VPB 121: Veterinary Physiology - II (2+1)

MUSCLE FATIGUE

• Fatigue is the decrease in the working capacity of a muscle or tiredness of the muscle when it is continuously stimulated.
• It is characterised by diminished force of contraction, increased latent period and contraction period and prolonged relaxation period. The relaxation is incomplete i.e., the muscle is in a state of contracture‚ (the muscle contracts even without the presence of action potential). At this stage the muscle cannot be excited or do any more work.
• In the locomotor system of the skeletal muscle, there are three sites, which are easily fatigued.
  • Synapses of CNS.
  • Neuromuscular junction.
  • The muscle.

Causes of fatigue

• Continuous stimulation of the nerve fibre or continuous stimulation of the muscle that too under ischemia (impaired blood supply) causes,
• Lack of energy sources: Depletion of O₂ from myoglobin; depletion of muscle glycogen; depletion of ATP
• Depletion of neurotransmitters: Prolonged muscle activity diminishes neurotransmitter acetylcholine concentration in the neuromuscular junction, thereby reduces the transmission of the impulses to muscle which causes diminished muscle contraction.
• Accumulation of metabolic end products such as lactic acid: In oxygen lack, the muscle glycogen broken down by anaerobic glycolysis to give pyruvic acid which is then reduced to lactic acid by the action of lactic dehydrogenase. But in an intact muscle the blood supply minimises the above effect by continuously supplying the O₂ and nutrients to a functioning muscle.