7DefinitionsSarcolemma: the specific name for the plasma membrane of a muscle cell; surrounded by endomysium.
8DefinitionsMyofibril: contractile organelles found in the cytoplasm of muscle cells.
9DefinitionsSarcomere: tiny contractile units that make up the myofibril; aligned end-to-end.
10DefinitionsMyofilaments: filaments composing the myofibrils; the two types are actin (thin) and myosin (thick).
11Sarcoplasmic reticulum: specialized smooth ER that surrounds myofibrils; stores calcium and releases on demand
12There are also multiple nuclei and mitochondria scattered throughout the muscle fiber, surrounding the myofibrils.
13So, in summary, as you work your way smaller in a muscle…. Muscle – surrounded by epimysiumFascicle – surrounded by perimysiumMuscle fiber (cell) – surrounded by endomysium/sarcolemmaMyofibril – sections called sarcomeres, surrounded by sarcoplasmic reticulum, mitochondria, and nucleiMyofilaments – 2 types are actin & myosin
14The light band is also called the I band, and has a dark midline interruption called the Z disc. The Z disc marks the end of the sarcomere, and is formed by a disc-like membrane.
15The dark band is also called the A band, and has a lighter central area called the H zone (which contains the M line).The H zone lacks thin filament, so it looks a bit lighter than the rest of the A band.
16Notice that the I and A bands are responsible for the striations of skeletal muscle tissue! So, striations are formed by the locations of actin and myosin filaments.
17Zooming in on a sarcomere… The bands of myofilaments are actually formed by many myofilaments packed together.
19THICK FILAMENTS…Contain myosin proteinATPase help split ATP to generate power for muscle contractionForm the “A” bandHave projections that are myosin “heads” – called cross bridgesCross bridges link thick and thin filaments together in contraction of muscle
21THIN FILAMENTS…Contain actin proteinAlso contain tropomyosin protein filaments (which block specific binding sites) and troponin proteinThin filaments anchored to the Z disc (a disc-like membrane separating the sarcomeres)The zone where thin filaments do not exist in the sarcomere is called the H zone
30Nerve endings and muscle fibers don’t physically touch… Neuromuscular juction – where axon terminals match up with muscle fibersSnyaptic cleft – space between nerve endings and muscle fibers; chemical impulses travel here between nerve endings and muscle
31What steps occur to stimulate muscle movement? 1. Nerve impulse reaches axon terminals2. Chemical Neurotransmitter (ACh – acetylcholine) released3. ACh diffuses across synaptic cleft and attaches to receptors
32What steps occur to stimulate muscle movement? 4. ACh causes the sarcolemma to become temporarily permeable to Na+5. Na+ rush into the muscle cell6. Excess of positive ions creates electric current (action potential)7. Muscle contracts (another whole set of steps!)
33So, we know how muscle contraction is stimulated… but now we need to know the steps that help muscle contraction to happen!Called the Sliding Filament Theory
34The Sliding Filament Theory Muscle fibers activated by nervous system due to action potentialCalcium ions (Ca+2) happen to be releasedDo you remember which structure releases those calcium ions???
35The Sliding Filament Theory Muscle fibers activated by nervous system due to action potentialCalcium ions (Ca+2) happen to be releasedDo you remember which structure releases those calcium ions???THE SARCOPLASMIC RETICULUM
37The Sliding Filament Theory Release of Ca+2 allows troposin & tropomyosin to stop blocking binding sites on the actin… which allows the cross-bridges on the thick filaments to attach to the binding sites on the thin filamentsLet the sliding begin!
39The Sliding Filament Theory Energized by energy from ATP, cross-bridges attach and detach from thin filamentsWorks like an oar to keep moving thin filaments closer and closer together (Attach, pull, detach!)
42Net entry of Na+ Initiates an action potential which ADPPiNet entry of Na+ Initiatesan action potential whichis propagated along thesarcolemma and downthe T tubules.T tubuleSarcolemmaSR tubules (cut)SynapticcleftvesicleAxon terminalAChNeurotransmitter released diffusesacross the synaptic cleft and attachesto ACh receptors on the sarcolemma.Action potential inT tubule activatesvoltage-sensitive receptors,which in turn trigger Ca2+release from terminalcisternae of SRinto cytosol.Calcium ions bind to troponin;troponin changes shape, removingthe blocking action of tropomyosin;actin active sites exposed.Contraction; myosin heads alternately attach toactin and detach, pulling the actin filaments towardthe center of the sarcomere; release of energy byATP hydrolysis powers the cycling process.Removal of Ca2+ by active transportinto the SR after the actionpotential ends.SRTropomyosin blockage restored,blocking myosin binding sites onactin; contraction ends andmuscle fiber relaxes.Ca2+123456
43The Sliding Filament Theory As this process is happening in every sarcomere throughout the muscle, the muscle itself is contracting!The whole series of events (beginning with the nervous system signal) takes just a few thousandths of a second!!!
