Muscle Contraction

Muscle Stimulation & Fatigue

In this experiment, you will explore muscle function through stimulation and fatigue. You will electrically stimulate the nerves in the forearm to demonstrate recruitment, summation, and tetanus.

Written by staff of ADInstruments and modified by Drs. Davis and Moeller. Revised 1 Nov 2011

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. Skeletal muscle is composed of long, multinucleate cells called fibers grouped into fascicles (Figure 1). Two or more muscles usually work antagonistically. In this arrangement, a contraction of one muscle stretches, or elongates, the other.

Figure 1. Skeletal Muscle Organization

A single motor neuron, and all the muscle fibers that it innervates, is known as a motor unit. An action potential in a motor neuron induces an action potential in the muscle fibers it innervates by releasing the neurotransmitter acetylcholine into the neuromuscular junction. This muscle action potential causes a brief increase in the intracellular concentration of calcium ions, [Ca2+], and activates the contractile molecular machinery inside the fiber. This requires the use of intracellular supplies of adenosine triphosphate (ATP) as the energy source. 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 in which 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 intervals...