Human Biology Muscles Chapter Summary Guide

Muscles chapter 9.1 Muscles are distinguished by their ability to transform chemical energy (ATP) into directed mechanical energy Types of muscles: 1. Skeletal 2. Smooth 3. Cardiac Skeletal and smooth muscle cells are elongated, and are called muscle fibers. Sarcolemma: plasma membrane of muscle cells Sarcoplasm: muscle cell cytoplasm skeletal Skeletal, striated, and voluntary skeletal muscles, organs that attach to and cover the skeleton. Skeletal muscle fibers are the longest muscle cells Cardiac cardiac, striated, and involuntary occurs only in the heart smooth visceral, nonstriated, and involuntary found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages. also forms valves to regulate the passage of substances through internal body openings, dilates and constricts the pupils of your eyes, and forms the arrector pili muscles attached to hair follicles. Characteristics of Muscle Tissue: 1. Excitability (responsiveness): ability of a cell to receive and respond to a stimulus by changing its membrane potential 2. Contractility: ability to shorten 3. Extensibility: ability to extend or stretch. Muscle cells shorten when contracting, but they can be stretched, even beyond their resting length, when relaxed. 4. Elasticity: ability of a muscle cell to recoil and resume its resting length after stretching Muscle functions: 1. Produce movement: • Locomotion / manipulation (skeletal) • Blood pressure (cardiac) • Propulsion (smooth) 2. Maintain posture and body position: skeletal muscles that maintain body posture 3. Stabilize joints: strengthen and stabilize the joints of the skeleton 4. Generate heat: Muscles generate heat as they contract, which plays a role in maintaining normal body temperature 9.2 A skeletal muscle is made up of muscle fibers, nerves, blood vessels, and connective tissues  Nerve and Blood Supply one nerve, one artery, and one or more veins serve each muscle. cardiac and smooth muscle tissues, which can contract without nerve stimulation every skeletal muscle fiber is supplied with a nerve ending that controls its activity. Skeletal muscle has a rich blood supply. This is understandable because contracting muscle fibers use huge amounts of energy and require almost continuous delivery of oxygen and nutrients via the arteries. Muscle cells also give off large amounts of metabolic wastes that must be removed through veins Capillaries, straighten when the muscle stretches and contort when the muscle contracts  Connective Tissue Sheaths there are several different connective tissue sheaths. Together these sheaths: 1. support each cell and reinforce and hold together the muscle 2. preventing the bulging muscles from bursting during exceptionally strong contraction connective tissue sheaths: Epimysium dense irregular connective tissue that surrounds the whole muscle. Perimysium and fascicles Within each skeletal muscle fascicles that resemble bundles of sticks. Surrounding each fascicle is a layer of dense irregular connective tissue called perimysium Endomysium consists of fine areolar connective tissue. surrounds each individual muscle fiber. tendons join muscles to bones When muscle fibers contract, they pull on these sheaths, which transmit the pulling force to the bone to be moved. sheaths contribute somewhat to the natural elasticity of muscle tissue, and also provide routes for the entry and exit of the blood vessels and nerve fibers that serve the muscle.  Attachments: Muscle attachments may be direct or indirect: 1. Direct (connects muscle to bone directly), or fleshy, attachments: epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage. 2. indirect attachments: muscle's connective tissue wrappings extend beyond the muscle either as a ropelike tendon or as a sheetlike aponeurosis, tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles. Indirect attachments are much more common because of their durability and small size Tendons are mostly tough collagen fibers 9.3 skeletal muscle fiber is a long cylindrical cell with multiple oval nuclei just beneath its sarcolemma (plasma membrane) muscle cell contains large amounts of glycosomes (granules of stored glycogen that provide glucose during muscle cell activity for ATP production) and myoglobin, a red pigment that stores oxygen. muscle cell contains three specialized structures: myofibrils, sarcoplasmic reticulum, and T tubules. they play important roles in muscle contraction myofibrils:  run parallel  densely packed in the fiber that mitochondria and other organelles appear to be squeezed between them. Striations: a repeating series of dark (A bands) and light (I