ihslec

• Muscular System


  • Hierarchical organization is an excellent illustration of the relationship between form and function
• Electrical impulses must travel to millions of tiny motors (muscle cells) throughout the body to enable movement, vision, and breathing
• Types of muscle tissue

  • Skeletal
  • Smooth
  • Cardiac
• Skeletal muscle


  Voluntary muscle that constitutes about 40% of the body's weight, responsible for body movements
• Smooth muscle


  Involuntary muscle found in walls of hollow organs and tubes, responsible for moving materials through the body

  
• Cardiac muscle


  Involuntary muscle found only in the heart, responsible for pumping blood through the circulatory system
• Functions of the muscular system

  • Movement of the body
  • Maintenance of posture
  • Respiration
  • Production of body heat
  • Communication
  • Constriction of organs and vessels
  • Contraction of the heart

  
• Contractility


  The ability of muscle to shorten forcefully, or contract
• Excitability


  The capacity of muscle to respond to an electrical stimulus
• Extensibility


  A muscle can be stretched beyond its normal resting length and still be able to contract
• Elasticity


  The ability of muscle to spring back to its original resting length after it has been stretched
• Connective tissue components of skeletal muscle

  • Epimysium
  • Perimysium
  • Endomysium

  
• Skeletal muscles have a rich supply of blood vessels and nerves
• Motor neurons


  Specialized nerve cells responsible for stimulating skeletal muscle contraction
• Every skeletal muscle fiber is controlled by a branch of a motor neuron

  
• Skeletal muscle fibers develop from the fusion of several hundred embryonic cells called myoblasts

  
• Muscle hypertrophy


  Enlargement of muscles due to an increase in the size of each muscle fiber, not an increase in the number of muscle fibers
• Skeletal muscle fibers have a striated (banded) appearance due to alternating light and dark bands

  
• The number of skeletal muscle fibers remains relatively constant after birth

  
• Muscle fibers


  • As much as 500 μm wide
  • Large muscles contain many large-diameter muscle fibers
  • Small, delicate muscles contain many small-diameter muscle fibers
  • Most muscles contain a mixture of small- and large-diameter muscle fibers

  
• Skeletal muscle fibers in longitudinal section have alternating light and dark bands that give the muscle fiber a striated (banded), or striped, appearance

  
• Muscle hypertrophy


  Enlargement of muscles in children and adults, resulting from an increase in the size of each muscle fiber, not from a substantial increase in the number of muscle fibers
• Hypertrophy of muscles in response to exercise is due mainly to an increase in muscle fiber size, rather than an increase in number
• Sarcolemma


  The plasma membrane 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.

  
• Triad


  Two terminal cisternae of the sarcoplasmic reticulum and their associated T tubule, a critical structure for muscle contraction
• Muscle fibers also contain numerous mitochondria and energy-storing glycogen granules, which constitute the cytoplasm (sarcoplasm) in muscle fibers

  
• Myofibrils


  Bundles of protein filaments in the sarcoplasm of muscle fibers

  
• Myofilaments


  Two types: actin (thin) myofilaments and myosin (thick) myofilaments, arranged into highly ordered units called sarcomeres
• Sarcomeres


  The structural and functional units of skeletal muscles, containing the myofilaments that provide the mechanical aspect of muscle contraction

  
• Sarcomeres


  • Extend from one Z disk to the next Z disk
  • Contain two lighter-staining I bands and a central darker-staining A band
  • The A band contains both actin and myosin myofilaments overlapping, except in the center (H zone) which contains only myosin myofilaments
  • The M line in the middle of the H zone consists of delicate filaments that hold the myosin myofilaments in place
  • Titin protein gives muscle the ability to stretch and recoil

  
• Actin myofilaments


  Composed of G actin, tropomyosin, and troponin
• Myosin myofilaments


  Composed of many elongated myosin molecules shaped like golf clubs, with two myosin heads that extend laterally and bind to actin to form cross-bridges during contraction
• Neuromuscular junction


  The point of contact of motor neuron axon branches with the muscle fiber, consisting of the axon terminals (presynaptic terminal) and the area of the muscle fiber sarcolemma they innervate (motor end-plate)
• Synaptic vesicles


  Small, spherical sacs in the presynaptic terminal that contain the neurotransmitter acetylcholine
• Ligand-gated ion channels


  Specialized membrane transport proteins that are opened or closed by specific molecules (ligands) such as neurotransmitters
• Sliding filament model


  When a muscle contracts, the actin and myosin myofilaments in the sarcomere slide past one another and shorten the sarcomere, causing the entire muscle fiber to shorten
• Sarcomere shortening


  1. Actin and myosin myofilaments slide past one another

  2. Sarcomere shortens

  3. Myofibrils shorten

  4. Muscle fiber shortens

  5. Muscle fascicles and whole muscle shorten
• During muscle relaxation, sarcomeres lengthen due to an external force applied to the muscle
• Muscle contraction causes a joint to flex, and muscle relaxation causes extension of the joint