Muscle Physiology Protocol
I. Goals for this lab
A.To increase your understanding of muscle physiology - tonus, motor unit recruitment and fatigue. B.Learn how to conduct and analyze an EMG (electromyogram)
C.To gain more experience with the scientific method, experimental design, making predictions, critical analysis of results, and interpretation of your results.
Human skeletal muscle consists of hundreds of individual cylindrically shaped cells (called fibers or myofibers) bound together by connective tissue. In the body, these muscles are stimulated to contract by somatic motor nerves that carry signals in the form of nerve impulses from the brain or spinal cord to the skeletal muscles (Fig. 1.1). Nerve cells that innervate skeletal muscle are called somatic motor neurons. They are located in the gray matter of the spinal cord and brain. Axons (or nerve fibers), which are long cylindrical extensions of the neurons, leave the spinal cord via spinal nerves and the brain via cranial nerves, and are distributed to appropriate skeletal muscles in the form of a peripheral nerve, which is a cable-like collection of individual nerve fibers. Upon reaching the muscle, each nerve fiber branches and innervates multiple individual muscle fibers.
Figure 1.1 Motor units
Although a single motor neuron can innervate several muscle fibers, each muscle fiber is innervated by only one motor neuron. The combination of a single motor neuron and all of the muscle fibers it controls is called a motor unit (Fig. 1.1). When a somatic motor neuron is activated, all of the muscle fibers it innervates respond to the neuron’s impulses by generating their own electrical signals that lead to contraction of the activated muscle fibers. When a motor unit is activated, the component muscle fibers generate and conduct their own electrical impulses that ultimately result in contraction of the fibers. Although the electrical impulse generated and conducted by each fiber is very weak (less than 100 microvolts), many fibers conducting simultaneously induce voltage differences in the overlying skin that are large enough to be detected by a pair of surface electrodes. The detection, amplification, and recording of changes in skin voltage produced by underlying skeletal muscle contraction is called electromyography. The recording thus obtained is called an electromyogram (EMG). The changes in amplitude of the recording indicate changes in the amount of electrical activity in the muscle. The amount of tension generated by a muscle, and therefore the force of its contraction, depends on how stretched or contracted it was before it was stimulated, among other factors. This principle is called the length–tension relationship. The reasons for it can be seen in figure 1.2. If a fiber is overly contracted at rest, its thick filaments are rather close to the Z discs. The stimulated muscle may contract a little, but then the thick filaments butt against the Z discs and can go no farther. The contraction is therefore a weak one. On the other hand, if a muscle fiber is too stretched before it is stimulated, there is relatively little overlap between its thick and thin filaments. When the muscle is stimulated, the myosin heads cannot “get a good grip” on the thin filaments, and again the contraction is weak. FIGURE 1.2 The Length–Tension Relationship
Between these extremes, there is an optimum resting length at which a muscle responds with the greatest force. The central nervous system continually monitors and adjusts the length of the resting muscles, maintaining a state of partial contraction called muscle tone. This maintains optimum length and makes the muscles ideally ready for action. The elastic filaments of the sarcomere also help to maintain enough myofilament overlap to ensure an effective contraction when the muscle is called into action. It should not...