Muscular System

• Muscles make up 40-50% of our body weight
• Types of muscles


  • Skeletal muscles
  • Smooth muscles
  • Cardiac muscles
• Skeletal muscles


  • Helps with movement and breathing
  • Cells are voluntary, striated, multinucleated, and much longer than their width
  • Also called muscle fibers
• Smooth muscles


  • Push food through the intestines
  • Contain blood in arteries and veins
  • Push urine down our ureters
• Cardiac muscles


  • Pump blood through the heart and blood vessels
  • Maintain blood pressure
  • Cardiac muscle cells are involuntary, but striated and uninucleated
  • They don't look like fibers but have extensions or branches
• Sarcoplasm


  Cytoplasm of a muscle cell, stores oxygen, and glycogen
• Sarcolemma


  Plasma membrane of a muscle cell
• Myofibril


  Contractile units of muscle fiber
• Sarcoplasmic reticulum


  In the smooth ER, stores/regulates Calcium
• Skeletal muscle cell


  • Surrounded by a cell membrane called the sarcolemma
• Muscle


  • Consists of skeletal muscle bundles called fascicles
  • Each fascicle is composed of muscle fibers or cells
  • Each fiber or cell is surrounded by connective tissue called the endomysium
  • Each fascicle is surrounded by connective tissue called the perimysium
  • The perimysium is covered with connective tissue called epimysium
  • A layer of tissue called fascia is on top of the epimysium
• Skeletal muscle under microscope

  • Have cross-striations due to the overlap of dark bands of thick protein myosin (A bands) and light bands of thin protein actin (I bands)
  • The middle of an I band is a z-line
  • The middle of an A band is an H line or zone
  • Area between two adjacent Z lines is called a sacromere
• Muscle fibrils


  • Actin and myosin fibrils that make up a muscle cell are surrounded by a sarcotubular system composed of T-tubules and an irregular curtain called the sarcoplasmic reticulum
• Function of T-tubules


  Rapid transmission of a nerve impulse to all the fibrils in a cell
• Function of sarcoplasmic reticulum

  Stores calcium ions
• All muscle cells or fibers are innervated by the same motor neuron called a motor unit
• Properties of muscle cells


  • Excitability: ability to react to a stimulus through their cytoplasm
  • Conductivity: ability to conduct an electric current
  • Contractability: reaction to stimulus by shrinking
  • Elasticity: ability to allow cell to return to original shape after contraction
• Muscle contraction


  1. Neurochemical (voltage)

  2. Chemical (NT)

  3. Energy (ATP)
• Resting potential


  Outside of muscle cell is positively charged electrically and inside is negatively charged
• Muscle contraction process


  1. Motor neuron innervates muscle cell

  2. Acetylcholine is secreted from axon terminals into neuromuscular junction

  3. Na+ ions rush inside cell, creating electrical potential (inside turns from negative to positive)

  4. Potassium move outside cell to restore resting potential

  5. Influx of positive sodium ions causes T-tubules to transmit stimulus deep into muscle cell, creates action potential

  6. Action potential causes sarcoplasmic reticulum to release calcium ions into fluids surrounding myofibrils of myosin and actin

  7. Calcium causes myosin to become activated myosin, links up with actin filaments
• Muscle relaxation


  1. Na-K pump operates, pumping sodium out and potassium in to restore resting potential

  2. Calcium ions are reabsorbed by sarcoplasmic reticulum
• ATP


  Energy source for muscle contraction
• ATP is produced during cellular respiration and fermentation. If oxygen is available, more ATP is produced
• Smooth muscle


  • Found in hollow structures like intestines, arteries, veins, and bladder
  • Under control by autonomic nervous system
  • Arranged in outer longitudinal layer (muscularis) and inner circular layer, this results in material being pushed forward in tub by simultaneous contraction of both of these layers
• Cardiac muscle


  • Only found in heart
  • Controlled by the autonomic nervous system
  • Can receive an impulse, contract, immediately relax, and receive another impulse, this occurs 75 times a minute