Chapter 6: THE MUSCULAR SYSTEM

6 the muscular system Chapter Outline THE BODY’S THREE KINDS OF MUSCLE The three kinds of muscle have different structures and functions the structure and function of skeletal muscles A whole skeletal muscle consists of bundled muscle cells Bones and skeletal muscles work like a system of levers Many muscles are arranged as pairs or in groups Skeletal muscles include “fast” and “slow” types HOW MUSCLES CONTRACT A muscle contracts when its cells shorten Muscle cells shorten when actin filaments slide over myosin how THE NERVOUS SYSTEM CONTROLS MUSCLE CONTRACTION Calcium ions are the key to contraction Neurons signal muscle cells at neuromuscular junctions Ways MUSCLE CELLS GET ENERGY properties of whole muscles Several factors determine the characteristics of a muscle contraction Tired muscles can’t generate much force diseases and disorders of the muscular system Muscle injuries include strains and tears Cramps and spasms are abnormal contractions Muscular dystrophies destroy muscle fibers Bacterial infections can interfere with nerve signals to muscle Cancers may develop in muscle tissue MAKING THE MOST OF MUSCLES ConnECTIONS: Muscles and THE muscular System IN Homeostasis Summary Review Questions Self-Quiz Critical Thinking Explore on Your Own Your future Objectives Describe the basic difference in structure and function for the three types of muscle tissue. Explain the structure of muscles, from the molecular level to the organ systems level. Be able to make a simple diagram of a muscle fiber, a myofibril, and a sarcomere. List the events that occur, in order, in contraction in a skeletal muscle fiber (cell). Define motor unit and name approximate numbers of muscle fibers for sample motor units. Make a simple drawing showing the arrangement of myosin and actin in a myofibril. Describe the specific action of myosin and actin in muscle contraction. Explain how muscle works with bone to cause movement and how antagonistic muscle action refines movements. Explain the process by which muscles become larger and stronger. Describe the differences in the two types of skeletal muscle and give examples of exercises that would target each type. Explain the differences in how the nervous system interacts with muscle to control contraction and the force produced. Explain the role of ATP in muscle contraction. List the sources of energy for muscle contraction. Demonstrate how muscle disorders impact the function of both the muscular and skeletal systems. Explain how muscles can pull but not push bones. Define muscle tension. Define muscle fatigue. Explain how bones and muscles are attached to each other. Key Terms skeletal muscle smooth muscle cardiac muscle muscular system myofibrils bursae origin insertion sarcomere actin myosin sliding filament model rigor mortis motor neuron neuromuscular junctions neurotransmitter muscle fatigue oxygen debt motor unit muscle twitch tetanus muscle tone muscle tension muscle spasm muscle cramp muscular dystrophies Duchenne muscular dystrophy (DMD) myotonic muscular dystrophy botulism tetanus (disease) sarcoma aerobic exercise strength training Lecture Outline Muscular mass can be achieved through hard work or by chemical shortcuts. Anabolic steroids and human growth hormone are commonly used by athletes. The intricate movements of the human body are the result of interactions between the skeleton and muscle. The Body’s Three Kinds of Muscle The three kinds of muscle have different structures and functions. Skeletal muscle, composed of long thin cells called muscle “fibers,” allows the body to move. Smooth muscle is found in the walls of hollow organs and tubes; the cells are smaller than those of skeletal muscle and are not striated. The heart is the only place where cardiac muscle is found. Although involuntary, it is striated. Cardiac muscle and smooth muscle are considered involuntary muscles because we cannot consciously control their contraction; skeletal muscles are voluntary muscles. Skeletal muscle comprises the body’s muscular system. The Structure and Function of Skeletal Muscles A whole skeletal muscle consists of bundled muscle cells. Inside each cell are threadlike myofibrils, made out of proteins, which are critical to muscle contraction. The cells are bundled together with connective tissue that extends past the muscle to form tendons, which attach the muscle to bones, in sacs called bursae. Bones and skeletal muscles work like a system of levers. The human body’s skeletal muscles number more than 600. The origin end of each muscle is designated as being attached to the bone that moves relatively little; whereas