Anatomy & Physiology Final Study Guide Chapter 10 Remember: ➔ -lemma = sheath ➔ 1 micrometer (µm) = one millionth of a meter ➔ 1 nanometer (nm) = one billionth of a meter ◆ 1 µm = 1,000 nm ➔ Invagination: a cavity or pouch formed by being turned inside out or folded back ➔ Hydrolysis: the chemical breakdown of a compound due to reaction with water ➔ Catalyze: cause or accelerate (a reaction) by acting as a catalyst ➔ Catabolism: the breakdown of complex molecules in living organisms to form simpler ones, together with the release of energy ➔ Varicose veins are enlarged, twisted veins (most common in legs) 3 Types of Muscle Tissue and their Characteristics ★ Skeletal ○ Move the bones of the skeleton ○ Striated ■ Alternating light and dark protein bands ○ Voluntary (some subconscious, ex: diaphragm, posture) ★ Cardiac* ○ Only in heart (wall) ○ Striated ○ Involuntary, unconscious, “autorhythmicity” ■ Can be adjusted by hormones & neurotransmitters ★ Smooth* ○ In the walls of hollow internal structures ■ Blood vessels ■ Airways ■ Abdominopelvic organs ○ Also in skin and hair follicles ○ Not striated (that’s why it’s “smooth”) ○ Involuntary ■ Digestive canal = autorhythmic * Both cardiac muscle and smooth muscle are regulated by neurons that are part of the autonomic (involuntary) division of the nervous system and by hormones released by endocrine glands. 4 Characteristics (Properties) of Muscle Tissues ★ Electrical excitability ○ Ability to respond to stimuli ■ Produces action potentials (electrical signals) ○ Muscle and nerve cells ■ Muscle action potentials ● 2 main types of stimuli that trigger action potentials ○ Autorhythmic electrical signals ■ Arise in muscular tissue ■ Heart’s natural pacemaker ○ Chemical stimuli ■ Neurotransmitters released by neurons ■ Hormones distributed by the blood ■ Local changes in pH Nerve action potentials ■ ★ Contractility ○ Ability to contract forcefully when stimulated by a nerve impulse ○ Generates tension (force of contraction) & pulls on attachment points ■ Muscle shortens = movement (ex: lifting a book) ■ Some contractions don’t shorten (ex: holding book in outstretched arm) ★ Extensibility ○ Ability to stretch, within limits, without being damaged ■ Connective tissue within muscle limits the range of extensibility and keeps it within the contractile range of the muscle cells ○ Smooth muscle = greatest amount of stretching ■ Stomach fills with food ○ Cardiac muscle stretches each time the heart fills with blood ★ Elasticity ○ The ability of muscular tissue to return to its original length and shape after contraction or extension 6 Functions of the Muscular System ★ Producing body movements ○ Whole body ■ Walking ■ Running ○ Localized ■ Grasping a pencil ■ Keyboarding ■ Nodding the head ★ Stabilizing body positions ○ Stabilize joints ○ Maintain body positions ■ Standing ■ Sitting ○ Postural muscles contract continuously when you are awake ■ Sustained contractions of neck muscles hold head upright ★ Storing and moving substances within the body ○ Sphincters ■ Ringlike bands of smooth muscle ■ Sustained contraction of sphincters = storage ■ Prevent outflow of the contents of a hollow organ ● Stomach ● Urine ● Anal sphincter ○ Cardiac muscle contractions ■ Wall of the heart ● Pump blood through vessels ■ Wall of blood vessels ● Adjust blood vessel diameter ● Regulate rate of blood flow ○ Smooth muscle contractions ■ Move food and substances through digestive canal ● Bile ● Enzymes ■ Push gametes (sperm & oocytes) through genital system passageways ■ Propel urine through urinary system ○ Skeletal muscle contractions ■ Promote the flow of lymph plasma ■ Aid the return of blood in veins to heart ★ Generating heat ○ Thermogenesis ■ Muscular tissue contracts = produces heat ○ Most heat generated by muscle is used to maintain normal body temperature ○ Shivering ■ Involuntary contraction ■ Increases the rate of heat production ★ Only 4 listed in textbook? Macroanatomy of a Muscle ★ Connective tissue (CT) surrounds and protects muscular tissue ★ A skeletal muscle consists of individual muscle fibers bundled into muscle fascicles and surrounded by three connective tissue layers that are extensions of the