You and your lab partner use a living nerve and muscle preparation to study muscle contraction. When you stimulate the nerve electrically, the muscle contracts; why? When you turn off the stimulator but add a high concentration of caffeine to the fluid surrounding the tissues, the muscle contracts; why (speculate on possible sites/mechanisms of action)? When curare (which blocks neuromuscular

Question

transmission) is added with caffeine, contraction occurs; why (speculate, and how does this alter your previous answer)? Can you identify a single site of action from this information? If not, what additional experiment would narrow your choices?

What will be an ideal response?

Answer 1

The applied electrical current opens voltage -gated ion channels in the nerve, causing an action potential, which
leads to the release of the neurotransmitter acetylcholine to stimulate an action potential in the skeletal muscle. This
activates excitation contraction coupling to increase intracellular calcium and thereby allow actin and myosin to
interact. If the muscle contracts to caffeine, then it must have done so by increasing intracellular calcium as that is
required to generate force. How caffeine increases intracellular calcium can not be determined yet. However, since
the addition of curare does not affect contraction, then caffeine must be acting directly on the skeletal muscle as the
affect of acetylcholine on the muscle cell is blocked by curare. Whether caffeine can increase intracellular calcium
by stimulating an action potential or some other way is not clear from the experiments. To determine whether the
action potential is required, tetrodotoxin could be used to block the action potential. In this case, the cell would still
be generating force which indicates that the site of action of caffeine is somewhere between the L type calcium
channel and the release of calcium from the sarcoplasmic reticulum (caffeine opens ryanodine receptor in the
sarcoplasmic reticulum).