Types of muscle Muscular system Cardiac: striated and involuntary. Only in heart Skeletal: striated, voluntary and long. Smooth: smooth and involuntary. In walls of hollow organs. Moves material inside body Characteristics of muscle Contractility shortens Excitability reacts Extensibility can be stretched Elasticity returns to original size Functions of Muscle Contraction allows movement contraction maintains posture Joint stabilization contraction generates heat (by-product of ATP metabolism) Each muscle (e.g. bicep, quad group ) is a bundle of muscle fibers + connective tissue+ nerve and blood vessels Skeletal muscle must have a nerve signal stimulus to contract 1
Skeletal muscle must have a blood supply to bring in nutrients and oxygen, to remove wastes Skeletal muscles cells are large running the entire length of the muscle (organbicep/quad) These large cells are the result of many embryonic cells merging Skeletal muscle cell cytoplasm contains myoglobin- a protein for storing oxygen, and stores of glycogen ( glycosomes ) Skeletal muscle cell endoplasmic reticulum is modified into the sarcoplasmic reticulum a storehouse of calcium (along with other functions) Skeletal muscle contains many (maybe thousands) long protein fibers called myofibrils Muscles (organs- like bicep) are wrapped in layers of connective tissue sheaths (see 278) The outermost wrapping is the epimysium Within the muscle are fascicles (bundles), wrapped in perimysium Each fascicle contains a number of muscle cells or muscle fibers, each individually wrapped in endomysium 2
The smallest functional unit of muscle An 3D arrangement of protein fibers, which as an whole is repeated end to end through the length of a muscle cell to form a myofibril Fibers include: Myosin proteins shaped like 2 golf clubs with handles wrapped against each other, heads protruding (see 279,282) These wrapped or braided proteins are then bunched, creating a bundle of the handles with many protruding heads. (together called the thick filament ) Fibers include: Thin filament Actually a composite of 3 separate structures: actin, troponin, tropomyosin Thin filament The actin component has subunits, each with a binding site for the myosin heads like a twisted double strand of pearls -281 Thin filament Tropomyosin- in the relaxed muscle- blocks actins' binding sites for myosin Thin filament Troponin is a combination of 3 proteins. One of these is a binding site for calcium. When calcium binds to troponin, it causes a position change in tropomyosin, removing the block to myosin binding sites 3
Fibers include: Elastic filaments titin Attached from Z disk through the thick filament to the M line. Helps the sarcomere return to resting shape from stretch or compression A sarcomere M line I band A band I band Z disk Z disk Z disk Z disk Myosin myosin Myosin myosin Myosin myosin Myosin myosin Z disk Z disk Z disk Z disk Muscle related terms: Sarcolemma: the membrane of the muscle T (transverse) tubules: holes or channels through the sarcolemma that pass all the way through. These contain extra-cellular fluid and help transmit the contraction signal to all the sarcomeres in the myofibril Cisternae: storehouses of intercellular Ca +. Part of the sarcoplasmic reticulum network. See p. 283 Depolarization Stimulation of the muscle cell by the motor neuron results in depolarization Depolarization The inside of the muscle cell is negative relative to the outside of the cell. There are relatively more negatively charged ions than positively charged ions Depolarization Acetylcholine binding at the motor end plate causes ion channels to open, and both Na and K move (in opposite directions) but More Na goes in than K goes out 4
Depolarization The membrane polarity changes first right around the motor end plate. This causes the opening of adjacent voltage gated channels (this means the key to opening is the voltage of the membrane). so the polarity changes nearby, triggering the next adjacent channels Depolarization repolarization The change in polarity causes Na channels to close and K channels to open--- Now K+ flows out, restoring polarity. Then, Na/K pumps (dependent on ATP) pump the ions back to their original sides. Summary: see 286, 287, 290, 291 Definition: Motor units A motor unit consists of a motor neuron and all the muscle fibers it innervates. Definition: Motor units Recruitment is the addition of motor units to equal or exceed the resistance All-or-none principle: A muscle cell will, when activated, contract to the greatest degree possible. If not activated, it does not contract at all. Muscle cells do not partially contract 5