fascia. ★ Fascia is a dense sheet or broad band of irregular connective tissue ○ Lines the body wall and limbs ○ Supports and surrounds muscles and other organs ○ Holds muscles with similar functions together ○ Allows free movement of muscles ○ Carries nerves, blood vessels, and lymphatic vessels ○ Fills spaces between muscles ★ Three layers of CT extend from fascia to protect and strengthen skeletal muscle ○ Epimysium ■ Outer layer ■ Encircles entire muscle ■ Dense irregular CT ○ Perimysium ■ Dense irregular CT ■ Surrounds groups of 10 to 100+ muscle fibers ● Bundles called fascicles ● Large enough to be seen with naked eye ● Characteristic “grain” of a cut of meat ○ Endomysium ■ Penetrates the interior of each muscle fascicle ■ Separates individual muscle fibers from one another ■ (mostly) Reticular fibers ○ All three layers are continuous with the connective tissue that attaches skeletal muscle to other structures, such as bone or another muscle. ■ Form ropelike tendon ● A white fibrous cord of dense regular connective tissue that attaches muscle to bone. ● Ex: calcaneal (Achilles) tendon of the gastrocnemius (calf) muscle, which attaches the muscle to the calcaneus (heel bone) ■ Or an aponeurosis ● A sheetlike tendon joining one muscle with another or with bone. ● When the CT extend as a broad, flat sheet ● Ex: epicranial aponeurosis on top of the skull between the frontal and occipital bellies of the occipitofrontalis muscle Microanatomy of a Muscle ★ Myoblasts ○ The fusion of 100+ small mesodermal cells called myoblasts fuse during embryonic development to form skeletal muscle fibers ○ Therefore, each mature skeletal muscle fiber has 100 or more nuclei ○ Once fusion occurs, muscle fibers cannot undergo cell division ■ Therefore, the # of skeletal muscle fibers is set before you are born ○ A few myoblasts persist in mature skeletal muscle as satellite cells ■ Satellite cells retain the capacity to fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers ● When the number of new skeletal muscle fibers that can be formed by satellite cells is not enough to compensate for significant skeletal muscle damage or degeneration, the muscular tissue undergoes fibrosis ○ The replacement of muscle fibers by fibrous scar tissue ★ Sarcolemma ○ Plasma membrane of a muscle fiber ○ Multiple nuclei of a skeletal muscle fiber are located just beneath sarcolemma ★ Sarcoplasm ○ Cytoplasm of a muscle fiber ○ Located within sarcolemma ○ Includes substantial amount of glycogen ■ Large molecule composed of many glucose molecules ■ Can be used for ATP synthesis ★ Myofibrils ○ Contractile organelles of skeletal muscle ○ Appear as little threads ○ Diameter = about 2 micrometers ○ Extend the entire length of muscle fiber ○ Prominent striations make entire skeletal muscle fiber appear striated ★ Myofilaments (also called filaments) ○ Thin filaments ■ Diameter = 8 nanometers ■ Length = 1-2 micrometers ■ Composed of the protein actin ● Individual actin molecules join to form an actin filament that is twisted into a helix ● Each actin molecule has a myosin-binding site ■ Also contain smaller amounts of two regulatory proteins ● Tropomyosin ○ In a relaxed muscle: ■ Blocks myosin from binding to actin ■ Strands of tropomyosin cover myosin-binding sites on actin ■ Strands held in place by troponin molecules ● Troponin ○ When calcium ions (Ca2+) bind to troponin: ■ Changes shape ■ Moves tropomyosin away from myosin-binding sites on actin molecules ■ Myosin binds to actin = muscle contraction begins ○ Thick filaments ■ Diameter = 16 nanometers ■ Length = 1-2 micrometers ■ Composed of the protein myosin ● Functions as a motor protein in all 3 types of muscle tissue ○ Skeletal muscle - 300 myosin molecules = 1 thick filament ● Production of force = Motor proteins pull various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion ● Shaped like two golf clubs twisted together ○ Myosin tail = golf club handles ○ Myosin heads = projections, or golf club heads ■ 2 binding sites ● Actin-binding site ● ATP-binding site ○ Both are involved in contractile process ○ 2 thin filaments for every thick filament (in regions of filament overlap) ○ Filaments are inside myofibrils ■ Do