6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs Week 2 Lab Exercise (Introduction) Introduction The muscular system is the largest group of tissues in the body. In fact, the muscular system typically accounts for approximately 50% of the averages person’s weight. The synergistic actions of muscle contractions in the body enable a full spectrum of motility. For example, muscles are used to move white blood cells through their environment, pump blood throughout the body, and enable skeletal movements such as jumping or picking up a glass of water. These movements are made possible by the muscle cells’ ability to contract and create tension within a muscle tissue. This tension creates pressure and propulsion, and enables movements ranging as broad as mastication to ambulation. Muscle disorders can be due to overuse or injury to a specific muscle or muscle group. Additional disorders to the muscular system can involve diseases affecting nerves that innervate muscles, infections, inflammation (myositis), medications or drugs and genetic disorders such as muscular dystrophy. https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 1/8 6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs Figure 1: The muscular system provides a mechanism for movement. Skeletal muscle facilitates locomotion while smooth and cardiac muscle work to transport materials throughout the body. Three Types of Muscles There are three major muscle types: skeletal, cardiac, and smooth. The majority of the muscles in the body are skeletal muscle, and are associated with the skeletal system. In fact, skeletal muscle is estimated to comprise approximately 40% of an average male’s weight, and approximately 32% of an average female’s weight. These muscles define the contour of the body, facilitate ambulation, and enable interaction with the world. Two other types of muscle, cardiac and smooth muscle, are found within organs and circulatory system and play an important role in transport within the body systems. The muscular system refers to skeletal muscle, and will be the focus of this week’s content. https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 2/8 6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs Figure 2: Gross muscle anatomy of the human body. Gross Muscle Anatomy Muscles are most easily identified through gross muscle anatomy. In other words, the location and structure of the muscle should be reviewed first. All muscle tissues are composed of individual fibers. Muscle tissues also have an origin and an insertion. The origin is defined as the tendinous connection of the muscle to a bone (usually the bone that is stabilized). Conversely, the insertion is defined as the tendinous connection of the muscle to a bone (usually the bone to be moved). Muscles are further defined as being pennate or not. Muscle fibers which are not pennate are all organized in the same direction and form a direct path between the origin and the insertion. Alternatively, pennate muscle fibers are organized at an angle relative to the insertion point (or, more precisely, relative to the point of action). As a result, pennate muscles accommodate more fibers per tissue and can create a stronger force than non-pennate muscles. However, pennate muscles also create a smaller, overall, length change due to the angularity of the fibers. Skeletal Muscle Fiber Skeletal muscle is surrounded by connective tissue called the epimysium. The muscle is made of bundles of fasciles, which themselves are bundles of myocytes, and are also sheathed by connective tissue called the perimysium. Each muscle fiber is also enclosed by a connective tissue sheath, called the endomysium. These bands of connective tissue converge to form a tendon connecting the muscle to its attachment site on the bone.Tendons and ligaments also have a hierarchical structure of parallel-aligned collagenous fibers, which contribute to their impressive tensile strength. Microscopic examination of skeletal muscle reveals some unique features, and illuminates why this tissue is also referred to as striated muscle. The long cylindrical cells, called muscle fibers or myocytes, assemble into the muscle https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 3/8 6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs tissue and appear striated (striped). Each muscle fiber, a single muscle cell, is an assembly of myofibrils with multiple oval nuclei interspersed throughout the bundle. It is covered by a specialized membrane called the sarcolemma which isolates individual muscle cells from each other. Figure 3: Microscopic anatomy of the muscle fiber. Histological Muscle Anatomy Muscle contractions occur through the interaction of two contractile proteins: actin and myosin. These proteins form myofilaments within each muscle fiber. Myofilaments are bundled into myofibrils which account for the majority of volume with the sarcoplasm. Myofilaments are generally divided into thick (dark) and thin (light) myofilaments. Thin filaments are composed of actin, and are anchored into the Z-disk of the sarcomere. Thick filaments are composed mainly of myosin. The striations visible in skeletal muscle are the result from the orderly alternations of thin and thick bands along the myofibrils. The actual contractile units within a muscle fiber are sarcomeres, found in the myofibrils. The sarcomere can be divided into discrete sections based on the banding pattern shown in Figure 3. The A band is a dark band that corresponds to the length of a bundle of myosin protein filaments. The light band are called the I bands, and are composed mainly of actin filaments. A protein disk, called the Z line, bisects and anchors the I band. The Z line is also where the I bands are anchored. In the middle of the A band is a zone corresponding to the area between thin filaments called the H-zone. Within the H zone is the M line which represents the proteins that anchor the myosin. https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 4/8 6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs Figure 4: Muscle fiber anatomy. Extensions of the sarcolemma called transverse tubules (or t-tubules) protrude into the sarcoplasm. These invaginations allow the muscle impulse, an electrical stimulus, to travel deep into the muscle fiber. Thus, the sarcolemma is more than a simple sheath that protects the muscle cell; it also provides a conduit for nerve signals to reach each sarcomere in the muscle via the t-tubules. The sarcoplasmic reticulum, a specialized endoplasmic reticulum, also surrounds the sarcomeres, and contains large stores of calcium which is vital to a muscle contraction. Within the sarcoplasmic reticulum, there are sac-like regions known as the terminal cisternae that store calcium ions. When two terminal cisternae are associated with a t-tubule, the structure is identified as a triad. When a muscle impulse reaches this region, calcium ions diffuse from the cistenae into the sarcoplasma.The calcium ions also affect the interaction of actin and myosin resulting in contractions. Sliding Filament Theory https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 5/8 6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs The sliding filament theory states that the thick filaments are pulled toward the sarcomere centers by cross bridge activity of the thick filaments. The length of the A band does not change as a muscle contracts, as the action is a result of the actin sliding past the myosin – not as a result of the myofilaments changing in size.Therefore, the width of the banding patterns changes as the degree of overlap changes and results in the shortening of the I band. Understanding how muscles generate a force explains why muscles can only pull, and never push. Action Potentials Muscle fiber contraction is regulated by the generation and transmission of action potentials along the sarcolemma. When this excitatory impulse arrives at the muscle fiber, a rapid depolarization occurs and initiates the physiologic response – contraction – by releasing calcium ions, Ca2+, from the sarcoplasmic reticulum. This is known as excitation-contraction coupling. ATP energizes this cycle, supplying chemical energy that is converted into mechanical energy. The first step involves ATP binding to a myosin. Here, myosin ATPases hydrolyze ATP into ADP and Pi, which remain bound to the myosin head. Intracellular Ca2+ binds with troponin, a protein associated with actin. The binding of Ca2+ to troponin causes a positional change in a second protein associated with actin, tropomyosin. Tropomyosin shifts to expose myosin binding sites on the actin filaments. This exposure of the actin filaments allows myosin heads to attach, forming cross bridges between actin filaments and myosin heads. This causes the ADP and Pi to be released and alters the shape of the myosin head. This shape change generates the sliding motion of the actin toward the center of the sarcomere and produces a power stroke. The cross bridge cycle ends when Ca2+ is pumped back into the sarcoplasmic reticulum. Figure 5: Actin and myosin filaments slide past each other to engage a muscle contraction. Figure 6: The actin-myosin crossbridge cycle. https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 6/8 6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs In sum, muscle contraction involves four phases: 1. 2. 3. 4. A latent period is required for the release of Ca2+ from the sarcoplasmic reticulum A contraction period represents the time of the actual cross bridge formation A relaxation period identifies the period during which Ca2+ stores are resupplied A refractory period is the time immediately following the stimulus during which the muscle fiber will not respond to another action potential. Motor Neurons A motor unit is one motor neuron and all the muscle fibers it innervates. The neuron’s axon has several terminal branches which each form a neuromuscular junction with one muscle fiber. The neuromuscular junction controls voluntary muscle function. Motor neurons are neurons that stimulate muscles. The action potential travels along the neuron until it reaches the end of the neuron. A gap, called a synapse, separates the neuron from the muscle fiber. Acetylcholine (ACh), a neurotransmitter, is released from the neuron and diffuses across the synapse. ACh binds to ACh receptors on the sarcolemma, generating a self-propagating action potential that travels, as a muscle impulse, down the t-tubules and initiates Ca2+ release causing the cross-bridge cycle to commence. Muscle Contraction Factors Many factors influence the force of the muscle contraction, including: the frequency of the stimuli, the strength of the stimulus, the length of the muscle fiber contraction, the type of contraction (isotonic vs. isometric), the type of muscle fiber (slow vs. fast twitch), as well as muscle tone and fatigue. An isotonic contraction occurs when muscle tension remains the same as muscle length changes (shortens). Lifting an object would involve an isotonic contraction. Alternatively, an isometric contraction occurs when muscle length remains the same as muscle tension increases. Pushing against an immovable object like a wall is an example of an isometric contraction. Twitch Fibers Muscle fibers are often defined as a short twitch fiber or a fast twitch fiber. Short twitch fibers are able to elongate the period during which action potentials are fired, and consume oxygen very efficiently. It takes longer for short twitch fibers to reach fatigue than it would for a fast twitch fiber. Therefore, they are more suitable for endurance sports such as marathon races. On the contrary, fast twitch fibers fire action potentials very quickly and are able to generate powerful bursts of energy. These fibers rely on anaerobic metabolism for energy (i.e., oxygen is not required). These muscle fibers are better suited for the type of motion needed for short distance sprinting. https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 7/8 6/30/2019 Week 2 Lab Exercise (Introduction) | eScience Labs Force Summation Skeletal muscles are able to