere


  Functional unit (made of actin and myosin), it is the smallest portion of a muscle that can contract, it is the arrangement of actin and myosin within sarcomeres that gives skeletal muscle its striated appearance

  
• Z discs


  • Form a stationary anchor for actin myofilaments, one sarcomere extends from one Z disk to the next Z disk
• I bands

  • Contain only actin myofilaments (thin filaments) and thus appear lighter staining
• A bands


  • The darker-staining band in the center of each sarcomere, each A band contains both actin and myosin myofilaments overlapping, except in the center
• H zone


  • The center of each A band, contains only myosin myofilaments
• M line


  • The middle of each H zone, consists of delicate filaments that hold the myosin myofilaments in place
• Myofibril


  A fundamental structure found in muscle cells, responsible for muscle contraction

  
• Types of muscle fibers (cells)

  • Type I (slow, oxidative fibers)
  • Type IIA (fast, oxidative-glycolytic fibers)
  • Type IIB (fast, glycolytic fibers)

  
• Endomysium


  • The delicate and innermost layer of connective tissue that surrounds individual muscle fibers within each fascicle, it permits only small-diameter nerve fibers and capillaries to pass through, and acts as a site for nutrient and waste exchange within the muscle tissue

  
• Perimysium


  • The thicker and middle layer that surrounds a group of muscle fibers, forming a bundle called a fascicle, it contains blood vessels and nerves that supply the muscle fibers within the fascicle

  
• Epimysium


  • The outermost connective tissue sheath of skeletal muscle, surrounding the entire muscle (organ), it is a layer of dense irregular connective tissue that separates muscle from other tissues and organs in the area, allowing the muscle to move independently

  
• Functions of skeletal muscle

  • Movement
  • Stabilize the body
  • Voluntary action or control
  • Protect internal organs and tissues
  • Move the eye
  • Store nutrients
  • Manage body temperature

  
• Skeletal muscles comprise 30-40% of our total body mass, they are found throughout the body, attached to bones via tendons, and are also present in the tongue, eye socket, diaphragm and upper esophagus

  
• Cardiac muscle


  • Also known as myocardium, has characteristics from both skeletal and muscle tissues, contractility can be altered by the Autonomic nervous system (ANS) and hormones, myofilaments are arranged in a similar pattern to skeletal muscle, which results in cross-striations

  
• Cardiomyocytes


  • Also known as cardiac muscle cells, contain one elongated nucleus that lies in the centre, the cytoplasm is called sarcoplasm and contains large and elongated mitochondria and glycogen granules
• Intercalated discs


  • Joins cardiac myocytes together and coincide them with Z line, provide attachment point which results to characteristic branched pattern, allow the cardiac muscle to function as a functional syncytium
• Intercalated discs


  Contain 3 types of cell junctions: Adherens junctions, Desmosomes, and Gap junctions
• Cardiac muscle function


  Involuntary in action, exists only in the heart and is responsible for the pumping action of the heart, capable of strong, continuous, and rhythmic contractions, consists of cells that respond to the impulses of the nervous system called the Pacemaker cells
• Location of cardiac muscle

  • Makes up the thick middle layer of the heart where it makes up the walls of the atria and ventricles and is surrounded by a thin outer layer called the epicardium or visceral pericardium and an inner endocardium