Muscular System

• Muscles


  • Make up about 40% to 50% of the body's weight
  • Mature skeletal or striated muscle cells are the longest and most slender muscle fibers, ranging in size from 1 to 50 mm in length and 40 to 50 µm in diameter
• Skeletal muscle cells


  Often referred to as skeletal muscle fibers; multinucleated; surrounded by a special membrane called sarcolemma
• Skeletal muscle structure


  1. Sarcolemma is surrounded by endomysium

  2. Muscle consists of skeletal muscle bundles called fasciculi surrounded by perimysium

  3. Perimysium connects with the coarse irregular connective tissue that surrounds the whole muscle called the epimysium

  4. Three layers of connective tissue act like cement holding all of the muscle cells and their bundles together
• Sarcoplasmic reticulum (SR)


  • Forms a tubular network around each myofibril
  • Specialized for the storage and release of calcium ions
• Myofibril


  • 192 mm in diameter and as long as the entire muscle fiber
  • Active shortening of myofibrils is responsible for skeletal muscle fiber contraction
• Myofilaments


  • Bundles of protein filaments in myofibrils
  • Thin filaments composed of actin; thick filaments composed of myosin
  • Titin, which are elastic myofilaments associated with thick filaments
  • With scattered mitochondria and granules of glycogen, the storage form of glucose
• Sarcomeres


  • Repeating functional units of the muscle fiber
  • Contains thick filaments, thin filaments, proteins stabilizing positions of the thick and thin filaments, and proteins that regulate the interactions between thick and thin filaments
• A band


  Region of overlapping actin and myosin
• M line


  • Center of the A band
  • Proteins of the M line connect the central portion of each thick filament to neighboring thick filaments. These dark-staining proteins help stabilize the positions of the thick filaments
• H band


  A lighter region on either side of the M line; contains thick filaments, but no thin filaments
• Zone of overlap


  A dark region where thin filaments are located between the thick filaments; where three thick filaments surround each thin filament, and six thin filaments surround each thick filaments
• I band


  • Region of the sarcomere that contains thin filaments but no thick filaments
  • Extends from the A band of one sarcomere to the A band of the next sarcomere
• Z lines


  Bisect the I bands and mark the boundary between adjacent sarcomeres; consist of proteins called actinins
• Myofilaments


  • Responsible for muscular contraction
  • Thin filaments – filamentous actin (F- actin), nebulin, tropomyosin, troponin
• Sliding filament theory of muscular contraction

  1. The H bands and I bands of the sarcomeres narrow

  2. The zones of overlap widen

  3. The Z lines move closer together

  4. The width of the A band remains constant
• These observations make sense only if the thin filaments are sliding toward the center of each sarcomere, alongside the thick filament
• Muscle cells possess four properties

  Excitability, conductivity, contractility, and elasticity
• Stimulus


  In our bodies, this stimulus is a nerve cell
• Conductivity


  Allows a response to travel throughout the cell depending on the type of tissue excited
• Elasticity


  Allows the muscle cell to return to its original shape after contraction
• Factors involved in muscle contraction

  • Neuroelectrical
  • Chemical interactions
  • Energy sources
• Smooth muscle


  • Cannot be controlled at will because it is under the control of the autonomic nervous system and also may be hormonally stimulated
  • Contains a single large nucleus and because its fiber is more delicate than skeletal muscle, cross-striation of the myosin and actin arrangements is not visible
• Smooth muscle contraction


  1. In hollow structures like the small intestine, the smooth muscle is arranged in two layers, an outer longitudinal layer and an inner circular layer

  2. Contraction of these two layers, with the circular layer contracting first, results in reducing both the length of the tube and the circumference of the tube pushing whatever is inside the tube in a forward direction
• Smooth muscle


  • Produce slower contractions than skeletal muscles but allows greater extensibility of the muscle
  • Actin and myosin fibers are not so regularly arranged in smooth muscle as in striated muscle
• Cardiac muscle


  • Cannot be influenced at will because it is under the control of the autonomic nervous system
  • Uninucleated; striated
• Cardiac muscle contraction


  1. If one muscle cell is stimulated, all the muscle cells or fibers are stimulated so all the muscle cells contract together

  2. The muscle cell that contracts the fastest will control the speed of other muscle cells, causing them all to contract at the faster rate

  3. Rapid rhythm - to receive an impulse, contract, immediately relax, and then receive another impulse; about 75 times a minute

  4. The period of an individual contraction is slower in cardiac (about 0.8 second) as opposed to skeletal muscle, which is much faster (about 0.09 second)

  5. Uncontrolled contraction of individual cells in the heart occurs, this is called fibrillation
• Muscles moving the humerus

  Originate on the bones of the shoulder girdle
• Muscles moving the humerus

  • Pectoralis major - flexes and adducts the arm
  • Latissimus dorsi muscle - extends, adducts, and rotates the arm medially
  • Teres minor - adducts and rotates the arm
  • Deltoid - abducts the arm and is also the muscle that receives injections
  • Supraspinatus - also abducts the arm
  • Infraspinatus - rotates the arm
• Rizzo, Donald C. (2015) Fundamentals of anatomy and physiology 4th ed