Chapter 10 MCB 244
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This 18 page Class Notes was uploaded by Jessica Logner on Sunday April 3, 2016. The Class Notes belongs to MCB 244 at University of Illinois at Urbana-Champaign taught by Dr, Chester Brown in Fall 2015. Since its upload, it has received 202 views. For similar materials see Human Anatomy and Physiology I in Biology at University of Illinois at Urbana-Champaign.
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Date Created: 04/03/16
Chapter 10: Muscle Tissue Muscle primary tissue type divided into: Skeletal Muscle (voluntary striated muscle, controlled by nerves of the central nervous system) Cardiac Muscle (involuntary striated muscle) Smooth Muscle (involuntary nonstriated muscle) 5 haracteristics of all muscle tissues: Specialized Cell (elongated; high density of myofilaments = cytoplasmic microfilaments of actin & myosin) Excitabili(receive & respond to stimulus) Contractili (shorten & produce force upon stimulation) Extensibilit(can be stretched) Elastici (recoil after stretch) Skeletal Muscles 44% of body mass attached to the skeletal system allow us to move organs composed of skeletal muscle cells (fibers), connective tissue, nerves, & blood vessels muscular system includes only skeletal muscles 6 unctions of Skeletal Muscle Tissue produce skeletal movement maintain posture & body position support soft tissues guard entrances & exits maintain body temperature store nutrient reserves (starvation only) Muscles: 3 layers of connective tissue epimysium, perimysium, & endomysium Epimysium covers the muscle (exterior colleen layer) and separates it from other tissues; composed of collagen; connects to deep fascia Perimysium composed of collagen & elasin, has associated blood vessels and nerves; bundles of muscle fibers in groups called fascicles (covered by perimysium) Endomysium composed of reticular fibers; contains capillaries, nerve fibers, and myosatellite cells (stem cells> repair), surrounds individual muscle fibers Skeletal Muscle Connective Tissue endomysium, perimysium, and epimysium come together at ends of muscles to form connective tissue attachment to bone matrix tendon (bundle) aponeurosis (sheet) Skeletal Muscle Nerves skeletal muscles are voluntary muscles controlled by nerves of the central nervous system; nerves branch extensively as every muscle fiber must be innervated Skeletal Muscle Blood Vessels muscles have an extensive capillary system that supplies large amounts of oxygen and nutrients & carries away wastes/ metabolic end products Skeletal Muscle Cells large and multinucleate formed but fusion of 100s of myoblasts nuclei of each myoblast retained to provide enough mRNA for protein synthesis in large fiber unfused myoblasts in adult = myosatellite cells myosatellite cells are capable of division and fusion to existing fibers for repair but cannot generate new fibers de novo sarcolemma (cell membrane): maintains separation of electrical charges (results in transmembrane potential) Na+K+ATPase pump maintains cell excitability Resting membrane potential ca. 85 mV (negative charge inside cell primarily from proteins) if membrane permeability is altered, Na+ will flow in causing a change in membrane potential Cytoplasm = sarcoplasm; rich in: glycosides (glycogengranules) & myoglobin (intracellularO2 carrier) Skeletal muscle fiber internal organization Transverse Tubules (T tubules) invaginations of sarcolemma that reach deep inside the cell to transmit changes in transmembrane potential to structures inside the cell transmit action potential through entire cell facilitates the contraction to the entire muscle fiber simultaneously (by carrying depolarization deep into cell) Myofibrils lengthwise subdivisions within muscle fiber; hundreds to thousands in each fiber made up of bundles of protein filaments (myofilaments) myofilaments are responsible for muscle contraction (80% of cell volume); thick (actin); thin (myosin) Sarcoplasmic Reticulum (SR) a membranous structure surrounding each myofibril; similar in structure to smooth endoplasmic reticulum Function: store calcium and help transmit action potential to myofibril Forms chambers (terminal cisternae) attached to T tubules Cisternae concentrate Ca2+ via ion pumps ReleaseCa2+ intosarco meres to begin muscle contraction SR has high density of Ca2+ pumps (SR [Ca2+] 1000x > than sarcoplasm) Triads located repeatedly alone the length of myofilaments ttubule wrapped around a myofibril sandwiched between two terminal cistern of SR (formed by 1 T tubule and 2 terminal cistern of SR) located at both ends of a sarcomere Sarcomere smallest functional unit of a myofibril contractile units of muscle; structural units of myofibrils; form visible patterns within myofibrils composed of : 1. Thick filaments (myosin) 2. Thin filaments (actin) 3. Stabilizing proteins (hold thick & thin filaments in place) 4. Regulatory proteins (contol interactions of thin & thick filaments) *** organization of thin & thick filaments = striated appearance Striated appearance of skeletal muscle alternating dark, thick myosin filaments and the light, thin actin filaments found in portions of the A band (A = Anisotropic = polarizes light); thin filaments are also found in I band = (I = Isotropic = does not polarize light) 6 components of the Sarcomere Aband, M line, Hband, Zone of overlap, Iband, Z lines/discs Aband whole width of thick myosin filaments, looks dark microscopically M line (at midline fo sarcomere) center of each thick filament, middle of Aband attaches neighboring thick filaments Hband lighter region on either side of the m line; contains thick filaments only Zone of overlap (triads encircle zones of overlap) ends of Abands; place where thin filaments intercalate between thick filaments Iband contains thin filaments outside zone of overlap; not whole width of thin filament Z lines/discs centers of the I bands; constructed of activins (anchor thin filaments and bind neighboring sarcomeres) and titin proteins (bind thick filaments to Zline, stabilize the filament) Sarcomere Function Triads (transverse tubule + 2 terminal cisternae of SR) encircle the sarcomere near zones of overlap Ca2+ from SR causes thin and thick filaments to interact Muscle Contraction Caused by interactions of thick and thin filaments Structures of protein molecules determine interactions Thin Filament Structure/Function Filamentous (F)actin: Composed of 2 twisted rows of globular (G)actin The active sites on strands of Gactin bind to myosin Nebulin: Holds Factin strands together Tropomyosin: Double stranded protein that covers the active sites on Gactin thereby preventing actinmyosin interaction Troponin: binds tropomyosin to Gactin Also has receptor for Ca2+: when Ca2+ binds to the troponin (C), it exposes the active sites on Factin for myosin binding, thereby initiating the muscle contraction Thick Filament Structure/Function Each contains about 300 twisted myosin subunits Titin anchors filaments to Zdisk and provides recoil Each myosin molecule is composed of 3 parts: Tail: Many tails bundled together to make length of thick filament Hinge: Flexible region; allows for movement in contraction Head: Made of two globular protein subunits; Reaches the nearest thin filament; Binds to actin in active site to form actinmyosin crossbridges Sliding Filament Theory of Muscle Contraction Contraction of skeletal muscle is due to thick filaments and thin filament sliding past each other (not compression of filaments). Thin filaments of sarcomere slide toward M line, alongside thick filaments Sliding causes shortening of every sarcomere in every myofibril in every fiber = shortening of whole skeletal muscle 1. Hband and Iband width decreases during contraction 2. Zones of overlap increase width 3. Zlines move closer together 4. Aband width remains constant Skeletal Muscle ExcitationContraction 1. Neural Stimulation results in the release of acetylcholine (ACh) at the neuromuscular junction 2. ExcitationContraction Coupling,that is the coupling of the neural excitation with muscle contraction via a series of molecular events involving Ca2+ 3. Muscle Contraction and the shortening of the sarcomeres and that of the muscle(s) involved, followed by... 4. Relaxation of the Muscle once the neural stimulation has ended and all Ca2+ has been resequestered by the sarcoplasmic reticulum (SR) Neuromuscular Junction (NMJ) Special intercellular connection between the nervous system and skeletal muscle fiber Couples neural excitation with skeletal muscle excitation in seven steps Muscle Contraction Cycle 6 Steps of the Contraction Cycle 1. Contraction Cycle Begins 2. ActiveSite Exposure 3. CrossBridge Formation 4. Myosin Head Pivoting 5. CrossBridge Detachment 6. Myosin Reactivation IMPORTANT!!! 1. Action potential―a transient allornone change [depolarization] in membrane potential―travels down a motor neuron and reaches the nerve terminal at the neuromuscular junction (NMJ). 2. The transient depolarization at the synaptic terminal opens Ca2+ channels and the influx of Ca2+ into the synaptic terminal causes the release of acetylcholine (ACh) into the synaptic cleft via exocytosis of synaptic vesicles. 3. ACh diffuses across the cleft and binds to receptors (nicotinic) on the motor end plate (sarcolemma membrane), resulting in a transient opening of nonselective cation channels. 4. Cations (primarily Na+) rush into the sarcoplasm resulting in a transient depolarization of the sarcolemma. The depolarization spreads across the entire sarcolemma and is transmitted deep into the muscle via the T Tubule system. 