not extend entire length of a muscle fiber ■ Arranged into compartments called sarcomeres ● Basic functional units of a myofibril ● Structure of Sarcomeres (organized into bands and zones) ○ Z discs separate sarcomeres ■ Narrow, plate shaped regions of dense protein ○ ○ ○ ○ ○ ★ Titin ○ ○ ○ ○ ○ ■ A sarcomere extends from one Z disc to the next A band ■ Extends entire length of thick filaments ■ Darker middle part of sarcomere ■ Zone of overlap ● Toward each end of A band ● Thick and thin filaments lie side by side I band ■ Contains the rest of the thin filaments, no thick ■ Lighter, less dense area ■ Z disc passes through the center of each I band ■ “I” is “thin” = thin filaments Alternating dark A bands and light I bands create the striations seen in myofibrils and whole skeletal and cardiac muscle fibers. H band ■ Contains thick filaments, no thin ■ In the center of each A band ■ Thick filaments are held together in the center of the H band by supporting proteins ■ “H” is “thick” = thick filaments M line ■ The supporting proteins that hold the thick filaments together at the center of the H band form the M line ■ M = Middle A structural protein ■ Contributes to the alignment, stability, elasticity, and extensibility of myofibrils Third most plentiful protein in skeletal muscle ■ After actin and myosin 50x larger than an average-sized protein ■ Molecular mass = about 3 million daltons Each titin molecule spans half a sarcomere (1 to 1.2 µm) Can stretch to at least 4x its resting length then spring back unharmed ■ “Elastic filament” ■ Titin accounts for much of the elasticity and extensibility of myofibrils ■ May help prevent overextension of sarcomeres ★ T tubules ○ Tiny tube-shaped invaginations of the sarcolemma ○ Thousands tunnel in from the surface toward the center of each muscle fiber ○ Open to the outside of the fiber ○ Filled with interstitial fluid ○ Muscle action potentials travel along the sarcolemma and through T tubules ■ Quickly spread throughout muscle fiber ■ Ensures that an action potential excites all parts of the muscle fiber at the same time ★ Sarcoplasmic reticulum ○ Fluid-filled system of membranous sacs ○ Encircle each myofibril ○ This system is similar to smooth ER in nonmuscular cells ○ Relaxed muscle fiber ■ Stores calcium ions (Ca2+) ○ Muscle contraction ■ Release of Ca2+ from terminal cisterns ★ Terminal cisternae ○ Dilated end sacs of the sarcoplasmic reticulum ■ “Butt” against the T tubule from both sides ★ Triad ○ A T tubule and the two terminal cisterns on either side of it Sliding Filament Model of Contraction The “story” of how a muscle contracts starting with the Action Potential from a motor neuron through the myosin attaching to actin and pulling it toward the M-line. ★ Sliding filament mechanism ○ The lengths of filaments are the same in relaxed and contracted muscle ○ Skeletal muscle shortens because the filaments slide past each other ○ Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line ○ Thin filaments slide inward & meet at center of sarcomere ■ They may even move so far inward that their ends overlap ■ I band and H zone narrow, then disappear when max contracted ■ Width of A band remains unchanged ○ Thin filaments are attached to Z discs ■ Slide inward = Z discs come closer together ■ Sarcomere shortens = shorten whole muscle fiber ■ Entire muscle shortened The Contraction (Cross Bridge) Cycle ★ Onset of contraction ○ Sarcoplasmic reticulum releases calcium ions (Ca2+) into the sarcoplasm ○ Calcium ions bind to troponin ○ Troponin moves away from the myosin-binding sites on actin ○ Once the binding sites are “free” the contraction cycle begins ■ A repeating sequence of events that causes the filaments to slide ★ 4 steps of contraction cycle 1. ATP hydrolysis ■ Backstory: Myosin head ATP-binding site functions as an ATPase ● ATPase = enzyme that hydrolyzes ATP into ADP ○ Adenosine diphosphate ○ Phosphate group ■ The energy generated from this hydrolysis reaction is stored in the myosin head for later use during the contraction cycle. ● Stored energy = energized ● Energized head = “cocked” position (90° angle) ■ Products of ATP hydrolysis still attached to the myosin head ● ADP ● Phosphate