the insertion is attached to the bone that moves the most. Because most muscle attachments are located close to joints, only a small contraction is needed to produce considerable movement of some body parts. Many muscles are arranged as pairs or in groups. Many muscles are arranged as pairs or grouped for related function. Some work antagonistically (in opposition) so that one reverses the action of the other. Others work synergistically, so the contraction of one stabilizes the contraction of another. Reciprocal innervation dictates that only one muscle of an antagonistic pair (e.g., biceps and triceps) can be stimulated at a time. Skeletal muscles include “fast” and “slow” types. Humans have two general types of skeletal muscles: “Slow” muscle is red in color due to myoglobin and blood capillaries; its contractions are slower but more sustained. “Fast” or “white” muscle cells contain fewer mitochondria and less myoglobin but can contract rapidly and powerfully for short periods. When athletes train, one goal is to increase the relative size and contractile strength of fast muscle fibers (sprinters), and/or the endurance capabilities of slow muscle fibers(distance swimmers). How Muscles Contract A muscle contracts when its cells shorten. Muscles are divided into contractile units called sarcomeres. Each muscle cell contains myofibrils composed of thin (actin) and thick (myosin) filaments. Each actin filament is actually two beaded strands of protein twisted together. Each myosin filament is formed from proteins with a double head (projecting outward) and a long tail, which are bound together with others. The arrangement of actin and myosin filaments gives skeletal muscles their characteristic striped appearance. Muscle cells shorten when actin filaments slide over myosin. Within each sarcomere there are two sets of actin filaments, which are attached on opposite sides of the sarcomere; myosin filaments lie suspended between the actin filaments. During contraction, the myosin filaments physically pull the two sets of actin filaments toward each other at the center of the sarcomere; this is called the sliding-filament model of contraction. In the presence of calcium a myosin head can form a cross-bridge with an adjacent actin filament. Binding causes change in the molecule shape, so it tilts toward the sarcomere’s center. As the molecule changes shape, the heads pull the actin filaments along toward each other. After pulling forward, the myosin heads detach from actin and bind to ATP. Energy from ATP drives the power stroke, which pushes the head back to the original position to prepare for another attachment. When a person dies, ATP production stops, myosin heads become stuck to actin, and rigor mortis sets in, making the body stiff. How the Nervous System Controls Muscle Contraction Calcium ions are the key to contraction. Skeletal muscles contract in response to signals from motor neurons of the nervous system. Signals arrive at the T tubules of the sarcoplasmic reticulum (SR), which wraps around the myofibrils. The SR responds by releasing stored calcium ions; calcium binds to the protein troponin, causing it to change shape and pulling another protein, tropomyosin, away from the biding site on the actin. The myosin can now form cross bridges with the actin. When nervous stimulation stops, calcium ions are actively taken up by the sarcoplasmic reticulum and the changes in filament conformation are reversed; the muscle relaxes. Neurons signal muscle cells at neuromuscular junctions. At neuromuscular junctions, impulses from the branched endings (axons) of motor neurons pass to the muscle cell membranes. Between the axons and the muscle cell is a gap called a synapse. Signals are transmitted across the gap by a neurotransmitter called acetylcholine (ACh). When the neuron is stimulated, calcium channels open to allow calcium ions to flow inward, causing a release of acetylcholine into the synapse. Botox blocks the release of ACh, preventing a muscle from contracting. Ways Muscle Cells Get Energy ATP supplies the energy for muscle contraction. Initiation of muscle contraction requires much ATP; this will initially be provided by creatine phosphate, which gives up a phosphate to ADP to make ATP. Cellular respiration provides most of the ATP needed for muscle contraction after this. During intense muscle action, glycolysis alone produces low amounts of ATP; lactic acid is also produced, which hinders further contraction. Muscle fatigue is due to the oxygen debt that results when muscles use more ATP than cellular respiration can deliver. Properties of Whole Muscles Several factors determine