LEC. 6 - BIO118

• Principal kinds of movement

  • ameboid
  • ciliary and flagellar
  • muscular
• Skeletal muscle


  • Striated
  • Voluntary
  • Innervated by somatic nerves
  • Neurogenic

  
• Skeletal muscle


  • Well-developed sarcoplasmic reticulum
  • Well-developed T-Tubule

  
• Sarcoplasmic reticulum


  Source of Calcium
• General organization of skeletal muscle tissue

  1. Muscle is anchored at each end by a tough strap of connective tissue - tendon

  2. Muscle consists of cylindrical, multinucleate cells - muscle fiber - arranged in parallel with one another

  3. Each fiber rises from many individual embryonic muscle cells - myoblasts

  4. Myoblasts fuse during embryonic development to form a myotube

  5. Each muscle fiber is composed of numerous parallel subunits - myofibril

  
• Sarcomere


  Functional unit of striated muscle

  
• Sarcomere


  • Composed of thin filaments consisting largely of the protein actin
  • Composed of thick filaments made up primarily of the protein myosin

  
• Sliding Filament Hypothesis


  1. Thin and thick filaments slide past each other

  2. Z-lines become closer together

  3. Causes shortening of sarcomeres

  4. Muscle contraction occurs when myosin cross-bridges bind to actin

  5. Muscle contraction requires ATP hydrolysis by myosin

  6. Calcium ions are needed to expose myosin binding sites on actin

  7. Calcium ions are released from sarcoplasmic reticulum in response to action potentials
• Actin filaments


  • One end attached to Z-plate at one end of sarcomere
  • Other end suspended in sarcoplasm

  
• Myosin filaments


  • Suspended in between Z-plates
  • Contain cross-bridges which pull actin filaments inward

  
• Sarcomeres stacked together in series and cause myofiber to shorten

  
• Working muscles require ATP, which is regenerated through cellular respiration during sustained exercise
• Motor unit


  A single motor neuron and all the muscle fibers it controls, the functional unit of skeletal muscle control
• Neuromuscular junction


  1. Synaptic contact between a nerve fiber and a muscle fiber

  2. Nerve impulses trigger release of neurotransmitter that signals the muscle fiber to contract
• Presynaptic portion of neuromuscular junction

  • Axon terminals of motor neuron
  • Contain vesicles of acetylcholine and mitochondria
• Postsynaptic portion of neuromuscular junction

  • Endplate membrane of muscle fiber
  • Contains nicotinic acetylcholine receptors
  • Contains acetylcholinesterase enzyme to hydrolyze acetylcholine
• Amyotrophic lateral sclerosis (ALS)


  Interferes with excitation of skeletal muscle fibers, usually fatal
• Myasthenia gravis


  Autoimmune disease that attacks acetylcholine receptors on muscle fibers, treatments exist
• Energy for muscle contraction comes from ATP, which is regenerated through aerobic metabolism of glucose and glycogen
• Synaptic cleft


  Hydrolyses ACh to choline and acetate to ensure that it works for only a brief period
• Regulation of skeletal muscle contraction

  1. Sarcomere

  2. Ca2+ released from SR

  3. Synaptic terminal

  4. T tubule

  5. Motor neuron axon

  6. Plasma membrane of muscle fiber

  7. Sarcoplasmic reticulum (SR)

  8. Myofibril

  9. Mitochondrion

  
• The stimulus leading to contraction of a muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber
• Regulation of skeletal muscle contraction

  1. Ca2+ ATPase pump

  2. Synaptic terminal of motor neuron

  3. Synaptic cleft

  4. T Tubule

  5. Plasma membrane

  6. Ca2+

  7. CYTOSOL

  8. SR

  9. ATP

  10. ADP

  11. Pi

  12. ACh
• Amyotrophic lateral sclerosis (ALS), formerly called Lou Gehrig's disease, interferes with the excitation of skeletal muscle fibers; this disease is usually fatal
• Myasthenia gravis is an autoimmune disease that attacks acetylcholine receptors on muscle fibers; treatments exist for this disease
• ATP


  Immediate source of energy
• Energy for Contraction


  1. Glucose broken down during aerobic metabolism

  2. Glycogen stores can supply glucose

  3. Muscles have creatine phosphate, an energy reserve (CP + ADP → ATP + creatine)
• Skeletal muscle fibers can be classified

  • As oxidative or glycolytic fibers, by the source of ATP
  • As fast-twitch or slow-twitch fibers, by the speed of muscle contraction

  
• Slow and fast oxidative fibers

  • Rely heavily on glucose and oxygen
• Fast glycolytic fibers


  • Rely on anaerobic glycolysis
• Muscles incur oxygen debt during anaerobic glycolysis
• Slow oxidative fibers (red muscles)

  • For slow, sustained contractions without fatigue
  • Contain extensive blood supply
  • High density of mitochondria
  • Abundant stored myoglobin (protein that binds oxygen more tightly than hemoglobin does)
  • Important in maintaining posture in terrestrial vertebrates

  
• Fast glycolytic fiber (white muscles)


  • Lacks efficient blood supply
  • Pale in color
  • Function anaerobically
  • Fatigue rapidly
• Fast oxidative fiber


  • Extensive blood supply
  • High density of mitochondria and myoglobin
  • Function aerobically
  • For rapid, sustained activities
• Subthreshold


  Not capable of inducing a response or muscle contraction
• Threshold


  Weakest stimulus capable of causing contraction
• Submaximal


  Intermediate between threshold and supramaxima
• Maximal


  Causes a maximal contraction wherein the overlap between the myofilaments is greatest
• Supramaximal


  Beyond maximal stimulus, no higher contractions are observed because all the motor units have been activated
• Twitch


  Results from a single action potential in a motor neuron