5. The depolarization affects dihydropyridine receptors (DHPRs) ― voltagesensitive calcium channels of the Ttubule system ― resulting in the release of a small amount of Ca2+ into the sarcoplasm. This release, along with the mechanical link known as triadic feet, which spans the sarcoplasmic gap between the Ttubule and the SR, transmits the signal to calcium channels in the SR known as ryanodine receptors (RyRs). Opening of these channels results in the release of a relatively large amount of Ca2+ into the sarcoplasma (known as the calcium transient [107→106 M]) 6. Ca2+ binds to troponin (specifically troponin C) on the thin filaments, which leads to a conformational change in the troponintropomyosin complex and the tropomyosin physically moving aside to uncover binding sites for myosin binding on the actin filament (known as the “active site”). 7. The myosin head binds to actin thereby forming a crossbridge. The myosin head pivots at the hinge towards the Mline as it undergoes a conformational shift known as the power stroke, pulling the actin toward the center of the sarcomere and thereby shortening the sarcomere (by ~0.5% for each stroke). 8. As soon as Ca2+ is released into the sarcoplasm it is actively taken up by Ca2+ATPase pumps in the SR. Similarly, as soon as ACh is released at the NMJ acetylcholinesterase degrades it. Both mechanisms minimize the latency period before another excitationcontraction coupling can occur. 9. With Ca2+ no longer bound to troponin C, tropomyosin slips back to its blocking position over the active sites on actin (for myosin biding). Contraction ends and actin slides back to its original “resting” position. 10. ATP is not only required for the SR Ca2+ATPase pumps but primarily for the release of the myosin head from actin and, thus, breakage of the crossbridge. As ATP is hydrolyzed by the myosin ATPase, the myosin head recocks making it ready for another power stroke. Key Concepts in Skeletal Muscle Contraction & Relaxation Cycle Skeletal muscle fibers shorten as thin filaments slide between thick filaments Free Ca2+ in the sarcoplasm triggers contraction SR releases Ca2+ when a motor neuron stimulates the muscle fiber Contraction is an active process Relaxation and return to resting length are passive Tension in a single muscle fiber depends on... The number of pivoting crossbridges The fiber's resting length at the time of stimulation The frequency of stimulation Tension in a group of muscle fibers depends on... The number of motor units involved and the tension generated both internally and externally (against elastic components) Tension Production by Muscle Fibers As a whole, a muscle fiber is either contracted or relaxed (allornone) LengthTension Relationships 1. Number of pivoting crossbridges depends on amount of overlap between thick and thin fibers 2. Optimum overlap produces greatest amount of tension; too much or too little reduces efficiency 3. Normal resting sarcomere length is 75% to 130% of optimal length 3 Phases of a Twitch Latent period(~ 2 msec) Delay before Ca2+ release as action potential moves through sarcolemma Contraction phase2 10's of msec) Calcium ions bind to troponin... Tension builds to peak Relaxation phase (~ 25 msec) Sarcoplasmic Ca2+ levels fall Active sites are covered; tension falls Frequency of Stimulation A single neural stimulation produces a single contraction or twitch (7100 msec) Sustained muscular contractions require repeated stimuli A single twitch will not produce normal movement (requires many cumulative twitches) Repeated stimulations result in higher tensions due to some Ca2+ remaining in the sarcoplasm from previous twitch 2 Types of Frequency Stimulation Treppe A stairstep increase in twitch tension Repeated stimulations im mediately after relaxation phase (stimulus frequency <50/second) Causes a series of contractions with increasing tension Resting tension reached between each twitch MOST SKELETAL MUSCLES DO NOT EXHIBIT TREPPE (CARDIAC MUSCLE DOES) Wave Summation Increasing tension or summation of twitches Repeated stimulations before the end of relaxation phase (stimulus frequency >50/sec) Next twitch arrives while some crossbridges still intact and some Ca2+ remains in sarcoplasm resulting in increasing tension or summation of subsequent twitches upon previous This is what typical skeletal muscle contractions looks like. Incomplete Tentanus Stimuli arrive at a frequency that prevents complete relaxation (Ca2+ reabsorption) so that tension builds upon the previous twitch Not complete tetanus because some relaxation occurs between twitches A type of wave summation that reaches a plateau in tension that is stimulus frequency dependent Complete Tetanus If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction Produces ~4x more tension than max treppe Stimulus frequency higher than Ca2+ pumps can work & thus no