MUSCLE & MOVEMENT C H A P T E R 3 3
KEY CONCEPTS 33.1 Muscle Cells Develop Forces by Means of Cycles of Protein Protein Interaction 33.2 Skeletal Muscles Pull on Skeletal Elements to Produce Useful Movements 33.3 Skeletal Muscle Performance Depends on ATP Supply, Cell Type, and Training 33.4 Many Distinctive Types of Muscle Have Evolved Intro Crash Course Video on Muscular System
INTRODUCTION Muscle tissue makes up a large portion of body mass, almost half of our body mass Muscles are important functionally as they are anatomically. Muscles are the basis for virtually all behavior, movement, breathing, swimming, flying, eating Many other physiological actions also depend on muscle contraction, such as blood circulation by the beating of the heart.
INTRODUCTION There are several types of muscle tissue in the animal kingdom The contractile mechanism is virtually the same in most muscles, however we will be focusing on the contractile mechanism in vertebrate skeletal muscle This is the type of muscle that is attached to the bones of skeleton and that provide power for walking and other forms of locomotion Crash Course Video
CONTRACTION OCCURS BY A SLIDING- FILAMENT MECHANISM Muscle contraction is the development of mechanical force. Sliding filament theory current theory of how muscle contracts This name describes what the process looks like and is useful in that way The motions of the filaments is not really sliding as the filaments are using ATP to develop mechanical forces to pull on each other The sliding of the filaments is a consequence of this energy-using, force-generating process Sliding filament video
ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION Muscle cells are called muscle fibers these are synonymous they are large and multinucleate. They form from fusion of embryonic muscle cells called myoblasts. One muscle consists of many muscle fibers bundled together by connective tissue.
A skeletal muscle is made up of many bundles of muscle fibres Each muscle fiber is a multinucleate cell containing numerous myofilbrils, which are highly ordered assemblages of thick myosin and thin actin filaments
ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION Muscle contraction is due to interaction of two contractile proteins: Actin thin filaments (actin filaments) Myosin thick filaments (myosin filaments) Each muscle fiber is made of several myofibrils bundles of actin and myosin filaments arranged in repeating units called sarcomeres.
One sarcomere, bounded by a Z line at each end. Sarcomeres are the units of contraction Cross section of one myofibril near the middle of the sarcomere. This cross section shows how actin and myosin filaments are organized in three dimension
ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION These structures give skeletal muscle a banded appearance.
THE STRUCTURE OF SKELETAL MUSCLE Where there are only actin filaments and titan molecules, the myofibril appears light; where there are both actin and myosin filaments, the myofibril appears dark. Bozeman Muscular System
ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION Each sarcomere starts and ends with a Z line, to which actin filaments attach. Myosin filaments extend out from the M band in the center. H zone and I band no overlap of actin and myosin, so they appear light in color.
ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION Titin the largest protein in the body, runs the full length of the sarcomere. Bundles of myosin filaments are held in the center of the sarcomeres by titin. Each titin molecule runs between the M band and a Z line, directly through a myosin filament.
ACTIN AND MYOSIN FILAMENTS SLIDE IN RELATION TO EACH OTHER DURING MUSCLE CONTRACTION When muscle contracts and shortens, sarcomeres shorten and band pattern changes. The actin and myosin filaments slide past each other, increasing the degree of overlap and shortening the muscle fiber.
ATP REQUIRING ACTIN MYOSIN INTERACTIONS ARE RESPONSIBLE FOR CONTRACTION Myosin molecule: two polypeptide chains coiled together, ending in a globular head A myosin filament is made of many molecules with heads projecting at the sides. Actin filaments are twisted chains of actin monomers. The protein tropomyosin is twisted around each actin filament, with molecules of troponin attached at intervals.
Tropinin has three subunits: one binds actin, one binds tropomyosin, and one binds Ca 2+ Animated tutorial Myosin heads have ATPase activity
ATP REQUIRING ACTIN MYOSIN INTERACTIONS ARE RESPONSIBLE FOR CONTRACTION So how to myosin and actin interact? Myosin and actin interact when the globular heads of myosin bind to actin, forming cross-bridges. When the myosin head binds, it changes shape, which pulls the actin filament towards the M band. It also hydrolyzes ATP, and the released energy causes the myosin head to disconnect and unbend, ready to bind to actin again and repeat the process. Cross Bridges Video Cross Bride Video # 2
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS Muscle cells are excitable the membranes can conduct action potentials (electrical impulses). Ordinarily, the outside of the cell membrane is more positive than the inside. An impulse, or action potential, is a region of reversed polarity, also called depolarization.
