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

SUMMATION OF MUSCLE CONTRACTION

• Summation is the added effect of individual muscle twitches to get strong and powerful muscle contraction.
• Summation occurs in two different ways
  • By increasing the number of motor units to contract simultaneously.
  • By increasing the rapidity of contraction of individual motor unit of a contracting muscle.
• Multimotor unit summation (Spatial summation)
  • The strength or the force of contraction of a muscle increases progressively with respect to increasing number of contracting motor units by increasing the strength of stimulation.
  • The strength of resulting contractions in a skeletal muscle is proportional to the size or the number of motor units contracted at the time, which in turn is directly proportional to the strength of the stimulus; i.e. increasing the strength of stimulus increases stimulation of motor units.
• Wave summation (Temporal summation)
  • The frequency of stimulation is increased to a motor unit or units in such a way that the successive stimuli stimulate additional motor units during its contraction phase.
  • When a motor unit is excited by a rapid succession of weak stimuli, it may evoke stronger contractions. Applying stimuli in rapid succession to a muscle increases the excitatory potential of the postsynaptic nerve ending. This increased excitatory potential is often referred to as excitatory postsynaptic potential (EPSP).
  • The second stimulus arrives before the completion of the first contraction, which initiates the second contraction. This gives greater strength or force, as well as duration to the already progressing contraction.