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 then 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 stimuli per minute are applied to the nerve muscle preparation then 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 then 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.