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Muscles Muscle cells are elongated Cells are called muscle fibers myo- and sarco- = muscle Cell membrane is sarcolemma cytoplasm is sarcoplasm Muscles Skeletal- striated, contracts rapidly, tires easily, multinucleate Cardiac- striated, intercalated disks, uninucleate Smooth- uninucleate, found in walls of organs Functions Movements Maintain posture Stabilize joints Heat- Skeletal muscle mass is 40 % of body mass- generates the most heat Characteristics Excitability Contractibility Extensibility Elasticity Structure Each fiber is surrounded by sheath of areolar CT= endomysium Fibers are gathered into bundles (fascicles) covered by collagen sheath called perimysium Fascicles are bound together by close fibrous CT called epimysium Structure Fibrous CT called fascia binds the epimysium to other muscle or bone Each skeletal muscle fiber has its own nerve ending Blood vessels supply each muscle fiber- muscle cells have constant supply of O2 and give off waste Attachments Muscles are attached at two locations Insertion at the movable bone Origin at immovable bone Direct and Indirect Attachments Direct attachments- epimysium is fused to the periosteum of a bone or the perichondrium of cartilage Indirect Attachments Fascia extends beyond the muscle as a tendon, or an aponeurosis- anchors the muscle to a bone, or to facia of other muscles Most common in body Fascicles Arranged in different patterns Parallel Pennate- fascicles are short and oblique to central spinelooks like a feather- can have unipennate and bipennate Convergent- all fascicles converge on single tendon- looks triangular Fascicles Patterns determine power Skeletal muscle shortens to 70 % of resting length Longer parallel fibers give greater range of motion, but not much power Power actually depends on number of cells Pennate pattern has most cells It doesn’t shorten as much, but has most power Structure Muscle cell is long and cylindrical 1-40 mm long, .01- .1 mm wide Multinucleate Sarcoplasm is similar to other cytoplasms but has larger amounts of stored glycogen and myoglobin Each cell contains myofibrils Myofibrils Contractile element of cell Comprise 80 % of volume of cell Contain smaller subunits called sarcomeres There are repeating sequences of alternating light and dark bands Myofibrils Dark bands are A bands (anisotropic) Polarize visible light Light bands are I (isotropic) Nonpolarizing Each A band has a light stripe down the middle (H band) (Hell band) H zones are only visible in relaxed cells Myofibrils Each H zone is bisected by a dark line- M line I band has a midzone line called Z line Sarcomere is inbetween Z lines Myofibril consists of two types of protein myofilaments Central thick filaments extends the length of the A band- myosin Myofibrils Thin filaments extend across the I band and partway into A band- actin Z line serves as point of attachment for actin- connects sarcomeres H zone in middle of A band has thin filaments that connect the thick filaments M band Myosin Each molecule has a tail and two heads The heads form cross- bridges to the actin Each thick filament contains 200 myosin molecules The heads contain ATP and ATP ases Actin Thin filaments contain binding sites which connect to myosin cross- bridges Contains regulatory proteins Tropomyosin- helps to stiffen actin chains +2 Troponin- binds Ca ions Sarcoplasmic Reticulum Similar to ER Surrounds myofibrils and forms terminal cisternae (saclike channels) in regions of A-I junctions At each junction, the sarcolemma dives into the cell interior to form a hollow tube called the T tubule Continuous with cell exterior and interior Sarcoplasmic Reticulum T tubule provides canals for fluids and ions to penetrate into the cell SR helps to regulate Ca+2 Contraction Filaments do not change length- they slide Actin slides past myosin filaments, causing the distance between the Z lines to shorten H zone disappears A bands move closer together Contraction Cross bridges on the myosin attach to the actin and then detach several times, like a ratchet +2 Attachment of the cross bridges requires Ca +2 Without Ca , the muscle cell is relaxed and the myosin binding sites on actin are blocked by tropomyosin Contraction With Ca+2, Ca+2 binds to troponin Troponin undergoes conformational change Causes tropomyosin to roll away from the binding site Causes exposure of myosin binding sites on actin, allowing binding of actin to myosin Contraction 1. Formation of cross bridges causes change of myosin from a high- energy conformation to a low energy, bent form 2. Formation of the bent form causes actin to slide to middle of the sarcomere ADP and Pi formed earlier are released from the myosin head Contraction 3. New ATP attaches to the myosin, causing the myosin to detach from the actin 4. ATP is hydrolyzed by ATPase, releasing energy and causing the mysoin head to cock (high energy position) Now we are back to the starting point Contraction Each stroke is only a 1 % shortening Total shortening is 30- 35 % Need many strokes Rigor Mortis Muscles begin to stiffen 3-4 hours after death Peak is reached 12 hours after death, then dissipates over next 48-60 hours +2 Dying cells cannot exclude Ca - ion flow promotes cross bridge formation Can’t make new ATP, so linkage is semi-permanent- later things break down Regulation of Contraction Nerve stimulates each skeletal muscle cell by propagating an action potential Nerve comes from brain or spinal cord, but axon reaches out to