bands) bands  dark A band has a lighter region in its midsection called the H zone  H zone is bisected vertically by a dark line called the M line  light I band also has a midline interruption, a darker area called the Z disc (or Z line) sarcomeres: region of a myofibril between two successive Z discs myofilaments: 1. myosin (thick filaments) extend the entire length of the A band 2. actin (thin filaments) extend across the I band 3. elastic filament Each thick filament contains about 300 myosin Actin has kidney-shaped polypeptide subunits, called globular actin or G actin. Each G actin has a myosinbinding site to which the myosin heads attach during contraction G actin subunits polymerize into long actin filaments called filamentous, or F, actin Thin filaments also contain several regulatory proteins: 1. Tropomyosin: rod-shaped protein help stiffen and stabilize actin they block myosin-binding sites on actin 2. Troponin: major protein in thin filaments, is a globular protein with bunits One subunit attaches troponin to actin. Another subunit binds tropo sition it on actin. The third subunit binds calcium ions. Both troponin and tropomyosin help control the myosin-actin interactions in n. elastic filament: composed of the giant protein titin, Titin extends from the Z ment, holds the thick filaments in place, maintaining the organization of the A band, and helps the muscle cell spring back into shape after stretching important structural protein is dystrophin, which links the thin filaments to the integral proteins of the sarcolemma proteins that bind filaments or sarcomeres together and maintain their alignment include nebulin, myomesin, and C proteins. Intermediate (desmin) filaments extend from the Z disc and connect each myofibril to the next throughout the width of the muscle cell Sarcoplasmic Reticulum and T Tubules: Skeletal muscle fibers contain two sets of intracellular tubules that help regulate muscle contraction: 1. sarcoplasmic reticulum  elaborate smooth endoplasmic reticulum. The SR regulates intracellular levels of ionic calcium, it stores calcium and releases it on demand when the muscle fiber is stimulated to contract (calcium provides the final "go" signal for contraction) 2. T tubules  sarcolemma of the muscle cell protrudes deep into the cell interior, forming an elongated tube called the T tubule  lumen (cavity) of the T tubule is continuous with the extracellular space. As a result, T tubules tremendously increase the muscle fiber's surface area. This allows changes in the membrane potential to rapidly penetrate deep into the muscle fiber  T tubule runs between the paired terminal cisterns of the SR, forming triads Muscle contraction is ultimately controlled by nerve initiated electrical impulses that travel along the sarcolemma. Because T tubules are continuations of the sarcolemma, they conduct impulses to the deepest regions of the muscle cell and every sarcomere. These impulses trigger the release of calcium from the adjacent terminal cisterns The roles of the T tubules and SR in providing signals for contraction integral proteins of the T tubule act as voltage sensors integral proteins of the SR form gated channels through which the terminal cisterns release Ca2+ Shortening only occurs if the cross bridges generate enough tension on the thin filaments to exceed the forces that oppose shortening Contraction ends when the cross bridges become inactive, the tension declines, and the muscle fiber relaxes In a relaxed muscle fiber, the thin and thick filaments overlap only at the ends of the A band sliding fila1nent model of contraction states that during contraction, the thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree. Neither the thick nor the thin filaments change length during contraction, how it works: 1) nervous system stimulates muscle fibers 2) myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments, and the sliding begins 3) cross bridge attachments form and break several times during a contract ion, acting like tiny ratchets to generate tension and propel the thin filaments toward the center of the sarcomere. 