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

PROPERTIES OF MUSCLE CONTRACTIONS

• When a muscle fibre is stimulated by minimal or maximal effective stimuli, the whole fibre contracts to the maximum or it will not contract at all. This law is applicable to single motor unit (motor nerve and all the muscle fibres it supplies).
• Whole cardiac muscle obeys this law because of syncytium.

Recruitment

• Because different motor neurons in a nerve has different threshold levels, an increase in intensity of the stimulus applied to the nerve evoke action potential in additional motor neurons and contraction in additional muscles fibers in the muscle served by that nerves..This increase in tension development of a muscle caused by gradual increase in intensity of the electrical stimulation is due to recruitment of additional motor units to the contractile state.
• Recruitment occurs because a nerve contains many different motor neurons having different threshold levels for initiating an action potential. Generally motor neurons having small diameters have low threshold values and are the first motor units recruited by a given stimulus. These small motor neurons are associated with somatic fibres. Largest motor neurons innervate FG fibers have highest threshold levels.

Simple muscle contraction (Single muscle twitch)

• Application of single threshold level of stimulus (electrical stimulation) causes a single muscle contraction followed by relaxation. The single muscle twitch has three phases.
• Latent period
• It is the period between the application of stimulus and beginning of contraction (about 10m.second).
• Period of contraction
• This period indicates the actual shortening (about 40m.second).
• Period of relaxation
• It is the period between the point of maximum contraction and the period of complete relaxation (about 50m.second).

Refractory period

• It is a brief period during which a muscle undergoing contraction for a first stimulus is unable to respond to a second stimulus. The earliest chemical change during muscular contraction is the breakdown of ATP. So long as the breakdown ATP is not resynthesised in adequate amounts, the muscle cannot be excited.
• Refractory period is that period during which this resynthesis of broken ATP takes place. Following stimulation of a motor neuron or a muscle cell a brief period exists during which the motor neurons or sarcolemma is unresponsive to the II stimulus is called resting potential.
• It is of two types
  • Absolute refractory period
    • It is the time interval between the twin impulses (less than 3millisecond) during which the muscle can not be stimulated even by stronger stimuli. This is due to the rising phase of the action potential when Na⁺ conductance to the interior of the cell is high.
  • Relative refractory period
    • It is the period of reduced excitability, which requires increased intensity of second stimulation to generate another action potential. It is characterised by after depolarization stage of action potential during which the K⁺ permeability is very high and the Na⁺ channels are recovering their excitability.

Treppe (Staircase phenomenon)

• When a stimulus of constant strength and duration is repeated once or twice per second (below tetanising frequency)causes increased contractions during the first few stimulations, which finally reach a constant response. This is due to development of physical and chemical changes i.e. reduced viscosity of the sarcoplasm, increased heat production, increased Ca⁺⁺ availability for binding with troponin during first few contractions, referred as beneficial effects.