The Muscular System PDF

The Muscular System Ryan R. Williams, M.D., Ph.D. Biology 122 California State University Dominguez Hills Overview Functions in: Movement of the entire organism Movement of materials within the organism That is, blood, food Three types of muscle tissue:...

The Muscular System Ryan R. Williams, M.D., Ph.D. Biology 122 California State University Dominguez Hills Overview Functions in: Movement of the entire organism Movement of materials within the organism That is, blood, food Three types of muscle tissue: smooth, cardiac, and skeletal The cells are called muscle fibers Overview Smooth muscle Fibers are: Shaped like cylinders with pointed ends (tapered). Single nucleus Arranged in parallel lines, forming sheets Not striated Located in the walls of hollow internal organs and blood vessels and causes these walls to contract Contraction is involuntary Although smooth muscle is slower to contract than skeletal muscle, it can sustain prolonged contractions and does not fatigue easily Overview Cardiac muscle Forms the heart wall Fibers are: Single nucleus, striated, and tubular Branched Interlock at intercalated disks Contain gap junctions that permit contractions to spread through the heart wall Relaxes completely between contractions, which prevents fatigue Contraction: Is rhythmic Occurs without nervous stimulation Is involuntary Overview Skeletal muscle Fibers are: Tubular, multinucleated, and striated Make up skeletal muscles, which are attached to the skeleton Very long Run the entire length of the muscle Is voluntarily controlled Overview Functions of skeletal muscles: Support—muscle contraction opposes gravity and allows us to remain upright Movements of bones and other body structures Arms, legs, eyes, facial expressions, and breathing Maintenance of a constant body temperature Contraction causes ATP to break down, releasing heat, which is distributed throughout the body Movement of fluids in the cardiovascular and lymphatic systems Keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels Protect the internal organs and stabilize joints Muscles pad the bones, and the muscular wall of the abdomen protects internal organs Muscle tendons hold bones together at joints Skeletal Muscle Structure Fascicle—bundle of skeletal muscle fibers Within a fascicle, each fiber is surrounded by connective tissue; the fascicle is also surrounded by connective tissue Fascia—connective tissue that covers muscles and extends to become its tendon Small, fluid-filled sacs called bursae (singular, bursa) can often be found between tendons and bones The bursae act as cushions, lubrication  muscle fiber fascicle fascia dense connective tissue tendon Skeletal Muscle Structure The origin of a muscle is the attachment site to the stationary bone The insertion is the attachment on the bone that moves When a muscle contracts, it pulls on the tendons at its insertion and the bone moves That is, when the biceps brachii contracts, it raises the forearm Skeletal Muscle Structure Skeletal muscles usually function in groups. Agonist (prime mover)—the muscle that does most of the work Synergist—assists the agonist Antagonist—the muscle that acts opposite to a prime mover That is, the biceps brachii and the triceps brachii are antagonists; biceps flexes the forearm, and the triceps extends the forearm If both contract at once, there would be no movement  tendon origin biceps brachii (contracted) triceps brachii (relaxed) radius humerus ulna insertion biceps brachii (relaxed) triceps brachii (contracted) Names and Actions of Skeletal Muscles The names of skeletal muscles often use the following terms to characterize them: Size: gluteus maximus, gluteus minimus Other terms used to indicate size: vastus (huge), longus (long), and brevis (short) Shape: deltoid (shaped like the Greek letter delta) Trapezius is shaped like a trapezoid Other terms used to indicate shape are latissimus (wide) and teres (round) Location: external obliques, internal obliques Frontalis muscle overlies the frontal bone Other terms used to indicate location are pectoralis (chest), gluteus (buttock), brachii (arm), and sub (beneath) Names and Actions of Skeletal Muscles Direction of muscle fibers: rectus abdominis (rectus = “straight”) Orbicularis oculi—circular muscle around the eye Other terms used to indicate direction are transverse (across) and oblique (diagonal) Attachment: sternocleidomastoid is attached to the sternum, clavicle, and mastoid process Brachioradialis—attached to the brachium (arm) and the radius (forearm) Number of attachments: biceps brachii has two attachments Quadriceps femoris has four origins Action: extensor digitorum extends the fingers (digits) Adductor longus adducts the thigh Other terms used to indicate action are flexor (to bend), masseter (to chew), and levator (to lift)  Orbicularis oculi: blinking, winking, Masseter: responsible for crow’s a chewing muscle; feet clenches teeth Orbicularis oris: Deltoid: “kissing” muscle brings arm away Pectoralis major: from the side of brings arm forward body; moves arm and across chest up and down in front Serratus Biceps brachii: anterior: bends forearm at pulls the scapula elbow (shoulder blade) forward, as in Rectus abdominis: pushing or bends vertebral punching column; compresses External abdomen oblique: Flexor carpi compresses group: abdomen bends wrist rotation of and hand trunk Adductor longus: moves thigh toward Quadriceps femoris: midline; raises thigh straightens leg at Sartorius: knee; raises thigh raises and laterally rotates Tibialis anterior: thigh; raises and rotates leg turns foot upward, as close to body; these combined when walking on heels actions occur when “crossing legs” or kicking across, as in soccer Extensor digitorum longus: Limbs raises toes; raises foot Arm: above the elbow Forearm: below the elbow Thigh: above the knee Leg: below the knee Trapezius: raises scapula, as when shrugging shoulders; pulls head backward Latissimus dorsi: brings arm down and backward behind the body Triceps brachii: straightens forearm at elbow Extensor carpi group: straightens wrist and hand Extensor digitorum: straightens fingers and wrist Gluteus maximus: extends thigh back Biceps femoris: bends leg at knee; extends thigh back Gastrocnemius: turns foot downward, as when standing on toes; bends leg at knee Achilles tendon Components of Muscle Fibers Sarcolemma—plasma membrane Sarcoplasm—cytoplasm Contains many myofibrils, the contractile parts of muscle fibers Contains glycogen, which provides energy for muscle contraction Contains myoglobin, which binds oxygen Sarcoplasmic reticulum—endoplasmic reticulum Calcium storage site T (transverse) tubules—penetrate the cells In close proximity to the sarcoplasmic reticulum Components of Muscle Fibers Muscle fibers are cylindrical in shape Grouped inside this larger cylinder are smaller cylinders called myofibrils Myofibrils run the entire length of the muscle fiber Made of a series of contractile units called sarcomeres Striations—stripes are created by the overlapping of myofilament within the sarcomeres of the myofibrils  Table 13.1 Anatomy of a Muscle Fiber. Name Function Sarcolemma The plasma membrane of a muscle fiber that forms T tubules Sarcoplasm The cytoplasm of a muscle fiber that contains the organelles, including myofibrils Myoglobin A red pigment that stores oxygen for muscle contraction T tubule An extension of the sarcolemma that extends into the muscle fiber and conveys impulses that cause Ca2+ to be released from the sarcoplasmic reticulum Sarcoplasmic The smooth endoplasmic reticulum (ER) of a muscle fiber reticulum that stores Ca2+ Myofibril A bundle of myofilaments that contracts Myofilament An actin or a myosin filament, whose structure and functions account for muscle striations and contractions A muscle contains bundles of muscle fibers, and a muscle fiber has many myofibrils. bundle of muscle cells (fibers) myofibril sarcolemma mitochondrion one myofibril sarcoplasm skeletal muscle myofilament cell (fiber) Z line one sarcomere Z line T tubule sarcoplasmic nucleus reticulum A myofibril has many sarcomeres. 3,900× Components of Muscle Fibers Myofibrils are further divided into a series of individual contractile units called sarcomeres Sarcomeres are formed by two types of myofilaments Thick myofilaments are made up of myosin Shaped like a golf club, with the straight portion of the molecule ending in a globular head, or cross-bridge Thin myofilaments are composed of two intertwining strands of the protein actin Associated with tropomyosin, and troponin Each of the sarcomere is called the Z line I band—light colored; made of only thin myofilaments A band—made of overlapping thin and thick myofilaments H band—centered within the A band contains only thick myofilaments  cross- bridge myosin actin Sarcomeres are relaxed. H band Z line A band I band Sarcomeres are contracted. The Sliding Filament Theory As the muscle fiber contracts as the sarcomeres shorten, thus shortening the myofibrils The myofilaments themselves remain the same length, and slide past each other The thin filaments slide past the thick filaments The I band shortens, the Z lines move inward, and the H band almost disappears ATP supplies the energy for muscle contraction The heads of the myosin binds