Mrs. Kaser's Science Page



Chapter 6 The Muscular Systemright0Because flexing muscles look like mice scurrying beneath the skin, some scientist long ago dubbed them muscles, from the Latin word mus meaning "little mouse." Indeed, the rippling muscles of professional boxers or weight lifters is often the first thing that comes to mind when one hears the word muscle. But muscle is also the dominant tissue in the heart and in the walls of other hollow organs of the body. In all its forms, it makes up nearly half the body's mass.The essential function of muscle is contraction, or shortening-a unique characteristic that sets it apart from any other body tissue. As a result of this ability, muscles are responsible for essentially all body movement and can be viewed as the "machines" of the body.Overview of Muscle Tissues Describe similarities and differences in the structure and function of the three types of muscle tissue and indicate where they are found in the body. Skeletal – single, very long, cylindrical, multinucleated cells with very obvious striations, voluntary via nervous system controls, attach to bones for movement, slow to fast contractions Cardiac – branching chains of cells, uninucleated, striated, involuntary, found only in the heart, slow rhythmic contractions Smooth – single, fusiform, uninucleated, no striations, involuntary, can stretch, found mostly in walls of hollow visceral organs but not the heart, very slow contraction Define muscular system. The function of muscle tissue is to contract or shorten to cause movement, maintain posture, stabilize joints, and generate heat The term muscular system applies specifically to skeletal muscleDefine and explain the role of the following: endomysium, perimysium, epimysium, tendon, and aponeurosis. Endomysium – delicate connective tissue sheath enclosing muscle fibers Perimysium – coarser fibrous membrane that wraps several sheathed muscle fibers to form a bundle of fibers called a fascicleEpimysium – even tougher overcoat of connective tissue that covers many fascicles bound together to form the muscle Tendon – strong, cords of dense fibrous tissue attaching a muscle indirectly to bones cartilages, or connective tissue coverings of each otherAponeurosis – sheet-like bundles of fibrous or membranous sheet connecting a muscle and the part it moves Microscopic Anatomy of Skeletal Muscle Describe the microscopic structure of skeletal muscle and explain the role of actin- and myosin-containing myofilaments. See diagram on page 167Sarcolemma – the many oval nuclei of a skeletal muscle cell Myofibrils – the long ribbon-like fibers that fill the cytoplasm – actually chains of tiny contractile units called sarcomeres – the alternating light and dark bands along the length of the aligned myofibrils give the muscle cells its striped appearance Light band has a midline interruptions, a darker area called the Z disk, and the dark A band has a lighter central area called the H zoneSarcomeres – functional unit of the muscle cellMyofilaments – found within the sarcomeres that actually produce the banding pattern – two types of threadlike proteinsThick filaments – myosin fibers – made of bundles molecules of the protein myosin and contain ATPase enzymes – extend the entire length of the dark A band – midparts of the thick filaments are smooth but ends are studded with small projections called myosin heads also called cross bridges because they link the thick and thin filaments together during contractionThin filaments – composed of the contractile protein actin and some regulatory proteins that play a role in allowing (or preventing) myosin head binding to actin – also called actin filaments, are anchored to the Z disk, a disc-like membrane) Light I band includes parts of two adjacent sarcomeres and contains only the thin filaments, and although the thin filaments overlap the ends of the thick filaments, they do not extend into the middle of a relaxed sarcomere, and this the central region (the H zone, which lacks actin filaments and looks a bit lighter) is sometimes called the bare zone When contraction occurs, and the actin-containing filaments slide toward each other into the center of the sarcomeres, these light zones disappear because the actin and myosin filaments are completely overlapped Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum with interconnecting tubules and sacs – the SR surround each and every myofibril – function is to sore calcium and to release it on demand Skeletal Muscle Activity Describe how an action potential is initiated in a muscle cell. One motor neuron may stimulate a few muscle cells or hundreds of them, depending on the particular muscle and the work it does One neuron and all the skeletal muscle cells it stimulates are a motor unit and when a long threadlike extension of the neuron, called the nerve fiber or axon, reaches the muscle, it branches into a number of axonal terminals, each of which forms junctions with the sarcolemma of a different muscle cell – these junctions