group 2. Attachment of myosin to actin ■ Energized myosin head attaches to myosin-binding site on actin ■ Releases previously hydrolyzed phosphate group ■ Myosin head = Cross-bridge = when a myosin head attaches to actin ■ Although a single myosin molecule has a double head, only one head binds to actin at a time. 3. Power stroke ■ After cross-bridge ■ Myosin head pivots ● Position changes from a 90° angle to a 45° angle ■ Myosin head pulls thin filament past the thick filament towards the center ● Generates tension (force) ■ Energy required comes from the energy stores in the myosin head from the hydrolysis of ATP ■ After power stroke ● ADP is released from myosin head 4. Detachment of myosin from actin ■ Cross bridge remains attached to actin ■ Then myosin head binds to another ATP molecule ■ ATP binding makes myosin head detach from actin ★ The contraction cycle repeats as the myosin ATPase hydrolyzes the newly bound molecule of ATP, and continues as long as ATP is available and the Ca2+ level near the thin filament is sufficiently high. ○ Each of the 600 cross-bridges in one thick filament attaches and detaches about five times per second. ○ At any one instant: ■ Some myosin heads are attached to actin ■ And others are detached, getting ready to bind again ○ Draws Z discs together ■ During a maximal muscle contraction, the distance between two Z discs can decrease to half the resting length. ○ Sarcomeres shorten > whole muscle fiber shortens ★ As the fibers of a skeletal muscle start to shorten: ○ They first pull on their connective tissue coverings and tendons ■ The coverings and tendons stretch and then become taut ○ The tension passed through the tendons pulls on the bones they’re attached to ○ The result is movement of a part of the body Isotonic vs Isometric Contraction ★ Isotonic contraction ○ The tension (force of contraction) developed in the muscle remains almost constant while the muscle changes its length ○ Used for body movements and moving objects ○ Two types: ■ Concentric ● Muscle shortens ● Pulls on another structure (like a tendon) to produce movement ● Reduces the angle at a joint ● Ex: Picking up a book from a table involves concentric isotonic contractions of the biceps brachii muscle in the arm. ■ Eccentric ● Muscle lengthens ● Ex: As you lower the book to place it back on the table, the previously shortened biceps lengthens in a controlled manner while it continues to contract ○ How can it contract & lengthen? ● The tension exerted by the myosin cross-bridges resists movement of a load (the book, in this case) and slows the lengthening process. ■ Repeated eccentric isotonic contractions (for example, walking downhill) produce more muscle damage and more delayed-onset muscle soreness than do concentric isotonic contractions. ★ Isometric contraction ○ The tension generated is not enough to exceed the resistance of the object to be moved, and the muscle does not change its length ○ Ex: holding a book steady using an outstretched arm ■ Book pulls arm downward, stretching shoulder and arm muscles ■ Isometric contraction of shoulder and arm muscles counteracts the stretch ○ Important for: ■ Maintaining posture ■ Supporting objects in a fixed position ■ Stabilizing joints as others are moved ○ Do not result in body movement ■ But energy is still expended ★ In an isotonic contraction, tension remains constant as muscle length decreases or increases; in an isometric contraction, tension increases greatly without a change in muscle length. Muscle Metabolism ★ Skeletal muscle fibers often switch between a low and high level of activity ○ Low level of activity ■ Relaxed ■ Using a modest amount of ATP ○ High level of activity ■ Contracting ■ Using ATP at a rapid pace ★ While muscle fibers are relaxed: ○ Produce more ATP than they need for resting metabolism ○ Most of the excess ATP is used to synthesize creatine phosphate ★ A huge amount of ATP is needed to: ○ Power the contraction cycle ○ To pump Ca2+ into the sarcoplasmic reticulum ○ And other metabolic reactions involved in muscle contraction ★ ATP present inside muscle fibers is enough to power contraction for only a few seconds. If muscle contractions continue