Background

The Sarcomere.

Skeletal muscle is composed of multinucleate cells called muscle fibers. Each fiber consists of numerous thread-like myofibrils which have a banded appearance created by a series of repeated units, called sarcomeres (see illustration), which are 2 to 3 µM long and are joined end-to-end. The ends of the sarcomere are marked by Z lines, the I band lies on either side of the Z line, and the A band is found in the center of the sarcomere. The I band is composed of thin filaments which project from the Z line while the A band has thick filaments and overlapping thin filaments (right)

Thick filaments are made up of myosin molecules which are composed of two polypeptide chains, each with a globular head and a long tail(right). Myosin molecules are laid down so that the tails form the body of the thick filament, with the heads hanging off. Thin filaments are composed primarily of actin, which forms a double helix and has binding sites for myosin. According to the sliding filament theory, the thick and thin filaments slide across one another to shorten the sarcomere. This is achieved by a reaction between the myosin heads and the actin molecules to form crossbridges. The rapid making and breaking of these bonds, together with the conformational (shape) changes in the myosin head, produces the tension required to create a contraction.

In skeletal neuromuscular systems, an action potential in a motor axon produces an action potential in the muscle fibers it innervates. The action potential travels into the muscle down the Transverse tubules, and causes calcium release from the sarcoplasmic reticulum. The calcium ions bind to troponin, a globular regulatory protein on the thin filaments. The resultant change in the shape of troponin moves tropomyosin, a second associated regulatory protein on the thin filaments, and exposes the myosin binding sites. Crossbridges between the myosin and actin are repeatedly made and broken to shorten the sarcomeres. Calcium is taken back up into the sarcoplasmic reticulum and the regulatory proteins return to their original shape and location, stopping crossbridge cycling.

Each skeletal muscle fiber receives synaptic input from only one motor neuron. This synapse is, therefore, the only source of excitation for the muscle fiber. An action potential in the axon creates an action potential in the muscle fibers it supplies, and a single contraction called a twitch. You have probably seen single twitches when you noticed spontaneous, involuntary, and often rhythmic movements of your fingers or eyelids.

A motor axon makes synaptic connections with a number of fibers in a muscle, and each muscle may be innervated by hundreds of motor axons. One way the nervous system can control the amount of movement is to control the number of motor axons firing, thus controlling the number of twitching muscle fibers. This process, called recruitment, will be studied in this laboratory.

A second way the nervous system controls the amount of muscle contraction is to control action potential frequency. An action potential in the muscle creates a transient increase in intracellular calcium levels which triggers a contraction. Two things must be considered here. There is a brief time after the action potential where the intracellular calcium levels slowly decline. During this time, a second action potential would trigger the entry of additional calcium into the sarcoplasm and produce an increased intracellular calcium level. Furthermore, this elevated calcium level would be maintained over a longer time period. Since the amount of contraction is dependent upon the level of intracellular calcium, this period of maintained, elevated calcium levels would cause a greater contraction. This type of greater contraction, seen when action potentials take place at high frequencies, is called tetanus.

In this laboratory you will stimulate the motor nerve that innervates the muscles of your forearm, causing your fingers to twitch. You will vary stimulus parameters to demonstrate recruitment, summation, where a twitch occurs before the muscle has relaxed, and tetanus, where the amount of contraction is greater than that seen during a single twitch.

Caution: This experiment involves applying mild electrical shocks through electrodes placed on the skin. Individuals with pacemakers or suffering from neurological or cardiac disorders should not volunteer for this lab. In addition, if any discomfort is felt during the exercises discontinue the experiment and consult the instructor.



Proceed to Equipment Setup.