4. Effect of two successive stimuli on skeletal muscle contraction
- When the second stimulus falls in the first half of latent
period of the first contraction it does not evoke any response
- When the second stimulus falls in the second half of the latent period or in the contraction
period, there is summation of contraction and the magnitude of the contraction
increases
- When the second stimulus falls in the relaxation period of
the first contraction, the relaxation is arrested and the second contraction
occurs and the height of the second contraction is more than the first one
because of the beneficial effect
- When the second stimulus falls immediately following the
relaxation of the first contraction a second twitch is recorded
Q.1 What is refractory period?
- After a tissue has responded to an effective stimulus, there
is a very brief interval of time, called the refractory period, during which
the tissue loses its excitability, i.e. it does not respond to a second
stimulus.
- It is divided into absolute refractory period (ARP) and
relative refractory period (RRP).
- During ARP, the tissue does not respond to another stimulus
howsoever strong it may be; while during RRP, the tissue responds to a stronger
than usual stimulus.
Q.2 What is the physiological basis of beneficial effect?
- In a single twitch, the Ca2+ released from the terminal
cisterns into the sarcoplasm is rapidly mopped up during relaxation. When there
is no relaxation, or incomplete relaxation, some Ca2+ remains in the sarcoplasm
for a longer time, and this, together with additional Ca2+ release by the
second stimulus, increases the duration of the active state.
- Decrease in the viscosity of the muscle resulting from the
first contraction decreases the elastic inertia of the muscle.
- Increase in H+ ion concentration.
- Increase in temperature.