past that time, the muscle fibers must make more ATP. ★ Three ways to produce ATP 1. Direct phosphorylation (from creatine phosphate) ■ An energy-rich molecule found in muscle fibers ■ The enzyme creatine kinase (CK) catalyzes the transfer of one of the high-energy phosphate groups from ATP to creatine ● Forms creatine phosphate and ADP ● Creatine ○ a small, amino acid–like molecule ○ synthesized in the liver, kidneys, and pancreas ○ transported to muscle fibers ■ Creatine phosphate is three to six times more plentiful than ATP in the sarcoplasm of a relaxed muscle fiber ■ Formation of ATP occurs very rapidly ● So creatine phosphate is the first source of energy ■ ■ ○ Provide enough energy for 15 secs of max muscle contraction Unique to muscle fibers By anaerobic glycolysis ■ Muscle activity continues, but supply of creatine phosphate depletes ■ Glucose is catabolized to generate ATP ● Passes easily from blood to contracting muscle fibers through facilitated diffusion ● Also produced by breakdown of glycogen within muscle fibers ■ Glycolysis breaks down each glucose molecule into two molecules of pyruvic acid ● Occurs in the cytosol ● Produces a net gain of 2 ATP molecules ● Does not require oxygen (anaerobic) ■ Pyruvic acid either: ● Enters mitochondria ○ Undergoes aerobic respiration ● Is converted to lactic acid ○ During heavy exercise ○ Not enough oxygen is available to skeletal muscle fibers ○ The entire process by which the breakdown of glucose gives rise to lactic acid when oxygen is absent or at a low concentration is referred to as anaerobic glycolysis. ■ Each molecule of glucose catabolized via anaerobic glycolysis yields 2 molecules of lactic acid and 2 molecules of ATP. ■ Lactic acid accumulation = muscle soreness ■ Slower than from creatine phosphate but faster than aerobic ■ Provides enough energy for 30 secs-2 mins of max muscle contraction ■ All body cells can make ATP this way ○ By aerobic respiration ■ Pyruvic acid either: ● Enters mitochondria ○ Undergoes aerobic respiration ■ Oxygen-requiring reactions ● Krebs cycle ● Electron transport chain ■ Produce ATP, CO2, H2O, and heat ● Is converted to lactic acid ■ Slower than anaerobic respiration, but yields much more ATP ● Each molecule of glucose= about 30 or 32 molecules of ATP ■ Muscular tissue has two sources of oxygen ● O2 that diffused into muscle fibers from the blood ● O2 released by myoglobin within muscle fibers ■ Oxygen-binding proteins ● Myoglobin - found only in muscle cells ● Hemoglobin - found only in red blood cells ● Bind oxygen when plentiful and release oxygen when scarce ■ Aerobic respiration provides enough ATP for muscles during periods of rest or light to moderate exercise ● Pyruvic acid ○ From glycolysis of glucose ● Fatty acids ○ From breakdown of triglycerides ● Amino acids ○ From breakdown of proteins ■ Aerobic respiration provides nearly all of the ATP for activities that last several minutes to an hour+ ■ What’s the difference? ■ All body cells can make ATP this way ★ Motor units ○ A somatic motor neuron + all skeletal muscle fibers it stimulates ○ One somatic motor neuron makes contact with an average of 150 skeletal muscle fibers ■ All fibers in unit contract in unison ■ Dispersed throughout muscle, not clustered together ○ Precise movements ■ Many small units ● Larynx (voice box) = 2-3 muscle fibers per motor unit ● Eye movements = 10-20 muscle fibers per motor unit ○ Large-scale powerful movements ■ Biceps brachii & calf = 2000-3000 muscle fibers per motor unit ★ 4 factors that affect the force of a muscle contraction 1. Motor unit recruitment - # of muscle fibers stimulated ■ The process in which the number of active motor units increases ■ Typically, the different motor units of an entire muscle are not stimulated to contract in unison ● Some contract, others relax ● Delays muscle fatigue ● Allows contraction to be sustained for long periods ■ Weak motor units recruited first, progressive stronger units added next ● Precise movements = small muscles ● Less precision, more tension = large motor units ■ Produces smooth movements instead of jerks 2. Size of the fibers/motor units ■ Since all of the muscle fibers of a motor unit contract