muscle Axon divides and fragments as it enters the muscle Each axonal ending goes to muscle fiber Contraction Cell membrane of axon does not touch muscle, but are separated by synapse Within axonal ending are synaptic vessicles containing acetylcholine Motor end plate in muscle contains Ach receptors +2 Nerve impulse reaches end of axon, opening Ca channels in membrane Contraction Ca+2 flows in from extracellular fluid, causing vessicles to release Ach into synapse Ach attaches to Ach receptors in sarcolemma Triggers action potential in sarcolemma Action Potential Resting cell is polarized- -60mV across the membrane + + K is on the inside, Na on the outside + + Attachment of Ach molecules opens Na gates and Na enters the cell Interior becomes less negative= depolarization and action potential is generated, if strong enough Action Potential 1. Depolarization- Na+ enters the cell and reduces the negative charge 2. Propagation- Positive charges inside cell spread to adjacent negative charges 3. Repolarization- Na+ gates close and K+ gates open + K leaves the cell Interior becomes more negative Action Potential During repolarization, cell is in refractory state- no further stimulus is possible Ionic balance has been restored- not the chemical balance + + Na - K pump restores the chemical balance Contraction All or nothing response for muscle cell Action potential is brief- 1-2 ms Contraction of muscle lasts 100ms ACh must be destroyed by Acetylcholinesterase AChE Curare binds to ACh receptors, cannot receive ACh Excitation- Contraction Coupling Period between action potential initiation and the beginning of muscle contraction= latent period 1. Action potential propagates down the T tubules, from ACh receptors 2. Terminal cisternae release Calcium into sarcoplasm Coupling 3. Calcium binds to troponin, causing tropomyosin to move 4. Filaments slide 5. Calcium signal ends, because calcium pump removes calcium 6. With low Ca+, tropomyosin moves back Contraction Sequence is repeated when new nerve impulse arrives Levels of calcium rise for stronger stimulus, and muscle cells don’t relax completely Calcium concentrations are regulated by calsequestrin, which releases it, and calmodulin, which binds it Motor Unit Each muscle is served by at least one motor neuron with hundreds of axonal endings One motor neuron and all the fibers that are served is a motor unit Average number of fibers is 150 Muscle Twitch Response of muscle to a single stimulus- twitch Three phases: 1. Latent period- no response seen 2. Period of contraction- 10- 100 ms 3. Period of relaxation- muscle tension relaxes Twitch Usually do not have one twitch- have waves of contraction Continuous contraction is tetanus Successive contractions are often stronger than earlier ones Heat has been released which allows enzyme systems to be more efficient Warm- up for sports- Called Treppe effect Muscle Tone Muscles are usually in some state of contraction Maintains fitness Spinal reflexes activate various groups of muscles ATP Muscles require ATP, but store very little Generate ATP in three ways: 1. ATP already in muscle is used up in 6 seconds High energy molecule, creatinine phosphate + ADP -> creatinine + ATP Energy lasts an additional 10 seconds ATP 2. Aerobic respiration- occurs in mitochondria - glu +O2-> water + carbon dioxide 3. Anaerobic respiration (glycolysis) can provide enough ATP for 30- 40 seconds Produces lactic acid Total energy can last for about a minute Fibers Force of contraction is affected by the number of fibers stimulated, and the size of the muscle The greater the cross section affected, the more power Fibers 1. Red slow- twitch- thin cells with slow acting myosin ATPases and contract slowly Color in red- twitch fibers comes from Fe in myoglobin Use lots of oxygen Fatigue resistant, high endurance Fibers 2. White fast- twitch- pale, wide fibers with fast- acting myosin ATPases Contract rapidly and powerfully Tires out Depend on glycolysis for ATP Fibers 3. Intermediate fast- twitch- can be red or pink Intermediate in size between the other two Have fast- acting ATPases like the white fibers, but require oxygen like the red fibers Fibers Can contract quickly, but not as fast as white fibers Fatigue resistant, but not as much as red fibers Marathon runners have 80 % slow twitch fibers Sprinters have 60 % fast twitch fibers Smooth Muscle Makes up walls of organ, except for heart Fibers are small 2-10 um wide and 50-200 long Skeletal is 1-40 mm long and 10- 100 um wide Uninucleate Poorly developed SR Nonstriated Characteristics Ratio of thick to thin filaments is lower in smooth than in skeletal No troponin- smooth muscle is always ready for contraction No sarcomeres Fibers occur in sheets, usually at cross angles, longitudinal and circular Allows peristalsis Characteristics Entire sheet responds to a single stimulus Individual cells can relay signal from cell to cell Contraction is slow and long, resistant to fatigue Low energy requirements, few mitochondria, uses anaerobic respiration Smooth Muscle Can stretch more than skeletal muscles-no sarcomeres Skeletal can contract or stretch 30 %- total 60 % Smooth can double in length (200 %) or contract to half (50 %) Miscellaneous Skeletal muscle is 36 % of body weight in women, 42 % in men Muscular dystrophy- lacks dystrophin- muscles atrophychild needs wheelchair by age 12- dies in 20’s Aging- CT in muscles increase, # fibers decrease, muscles become stringy Lose 50% muscle strength by 80- smooth muscle is ok
