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

FACTORS INFLUENCING THE STRENGTH OF CONTRACTION

• Size and the number of motor unit stimulated.
• Rapidity with which individual motor unit of a muscle is stimulated.

Starling's law

• The first factor is governed by the strength of stimulus. The second is mostly by the rate of effective stimuli. The last is by the initial length of the muscle fibre.
• In cardiac muscle, mechanical summation is impossible because of prolonged action potential (250msec.) almost completely overlaps with long twitch (300 m sec.).
• In smooth muscle, the action potential is relatively short (5 -10 m. sec.), hence no overlapping of twitches, thus summation is possible.
• Starling's law of muscle contraction
• The energy of contraction of the muscle is directly proportional to the initial length of the fibre. This law can be proved by the load and after load experiment on the gastrocnemius muscle of the frog.

Tetanisation

• It is the fusion of successive twitches when the frequency of stimuli is given at a rapid rate.
• Experimental tetanus
  • The experimental tetanus of a muscle may be defined as fusion of individual contractions caused by repeated experimental stimulation of a viable muscle, by which two kinds of tetanus can be produced by using gastrocnemius muscle of the frog.
• Complete tetanus
  • If the frequency of the successive stimulation to a muscle is so adjusted that the subsequent effective stimuli fall during the preceding contraction phase, a complete tetanus or complete fusion of contractions can be obtained (tonic contraction).
• Incomplete tetanus
  • When the subsequent effective stimuli are applied to fall during the relaxation phase of the preceding contraction, then incomplete tetanus or incomplete fusion of contractions can be obtained (clonic contraction).
  • Cardiac muscle fibres do not functionally tetanise, since, mechanical summation is not possible due to prolonged action potential (250msec.), which almost completely overlaps with long twitch (300msec.) duration.
• Critical frequency
  • It is the lowest frequency required to produce a complete tetanus.

Factors responsible for tetaniszation

• Viscose property
  • Viscose inertia of the sarcoplasm, sarcolemma, epimysium, perimysium and facia provide a resistance to change in the length immediately after contraction.
• Fusion of activation process
  • Successive pulsatile stimuli at rapid rate causes fusion of activation or the manifestation of muscle contractions by providing free Ca⁺⁺ continuously to the sarcoplasm.