EXCITABLE TISSUE The contractile property of the muscle

EXCITABLE TISSUE

• The contractile property of the muscle is studied by using the frog‘s gastrocnemious –sciatic nerve preparation. This is also called muscle –nerve preparation.

• When the stimulus with a threshold strength is applied, the muscle contracts and then relaxes. • These activities can be recorded graphically by using suitable instruments. • The contraction is recorded as upward deflection from base line. And the relaxation is recorded as downward deflection back to the base line.

• This simple contraction & relaxation is called a “simple muscle twitch” (SMT).

Simple muscle twitch

• PS- Point of stimulation. • PC- Point of contraction. • PMC- Point of maximum contraction (height of contraction) • PMR- Point of maximum relaxation. • LP- latent period. 10 ms • CP- Contraction period. 40 ms • RP- Relaxation period. 50 ms

Effect of Temperature on SMT

When warm ringer solution is applied to the muscle nerve preparation and a simple muscle twitch is recorded then we notice that • All the durations are shortened (latent period, contraction period, & relaxation period). . • The height of contraction has increased.

When cold ringer solution is applied to the muscle nerve preparation and a simple muscle twitch is recorded then we notice that • All the durations are prolonged (latent period, contraction period, & relaxation period). • The height of contraction has decreased.

• A fatigue curve is obtained when the nerve muscle preparation is stimulated repeatedly till a fatigue curve is obtained. • A fatigue curve has a prolonged refractory period, low height of contraction no point of • complete relaxation (contracture remainder). • Fatigue is a reversible phenomenon.

• Following fatigue if the nerve muscle preparation is rested for a while and then stimulated by a threshold stimulus then a normal simple muscle twitch is obtained. • The site of fatigue in an nerve muscle preparation is the neuromuscular junction. • The site of fatigue in an intact animal is the CNS.

Effect of multiple successive stimuli

Genesis of Tetanus

• Beneficial effect is obtained when the second stimulus falls after the complete relaxation of the first response. • Super position or super imposition occurs when the second stimulus falls during the relaxation period of the 1 st curve. • Summation occurs when the second stimulus falls during the contraction period of the previous curve.

• When multiple stimuli at the rate of 5 stimuli per minute are applied to the nerve muscle preparation the staircase phenomenon is recorded. • Here each successive contraction is recorded separately (there is complete relaxation). • Here each successive contraction is higher than the previous one due to beneficial effect.

When multiple stimuli at the rate of 10 -15 stimuli per minute are applied to the nerve muscle preparation the recorded phenomenon is called clonus, incomplete tetanus. • Here each successive contraction occurs during the relaxation of the previous one. • There is incomplete relaxation.

• When multiple stimuli at the rate of 30 stimuli per minute are applied to the nerve muscle preparation the recorded phenomenon is called Tetanus. • Tetanus is a state of sustained contraction. • This happens when successive stimuli falls during the contraction of the previous one. • Cardiac muscle cannot be tetanized due to prolonged absolute refractory period.