Definitions Sarcolemma: the specific name for the plasma membrane of a muscle cell; surrounded by endomysium.
Definitions Myofibril: contractile organelles found in the cytoplasm of muscle cells.
Definitions Sarcomere: tiny contractile units that make up the myofibril; aligned end-to-end.
Definitions Myofilaments: filaments composing the myofibrils; the two types are actin (thin) and myosin (thick).
Sarcoplasmic reticulum: specialized smooth ER that surrounds myofibrils; stores calcium and releases on demand
There are also multiple nuclei and mitochondria scattered throughout the muscle fiber, surrounding the myofibrils.
So, in summary, as you work your way smaller in a muscle…. Muscle – surrounded by epimysium Fascicle – surrounded by perimysium Muscle fiber (cell) – surrounded by endomysium/sarcolemma Myofibril – sections called sarcomeres, surrounded by sarcoplasmic reticulum, mitochondria, and nuclei Myofilaments – 2 types are actin & myosin
The 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.
The 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.
Notice 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.
Zooming in on a sarcomere… The bands of myofilaments are actually formed by many myofilaments packed together.
THICK FILAMENTS… Contain myosin protein ATPase help split ATP to generate power for muscle contraction Form the “A” band Have projections that are myosin “heads” – called cross bridges Cross bridges link thick and thin filaments together in contraction of muscle
THIN FILAMENTS… Contain actin protein Also contain tropomyosin protein filaments (which block specific binding sites) and troponin protein Thin 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
What functional properties allow a muscle to perform its duties? Irritability –Ability to receive and respond to a stimulus Contractility –Ability to shorten when adequate stimulus is received
What functional properties allow a muscle to perform its duties? Conductivity –Ability for impulse to travel along plasma membrane of muscle cell Elasticity –Ability to recoil and resume original length
What role does the nervous system play in muscle movement? Motor Unit – one neuron and all the skeletal muscle cells it stimulates
Within a motor unit… Muscle Fibers Axons Axon Terminals (neuromuscular junctions)
Nerve endings and muscle fibers don’t physically touch… Neuromuscular juction – where axon terminals match up with muscle fibers Snyaptic cleft – space between nerve endings and muscle fibers; chemical impulses travel here between nerve endings and muscle
What steps occur to stimulate muscle movement? 1. Nerve impulse reaches axon terminals 2. Chemical Neurotransmitter (ACh – acetylcholine) released 3. ACh diffuses across synaptic cleft and attaches to receptors
What steps occur to stimulate muscle movement? 4. ACh causes the sarcolemma to become temporarily permeable to Na + 5. Na + rush into the muscle cell 6. Excess of positive ions creates electric current (action potential) 7. Muscle contracts (another whole set of steps!)
So, 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
The Sliding Filament Theory Muscle fibers activated by nervous system due to action potential Calcium ions (Ca +2 ) happen to be released –Do you remember which structure releases those calcium ions???
The Sliding Filament Theory Muscle fibers activated by nervous system due to action potential Calcium ions (Ca +2 ) happen to be released –Do you remember which structure releases those calcium ions??? –THE SARCOPLASMIC RETICULUM
The 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 filaments Let the sliding begin!
The Sliding Filament Theory Energized by energy from ATP, cross-bridges attach and detach from thin filaments Works like an oar to keep moving thin filaments closer and closer together (Attach, pull, detach!)
http://highered.mcgraw- hill.com/sites/0072495855/stu dent_view0/chapter10/animati on__sarcomere_contraction.ht ml
ADP Pi Net entry of Na + Initiates an action potential which is propagated along the sarcolemma and down the T tubules. T tubule Sarcolemma SR tubules (cut) Synaptic cleft Synaptic vesicle Axon terminal ACh Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca 2+ release from terminal cisternae of SR into cytosol. Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process. Removal of Ca 2+ by active transport into the SR after the action potential ends. SR Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fiber relaxes. Ca 2+ 1 2 3 4 5 6
The 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!!!
The Sliding Filament Theory Notice in the contracted muscle, the H zone has disappeared The I band has shortened significantly (all that’s left is the Z disc) The A band (the dark striations!) have stayed the same thickness
Where’s all this energy coming from? As a reminder, energy comes from ATP because of breaking a phosphate bond Breaking a bond releases energy When this energy is used by your body, it releases heat Because 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
ATP Regeneration – 3 Sources Direct phosphorylation of ADP by creatine phosphate –When ATP used, changes to ADP –Creatine phosphate adds that missing phosphorous back on! –PROBLEM: only makes 1 ATP at a time… so not very much. And, only supplies energy for 15-20 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…..
ATP Regeneration – 3 Sources Aerobic respiration –Occurs in the mitochondria –Glucose 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 OXYGEN –But what if you’re out of oxygen??? Then your muscles will begin……..
ATP Regeneration – 3 Sources Anaerobic glycolysis and lactic acid formation –Glucose broken down to pyruvic acid, releasing 2 ATP –If 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 acid –PROBLEM: Buildup of lactic acid is not good… promotes muscle fatigue and soreness. And, only supplies energy for 30 seconds of activity!
ATP Regeneration 95% of ATP produced through aerobic respiration C 6 H 12 O 6 (aq) + 6O 2 (g) → 6CO 2 (g) + 6H 2 O + ATP If you don’t have proper blood circulation or breathing, muscles can’t get oxygen needed for aerobic respiration If they can’t get oxygen, they can’t produce enough ATP, which means muscles can’t contract!
Muscle Fatigue A 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
Muscle Fatigue FYI: 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!
Muscle Contraction Isotonic contractions –Myofilaments slide, shortening the muscle –Movement occurs – bending knee, rotating arms, smiling Isometric contractions –Myofilaments 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
Muscle Tone Muscle tone – sustained partial contraction of a muscle; muscle stays firm, healthy, and ready for action Muscle inactivity can lead to muscle weakness and wasting (this is why Range of Motion exercises on bedridden people is important!)
Effect of Exercise on Muscles Aerobic exercises include… –Running –Jogging –Biking –Elliptical
Effect 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 efficient Muscles do NOT increase in size!
Effect of Exercise on Muscles Resistance exercises include… –Pushing against wall –Contracting muscles (likes gluteus maximus) –Lifting weights Does 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!