and relax together, the total strength of a contraction depends, in part, on the size of the motor units and the number that are activated at a given time. ■ Hypertrophied muscles are capable of more forceful contractions ● Muscular hypertrophy: the muscle growth that occurs after birth occurs by enlargement of existing muscle fibers ● Due to increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles ● Results from very forceful, repetitive muscular activity, such as strength training 3. Frequency of stimulation ■ The number of impulses per second is the frequency of stimulation ■ Wave summation: ● stimuli arriving at different times cause larger contractions ● When a 2ns stimulus occurs after the refractory period of the 2st stimulus is over, but before the skeletal muscle fiber has relaxed, the second contraction will actually be stronger than the first ■ Unfused tetanus ● Muscle fiber stimulated @ rate of 20-30 times per second ○ Only partially relaxes between stimuli ● Result = sustained (but wavering) contraction ■ Fused tetanus ● Muscle fiber stimulated @ rate of 20-30 times per second ○ Does not relax at all ● Result = sustained contraction, individual twitches not detected ● Peak tension generated is 5 to 10 times larger than the peak tension produced during a single twitch ■ Wave summation & tetanus occur when additional Ca2+ is released from the sarcoplasmic reticulum by subsequent stimuli 4. Length-tension relationship ■ the forcefulness of muscle contraction depends on the length of the sarcomeres within a muscle before contraction begins ■ Zone of overlap is optimal when: ● sarcomere length ~ 2.0–2.4 µm ○ Very close to resting length of most muscles ● Muscle fiber can develop maximum tension ● Maximum tension (100%) occurs when zone of overlap between a thick and thin filament extends from the edge of the H zone to one end of a thick filament ● Sarcomeres stretched to longer length = zone of overlap shortens ○ Fewer myosin heads can make contact with thin filaments ○ Stretched to 170% of its optimal length ■ No overlap between thick & thin filaments ■ Myosin heads do not bind to thin filaments ■ Muscle does not contract ■ Tension = zero ● Sarcomeres become shorter than optimum ○ Tension still decreases ○ Thick filaments compressed by Z discs > crumple ■ Fewer myosin heads make contact w/ thin filaments ● Resting muscle fiber length is very close to optimum length Post Exercise Oxygen Consumption ★ Heavy breathing continues for a while after muscle contraction stops ○ Can be minutes to several hours ★ Oxygen debt (recovery oxygen uptake) ○ The added oxygen that is taken into the body after exercise ○ More than the resting oxygen consumption ○ Restores metabolic conditions to the resting level in 3 ways: 1. Convert lactic acid back to glycogen stores in liver ● Small amount ● Most is made from dietary carbs though 2. Resynthesize creatine phosphate and ATP in muscle fibers 3. Replace the oxygen removed from myoglobin “What is a “graded muscle response”? What are the 2 ways a muscle contraction can be “graded”?”’ ★ From Google ○ Graded Muscle Responses are variations in the degree or strength of muscle contraction in response to demand, required for proper control of skeletal movement. ■ Amount of force created can be graded (varied) in two ways ● Frequency of the stimulation ● Number of motor units stimulated ■ Not like “all or none” action potentials Muscle Atrophy ★ Progressive loss of myofibrils = a decrease in size of individual muscle fibers ★ Disuse atrophy ○ Muscles are not used ○ Ex: bedridden individuals; people with casts ○ The flow of nerve impulses to inactive skeletal muscle is greatly reduced ○ Reversible! ★ Denervation atrophy ○ Nerve supply is disrupted or cut ○ 6 months - 2 years > muscle fiber shrinks to about ¼ of its original size ○ Fibers are irreversibly replaced by fibrous connective tissue Muscle Fiber Types ★ Skeletal muscle fibers are not all alike ○ In composition or function ○ They vary in myoglobin content ■ Myoglobin: the red-colored protein that binds O2 in muscle fibers ○ Contract and relax at different speeds ○ Vary in metabolic reactions they use to generate ATP ■ Categorized as either