Student Handout Frog Skeletal Muscle
Introduction In this laboratory, you will investigate the ph ysiological properties of skeletal muscle using the isolated frog gastrocnemius. Concepts that you will explore include the single twitch, graded response and the relationship between muscle length and tension generated. You will also explore tetanus and muscle m uscle fatigue. These experiments illustrate the collective understanding of m uscle physiology gained from over 400 years of research.
Background The frog muscle preparation that you will use in the laboratory is the ea rliest isolated tissue preparation. The first experiments on muscle physiology appear to have been performed between 1661 and 1665 by Jan Swammerdam, who demonstrated that an isolated frog muscle could be made to contract when the sciatic nerve was irritated with a m etal object. Later, Luigi Galvani (1737 –1798) –1798) demonstrated that frog muscle responded to electrical currents. Here we focus on the mechanical properties of skeletal muscle. The invention of the kymograph (a rotating drum powered by a clockwork motor) in the late 1840s, attributed to either Carlo Matteucci (1811 –1868) –1868) or Carl Ludwig (1816 –1895), –1895), revolutionized experimental physiology for it enabled events such as muscle contra ctions to be recorded and analyzed for the first time. Today, the computer has taken the place of the kymograph but physiology students of the late 1800’s would recognize the experiments described here. These demonstrate some of the important functional characteristics of skeletal muscle. The basic unit of a muscle is the muscle cell, or fiber (Figure 1).
Figure 1. Skeletal muscle structure. Whole muscles are made up of bundles of these fibers. Unlike cardiac muscle cells, there are no gap junctions between adjacent cells. This means that each fiber behaves independently. A single muscle fiber has a very regular structure, defined by myofibrils.
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Student Handout Frog Skeletal Muscle Each myofibril consists of an arrangement of the contractile proteins actin and myosin, which are able to slide past each other in the presence of Ca2+ and ATP. A single motor neuron, and all the muscle fibers that it innervates, is known as a motor unit (Figure 2).
Figure 2. A motor unit. Skeletal muscle is similar to nerve tissue in that the fiber responds to a stimulus in an all or-none fashion. This response is called a twitch. One motor neuron supplies a number of muscle fibers to constitute a motor unit. Motor units vary greatly in size, from just a few muscle fibers innervated by a single neuron (small motor unit) up to thousands (large motor unit). The smaller the motor unit, the finer the control of movement in that muscle. Thus the muscles controlling the movements of the fingers and eyes have small motor units whereas those controlling the large limb muscles may have very large motor units. However, most muscle consists of a range of motor unit sizes. Depending on the intensity and frequency of stimulation, greater numbers of fibers are activated. The strength of a muscle contraction, therefore, can be increased in two ways: by increasing the number of active motor units (termed recruitment) and by stimulating existing active motor units more frequently. The absolute force that a muscle can generate
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Student Handout Frog Skeletal Muscle is dependent on the total number of muscle fibers. So muscles with large cross-sectional areas are able to generate larger forces than those with small cross-sectional areas. Motor nerves release the neurotransmitter acetylcholine from their terminals, called motor end plates.
Figure 3. A neuromuscular junction. The acetylcholine released into the junctional cleft binds to receptors on the muscle membrane that are directly coupled to cation-selective ion channels. Opening of these channels depolarizes the muscle fiber and leads to the release of intracellular calcium from the sarcoplasmic reticulum, a variant of smooth endoplasmic reticulum. The increased cytosolic calcium sets in motion the biochemical events that underlie contraction. The acetylcholine is rapidly hydrolyzed by acetylcholine esterase on the skeletal muscle membrane in this region and thus does not accumulate in the junctional cleft. Skeletal muscle can be studied under isometric (constant length) or isotonic (constant load) conditions. Here the force is measured isometrically. Action potentials in skeletal muscle, like those in nerve, last for only a few milliseconds (ms). In contrast the mechanical response of the muscle – the muscle twitch – last significantly longer. Page 3 of 6
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Student Handout Frog Skeletal Muscle
Figure 4. Temporal relationship between muscle action potential and consequent contraction. A second stimulus arriving before the muscle has relaxed again, c auses a second twitch on top of the first so that greater peak tension is developed. This is called summation. With increasing frequency of stimulation, there is less a nd less time for the muscle fiber to relax between stimuli and eventually the contractions fuse and a smooth powerful contraction – tetanus – is seen.
Figure 5. Effects of repeated frequency of stimulation. Indeed, normally skeletal muscles are activated by volleys of action potentials and operate in a state of fused contractions. The strength of muscle contraction is also influenced by the degree of stretch of the muscle. In considering the force of the response of a muscle to stimulation, it is necessary to separate out the passive and active forces. The passive forces reflect the contributions of elastic elements in the muscle both extracellularly and within the fibers themselves. The active force is that generated by the con tractile machinery when the fibers are stimulated.
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Student Handout Frog Skeletal Muscle
Figure 6. Skeletal muscle length - tension relationship. Experimentally, what is measured at different degrees of stretch is the total force during nerve stimulation and contraction, and the passive force in the absence of nerve stimulation. The difference between the two is the a ctive force at any muscle length. Skeletal muscle contraction requires metabolic energy. A depletion of energy stores results in fatigue. Some muscle fibers are more resistant to fatigue than others. These have a greater capacity for oxidative metabolism (Note, however, that in the intact animal, fatigue occurs primarily because the motor drive from the brain is reduced, rather than as a result of an appreciable depletion of the muscle energy reserves).
What you wil l do in t he laboratory There are five exercises that you will complete during this Laboratory. 1. Twitch recruitment. In this experiment, you will give the muscle a series of stimuli of increasing amplitude to examine the effects of stimulus amplitude on contraction force.
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Student Handout Frog Skeletal Muscle 2. Effect of stretch on contractio n force . Here you will increase the stretch of the muscle by raising a micropositioner. 3. Effect of stimulus frequency on contraction force . In this experiment, you will stimulate the muscle with twin pulses at different pulse interva ls and observe their effect on muscle contractions. 4. Tetanus. In this part of the laboratory, you will examine the muscle’s response to a repetitive stimulus at different frequencies. 5. Muscle fatigue. Here you will stimulate the muscle at a high frequency (50 Hz) for a long duration to examine the effects of muscle fatigue.
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