Muscular system

• Types of Muscular Tissue

  • Skeletal
  • Cardiac
  • Smooth
• Skeletal muscle tissue


  • Striated
  • Works mainly in a voluntary manner
  • Activity can be consciously controlled
  • Most are controlled subconsciously to some extent
• Cardiac Muscle Tissue


  • Found only in the walls of the heart
  • Striated like skeletal muscle
  • Action is involuntary
  • Contraction and relaxation of the heart is not consciously controlled
  • Contraction is initiated by a node of tissue called the “pacemaker”
• Smooth Muscle Tissue


  • Located in the walls of hollow internal structures
  • Lacks the striations of skeletal and cardiac muscle tissue
  • Usually involuntary
• Properties of Muscular Tissue

  1. Excitability: Ability to respond to stimuli

  2. Contractility: Ability to contract forcefully when stimulated

  3. Extensibility: Ability to stretch without being damaged

  4. Elasticity: Ability to return to an original length
• Skeletal Muscle Tissue Connective Tissue Components

  • Fascia/Fasicle: Dense sheet or broad band of irregular connective tissue that surrounds bundles of muscles fibers
  • Epimysium: The outermost layer, Separates 10-100 muscle fibers into bundles called fascicles
  • Perimysium: Surrounds numerous bundles of fascicles
  • Endomysium: Separates individual muscle fibers from one another
  • Tendon: Cord that attaches a muscle to a bone
  • Aponeurosis: Broad, flattened tendon
• Neurons that stimulate skeletal muscle to contract are: SOMATIC MOTOR NEURONS
• The axon of a somatic motor neuron typically branches many times, each branch extending to a different skeletal muscle fiber, each muscle fiber is in close contact with one or more capillaries
• The number of skeletal muscle fibers is set before you are born. Most of these cells last a lifetime.
• Muscle growth

  Hypertrophy: an enlargement of existing muscle fibers
• Testosterone and human growth hormone stimulate hypertrophy
• Satellite cells retain the capacity to regenerate damaged muscle fibers
• Atrophy is the reduction of muscle mass
• Sarcolemma


  The plasma membrane of a muscle cell
• Transverse (T tubules)


  Tunnel in from the plasma membrane
• Muscle action potentials travel through the T tubules
• Sarcoplasm


  The cytoplasm of a muscle fiber
• Sarcoplasm includes glycogen used for synthesis of ATP
• A red-colored protein called myoglobin binds oxygen molecules
• Myoglobin releases oxygen when it is needed for ATP production
• Myofibrils


  Thread-like structures which have a contractile function
• Sarcoplasmic reticulum


  • Membranous sacs which encircle each myofibril
  • Stores calcium ions (Ca++)
  • Release of Ca++ triggers muscle contraction
• Filaments


  Function in the contractile process
• Types of filaments

  • Thick (actin)
  • Thin (Myosin)
• Sarcomeres


  • Compartments of arranged filaments
  • Basic functional unit of a myofibril
• Z discs


  • Separate one sarcomere from the next
  • Thick and thin filaments overlap one another
• A band


  • Darker middle part of the sarcomereThick and thin filaments overlap
• I band


  • Lighter, contains thin filaments but no thick filaments
  • Z discs pass through the center of each this band
• H zone


  Center of each A band which contains thick but no thin filaments
• M line


  Supporting proteins that hold the thick filaments together in the H zone
• Proteins building myofibrils

  • Contractile proteins: Generate force during contraction
  • Regulatory proteins: Switch the contraction process on and off
  • Structural proteins: Align the thick and thin filaments properly, Provide elasticity and extensibility, Link the myofibrils to the sarcolemma
• Myosin


  • Thick filaments, Functions as a motor protein which can achieve motion, Converts ATP to energy of motion, Projections of each myosin molecule protrude outward (myosin head)
• Actin


  • Thin filaments, Actin molecules provide a site where a myosin head can attach, Tropomyosin and troponin are also part of the thin filament
• An increase in calcium ions (Ca++) concentration in the muscle

  Starts contraction
• A decrease in calcium ions (Ca++)

  Stops muscle contraction
• Action potentials


  Causes Ca++ to be released from the SR into the muscle cell
• Ca++ ions move tropomyosin away from the myosin-binding sites on actin


  Allowing cross-bridges to form
• As the Ca++ ions level in the cell drops Myosin-binding sites are covered, and the muscle relaxes
• The forcefulness of muscle contraction

  Depends on the length of the sarcomeres
• When a muscle fiber is stretched

  There is less overlap between the thick and thin filaments and tension (forcefulness) is diminished