slow or fast depending on how rapidly the ATPase in its myosin heads hydrolyzes ATP (SO, FOG, & FG) ○ How quickly they fatigue ★ 2 criteria for classification ○ Red muscle fibers ■ High myoglobin content ■ Appear darker (dark meat in chicken legs/thighs) ■ Also contain more mitochondria ■ Supplied by more blood capillaries ○ White muscle fibers ■ Low myoglobin content ■ Appear lighter (white meat in chicken breasts) ★ 3 classifications of muscle fibers 1. Slow oxidative (SO) fibers ■ Appear dark red ● High myoglobin content & many blood capillaries ■ Many large mitochondria ● Generate ATP mainly by aerobic respiration ■ Considered “slow” because ● ATPase in the myosin heads hydrolyzes ATP relatively slowly ● Slow contraction speed ● Twitch contractions = 100-200 milliseconds ● Take longer to reach peak tension ■ Resistant to fatigue ● Capable of prolonged, sustained contractions for many hours ■ Maintaining posture ■ Aerobic, endurance-type activities (running a marathon) ■ Small in diameter 2. Fast oxidative–glycolytic (FOG) fibers ■ Typically the largest fibers ■ These contain large amounts of myoglobin and many blood capillaries too ● Also have dark red appearance ■ Moderately high resistance to fatigue ● Can generate considerable ATP by aerobic respiration ■ High intracellular glycogen level ● Also generate ATP by anaerobic glycolysis ■ Considered “fast” because ● ATPase in their myosin heads hydrolyzes ATP 3-5x faster than SO ● Faster contraction speed ● Reach peak tension faster than SO ● But shorter peak durations – less than 100 milliseconds ■ Walking & sprinting ■ Large diameter 3. Fast glycolytic (FG) fibers ■ Appear white in color ● Low myoglobin content ● Relatively few mitochondria ■ Large amounts of glycogen ● Generate ATP mainly by glycolysis ■ Able to hydrolyze ATP rapidly ● FG fibers contract strongly and quickly ■ Intense movements of short duration (weight lifting, throwing a ball) ■ Fatigue quickly ■ Strength training programs increase the size, strength, and glycogen content of fast glycolytic fibers ● Enlargement from hypertrophy ● The FG fibers of a weight lifter may be 50% larger than those of a sedentary person or an endurance athlete ★ Most skeletal muscles are a mixture of all three types of skeletal muscle fibers ○ About half are slow oxidative ○ Proportions vary based on ■ Action of muscle ■ Person’s training regimen ■ Genetic factors ○ High proportion of SO fibers ■ Continually active postural muscles ● Neck, back, legs ○ High proportion of FG fibers ■ Not constantly active, briefly used now and then to produce large amounts of tension ● Shoulders and arms ○ Large numbers of SO and FOG fibers ■ Support the body & used for walking and running ● Legs ○ One motor unit = all fibers are the same type ■ Different motor units recruited in a specific order (depending on need) ■ Weak contractions suffice = SO motor units ■ More force needed = FOG motor units ■ Maximal force required = FG motor units + other 2 types Smooth muscle ★ Characteristics ○ Involuntary ○ Only endomysium (no epimysium or perimysium) ○ Two types: ■ Visceral (single-unit) smooth muscle tissue ● (More common) found in the skin & in tubular arrangements that form part of the walls of small arteries and veins of hollow organs ● Autorhythmic ● Connect by gap junctions ○ Form a network for muscle action potentials to spread ○ Stimulation of one visceral muscle fiber causes contraction of many adjacent fibers ■ Multi-unit smooth muscle tissue ● Individual fibers with their own motor neuron terminals and few gap junctions ● Stimulation of multi-unit fiber causes contraction of that fiber only ● Found in the walls of large arteries, airways to lungs, arrector muscles of hair, muscles of the iris that adjust pupil diameter, and in the ciliary body that adjusts focus of the lens in the eye ○ Relaxed smooth muscle fiber = ■ 30-200 micrometers long ■ Tapered ends and thick in the middle (3-8 micrometers) ○ One centrally-located, oval shaped nucleus ○ Sarcoplasm contains thick and thin filaments ■ Ratios = 1:10 and 1:15 ■ Not arranged in sarcomeres like striated muscle ○ Contain intermediate filaments ■ Protein filament, ranging from 8 to 12 nm in