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS In an excitable cell, if depolarization is initiated at one point, it travels the full length of the cell membrane:
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS Muscle contraction is initiated by nerve impulses. Each muscle fiber is in contact with an axon of a nerve cell. The point of contact is a neuromuscular junction. Excitation: when a nerve impulse arrives at the neuromuscular junction; an action potential is initiated in the muscle fiber membrane
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS Excitation contraction coupling: process by which excitation of a muscle fiber leads to contraction; Ca 2+ plays an important role Muscle fiber cell membranes extend inward to form T tubules (transverse tubules). Action potentials also travel along the T tubules. T tubules run close to the sarcoplasmic reticulum (endoplasmic reticulum), which surrounds every muscle fiber.
1. Black arrows symbolize an impulse of action potential. When an action potential is an axon arrives at a neuromuscular junction 2. It initiates an action potential in the muscle fiber cell membrane. The action potential spreads along the entire length of the cell membrane, and, as it does so, it spreads down the T tubules 3. Which causes the sarcoplasmic reticulum to release Ca 2+ from its internal stores of Ca 2+ 4. Release Ca 2+ diffuses into sarcoplasm bathing the myofibrils, stimulating muscle contraction. 5. After stimulation by the nerve cell ends, Ca 2+ is taken up from the sarcoplasm by the sarcoplasmic reticulum, terminating muscle contraction.
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS Ca 2+ pumps in the sarcoplasmic reticulum take up Ca 2+ from the sarcoplasm (cytoplasm of muscle cell) and store it inside the sarcoplasmic reticulum.
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS Two proteins span the space between T tubules and sarcoplasmic reticulum and are physically connected. The dihydropyridine (DHP) receptor on the T tubule membrane is voltage-sensitive. The ryanodine receptor in the sarcoplasmic reticulum membrane is a Ca 2+ channel.
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS When an action potential reaches the DHP receptor it changes conformation. Ryanodine receptor then allows Ca 2+ to leave the sarcoplasmic reticulum. Ca 2+ ions diffuse into the sarcoplasm and trigger interaction of actin and myosin and sliding of filaments.
4. In the power stroke, the myosin head changes conformation, bending so that the filaments are forced to slide past one another 8. When excitation stops, Ca 2+ is returned to the SR by Ca 2+ pumps using ATP, and the muscle relaxes. 1. Ca 2+ is released from the sarcoplasmic reticulum 2. Ca 2+ in the SR binds troponin and exposes myosin-binding sites on the actin filaments 6. ATP is hydrolyzed. The myosin head returns to its unbent, extended conformation 7. As long as Ca 2+ remains available, the cycle repeats and muscle contraction continues 5. ADP is released; ATP binds to myosin, causing it to release actin 3. Myosin heads bind to actin; release of Pi initiates power stroke
EXCITATION LEADS TO CONTRACTION, MEDIATED BY CALCIUM IONS When Ca 2+ binds to troponin, a conformation change exposes the myosin binding site on the actin. As long as Ca 2+ remains available, the cycle of myosin binding and release repeats. When excitation ends, calcium pumps remove Ca 2+ from the sarcoplasm. Troponin and tropomyosin return to their original state, tropomyosin blocks binding of myosin heads to actin, and contraction stops.
S K E L E T A L M U S C L E S P U L L O N S K E L E T A L E L E M E N T S T O P R O D U C E U S E F U L M O V E M E N T S 33.2
INTRODUCTION Skeletal systems are the rigid supports against which muscles can pull. Two types of skeletal systems in animals exoskeletons and endoskeletons. Vertebrates deep inside bodies endoskeleton Arthropods skeleton encases the rest of the body - exoskeleton
IN VERTEBRATES, MUSCLES PULL ON THE BONES OF THE ENDOSKELETON Vertebrate endoskeletons are mostly composed of bone an extracellular matrix of collagen fibers with insoluble calcium phosphate crystals. Also has living cells that remodel and repair bones throughout the life of an animal. Bones also serve as a reservoir for calcium. Exchange of calcium with the rest of the body is under control of hormones such as parathyroid hormone and vitamin D.