relaxation— only summation Fatigues easily at this rate of stimulation After a long period of tetanus, tension will fall off (below max tension). Tension Produced By A Whole Skeletal Muscle Depends On... 1. Internal tension produced by sarcomeres Not all tension is transferred to the loadsome of it is lost due to the elasticity of muscle tissues 2. External tension exerted by muscle fibers on elastic extracellular fibers Tension applied to the load 3. Total number of muscle fibers stimulated Motor Unit All fibers controlled by a single motor neuron (avon branches to contact each fiber * each skeletal muscle has thousand of fibers organized into motor units * number of fibers in a motor unit depends on the function (fine control : 4/unit; gross control 200/unit) * fibers from different units are intermingled in the muscle so that the activation of one unit will produce equal tension across the whole muscle Recruitment (multiple motor unit summation) In a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated Order of activation of motor units is important ***Slower, weaker units activated first; stronger units are added to produce steady increases in tension as required Maximum tension achieved when all motor units reach tetanus; can be sustained only a very short time Sustained Tension Typically less than maximum tension Allows motor units rest in rotation Force is increased by increasing the number of motor units (recruitment) Muscle Tone Resting tension Maintains shape/definitionsome units always contracting Braces skeleton & acts as shock absorber; accelerates recruitment Exercise increases the number of contractual units→increases metabolic rate (even at rest) → increases speed of recruitment (better tone) Contractions are classified based on. .. pattern of tension production Isotonic Contraction Skeletal muscle changes length: resulting in motion If muscle tension > load: muscle shortens (concentric contraction) If muscle tension < load: muscle lengthens (eccentric contraction) Isometric Contraction Skeletal muscle develops tension, but is prevented from changing length (e.g., postural muscles) Load (resistance) & Contractile Speed Are inversely related The heavier the load on a muscle the longer it takes for shortening to begin and the less the muscle will shorten As load ↑, speed of crossbridge pivoting ↓ Muscle Relaxation passively returns to resting length Elastic recoil The pull of elastic elements (tendons and ligaments) Expands the sarcomeres to resting length Opposing muscle contractions Reverse the direction of the original motion (e.g., bicep vs. tricep brachii) Gravity Opposes muscle contraction to return a muscle to its resting state Muscles fatigue when they can no longer perform a required activity due to... 1. Depletion of reserves (glycogen, ATP, CrP) 2. Decreased pH due to lactic acid accumulation this decreases calcium binding to troponin and alters enzyme activities 3. Damage to sarcolemma and sarcoplasmic reticulum 4. Sense of weariness and decreased desire to continue as a result of pain *The higher the degree of fatigue, the longer the recovery time: replenishment nutrients include ATP, CrP, glycogen, oxygen, and Cori cycle in liver (lactate → pyruvate → glucose). • repair of damage Force amount of tension produced Power amount of tension produced per unit time Endurance amount of time an activity can be sustained Force, Power, and Endurance Depend On.... 1. the types of skeletal muscle fibers 2. physical conditioning Fast Glycolytic Fibers Fast twitch/ Type IIb Myosin ATPase works quickly (fast cycling) ATP production via glucose fermentation (anaerobic glycolysis) Large diameter fibers Many myofilaments and high glycogen supply Few mitochondria Fast to act, powerful, but quick to fatigue Slow Oxidative Fibers Slow twitch/Type I Myosin ATPases work slowly Specialized for aerobic respiration: many mitochondria, extensive blood supply (capillarity), & myoglobin (red pigment, binds oxygen) Smaller fibers (for better diffusion) Slow to contract, produce lower tension, but resist fatigue Catabolize lipids, glucose, and amino acids Intermediate or Fast Oxidative/Glycolytic Fibers Intermediate twitch/ Type IIa Qualities of both fast glycolytic and slow oxidative fibers Fast acting but perform aerobic respiration so slow to fatigue Intermediate levels of myoglobin and capillarity Physical conditioning can convert some fast fibers into intermediate fibers for stamina Extra Chapter 10 Details White Muscles mostly fast fibers; pale (chicken breast) Red Muscles mostly slow fibers; dark (chicken legs Most human muscles are... pink and contain a mix of fibers Muscle Hypertrophy Muscle growth from activity (e.g., training, especially when repeatedly near maximal tension) May increase diameter of muscle fibers, number of myofibrils, and/or mitochondria