XIIYOUTUBE 16. Skeleton and Movements - part 03- Relaxation of muscle fibres and Properties of Muscles on Electrical Stimulation

 

16. Skeleton and Movements - part 03- Relaxation of muscle fibres and Properties of Muscles on Electrical Stimulation


Relaxation of muscle fibres :

  • During relaxation all the events occur in reverse direction. 
  • When stimulation is terminated, actomyosin complex is broken down and myosin head gets detached from actin filaments. 
  • This process involves use of ATP. 
  • At the same time calcium ions return back. This is also an active process that uses energy. 
  • Due to disappearance of calcium ions, troponin – tropomyosin complex is restored again. This complex covers the active sites of actin filament. 
  • Due to this the interaction between actin and myosin ceases to occur and the actin filaments return back to their original position. This results in muscular relaxation. 
  • Like contraction, relaxation is also an active process 

Oxygen debt is used in oxidizing the accumulated lactic acid aerobically and in restoring the depleted creatine phosphate and ATP.

Rigor Mortis :

  • Usually, some hours after the death of an individual, its muscles are stiffened. This muscular stiffening, after death is rigor mortis. 
  • It helps in fixation of hours of death after a murder.
  • After death, the fresh supply of ATP to muscles becomes impossible. 
  • Therefore once the local store of ATP is finished, the detachment of myosin from actin can not take place. 
  • This results in permanent state of contraction of the muscle.


Properties of Muscles on Electrical Stimulation:

A. Single muscle twitch : 

  • A muscle contraction initiated by a single brief-stimulation is called a single muscle twitch. 
  • It occurs in 3 stages : a latent period of no contraction, a contraction period and a relaxation period.

B. Summation : 

  • If the muscle is stimulated before the end of the twitch, it generates greater tension i.e., summation or addition of effect takes place. 
  • Repeated stimuli will produce increasing strength of contraction (stair case phenomenon).

C. Tetanus : 

  • If stimulation is very rapid and frequent the muscle does not have time to relax. It remains in a state of contraction called tetanus.

D. Refractory period : 

  • Immediately after one stimulus, the muscle fibre cannot respond to another stimulus. 
  • This resting or refractory period is about 0.02 seconds.

E. Threshold stimulus : 

  • For a muscle fibre to contract, a certain minimum strength or intensity of stimulus is required. This is called threshold stimulus.

F. All or none principle :

  •  A stimulus below threshold will not result in contraction. 
  • A threshold stimulus will result in contraction.This contraction leads to maximum force.
  • Higher stimulus will not increase force of contraction i.e. a muscle fibre contracts either fully or not at all. This is 'all or none' principle.
  • All types of muscle fibres and nerve fibres obey this law.

G. Oxygen debt : 

  • During strenuous exercise, muscle’s oxygen supply rapidly becomes insufficient to maintain oxidative phosphorylation of respiratory substrate. 
  • At this stage, muscles contract anaerobically and accumulate lactic acid produced by anaerobic glycolysis.
  •  Lactic acid produces less ATP and is toxic. 
  • It causes tiredness, pain and muscle cramps. 
  • During recovery, oxygen consumption of the muscle far exceeds than that in the resting state. 
  • This extra oxygen consumed during recovery is called oxygen debt of the muscle.
We can compare this unit with lever. Where joint acts as fulcrum, respective muscle generates the force required to move the bone associated with joint.

Class I lever : 


  •  The joint between the first vertebra and occipital condyle of skull is an example of first class lever. 
  • The force is directed towards the joints (fulcrum); contraction of back muscle provides force while the part of head that is raised acts as resistance.




Class II lever :


  •  Human body raised on toes is an example of second class lever. 
  • Toe acts as fulcrum, contracting calf muscles provides the force while raised body acts as resistance.


Class III lever : 

  • Flexion of forearm at elbow exhibit lever of class III. 
  • Elbow joint acts as fulcrum and Radius and ulna provides resistance.
  • Contracting biceps muscle provides force for the movement.

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