4) muscle cell shortens, following things occur as a muscle cell shortens: I. I bands shorten II. distance between successive Z discs shortens III. H zones disappear IV. contiguous A bands move closer together, but their length does not change 9.4 Motor neurons are the way that the nervous system connects with skeletal muscles and "tells" them to contract neurons and muscles are excitable cells That is, they respond to external stimuli by changing their resting membrane potential. changes in membrane potential act as signals. One type of electrical signal is called an action potential (AP ; sometimes called a nerve impulse) An AP is a large change in membrane potential that spreads rapidly over long distances within a cell. Generally, APs don't spread from cell to cell. For this reason, the signal has to be converted to a chemical signal-a chemical messenger called a neurotransmitter Neurotransmitter: diffuses across the small gap between excitable cells to start the signal again The neurotransmitter that motor neurons use to "tell" skeletal muscle to contract is acetylcholine or Ach Ion Channels: The movement of ions through these ion channels changes the membrane voltage. two classes of ion channels are important for excitation and contraction of skeletal muscle:  Chemically gated ion channels: opened by chemical messengers, creates small local changes in the membrane potential, acetylcholine are an example, ACh receptor is a single protein in the plasma membrane that is both a receptor and an ion channel  Voltage-gated ion channels: open or close in response to changes in membrane potential chemically gated ion channels cause a small local depolarization (a decrease in the membrane potential) that then triggers the voltage-gated ion channels to create an action potential. Motor neurons reside in the spinal cord Each neuron has a long threadlike extension called an axon that extends from the cell body in the spinal cord to the muscle fiber it serves The axon of each motor neuron branches profusely as it enters the muscle so that it can innervate multiple muscle fibers neuromuscular junction is the region where the motor neuron contacts the skeletal muscle. It consists of multiple axon terminals and the underlying junctional folds of the sarcolemma. Each muscle fiber has only one neuromuscular junction, located approximately midway along its length Within the axon terminal are synaptic vesicles, small membranous sacs containing the neurotransmitter acetylcholine. junctional folds provide a large surface area for the thousands of ACh receptors located there synaptic cleft: filler between axon terminal and junctional folds result of the events at the neuromuscular junction is a transient change in membrane potential that causes the interior of the sarcolemma to become less negative (a depolarization). This local depolarization is called an end plate potential (EPP) After ACh binds to the ACh receptors, its effects are quickly terminated by acetylcholinesterase (enzyme located in the synaptic cleft breaks down ACh to its building blocks) end plate potential (EPP) is generated at the neuromuscular junction action potential is the result of a predictable sequence of electrical changes Three steps are involved in triggering and then propagating an action potential: 1) generation of an end plate potential 2) action potential depolarization 3) repolarization During repolarization, a muscle fiber is said to be in a refractory period, because the cell cannot be stimulated again until repolarization is complete. repolarization restores only the electrical conditions of the resting (polarized) state. action potential is brief and ends well before any signs of contraction are obvious. Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an act ion potential along the sarcolemma causes myofilaments to slide Myosin binding to actin forms cross bridges and contraction begins, E-C coupling is over intracellular calcium levels are low, the muscle cell is relaxed because tropomyosin molecules physically block the myosin-binding sites on actin Two calcium ions must bind to a troponin, causing it to change shape tropomyosin blockade is removed when sufficient calcium is present. Once binding sites on actin are exposed, the events of the cross bridge cycle occur The thin filaments continue to slide as long as calcium and adequate ATP are present myosin cross bridge attaches and detaches many times during a single contraction 9.5 The force exerted by a contracting muscle on an object is called muscle tension. The opposing force exerted on the muscle by the weight of the object to be moved is called the load. nerve-muscle functional unit called a motor unit A motor unit consists of one motor neuron and all the muscle fibers it innervates, or supplies myogram, a recording of contractile activity consisting of one or more recorded lines called tracings muscle twitch is the response of a muscle to a single stimulation, has three distinct phases: 1) latent period: is the first few milliseconds cross bridges begin to cycle but muscle tension is not yet measurable so the myogram does not show a response 2) Period of contraction: cross