the characteristics of a muscle contraction. A motor neuron and the muscle fibers under its control are a motor unit; the number of fibers in a motor unit depends on the precision of the muscle control needed. A single, brief stimulus to a motor unit causes a brief contraction called a muscle twitch. Repeated stimulation makes the twitches run together in a sustained contraction called tetanus (tetany), the maximum force a muscle can produce. Not all fibers in a muscle contract at the same time. The number of motor units that are activated determines the strength of the contraction: small number of units = weak contraction; large number of units at greater frequency = stronger contraction. Muscle tone is the continued steady, low level of contraction that stabilizes joints and maintains general muscle health. Muscle tension is the force a contracting muscle exerts on an object; to contract, a muscle’s tension must exceed the load opposing it. A concentric isotonically contracting muscle shortens and moves a load. An isometrically contracting muscle develops tension but does not shorten. Tired muscles can’t generate much force. Muscles fatigue when they cannot match ATP production with ATP use, such as when a strong stimulation keeps a muscle in a state of tetanus too long. After resting, muscles will be able to contract again; muscles may need to rest for minutes or up to a day to recover fully. Diseases and Disorders of the Muscular System Muscle injuries include strains and tears. Muscle strains come from movement that stretches or tears muscle fibers; ice, rest, and anti-inflammatory drugs (e.g., ibuprofen) allow damage to repair. If the whole muscle is torn scar tissue may develop, shortening the muscle and making it function less effectively. Cramps and spasms are abnormal contractions. A muscle spasm is a sudden, involuntary contraction that rapidly releases, while cramps are spasms that don’t immediately release; cramps usually occur in calf and thigh muscles. Tics are minor, involuntary twitches of muscles in the face and eyelids. Muscular dystrophies destroy muscle fibers. Muscular dystrophies are genetic diseases leading to breakdown of muscle fibers over time. Duchenne muscular dystrophy (DMD) is common in children; a single mutant gene interferes with sarcomere contraction. Myotonic muscular dystrophy is usually found in adults; muscles of the hands and feet contract strongly but fail to relax normally. In these diseases, muscles progressively weaken and shrivel. Bacterial infections can interfere with neural signals. The botulinum toxin from Clostridium botulinum, which causes botulism food poisoning, interferes with ACh release and causes muscle paralysis. A different but related bacterium (Clostridium tetani) produces a toxin that causes the disease tetanus. This is different from the maximum tension a muscle can produce. This toxin blocks signals in the spinal cord. It can result in death if signals to the heart and respiratory muscles are being blocked. There is a vaccine available, which requires “booster shots” occasionally. Cancer may develop in the muscle tissue. Muscle cancers are relatively rare. Most commonly, younger individuals develop rhabdomyosarcoma. Although demanding, the therapy has a high chance of success. Making the Most of Muscles. Muscles that are damaged or that go unused for prolonged periods of time will atrophy (waste away). Aerobic exercise improves the capacity of muscles to do work. Walking, biking, and jogging are examples of exercise that increase endurance. Regular aerobic exercise increases the number and size of mitochondria, the number of blood capillaries, and the amount of myoglobin in the muscle tissue. C. Strength training improves function of fast muscle but does not increase endurance. D. Even modest activity slows the loss of muscle strength that comes with aging. 6.9 Connections: Muscles and the Muscular System in Homeostasis A. The muscular system is responsible for macroscopic and microscopic movements of and within the body. B. Muscles also stabilize joints and generate metabolic heat. Suggestions for Presenting the Material The value of learning the names of the muscles may be questionable, but the popularity of body building has made the names of major muscles almost common knowledge. Make use of Figure 6.2 and its animation to help present this material. It is nearly impossible to present the ultrastructure of muscle without visuals similar to Figures 6.3, 6.7, 6.8, 6.9, 6.10, and 6.11. Using the analogy of a rope (see the “Enrichment” section below) is very helpful. Figures 6.7, 6.8, and 6.9, are also animated. Don’t miss