Question

Explain the phenomenon of "twitch summation."

Answer 1

Twitch summation or treppe is the addition of a second twitch, resulting in greater tension, and it results from stimulating the muscle before it has a chance to relax completely. Tetanus is prolonged contraction without relaxation and results from repeating stimulation before the muscle has a chance to relax at all.

Explanation:

Before we examine how postural muscles maintain a contraction, we need to first look at a single muscle twitch, which is a single contraction in response to a brief threshold stimulation. Let me quickly note that a twitch in this context is different from involuntary twitching of muscle that we experience on occasion. A threshold stimulation is the smallest amount of stimulation that will actually result in a contraction.

Think of a threshold as being like a minimum amount of pressure needed to activate a key on your computer. Just resting your fingers on the keyboard has no effect. You need to apply a slight pressure in order to get a response. Likewise, a minimum amount of stimulation is needed to induce a muscular contraction.

We can observe a muscle twitch by removing the gastrocnemius from a frog and administering a small stimulus to a muscle. Let's do an experiment using the gastrocnemius of a frog. First, remove the muscle from the frog. Hook the muscle up to a force transducer, which will then record force or tension development when the muscle contracts.

We can stimulate the muscle with an external power source. By gradually increasing the amount of stimulus strength, we can find the threshold level of stimulation. Remember, the tension developed from the threshold stimulus is a single muscle twitch, and that's what we're looking for. Ah, there it is - a muscle twitch.

The image you see on the screen now illustrates the different parts of a single muscle twitch, with time on the x-axis and tension or force development on the y-axis. The red arrow you see at the bottom marks the time at which the muscle is stimulated. Experimentally, muscle can be stimulated with a small electrical shock.

As you look closely, you will see a short period of time between stimulation and tension development, and that's referred to as the latent period. This latent period is due to slack in the muscle that must be removed before the tension can develop and therefore be realized. Once the slack is removed, the muscle contraction develops tension, and we call this the contraction period. The relaxation period is the time during which tension is removed from the muscle. Note that the relaxation period is longer than the contraction period and the entire twitch is over in a matter of milliseconds.