bridges are active myogram tracing rises to a peak 3) Period of relaxation: due to pumping of Ca2+ back into the SR number of active cross bridges is declining, contractile force is declining. Muscle tension decreases to zero muscle contraction can be graded in two ways: 1) increase in the frequency of stimulation causes temporal summation. The higher the frequency, the greater the strength of contraction of a given motor unit. 2) increase in the strength of stimulation causes recruitment. The stronger the stimulation, the more motor units are activated, and the stronger the contraction. if two identical stimuli (electrical shocks or nerve impulses) are delivered to a muscle in rapid succession, the second twitch will be stronger than the first temporal, or wave, summation. It occurs because the second contraction begins before the muscle has completely relaxed. The second contraction is greater than the first because the muscle is already partially contracted and because even more calcium is squirted into the cytosol. Higher stimulation frequency results in unfused tetanus stimulation frequency continues to increase, muscle tension increases until it reaches maximal tension. At this point all evidence of muscle relaxation disappears and the contractions fuse into a smooth, sustained contraction plateau called fused or complete tetanus Stimuli that produce no observable contractions are subthreshold stimuli. The stimulus at which the first observable contraction occurs is called the threshold stimulus maximal stimulus is the strongest stimulus that increases contractile force increasing the stimulus intensity beyond the maximal stimulus does not produce a stronger contraction. Muscle tone does not produce active movements, but it keeps the muscles firm, healthy, and ready to respond to stimulation. Skeletal muscle tone also helps stabilize joints and maintain posture two main categories of contractions-isotonic and isometric, depending on whether a muscle changes length (isotonic) or not (isometric). Isotonic: (1) Concentric contractions (muscle shortens and does work) (2) eccentric contractions (muscle generates force as it lengthens, are equally important for coordination and purposeful movements) ln isotonic contractions, the thin filaments slide. In isometric contractions, the cross bridges generate force but do not move the thin filaments 9.6 As a muscle contracts, ATP supplies the energy to move and detach cross bridges, operate the calcium pump in the SR, and operate the Na+-K+ pump in the plasma membrane. after ATP is hydrolyzed to ADP and inorganic phosphate in muscle fibers, it is regenerated within a fraction of a second by one or more of the three pathways: 1) direct phosphorylation of ADP by creatine phosphate (1ATP per CP)  creatine phosphate (CP): high-energy molecule stored in muscles, is tapped to regenerate ATP, CP reaction with ADP, catalyzed by the enzyme creatine kinase  ATP and CP provide for maximum muscle power  The coupled reaction is readily reversible, and to keep CP available 2) anaerobic glycolysis, which converts glucose to lactic acid (2ATP per glucose)  ATP is generated by breaking down glucose obtained from the blood  it does not use oxygen  During glycolysis, glucose is broken down to two pyruvic acid molecules, releasing enough energy to form small amounts of ATP  when blood flow and oxygen delivery are impaired during muscle contraction, most of the pyruvic acid is converted into lactic acid (end product of glucose metabolism during anaerobic glycolysis) + Most of the lactic acid diffuses out of the muscles into bloodstream  liver cells can reconvert it to pyruvic acid or glucose and release it back into the bloodstream for muscle use or convert it to glycogen for storage 3) aerobic respiration (32 ATP per glucose)  Aerobic respiration requires oxygen and mitochondria, and involves a sequence of chemical reactions that break the bonds of fuel molecules and release energy to make ATP  begins with glycolysis and is followed by reactions in the mitochondria. It breaks down glucose entirely to water and carbon dioxide and generates large amounts of ATP.  glucose, pyruvic acid from glycolysis, and free fatty acids are the major sources of fuels  it is slow because of its many steps and it requires continuous delivery of oxygen and nutrient fuels length of time a muscle can continue to contract using aerobic pathways is called aerobic endurance the point at which muscle metabolism converts to anaerobic glycolysis is called anaerobic threshold Muscle fatigue is a state of physiological inability to contract even though the muscle is still receiving