the opportunity when discussing the sliding-filament model to emphasize that the molecular explanation of movement explains why individual muscles can only pull, not push. Again the rope analogy is helpful. The dual role of calcium as the provider of bone hardness and as muscle facilitator (both contraction and synapse function) should be made clear. Emphasize that ATP is required for both muscle contraction and relaxation as well as for the active transport of calcium back into the sarcoplasmic reticulum. Students may be familiar with the “white” and “dark” meat in chicken. This can be a useful analogy when explaining the difference between red fibers and white. Chickens walk around all day long on the “dark meat” of the legs and thighs. But they rarely fly, and then only a short distance using the “white” meat of the breast fibers to move the wings. Classroom and Laboratory Enrichment Students can visualize the ultrastructure of muscles more readily if the comparison to a large rope is made. The best is one used for boat anchorage because it is made of many subunits. Demonstrate the action of muscle by using a “muscle contraction kit” available from biological supply houses. Obtain a fresh beef knee joint from a local butcher to demonstrate the structure and tissues of that joint. If available, use a model of a sarcomere to facilitate the students’ comprehension of that microscopic structure. Arrange for a body builder to appear before the class (more effective if unannounced). Describe the origin, insertion, and function of some major muscles as they are flexed. How muscles work and their importance to daily activities, as well as sports, can be illustrated by presenting a list of famous athletes and their probably even more famous injuries. Classroom Discussion Ideas Discuss the negative aspects of “self-medication” with supplements designed to enhance muscle performance. What motivates young athletes to take enhancing drugs? What could be bad about taking a chemical such as creatine, which is produced by the body normally? If creatine could help slow the progression of Duchenne muscular dystrophy in children, would this be an acceptable use of the chemical? Why then would it be bad for a body builder to take it? Building big, bulky muscles requires testosterone, yet it is common today for female body builders to have as much bulk as males. How is this possible? Would you recommend this for either sex? Consider the present evolutionary state of the human knee. Is it sufficient for the punishment modern athletic activity places on it? (Note: This is similar to an idea to discuss for bones.) What technological and research developments allowed the sliding filament theory of the 1960s to become the sliding-filament mechanism of today? Evaluate this statement: “If muscles can only pull, how can you push a door open?” Which muscles are collectively called the “hamstrings”? In which sports are they most likely to be injured, and why? Term Paper Topics, Library Activities, and Special Projects Investigate the difference(s) in the development of muscles for power (weight) lifting versus development of muscles for body sculpting and exhibition. Document the development of the sliding-filament model of muscle contraction and the research evidence that supports it. Report on the technique of arthroscopic surgery. Discuss the muscle disease called myasthenia gravis, its suspected cause, and the type of treatment currently used. Investigate more deeply the genetic basis for Duchenne muscular dystrophy. Videos, Animations, and Websites VIDEOS Films for the Humanities and Sciences Muscular System at Work: The Inner Athlete http://ffh.films.com/id/10137/Muscular_System_at_Work_The_Inner_Athlete.htm The Sliding Filament Theory Overview of the sliding filament theory. http://www.youtube.com/watch?v=EdHzKYDxrKc Muscle Contraction A view of muscle contraction on a molecular level. http://www.youtube.com/watch?v=EdHzKYDxrKc ANIMATIONS PBS NOVA – Discovering Exercise in a Pill Interactive animation of the effects of exercise. http://www.pbs.org/wgbh/nova/body/exercise-drug.html BBC – Human Body Interactive game of the muscular system. http://www.bbc.co.uk/science/humanbody/body/interactives/3djigsaw_02/index.shtml?muscles WEBSITES PBS NOVA – Fit to Go the Distance Overview of the benefits of exercise. http://www.pbs.org/wgbh/nova/body/fit-distance.html Possible Responses to Review Questions Skeletal muscles are attached to bone in pairs and groups; muscles move bones around joints by contracting and releasing. Refer to Figures 6.3 and 6.7. Answers from top of figure down: muscle outer sheath (connective