are called neuromuscular junctions Even though the nerve endings and the muscle cells’ membranes are very close, they never touch leaving a gap between them called the synaptic cleft, which is filled with tissue fluid, interstitial fluid When the nerve impulse reaches the axonal terminals, a chemical called neurotransmitter (acetylcholine) is released and diffuses across the synaptic cleft and attaches to membrane protein receptors attached to the sarcolemma – if enough acetylcholine is released, the sarcolemma becomes temporarily permeable to sodium ions (Na+), which rushes into the muscle cell giving the interior of the cell a positive charge generating the electrical current called an action potential – once begun, this action potential is unstoppable (all or none response) and it travels over the entire surface of the sarcolemma, conducting the electrical impulse from one end of the cell to the other resulting in the contraction of the muscle cellThe events that return the cell to its resting state include diffusion of potassium ions (K+) out of the cell and the operation of the sodium-potassium pump, the active transport mechanism that moves the sodium and potassium ions back to their initial positions Describe the events of muscle cell contraction. When muscle fibers are activated by the nervous system, the cross bridges attach to myosin binding sites on the thin filaments, and sliding beginsEnergized by ATP, each cross bridge attaches and detaches several times during contraction, pulling the thin filaments toward the center of the sarcomere – this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens The attachment of myosin cross bridges to actin requires calcium ions, and the action potential leading to contraction causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasmWhen the action potential ends, calcium ions are reabsorbed into the SR storage areas, and the muscle cell relaxes and settles back to its original length Acetylcholine is being broken down by enzymes on the sarcolemma making sure there is only one contraction preventing continued contraction without a nerve impulse Define graded response, tetanus, isotonic and isometric contractions, and muscle tone as these terms apply to a skeletal muscle. Graded response – the all-or-none law of muscle contraction applies to the muscle cell not the whole muscle – whole muscles will react to stimuli with different degrees of shorteningGrades response is done in two waysBy changing the frequency of muscle stimulationBy changing the number of muscle cells being stimulatedMuscle twitches – single, brief, jerky contractions that occur as a result of nervous system problems Tetanus – two types – not to be confused with the bacterium toxin that can lead to deathFused or complete tetanus – when the muscle is stimulated so rapidly that no evidence of relaxation is seen and the contractions are completely smooth and sustainedUnfused or incomplete tetanus – stage muscles are in until complete tetanus state is achievedIsotonic contractions – same tone or tension contractions – they myofilaments are successful in sliding movements, the muscle shortens, and movement occurs like when you lift a bookIsometric contractions – same measurement contractions – contractions when the muscle do no shorten – the myosin myofilaments are trying to slide and the tension in the muscle keeps increasing because the muscle is pitted against some immoveable object like when you push against the wallMuscle tone – state of continuous partial contraction – the result of different motor units scattered throughout the muscle being stimulated by the nervous system in a systematic way – not consciously controlled Describe three ways in which ATP is regenerated during muscle activity. Direct phosphorylation of ADP by creatine phosphate – high energy molecule creatine phosphate (CP) transfers a phosphate to ADP to make 1 ATP/CP – CP supplies exhausted soon but no oxygen is required Aerobic respiration – occurs in mitochondria involving the oxidative phosphorylation pathways starting with glucose releasing carbon dioxide and water as well as 36 ATP/glucose molecule – slow and requires continuous supply of oxygen and nutrients Anaerobic glycolysis and lactic acid formation – no oxygen required in the first steps of glucose breakdown which occurs in the cytosol to produce pyruvic acid and 2 ATP/glucose – occurs during intense muscle activity when oxygen is absent resulting in pyruvic acid being converted to lactic acid instead of going through the rest of aerobic respiration Define oxygen debt and muscle fatigue and list possible causes of muscle fatigue. Oxygen debt – occurs during prolonged muscle activity when there is not enough oxygen being taken in fast enough to keep cell respiration making ATP Muscle fatigue – occurs when muscles strenuously for a long time – a muscle is fatigued when it is unable to contract even though it is still being stimulated – will begin to contract more weakly until it finally ceases reacting and stops