Faster than a twitching muscle.. A twitch is a single signal cycle (say that 5 times fast) This happens pretty fast-- 1/10th to 1/100th of a second. Some Muscle Definitions: (see 297) Latent period: The (brief) pause between initiation of the action potential and muscle contraction. Contraction phase: Cross-bridge formation and shortening of muscle Relaxation phase: Cross-bridges decrease in #; calcium is removed from sarcoplasm. Smoothing the movements Graded muscle responses Rather than appearing to be twitchy most of our movements are smooth. Smoothing the movements Graded muscle responses Rather than appearing to be twitchy most of our movements are smooth. This is accomplished by managing 2 variablesthe frequency of the stimulus and the strength of the stimulus Smoothing the movements Stimulus frequency: if a muscle is sent the message to contract twice in quick succession, the 2 nd response results in greater contraction, since the muscle has not completely relaxed from the 1 st contraction. This is called wave summation - the additive effect of the consecutive stimuli more Some Muscle Definitions: Imagine a twitch, followed by another, followed by a third, with little time for the muscle to relax in between. The tension will increase with each twitch, to some maximal tension level This is called summation Think of kicking a soccer ball, while walking up a hill. You kick the ball, it goes up hill, you walk up after it while it starts to roll back down. You kick it again, this time from higher up the hill, so it progresses a little farther each time 6
more Some Muscle Definitions: If peak muscle tension is achieved by these cycles of contraction incomplete relaxation contraction it is called incomplete tetanus more Some Muscle Definitions: If these twitches are in such rapid succession that there is no time for relaxation between stimuli is said to be in complete tetanus Smoothing the movements Stimulus strength: a stronger stimulus activates more motor units within the muscle (organ) and therefore more contractile force. The size principle states that smaller motor units are more easily excitable, which is why a smaller stimulus results in less contraction a smaller number of muscle fibers were activated. (see 299) Types of contractions A muscle can shorten while being activated -a concentric contraction A muscle can lengthen while being activated -an eccentric contraction iso"= same, metric = measure or length, tonic = tone or force Types of contractions A muscle can activate and have some change in length -an isotonic contraction a muscle can activate and not change in length -an isometric contraction ATP- adenosine triphosphate: a bond is broken, with the stored energy being utilized by other systems In muscles, is required for (at least) unlocking the myosin head from actin, powering the Na/K pump for returning to resting potential and pumping Ca back into the SR 7
Creatine Phosphate: donates a phosphate to replenish dwindling ATP supplies. This is an anaerobic process, being sustained only a few seconds. energy yielding nutrients Fats Carbs Protein Examine figure p.20 on 303 for energy usage patterns comparing activity to fuel source Glucose: as glycogen stores: These are what fuel short duration, high intensity workouts. High carbohydrate diets apparently support storage of more glycogen, being supportive of long duration workouts. Glucose breaks down to pyruvate. In the absence of mitochondria/o 2 capacity, pyruvate is converted to lactic acid. (glance at appendix page A-5,A-6, A-7) Rising acidity will reduce muscle function Lactic acid is carried through the blood to the liver, where it is converted to glucose, then put back into circulation. 8
Glycogen stores supply energy for 1st part of strenuous activity. Fat use increases. With training, glycogen stores increase, reliance on fat increases, efficiency of energy use increases The liver makes new glucose from amino acids and lactic acid - fat Fat cells release free fatty acids to the blood, which are picked up by cells requiring fuel. As blood levels fall, epinephrine (from adrenals) causes release of more FAs With training, fat is accessed more efficiently, and relied on for fuel more. Activities of greater than 20 minutes are those more likely to reduce fat stores. - protein Generally, proteins best use is for building structures. It is the least used for energy in regular exercise High carbohydrate diets tend to spare protein. Aerobic athletes training towards greater endurance increase use of protein for fuel. 9
Anaerobic athletes (bodybuilders) tend to decrease protein use (for fuel) with training. Protein requirements for building strength still exist. Levers Levers are tools to make work easier A lever system consists of a force (or effort), a weight (or load) and a fulcrum The relative position of these 3 elements determines the class of the lever As usual, you want 1st class when you can get it! 10