tissue); two bundles of muscle cells (each with own connective tissue sheath); one muscle cell; one myofibril; one sarcomere. Sliding filament model: 1) ATP energizes myosin so its heads can attach to actin when calcium is present (released from SR); 2) Myosin heads pull toward the center of the sarcomere, sliding the actin forward to pull the Z lines at the end of sarcomeres toward one another; 3) More ATP binds, making myosin release actin and reset to grab actin again. As long as there is sufficient calcium, contraction continues. ATP is needed to continue the cycle and detach the filaments from one another. Oxygen debt is incurred when muscles use more ATP than cellular respiration can produce. Muscle fatigue is when ATP production cannot match ATP use and the muscles can’t contract anymore—it involves a depletion of glycogen and a buildup of lactic acid in the muscles. The sarcoplasmic reticulum stores and releases calcium in the cell needed for contraction. Calcium allows cross-bridge formation between myosin and actin filaments; ACh is released at neuromuscular junctions and paves the way for the SR to release calcium. A motor unit is a motor neuron and the muscle cells associated with it at a synapse. A rapid series of muscle twitches produces a stronger contraction because the muscle is able to sum up the signals. If the second signal arrives before the muscle has relaxed, the force produced from the second twitch is added to that of the first. Each twitch produces the same amount of force, but if they occur close together, each subsequent twitch starts at a higher level, creating increased force overall. Slow muscle is red from more myoglobin and capillaries and it has more mitochondria; it makes more ATP and contracts slowly and over a longer period of time. Fast muscle is white from less myoglobin and fewer capillaries and mitochondria; it allows rapid contraction but only over a short period of time. These fibers tend to be thicker, which lends to their increased ability to produce force. Possible Responses to Critical Thinking Questions If training runners to compete in the 100-meter dash, you would stress strength training to build up a powerful muscle that can unleash a burst of energy over a short period of time. You would want to increase the size of the fibers; add more myofibrils. On the other hand, if you were training a long-distance swimmer, you would emphasize aerobic exercise, which would increase: (a) the number of mitochondria (ATP), (b) the number of blood capillaries supplying muscle tissue, and (c) the oxygen-binding pigment myoglobin. No, if the calf muscle is indeed torn, continued yard work will potentially exacerbate the tear, and will definitely delay healing. All muscle injuries need rest to heal. The poison curare blocks the binding of acetylcholine that is released by the axon into the synapse between neuron and muscle cell to transmit the message instructing the muscle to contract. If this drug reaches the diaphragm, it will be paralyzed and unable to contract, resulting in no increase and decrease of the chest cavity, which is the cause of lung inflation and deflation, respectively. After Sean completes his 15 minutes of vigorous climbing on the stair machine, he experiences rapid breathing because his blood has not been able to supply oxygen to the muscles for the production of ATP at a rate commensurate with its usage. Therefore, he has incurred an “oxygen debt,” which will have to be paid off in the next several minutes. The aching in his quadricep muscles is similarly temporary due to the buildup of lactate, which is formed from pyruvate when there is not sufficient oxygen to “pull” the pyruvate into the Krebs cycle and electron transport chain. Over the next few minutes the lactate will revert back to pyruvate and enter the Krebs cycle. Maria’s plan is not a good one; proper training is far safer than taking performance-enhancing drugs, which have not been approved for use in this manner. Additionally, should she place well and become internationally recognized she is likely to be tested and her use of the drugs will be exposed. Possible Responses to Explore on Your Own Questions The flexor digitorum superficialis should be easily felt and, in those with little body fat, readily seen. For facial muscles, these can be easily felt by an individual as well as seen—the shape of the face will change some depending on whether you frown or smile. 84 Chapter Six The Muscular System 63 84 Chapter Six The Muscular System 24 84 Chapter Six The Muscular and Skeletal Systems 75 84 Chapter Six The Muscular System 24 84 Chapter Six The Muscular System 83