contracting Describe the effects of aerobic and resistance exercise on skeletal muscles and other body organs. Aerobic or endurance exercise – result in stronger, more flexible muscles with greater resistance to fatigue due in part to increased blood supply to muscles and the individual muscle cells produce more mitochondria and store more oxygen – does not cause the muscle to increase much in size – benefits metabolism, digestion, neuromuscular coordination and skeleton as well as the heart, reduced fat deposits in vessels, and lungs Resistance or isometric exercise – results in the bulging muscles seen in body builders – occurs when muscles are pitted against some immoveable object such as a wall – key is to force the muscle to contract with as much force as possible – increased the size and strength is due to enlargement of individual muscles cells as they make more contractile filaments – the amount of connective tissue that reinforces the muscle also increases Muscle Movements, Types, and Names Define origin, insertion, prime mover, antagonist, synergist, and fixator as they relate to muscles. Origin – attached to the immovable or less moveable boneInsertion – attached to the movable bone and when the muscle contracts, the insertion moves toward the originPrime mover – the muscle that has the major responsibility for causing a particular movement Antagonist – muscles that oppose or reverse a movement – when a prime mover is active, its antagonist is stretched and relaxed – antagonists can be prime movers Synergist – help prime movers by producing the same movement or by reducing undesirable movements such as when a muscle crosses two or more jointsFixator – specialized synergists that hold a bone still or stabilize the origin of a prime mover so all the tension can be used to move the insertion bone The actions of antagonistic and synergistic muscles are important for smooth, coordinated, precise movements Demonstrate or identify the different types of body movements. Flexion – movement that decreases the angle of the joint and brings two bones closer together – hinge joints and ball-and-socket joints Extension – movement that increases the angle between two bones – straightening the knee or elbowRotation – movement of a bone around its longitudinal axis – ball-and-socket joints, movement of the atlas around the dens of the axis (shaking your head no)Abduction – moving a limb away from the midline, or median plane, of the body – also the fanning movement of fingers and toesAdduction – movement of limbs toward the body midline Circumduction – combination of flexion, extension, abduction, and adduction – seen in ball-and-socket joints such as shoulder where the proximal end of the limb is stationary and the distal end moves in a circle Dorsiflexion and plantar flexion – up and down movement of the foot at the ankle – lifting the foot = dorsiflexion while depressing the foot = plantar flexion Inversion and eversion – inversion of the foot turns it medially (inward toward the other foot) – eversion of the foot turns it laterally (outward and away from the other foot) Supination and pronation – refer to movements of the radius around the ulna – supination occurs when the forearm rotates laterally so that the palm faces anteriorly and the radius and ulna are parallel while pronation occurs when the forearm rotated medially so that the palm faces posteriorly and the radius crosses the ulna so the two bones form an X Opposition – the action by which you move your thumb to touch the tips of the other fingers on the same hand List some criteria used in naming muscles. Direction of the muscle fibers – rectus = straight – oblique = slantedRelative size of the muscle – maximus vs. minimus and longus Number of origins – bi, tri, quadLocation of the muscle’s origin and insertion Shape of the muscle Action of the muscle – flexor, adductor Gross Anatomy of Skeletal Muscles Name and locate the major muscles of the human body (on a torso model, muscle chart, or diagram) and state the action of each. See diagrams on pages 182 – 192 Developmental Aspects of the Muscular System Explain the importance of a nerve supply and exercise in keeping muscles healthy. If the nerve supply to a muscle is destroyed and the muscle is no longer stimulated, it loses tone and becomes paralyzed and soon after, it becomes flaccid, or soft and flabby, and begins to atrophy Describe the changes that occur in aging muscles. Increasing muscular control reflects the maturation of the nervous system – muscle control is achieved in a cephalic/caudal and proximal/distal direction As we age, the amount of connective tissue in the muscle increases and the amount of muscle mass decreases and the muscles become more stringier (sinewy) but exercise helps retain muscle mass and strength – lose of muscle mass leads to weight lose – loss in muscle mass also leads to a decrease in muscle strength (strength decreases by about 50% by the age 80) ................
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