Factors influencing force of contracton

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Factors influencing force of contracton

  1. 1. FACTORS INFLUENCING FORCE OF CONTRACTION MOTOR UNIT RECRUITMENT FATIGUE RK Goit, Lecturer Department of Physiology
  2. 2. The Muscle Twitch  the mechanical response of a muscle fiber to a single action potential  starts about 2 ms after the start of depolarization of the membrane
  3. 3.  Peak tension  an ocular muscle- 7.5 msec (fast twitch fibers)  gastrocnemius muscle- 40 msec (mixture of fast & slow twitch fibers)  soleus muscle- about 90msec (slow twitch fibers)  ocular movements must be extremely rapid to maintain fixation of the eyes on specific objects  gastrocnemius muscle must contract moderately rapidly to provide sufficient velocity of limb movement for running & jumping  soleus muscle is concerned with slow contraction for continual, long-term support of the body
  4. 4. Classification of skeletal muscle fiber types Type I Type II Other names Slow; Oxidative; Red Fast; Glycolytic; White Myosin ATPase activity Low High Ca2+ Pumping capacity of sarcoplasmic reticulum Moderate High Diameter Moderate Large Glycolytic capacity Moderate High Oxidative capacity High Low Twitch duration 100 ms 7.5 ms Number of fibers Few (100-500) Many (1000-2000) Contraction velocity Moderate Fast Fatigability Low high Motor unit size Small Large Size of motor neuron Small Large Function Concerned with strong, gross, sustained movements Concerned with fine, rapid, precise movement
  5. 5. Frequency-tension relation
  6. 6.  increase in muscle tension from successive action potentials -summation  a maintained contraction in response to repetitive stimulation -tetanus  at low stimulation frequencies, the tension may oscillate as the muscle fiber partially relaxes between the stimuli-unfused tetanus  a fused tetanus, with no oscillation, is produced at higher stimulation frequencies
  7. 7. Fatigue  decrease in performance due to continuous & prolonged activity  can occur in the whole organism or in isolated preparations  is a reversible phenomenon & there is no permanent functional or structural damage to the tissue  if a muscle is allowed to rest after the onset of fatigue, it can recover its ability to contract upon restimulation
  8. 8. Major sites of fatigue  Central nervous system  encouragement & motivation increases the performance of the subject  encouragement stimulates the frontal lobe of the cortex that increases the activity in the motor cortex  Neuromuscular junction  exhaustion of ACh  fast twitch muscle fibers seem to be more prone to fatigue while slow fibers get fatigued later on
  9. 9.  Muscle Depletion of energy reserves  Depletion of creatine phosphate & glycogen Accumulation of metabolites  ↑ lactic acid concentration  ↓ pH ↓ affinity of Ca++ for troponin Inhibits some key glycolytic enzymes (glycogen phosphorylase & phosphofructokinase) Accumulation of Ca++ in T-tubules  Motor nerve  nerve is theoretically unfatiguable
  10. 10. Recruitment  the process of increasing the number of motor units that are active in a muscle at any given time  slow twitch motor units tend to be small (100 to 500 muscle fibers) & is easily excited  fast twitch motor units tend to be large (1000 to 2000 muscle fibers) & are more difficult to excite  slow twitch motor units tend to be recruited first  fast twitch motor units are recruited when more & more force is needed
  11. 11.  process of increasing the force of contraction by recruiting additional motor units is called summation
  12. 12. Factors determining force of contraction 1. Action potential frequencies (frequency- tension relation) 2. Fiber length (length-tension relation) 3. Fiber diameter 4. Fatigue 5. Number of active fibers
  13. 13. References  Berne & Levy Physiology, 6/E  Ganong Review of Medical Physiology, 23/E  Textbook of Medical Physiology, 12/E Guyton & Hall  Understanding Medical Physiology, 4/E Bijlani & Manjunatha  Vander’s Human Physiology: The Mechanisms of Body Function, 11/E
  14. 14. Thank You

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