32-53

• Resting membrane potential of nerve fibers

  90 millivolts (mv)
• Nerve Action Potential


  Rapid changes in the membrane that transmit nerve signals
• Stages of the Action Potential

  1. Resting stage (Polarized)

  2. Depolarization stage (Na influx)

  3. Repolarization stage (Potassium efflux)
• Stages of Spike Potential and After-Potentials

  1. Spike Potential (Depolarization)

  2. Negative After-Potential (After-depolarization)

  3. Positive After-Potential (After-hyperpolarization)
• Negative After-Potential


  Delay of the membrane potential to return to its resting level due to the build-up of K ions outside the membrane
• Positive After-Potential


  Membrane potential returns to its normal resting stage, a little more negatively
• Voltage-Gated Sodium and Potassium Channels

  • Propagation of the Action Potential
  • All or Nothing Principle
• Excitation Process of Eliciting Action Potential

  1. Chemical Stimulation

  2. Mechanical Stimulation

  3. Electrical Stimulation
• Rheobase


  Least possible voltage at which a nerve fiber will contract
• Utilization Time


  Time required for the rheobase voltage to stimulate the nerve fiber
• Chronaxie


  Time required to stimulate the nerve fiber when the voltage is double that of the rheobase
• Refractory Period


  Time where a second action potential cannot occur as long as the membrane is still depolarized from the preceding action potential
• Absolute Refractory Period


  Time wherein a second action potential cannot be elicited even with a very strong stimulus
• Relative Refractory Period


  Time wherein a strong than normal stimulus can excite the fiber
• Membrane Stabilizing Factors


  • High extracellular fluid calcium ion concentration
  • Low potassium ion concentration
  • Local anesthetics which prevents opening of Na channels
• Local anesthetics


  • Cocaine
  • Procaine
  • Etracaine
  • Analgesics
• Skeletal Muscle


  Voluntary control, comprises the great mass of somatic musculative (10%), has well-developed cross-striations, cannot contract on their own

  
• Smooth (Visceral) Involuntary Muscle

  Lacks cross-striation, found in most hollow viscera, can contract on their own

  
• Cardiac (Heart) Muscle


  Has cross-striations like skeletal and is involuntary like smooth muscles, contracts rhythmically even in the absence of external stimulus

  
• Skeletal Muscle


  • Made up of individual muscle fibers arranged in parallel between the tendinous ends
  • Begins and ends in tendons
  • Enveloped by a cell membrane called Sarcolemma which consist of a true cell membrane called the plasma membrane
  • Each muscle fiber is made up of myofibrils divided into individual filaments lying side by side. 1 myofibril = 1500 myosin (thick) filaments and 3000 action (thin) filaments

  
• Actin Filament


  Light band, I band, Composed of Troponin Strands (protein tropomyosin) and Troponin Molecule
• Troponin Strands


  At resting stage, it covers the active sites of actin strands so that interaction with myosin cannot occur
• Troponin Molecule


  Attached to this molecule are 3 types of troponin: Troponin I (affinity to actin), Troponin T (affinity for tropomyosin), Troponin C (affinity to calcium which indicates the contraction)

  
• Myosin Filament


  Dark band, A band, has projections at the sides called cross-bridges or myosin heads that link actin and myosin together during contraction

  
• Walk-Along Theory of Muscle Contraction


  1. Discharge of motor neuron

  2. Release of transmitter (acetylcholine)

  3. Binding of acetylcholine to acetylcholine receptors (nicotinic acetylcholine)

  4. Increased permeability of end-plate membrane to sodium and potassium

  5. Generation of end-plate potential

  6. Generation of action potential (depolarization)

  7. Inward spread of depolarization along the T tubules

  8. Release of calcium from lateral sacs of sarcoplasmic reticulum and diffusion to thick and thin filaments

  9. Binding of calcium to troponin C, uncovering myosin binding sites

  10. Formation of cross-linkages between actin and myosin and sliding of thin and thick filaments, producing shortening or contraction

  
• Steps in Relaxation


  1. Calcium pumped back into sarcoplasmic reticulum

  2. Release of calcium from troponin

  3. Cessation of interaction between actin and myosin

  4. Acetylcholine which started the process will be destroyed by the enzymes in the sarcolemma (acetylcholinesterase)
• Factors Affecting Mechanical Response of Muscles

  • Strength of Stimulus (directly proportional)
  • Length of Sarcomere (Tension) - 2.0 - 2.2 microns (highest degree of contraction)
  • Stretch - too much = less contraction
  • Velocity (Load) inversely proportional - ↑ load = ↓ contraction

  
• Isotonic Contraction


  Requires much sliding among myofibrils (Shortening), contraction lasts longer, performs external work, has same fusion, move limbs in running

  
• Isometric Contraction


  Does not require much sliding of myofibrils (no shortening), same length, contraction shorter in duration, no work, doesn't move load, keep limbs stiff when legs hit the ground in running

  
• Fast Fibers (Type II) White Muscle


  • Muscles that react rapidly, much larger fibers for great strength of contraction, extension sarcoplasmic reticulum for rapid release of Ca ions, large amounts of glycolytic enzyme, less extensive blood supply, fewer mitochondria, less myoglobin, adapted for rapid, twitch duration 7.5 ms, powerful contraction

  
• Slow Fibers (Type 1) Red Muscle

  • Muscle that respond slowly but with prolonged period of contraction, smaller fibers, innervated by small nerve fibers, more extensive blood supply to extra amount of oxygen, greatly contain large amounts of myoglobin, greater number of mitochondria, prolonged muscle activity - support against gravity, respond slowly, twitch duration - 100 ms

  
• Special Features and Abnormalities of Skeletal Muscle Function

  • Muscle Hypertrophy
  • Muscle Atrophy
  • Rigor Mortis (After death)
  • Familiar Periodic Paralysis
  • Muscle Fibrillation (Fine irregular contractions)
  • Muscle Fasciculation (Jerky, visible contractions)

  
• 50% of the body is made up of Muscles