Question

You are conducting typical force-voltage, force-frequency and length-tension experiments on the toad sciatic nerve/gastrocnemius muscle preparation. Draw and explain the twitch characteristics, recruitment and length-tension results obtained during these experiments. How might the results differ if SR density was increased?

Answer 1

In addition to recruitment of motor units by increasing stimulus amplitude (i.e. voltage,) variation in contraction can

be achieved by temporal summation. Action potentials in the motor nerve fibers elicit the release of a chemical

neurotransmitter called acetylcholine (ACh) from the axon endings. This transmitter combines with nicotinic

receptors to produce action potentials on the membrane of the skeletal myocyte. An electrical impulse in the

muscle cell, in turn, causes Ca++ to be released from the sarcoplasmic reticulum (SR). At the molecular level, for

contraction, the release of Ca++ into the cytosol permits actin and myosin to interact in the presence of ATP.

Actin-myosin interactions and contractions are relaxed when Ca++ is re-absorbed into the SR by a Ca++ pump.

The rate at which myocytes are stimulated influences how long Ca++ lingers in the cytoplasm. If a myocyte

receives a single, submaximum stimulus, Ca++ is released and quickly re-absorbed. If the myocyte receives a

second impulse before relaxation is fully affected, the amount of cytoplasmic Ca++ in the second contraction is

greater than the first. Consequently, the strength (i.e. force in grams) of the 2nd contraction is greater.