44The Sliding Filament Theory Notice in the contracted muscle, the H zone has disappearedThe I band has shortened significantly (all that’s left is the Z disc)The A band (the dark striations!) have stayed the same thickness
45Where’s all this energy coming from? As a reminder, energy comes from ATP because of breaking a phosphate bondBreaking a bond releases energyWhen this energy is used by your body, it releases heatBecause ATP is the only energy source that can be used to move the cross-bridges back and forth (which contract the muscle), ATP must be regenerated continuously
46ATP Regeneration – 3 Sources Direct phosphorylation of ADP by creatine phosphateWhen ATP used, changes to ADPCreatine phosphate adds that missing phosphorous back on!PROBLEM: only makes 1 ATP at a time… so not very much. And, only supplies energy for seconds of activity!Your body will always do this, but it’s not very effective. Therefore, we have to have other ways of supplying energy…..
47ATP Regeneration – 3 Sources Aerobic respirationOccurs in the mitochondriaGlucose broken down to pyruvic acid (releasing 2 ATP), and then into carbon dioxide and water (releasing 34 ATP)36 ATP made for 1 glucose! A lot of energy! And, can supply energy for hours at a time!PROBLEM: NEEDS OXYGENBut what if you’re out of oxygen??? Then your muscles will begin……..
48ATP Regeneration – 3 Sources Anaerobic glycolysis and lactic acid formationGlucose broken down to pyruvic acid, releasing 2 ATPIf oxygen present, process continues to the rest of aerobic respiration…BUT… if oxygen is inadequate, or muscle activity is intense, pyruvic acid is instead changed to lactic acidPROBLEM: Buildup of lactic acid is not good… promotes muscle fatigue and soreness. And, only supplies energy for 30 seconds of activity!
49ATP Regeneration 95% of ATP produced through aerobic respiration C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O + ATPIf you don’t have proper blood circulation or breathing, muscles can’t get oxygen needed for aerobic respirationIf they can’t get oxygen, they can’t produce enough ATP, which means muscles can’t contract!
50Muscle FatigueA muscle is fatigued when it is unable to contract even though it is being stimulated – means you don’t have ATP to move the cross-bridges!Lack of oxygen can cause…Lactic acid buildup (anaerobic glycolysis)ATP supply low (production can’t keep up with usage)Muscle will contract less and less effectively, eventually stopping contraction completely
51Muscle FatigueFYI: When you breathe heavy after physical activity, your muscles are trying to get enough oxygen for aerobic respiration to replace all of the ATP you used!
53Muscle Contraction Isotonic contractions Myofilaments slide, shortening the muscleMovement occurs – bending knee, rotating arms, smilingIsometric contractionsMyofilaments trying to slide, but can’t – just building up tension (crossbridges are “rowing”, but actin is not moving together)Movement doesn’t occur – object too heavy to lift, push against wall
54Muscle ToneMuscle tone – sustained partial contraction of a muscle; muscle stays firm, healthy, and ready for actionMuscle inactivity can lead to muscle weakness and wasting (this is why Range of Motion exercises on bedridden people is important!)
55Effect of Exercise on Muscles Aerobic exercises include…RunningJoggingBikingElliptical
56Effect of Exercise on Muscles Increase endurance of muscles because muscle cells will form more mitochondria and store more oxygen (meaning more energy for the muscles)Also – improves body metabolism, improve digestion, enhance coordination, strengthens skeleton, heart & lungs more efficientMuscles do NOT increase in size!
57Effect of Exercise on Muscles Resistance exercises include…Pushing against wallContracting muscles (likes gluteus maximus)Lifting weightsDoes increase muscle size!Due to enlargement of individual muscle cells (makes more myofilaments)You don’t add more muscle cells – you just bulk up the ones you already have!!!NEED BOTH TYPES OF EXERCISES IN ANY TRAINING PROGRAM!