stimuli, chemical changes may be involved: a. Ionic imbalances: reduce the size of the action potential, which reduces the movement of the voltage sensitive proteins in the T tubules and so reduces the amount of Ca2+ released from the SR. b. Increased inorganic phosphate c. Decreased ATP and increased magnesium (Mg2+) d. Decreased glycogen Lactic acid has Jong been assumed to be a major cause of fatigue For a muscle to return to its pre-exercise state, the following must occur: a. oxygen reserves (stored in myoglobin) must be replenished b. accumulated lactic acid must be reconverted to pyruvic acid c. Glycogen stores must be replaced. d. ATP and creatine phosphate reserves must be resynthesized The extra amount of oxygen that the body must take in for these restorative processes is called the excess postexercise oxygen consumption (EPOC) 9.7 The force of muscle contraction depends on the number of myosin cross bridges that are attached to actin. The higher the frequency of muscle stimulation, the greater the force the muscle exerts The more motor units recruited, the greater the force The large fibers of large motor units produce the most powerful movements. If a muscle is stretched so much that the filaments do not overlap, the myosin heads have nothing to attach to and cannot generate tension. Muscles vary in how fast they can contract and how long they can continue to contract before they fatigue Muscle Fiber Type: a. Speed of contraction: there are slow fibers and fast fibers, Contraction duration also varies with fiber type and depends on how quickly Ca2+ moves from the cytosol into the SR b. Major pathways for forming ATP: cells that rely mostly on the oxygen-using aerobic pathways for ATP generation are oxidative fibers. Those that rely more on anaerobic glycolysis and creatine phosphate are glycolytic fibers we can classify skeletal muscle cells as: slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers slow oxidative fiber:  Contracts slowly  Depends on oxygen delivery and aerobic pathways  Resists fatigue and has high endurance  many mitochondria (actual sites of oxygen use)  rich capillary supply (the better to deliver O2)  best suited to endurance-type activities fast glycolytic fiber:  Contracts rapidly  Uses little oxygen  Depends on plentiful glycogen reserves for fuel  Tires quickly because glycogen reserves are short- lived  Has a large diameter, indicating both the plentiful myofilaments that allow it to contract powerfully  Has few mitochondria, little myoglobin, and few capillaries (making it white)  best suited for short-term, rapid, intense movements fast oxidative fibers:  intermediate between the other two types  contract quickly  oxygen dependent and have a rich supply of myoglobin and capillaries  fatigue resistance all muscle fibers in a particular motor unit are of the same type muscles contain mixtures of the three fiber types A greater load results in a longer latent period, slower shortening, and a briefer duration of shortening 9.8 Forcing a muscle to work hard increases its strength and endurance. As muscles adapt to greater demaJ1d, they must be over-loaded to produce further gains. Aerobic, or endurance, exercise such as swimming, running, fast walking, and biking results in several recognizable changes in skeletal muscles: a. capillaries surrounding the muscle fibers increases b. mitochondria within the muscle fibers also increases + fibers synthesize more myoglobin Regular endurance exercise may convert fast glycolytic fibers into fast oxidative fibers resistance exercise (typically under anaerobic conditions) such as weightlifting or isometric exercise, which pits muscles against high-resistance or immovable forces  additional muscle bulk largely reflects the increased size of individual muscle fibers homeostatic imbalance muscular dystrophy:  muscle destroying diseases that generally appear during childhood  The affected muscles initially enlarge  most common and serious form is Duchenne muscular dystrophy (DMD), diagnosed when the boy is between 2 and 7 years old myasthenia gravis (asthen = weakness; gravi = heavy), a disease characterized by drooping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness, involves a shortage of ACh receptors. Myasthenia gravis is an autoimmune disease in which the immune system destroys ACh receptors Rigor mortis (death rigor) illustrates the fact that cross bridge detachment is ATP driven. Most muscles begin to stiffen immobilization due to enforced bed rest or loss of neural stimulation results in disuse atrophy (degeneration and loss of mass), which begins almost as soon as the muscles are immobilized.