to actin The heads of the myosin break down ATP, and their cross-bridges pull the actin filament toward the center of the sarcomere The Sliding Filament Theory Two other proteins associate with thin filaments (actin): Threads of tropomyosin wind around the strands of actin, covering binding sites for myosin Troponin occurs at intervals along the threads When Ca2+ is released from the sarcoplasmic reticulum, it binds to troponin The tropomyosin threads move, exposing myosin- binding sites  The Sliding Filament Theory Myosin heads have ATPase activity When ATP binds to the myosin heads, it splits into ADP and P This changes the angle of the myosin head, and allows the myosin heads to attach to actin and form temporary bonds called cross- bridges Then ADP and P are released, and the heads bend back into their original shape This is the power stroke that pulls the actin filament toward the center of the sarcomere Then ATP binds to myosin again and this breaks the cross-bridges Myosin detaches from actin and splits the ATP again The cycle continues and shortens the sarcomeres (and therefore the muscle) Rigor mortis—relaxing the muscle is impossible, because ATP is needed to break the cross-bridges Skeletal Muscle Contraction Muscle Fiber Contraction Motor neuron—a type of nervous system cell that stimulates muscle fibers to contract Nerve—group of axons Axon—the long thin extension of a neuron that stimulates a muscle fiber Branches, so can stimulate several muscle fibers Muscle Fiber Contraction Neuromuscular junction Where an axon terminal (end of an axon) forms a synapse with the sarcolemma Synaptic cleft—the space that separates the axon terminal and the sarcolemma Axon terminals contain synaptic vesicles filled with the neurotransmitter acetylcholine (ACh) When an action potential traveling down the axon arrives at an axon terminal, synaptic vesicles release ACh into the synaptic cleft ACh diffuses across the cleft and binds to receptors in the sarcolemma This generates electrical signals that spread across the sarcolemma and down the T tubules This causes calcium to be released from the sarcoplasmic reticulum  skeletal muscle fiber axon branch axon terminal synaptic vesicle synaptic One motor axon goes to cleft several muscle fibers. acetylcholine (ACh) axon terminal synaptic vesicle synaptic cleft folded sarcolemma sarcolemma ACh receptor A synaptic cleft exists between an Neurotransmitter (ACh) diffuses across synaptic cleft and binds axon terminal and a muscle fiber. to receptors in sarcolemma. Motor Units and Muscle Twitch Motor unit—a nerve fiber and all the muscle fibers it innervates All muscle fibers in a motor unit are stimulated at once; they all either contract or don’t The number of muscle fibers within a motor unit varies Some motor units, like the ones that require fine control, contain only a few fibers Motor units responsible for strength contain many fibers Muscle twitch—a single contraction of a muscle fiber; lasts only a fraction of a second Motor Units and Muscle Twitch A muscle twitch is the smallest degree of contraction and may be divided into three stages: Latent period—the time between stimulation and initiation of contraction ACh diffuses across the synaptic cleft, causing an action potential to spread across the sarcolemma and down the T tubules Contraction period—calcium leaves the sarcoplasmic reticulum; cross-bridges form Relaxation period—cross-bridges are broken; calcium returns to the sarcoplasmic reticulum. Force diminishes as the muscle returns to its former length Motor Units and Muscle Twitch Summation, Tetanus, and Fatigue Summation—increased muscle contraction due to increased Ca2+ Can occur until reaches maximal sustained contraction, called tetanus Tetanus continues until fatigue due to depletion of energy reserves (ATP) Fatigue—when a muscle relaxes, even though stimulation continues Recruitment—as the intensity of nervous stimulation increases, more motor units in a muscle are activated Maximum contraction requires that all motor units be in tetanus This rarely happens, because they would all fatigue at the same time Instead, some motor units contract maximally while others rest Muscle tone—muscle firmness, is dependent on muscle contraction Some motor units are always contracted, but not enough to cause movement Summation, Tetanus, and Fatigue Energy for Muscle Contraction Muscles store energy sources as glycogen and triglycerides Muscles acquired energy sources from blood as glucose and fatty acids As time of exercise increases muscle energy stores decreases and use of energy sources from the blood increases Muscle cells store limited