generate different levels of force through a concept called Force Summation.Essentially, skeletal muscles are able to employ more (or less) twitch muscle contractions at faster (or slower) frequencies. Individually, twitch muscle contractions are the weak byproduct of a single action potential. They are not long or strong enough to create a useful contraction. Successive stimuli can generate additional tension, depending on the amount of Ca2+ accumulated between stimulation, and result in a more or less forceful muscle contraction overall. The term multiple fiber summation describes when the amount of twitch contractile units is increased; while the term frequency summation describes when the number of action potentials fired at the twitch contractile units is increased. Twitch contractions are uncommon in the body as muscle fibers typically act in concert to generate an effective contraction. As you have learned, ATP is the energy source for muscle contraction. It is obtained from a coupled reaction of creatine phosphate and ADP, as well as from aerobic and anaerobic metabolism of glucose (which results in lactic acid accumulation and oxygen depletion). When ATP is depleted, muscle fatigue occurs. Muscles are classified as prime movers or agonists, antagonists, synergists, and fixators. Prime movers or agonists are the main muscle responsible for a movement. Antagonists cause the opposite movement of the prime mover, while synergists aid in the movement of the prime mover. Fixators aid in the stability of joints. The criteria for naming muscles include a muscle’s location, shape, relative size (maximus, minimus, longus or brevis), fascicle direction (rectus, transverses, oblique), number of origins, attachment sites, and action (flexor, extensor, abductor, adductor, pronator or supinator). https://esciencelabs.com/studentresource/week-2-lab-exercise-introduction/555 8/8 WEEK 2 ASSIGNMENT: MUSCLE FATIGUE Submission Instructions Please complete your answers to the lab questions on this form. Please complete your answers, and SAVE the file in a location which you will be able to find again. Then, attach and submit the completed form to the Week 2 Laboratory dropbox in the Ashford University classroom. Result Tables Table 1: Muscle Fatigue Data Trial Time (Seconds) Trial 1 Trial 2 Trial 3 Post-Lab Questions 1. Did you notice any changes in the amount of time you could perform each wall sit, or how your legs felt after each of the trials? 2. Explain the actions that were occurring at the molecular level to produce this movement. Include sources of energy and any possible effect of muscle fatigue. 3. Hypothesize what would happen if blood flow was restricted to the leg when this experiment is performed. 4. How do banding patterns change when a muscle contracts? © eScience Labs, 2013 5. What is the difference between a muscle organ, a muscle fiber, myofibril and a myofilament? 6. Outline the molecular mechanism for skeletal muscle contraction. At what point is ATP used and why? © eScience Labs, 2013 WEEK 2 ASSIGNMENT: MUSCLE FATIGUE Muscle contractions are essential for muscles to function properly. Energy is released when biomolecules such as sugars and fats are broken down, and is stored in the form of ATP. ATP enables muscle contraction, but can only be stored in relatively small amounts. For this reason, the body must continually metabolize new ATP molecules. Muscle fatigue occurs if the local ATP reservoir for a muscle becomes depleted. The inability of a muscle to maintain tension is muscle fatigue. Failure to contract may occur because of the accumulation of lactic acid, a lack of ATP, or decreased blood flow. This is a common result of strenuous exercise in which ATP is consumed at a faster rate than it is produced. At this point, muscles may fail to contract and the intensity of an exercise must decrease. In this experiment, you will test how long it takes your muscles to fatigue. Materials You must provide the following items to complete your experiment:      Participant Stopwatch Sturdy Wall to Stand Against Computer Access Internet Access Note Please ask a partner to volunteer for you if you have a medical condition that does not permit you to perform this activity. Procedure 1. Before you begin the experiment, review the virtual demonstrations regarding the muscular system in the introductory section of the eScience Labs Student Portal. 2. Find a wall that is strong enough for you to push against. A temporary wall (such as a partition panel) is not suitable. 3. Find a stopwatch and adjust the settings so it is ready to operate. You may also use an online stopwatch, such as the one located here, if you are able to perform the experiment near a mobile playing device (such as an iPad or Tablet). © eScience Labs, 2013 4. Stand with your back pressed up against the wall, and lower yourself into a “wall-sit”. To do this: a. Align the backs of your heels, hips, and shoulders with the wall. b. Keeping your back pressed against the wall, take a few small steps forward (your upper half will lower as you walk your feet out). c. Lower yourself into a sitting position, keeping your back flat against the wall, until your knees form a 90 degree angle. d. Steady this position by focusing the majority of your weight in your heels. Do not allow your lower back to pull away from the wall. 5. Start the stopwatch and time how long you are able to hold the wall-sit position. The amount of time will vary, but will likely fall within approximately 30 - 120 seconds. When you are tired, check the time on the stopwatch, and move out of the position by slowly lowering your body down to the floor or standing up. 6. Record how long you were able to hold the wall-sit in Table 3. 7. Allow your muscles to rest for approximately two minutes, reset the stopwatch, and repeat Steps 3 - 6. 8. Again, allow your muscles to rest for approximately two minutes, reset the stopwatch, and repeat Steps 3 - 6. You should have three trials of data. © eScience Labs, 2013 
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