diameter, that may provide structural reinforcement, hold organelles in place, and give shape to a cell ○ Lack T tubules ■ Caveolae instead ■ Takes longer for Ca2+ to reach the filaments in the center of the fiber and trigger contractile process ○ Small amount of sarcoplasmic reticulum (Ca2+ storage for muscular contraction) ○ Thin filaments attach to dense bodies ■ Similar to Z discs ■ Some dispersed through sarcoplasm, others attached to sarcolemma ○ Bundles of intermediate filaments also attach to dense bodies ■ Stretch from one dense body to another ■ Sliding filament mechanism tension transmitted to intermediate filaments ■ Intermediate filaments pull on dense bodies = shortening/contraction ■ Twists into helix during contraction ★ Functions ○ Contractions start more slowly and last much longer than skeletal muscle fibers ○ Can shorten and stretch to a greater extent than other muscle types ○ Sarcoplasmic reticulum is found in small amounts ○ Calcium ions flow into smooth muscle sarcoplasm from both the interstitial fluid and sarcoplasmic reticulum (slowly) ○ Several mechanisms regulate contraction/relaxation ■ Calmodulin (a regulatory protein) binds to Ca2+ in the sarcoplasm ● Troponin takes this role in striated ■ calmodulin activates myosin light chain kinase (enzyme) ■ This enzyme uses ATP to add a phosphate group to a portion of the myosin head ■ Phosphate group attaches > myosin head binds to actin > contraction ■ Slow because myosin light chain kinase works slowly ○ Calcium ions also exit muscle fiber slowly = relays relaxation ○ Smooth muscle tone (a state of partial contraction) comes from the prolonged presence of Ca2+ in the cytosol ■ Can sustain long-term tone ■ Important in digestive canal ● Walls maintain a steady pressure on the contents of the canal ● and hollow organs like stomach, intestines, bladder ■ And in the walls of blood vessels (arterioles) ● Which maintain a steady pressure on blood ○ Most smooth muscle fibers contract or relax in response to ■ ■ ■ ○ Nerve impulses from the ANS Stretching, hormones Local factors ● Changes in pH ● Oxygen and carbon dioxide levels ● Temperature ● Ion concentration Stress-relaxation response ■ When smooth muscle fibers are stretched, they initially contract ● This increases tension ● After about a minute, the tension decreases ■ Allows smooth muscle to undergo great changes in length, but still contract Extra: - Muscles constitute 40–50% of total body weight - The prime function of muscle is changing chemical energy into mechanical energy to perform work - Cardiac muscle tissue remains contracted 10 to 15 times longer than skeletal muscle tissue due to prolonged delivery of Ca2+ into the sarcoplasm - Cardiac muscle depends greatly on aerobic respiration to generate ATP - Skeletal muscle fibers cannot divide and have limited powers of regeneration - Cardiac muscle fibers can regenerate under limited circumstances - Smooth muscle fibers have the best capacity for division and regeneration - 3 Ingredients required for muscle contraction Neural Stimulation- Stimulates muscle fiber —> for muscle fiber Action Potential Free Ca++ in sarcoplasm - uncovers blocked binding sites on actin Available ATP- Giving myosin head “energy to pull - Most muscles develop from mesoderm Mesoderm segments into cube-shaped structures called somites Somites: segmental axial structures of vertebrate embryos that give rise to vertebral column, ribs, skeletal muscles, and subcutaneous tissues - “Graded Muscle Response” (varied contraction strength) Multiple Motor Unit Summation: More fibers in each muscle are stimulated when force/tension needs to be increased Temporal Summation- The rate of stimulation from the motor nerve is increased to increase contraction strength - With aging, there is a slow, progressive loss of skeletal muscle mass, which is replaced by fibrous connective tissue and fat Between ages 30-50 - Estimated 10% of muscle mass lost Between ages 50-80 - Another 40% Could be due to decreased physical activity - Aging also results in a decrease in muscle strength, slower muscle reflexes, and loss of flexibility - Also aging # of SO fibers increases Lower limbs usually weaken before upper limbs