IN VERTEBRATES, MUSCLES PULL ON THE BONES OF THE ENDOSKELETON Cartilage is flexible skeletal tissue. Joints: where two or more bones come together Muscles attach to bones by bands of flexible connective tissue called tendons, which often extend across joints. In the tuna, forces developed by the swimming muscles are transmitted to the tail by tendons.
The swimming muscles are in the middle of the body Tendons run from the swimming muscles to the tail The tail beats back and forth with enormous strength while the rest of the body is stiff
IN VERTEBRATES, MUSCLES PULL ON THE BONES OF THE ENDOSKELETON Muscles can exert force in only one direction, so they must work in antagonistic pairs when one contracts, the other relaxes. Different sets of muscles work together to control complex movements. Movement generated by contraction of a muscle depends on the state of contraction or relaxation of other muscles.
IN ARTHROPODS, MUSCLES PULL ON INTERIOR EXTENSIONS OF THE EXOSKELETON Arthropods have an exoskeleton composed of chitin that covers the entire body. It is hardened by calcium minerals in crabs and lobsters. The exoskeleton protects the soft tissues, but the animal cannot grow after it is formed. It must be shed periodically and replaced with a larger one.
IN ARTHROPODS, MUSCLES PULL ON INTERIOR EXTENSIONS OF THE EXOSKELETON The hard exoskeleton provides the structure against which muscles can pull. Muscles are attached to inward projections of the exoskeleton called apodemes.
HYDROSTATIC SKELETONS HAVE IMPORTANT RELATIONSHIPS WITH MUSCLE Hydrostatic skeleton: body or a part of the body becomes stiff and skeleton-like because of high fluid pressure inside Earthworms use muscles in the body wall to create a hydrostatic skeleton. When muscles oriented in one direction contract, the fluid-filled body cavity bulges out in the opposite direction.
S K E L E T A L M U S C L E P E R F O R M A N C E D E P E N D S O N AT P S U P P LY, C E L L T Y P E, A N D T R A I N I N G 33.3
INTRODUCTION Skeletal muscles vary in performance. Examples: Postural muscles sustain loads steadily over long periods of time; back and gravity Finger muscles typically contract quickly for brief periods. The rate of doing work (power output) can be only as high as the rate at which ATP is supplied.
MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE S CURRENT RATE OF ATP SUPPLY Muscles have three ways to supply ATP: Immediate system uses preformed ATP and creatine phosphate Glycolytic system synthesizes ATP by anaerobic glycolysis, producing lactic acid Oxidative system synthesizes ATP from food molecules by aerobic respiration
Immediate system: Preformed ATP is immediately available but quickly exhausted Glycolytic System: Anaeorbic glycolysis accelerates its synthesis of ATP to its peak rate within seconds but is self-limiting Oxidative System: Production of ATP by aerobic metabolism ramps up in the first minute and can be sustained indefinitely
MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE S CURRENT RATE OF ATP SUPPLY Immediate system supplies ATP quickly when muscles begin to do work. But only a small amount of preformed ATP is present, so this system works for a brief time. The glycolytic system can supply ATP at a high rate, but it is self-limiting, probably due to lactic acid build-up. Oxidative system produces ATP at a slower rate, but it can be sustained for long periods.
MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE S CURRENT RATE OF ATP SUPPLY In terms of power output: The immediate system permits a muscle cell to reach highest power output The oxidative system permits lowest power output The glycolytic system permits intermediate output In terms of endurance, the three systems rank in the opposite order.
MUSCLE POWER OUTPUT DEPENDS ON A MUSCLE S CURRENT RATE OF ATP SUPPLY Athletics Sports such as sprints require high power output for a short time depend on the immediate system. Long-endurance sports such as marathons must depend on the oxidative system. Jogging and swimming are thus called aerobic exercise.
MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE Different muscles use different systems: Slow oxidative cells (slow-twitch, or red ) Fast glycolytic cells (fast-twitch, or white ) The cell types are usually mixed in one muscle in mammals, but in other animals a muscle may be nearly all red or all white.
MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE Slow oxidative cells use the oxidative system of ATP synthesis. They have high levels of the hemoglobin-like compound myoglobin, which makes them red. Myoglobin speeds entry of O 2 into cells. They also have many mitochondria and the enzymes required for oxidative metabolism. They contract and develop tension slowly, but sustain low power output for long periods.
MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE Fast glycolytic cells have high levels of the enzymes for anaerobic glycolytic ATP synthesis. ATP can be produced rapidly. They contract and develop tension rapidly but also fatigue rapidly. They have relatively few mitochondria and lack myoglobin.
MUSCLE CELL TYPES AFFECT POWER OUTPUT AND ENDURANCE Individuals vary in the proportion of red and white muscle cells. Champion athletes in endurance sports (e.g., long-distance running) have more red cells. Champion sprinters, wrestlers, etc. have more white cells.
Competitors in sustained aerobic events have high proportions of slow oxidative cells
TRAINING MODIFIES MUSCLE PERFORMANCE Skeletal muscle also shows phenotypic plasticity, for instance, muscles become larger with training such as lifting weights. The muscle cells increase the amount of actin and myosin present.
TRAINING MODIFIES MUSCLE PERFORMANCE Endurance training long distance running or cycling Cells increase number of mitochondria. Some cells may transform from fast glycolytic to slow oxidative. Growth of capillaries in muscles is stimulated. Investigators are searching for transcription factors that are upregulated by endurance exercise.
TRAINING MODIFIES MUSCLE PERFORMANCE Resistance exercise generates large forces, such as weight lifting Amount of actin and myosin increases Causes some muscle cells to transform from slow oxidative to fast glycolytic
M A N Y D I S T I N C T I V E T Y P E S O F M U S C L E H AV E E V O LV E D 33.4
VERTEBRATE CARDIAC MUSCLE IS BOTH SIMILAR TO AND DIFFERENT FROM SKELETAL MUSCLE Cardiac muscle is also striated, but cells are smaller than skeletal muscle and have one nucleus. Adjacent cells are electrically connected by gap junctions in intercalated discs. At gap junctions, the sarcoplasms of the two cells are continuous with each other. Two adjacent cells are shown here pulled apart. When together, the pores connect their sarcoplasms, and the overall structure joining them is called an intercalated disc
VERTEBRATE CARDIAC MUSCLE IS BOTH SIMILAR TO AND DIFFERENT FROM SKELETAL MUSCLE Gap junctions allow electrical impulses to spread rapidly, so all cells are excited at about the same time and contract at the same time. Some cardiac cells are modified to generate the heartbeat rhythm. Be Still My Beating Stem Cell Heart
VERTEBRATE SMOOTH MUSCLE POWERS SLOW CONTRACTIONS OF MANY INTERNAL ORGANS Smooth muscle does not appear striated; the filaments are not regularly arranged.
VERTEBRATE SMOOTH MUSCLE POWERS SLOW CONTRACTIONS OF MANY INTERNAL ORGANS Smooth muscle is in most internal organs and in the walls of blood vessels. Smooth muscle cells are arranged in sheets and have electrical contact via gap junctions. Action potential in one cell can spread to all others in the sheet.
SOME INSECT FLIGHT MUSCLE HAS EVOLVED UNIQUE EXCITATION CONTRACTION COUPLING Some insects have evolved asynchronous flight muscles each excitation results in many contractions. At high frequencies of contraction, asynchronous muscle maintains higher efficiency and greater power output than synchronous muscle. It has evolved independently several times and occurs in about 75% of insect species.
MANY DISTINCTIVE TYPES OF MUSCLE HAVE EVOLVED Scallops and clams have powerful adductor muscles to hold the two halves of the shell together.
CATCH MUSCLE IN CLAMS AND SCALLOPS STAY CONTRACTED WITH LITTLE ATP USE The adductor muscles sometimes need to remain contracted for long periods of time when predators are nearby. The muscle enters a specialized state called catch in which high contractile force is maintained with almost no use of ATP. The mechanism of catch is not well understood.
FISH ELECTRIC ORGANS ARE COMPOSED OF MODIFIED MUSCLE Electric fish can produce external high voltage pulses to stun their prey. The electric organs consist of modified muscle cells with little actin or myosin. The tiny voltage differences across the cell membranes of many cells all add together when the cells are excited during an electric pulse.