Muscle Twitch

Muscle

In this experiment, you will explore how muscles work. You will also examine some of the properties of muscle fatigue. In this experiment, you will electrically stimulate the nerves in the forearm to demonstrate recruitment, summation, and tetanus.

Written by staff of ADInstruments.

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Background

The skeleton provides support and articulation for the body. Bones act as support structures and joints function as pivot points. Skeletal, or striated, muscles are connected to the bones either directly or by tendons, strong bundles of collagen fibers. Two or more muscles usually work antagonistically. In this arrangement, a contraction of one muscle stretches, or elongates, the other. Skeletal muscle is composed of long, multinucleate cells called fibers. These fibers are innervated by motor nerves. An action potential in a motor axon produces an action potential in the muscle fibers it innervates. This muscle action potential allows for a brief increase in the intracellular concentration of calcium ions ([Ca2+]), and activates the contractile molecular machinery inside the fiber. The result is a brief contraction called a twitch.

A whole muscle is controlled by the firing of up to hundreds of motor axons. These motor nerves control movement in a variety of ways. One way the nervous system controls a muscle is by adjusting the number of motor axons firing, thus controlling the number of twitching muscle fibers. This process is called recruitment. A second way the nervous system controls a muscle contraction is to vary the frequency of action potentials in the motor axons. At stimulation frequencies of less than 5 Hz, intracellular [Ca2+] returns to normal levels between action potentials: the contraction consists of separate twitches. At stimulation frequencies between 5 and 15 Hz, [Ca2+] in the muscle has only partly recovered when the next action potential arrives. The muscle fiber produces a pulsing tension called a summation...