amounts of ATP, once it is used up, they have three ways to produce more: The creatine phosphate (CP) pathway Fermentation Cellular respiration Mitochondria uses oxygen, so it is aerobic Neither the CP pathway nor fermentation requires oxygen (they are anaerobic) Production of ATP by Muscles Energy for Muscle Contraction The creatine phosphate pathway—the simplest and fastest way for muscle to make ATP; it consists of only one reaction: Creatine phosphate is formed only when a muscle cell is resting, and only a limited amount is stored The CP pathway is used at the beginning of exercise That is, the energy to complete a play in a football game comes principally from the CP system Intense activities lasting longer than 5 seconds also make use of fermentation Energy for Muscle Contraction The anaerobic processes of glycolysis and fermentation produce two ATPs from the breakdown of glucose to lactate Hormones signal cells to break down glycogen, making glucose available as an energy source Fermentation, like the CP pathway, is fast-acting, but results in the buildup of lactate Lactate produces short-term muscle aches and fatigue Oxygen debt—heavy breathing following strenuous exercise is required to complete the metabolism of lactate and restore cells to their original energy state Energy for Muscle Contraction Cellular respiration—the slowest of all three mechanisms used to produce ATP, but the most efficient Occurs in the mitochondria Myoglobin—a protein in muscle cells that delivers oxygen directly to the mitochondria Can use glucose from stored glycogen, glucose in the blood, and fatty acids Fast-twitch fibers rely on the creatine phosphate pathway and fermentation (are anaerobic) Slow-twitch fibers tend to prefer cellular respiration, which is aerobic Energy for Muscle Contraction Fast-twitch fibers (white) Designed for strength; their motor units contain many fibers Provide explosions of energy Are most helpful in sports such as sprinting and weightlifting Are light in color because they have fewer mitochondria, little myoglobin, and fewer blood vessels than slow-twitch fibers Develop maximum tension faster than slow-twitch fibers Maximum tension is greater Their dependence on anaerobic energy leaves them vulnerable to an accumulation of lactate, which causes them to fatigue quickly Energy for Muscle Contraction Slow-twitch fibers (red) Motor units have fewer muscle fibers Have more stamina Most helpful in endurance sports: long-distance running, biking, jogging, and swimming Produce most of their energy aerobically; they tire only when their fuel supply is gone Have many mitochondria and are dark in color because they contain myoglobin Tension develops slowly Highly resistant to fatigue Have a reserve of glycogen and fat so their mitochondria can maintain a steady, prolonged production of ATP when oxygen is available  slow-twitch fast-twitch fibers fibers Fast-twitch muscle fiber Slow-twitch muscle fiber is anaerobic is aerobic has explosive power has steady power fatigues easily has endurance Common Muscular Conditions Spasms—sudden, involuntary muscle contractions, accompanied by pain Can occur in smooth and skeletal muscles Convulsion—multiple spasms of skeletal muscles Cramps—strong, painful spasms due to strenuous activity Facial tics—spasms in the face that can be controlled voluntarily, but only with great effort Strain—stretching or tearing of a muscle Sprain—twisting of a joint, leading to swelling and injury of muscles, ligaments, tendons, blood vessels, and nerves Tendinitis—inflammation of a tendon May irritate the bursae underlying the tendon, causing bursitis Muscular Diseases Myalgia—achy muscles; most often caused by overuse of a muscle Myositis—inflammation of the muscles Caused by a viral infection or an immune system disorder Fibromyalgia—a chronic condition Pain, tenderness, and stiffness of muscles Precise cause is not known Sarcomas—cancers that originate in muscle or the connective tissue associated with muscle Sarcomas also occur in bone, adipose, and cartilage Muscle Diseases Muscular dystrophy—a group of disorders characterized by progressive degeneration and weakening of muscles Duchenne muscular dystrophy—the most common type Inherited; a lack of the protein dystrophin Calcium leaks into the cell and activates an enzyme that dissolves muscle fibers Myasthenia gravis—autoimmune disease Weakness in the muscles of the eyelids, face, neck, and extremities The immune system mistakenly produces antibodies that destroy acetylcholine receptors Treatment includes drugs that inhibit the enzyme that digests acetylcholine, so ACh accumulates in neuromuscular junctions