number, myoglobin content, capillary density, glycogen reserves Does NOT increase number of muscle fibers Muscle Atrophy Muscle loss due to inactivity Reduced muscle size, tone, and power Aerobic Activities Endurance training/ prolonged activity is supported by mitochondrial metabolism & requires oxygen and bloodborne nutrients Improves endurance by training fast fibers to be more like intermediate fibers Improves cardiovascular performance Does NOT result in muscle hypertrophy Training Increases Aerobic Endurance/Efficiency by... Increasing capillary density as well as mitochondria and myoglobin content Anaerobic Activities Uses fast fibers, which fatigue quickly with strenuous activity (weightlifting) Improved by frequent, brief, intensive workouts Training causes hypertrophy (fibers increase in diameter, NOT NUMBER; increase number of myofibrils & myofilaments, thereby increasing the tension that can be generated) Training Increases Anaerobic Endurance/ Efficiency by... Increasing glycogen supply, amount of ATP and CrP available and tolerance to lactic acid generation & increases power output and sustainable anaerobic duration Exercise is important because... What you don't use, you lose Muscle tone indicates base activity in motor units of skeletal muscles Muscles become flaccid when inactive for days or weeks Muscle fibers break down proteins, become smaller and weaker With prolonged inactivity, fibrous tissue may replace muscle fibers Crosstraining, that is training by alternating aerobic and anaerobic activities, enhances health by increasing both muscle mass and aerobic endurance Cardiac and Smooth muscle Cardiac Muscle Tissue Cells are striated and only found in the heart Striations are similar to that of skeletal muscle because the internal myofilament arrangement is similar 7 Characteristics of Cardiomyocytes 1. Are small 2. Have a single nucleus (typicallysometimes 2) 3. Have short, wide T tubules that surround Zline 4. Have no triads 5. Have SR with no terminal cisternae 6. Are highly aerobic (high in myoglobin & mitochondria) 7. Have intercalated discs (gap junctions) and desomosomes which enhances molecular and electrical (AP) connections Intercalated discs link cardiocytes mechanically, chemically, & electrically Heart functions like a single, fused mass of cells Intercalated Discs Are specialized contact points between cardiomtocytes Join cell membranes of adjacent cardiomyocytes (gap junctions, desmosomes) Maintain structure, enhance molecular and electrical connections, & conduct action potentials Coordination of cardiomyocytes (because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells) Functional Characteristics of Cardiac Muscle Tissue 1. Automaticity Contraction without neural stimulation Controlled by pacemaker cells (cells that generate action potentials spontaneously found in the sinoatrial and atrioventricular nodes and Purkinje fibers) 2. Variable contraction tension and pace Controlled by nervous system 3. Extended contraction time Ten times as long as skeletal muscle 4. Prevention of wave summation and tetanic contractions by cell membranes Long refractory period; only twitches Smooth Muscle Tissue nonstriated muscle tissue with different internal organization of actin and myosin and different functional characteristics than that of striated muscle lines hollow organs and regulates blood flow and movement of materials in organs organized in 2 layers: circular & longitudinal Characteristics of Smooth Muscle Tissue Long, slender, and spindle shaped Have a single, central nucleus Have no T tubules, myofibrils, or sarcomeres Have no tendons or aponeuroses Have scattered myosin fibers Myosin fibers have more heads per thick filament Have thin filaments attached to dense bodies Dense bodies transmit contractions from cell to cell Structurally different than striated muscle: no troponin; active sites on actin are always exposed Smooth Muscle Contraction Events 1. Stimulation causes Ca release from SR 2. Ca2+ binds to calmodulin in the sarcoplasm Calmodulin = CALcium MODULated proteIN 3. Calmodulin activates myosin light chain (MLC) kinase; this complex phosphorylates myosin 4. MLC Kinase converts ATP > ADP to cock myosin head 5. Cross bridge form > contraction, cells pull toward center Length Tension Relationships Smooth Muscle Tissue Thick and thin filaments are scattered Resting length not related to tension development Functions over a wide range of lengths (plasticity) Control of Contractions Smooth Muscle Tissue Multiunit smooth muscle cells that are connected to motor neurons Visceral (single unit) smooth muscle cells that are not connected to motor neurons and produce rhythmic cycles of activity controlled by pacesetter Modified by neural, hormonal or chemical factors
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