Muscles & Muscle Tissue

Describe the gross anatomy of a muscle, beginning with the myofibrils.

Nerve and Blood Supply- One nerve, one artery, and one or more veins serve each muscle that all enter or exit near the canal part of the muscle and branch profusely through its connective tissue sheaths Each fiber is supplied with a nerve ending that controls its activity Rich blood supply Metabolic wastes removed through veins Capillaries straighten when muscle stretches and contort when muscle contracts Connective Tissue Sheaths- When muscles contract, they pull on these sheaths, transmitting the force to the bone to be moved & contributed to elasticity and provide entry and exit route for nerves and blood vessels Epimysium: an "overcoat" of dense irregular CT that surrounds the whole muscle, sometimes blends with deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin Perimysium and fascicles: muscle fiber groups grouped into fascicles that resemble a bundle of sticks, surrounded by perimysium (layer of fibrous CT) Endomysium: a wispy sheath of fine areolar CT that surrounds each individual muscle fiber Attachments- When a muscle contracts, the movable bone, the muscle's insertion moves toward the immovable or less movable bone, the muscle's origin Microscopic Anatomy- Each skeletal muscle fiber is a long cylindrical cell with multiple oval nuclei just beneath its sarcolemma: plasma membrane Sarcoplasm: the cytoplasm of a muscle cell, contains large amounts of glycosomes (stored glucogen) and myoglobin Myofibrils: a single muscle fiber contains hundreds to thousands that run parallel to its length, densely packed in the fiber that mitochondria and other organelles appear squeezed, 80% of cell volume Striations: the dark A and light I bands are aligned Each dark A band has a lighter region in its midsection called the H zone Each H zone is bisected vertically by a dark line called the M line formed by molecules of the protein myomesin Each light I band has a midline interruption, a darker area called the Z disc Sarcomere: region between two successive Z disc, smallest contractile unit of a muscle fiber, contains an A band flanked by half an I band at each end Myofilaments: arise from sarcomeres, play a role in motility and shape change in all cell of the body, reaches highest development in the contractile muscle fibers Thick filaments contain the protein myosin Thin filaments contain the protein actin, tropomyosin, and troponin Elastic element contains the protein titin, holds thick filaments in place, maintains A band, and helps the muscle spring back into shape after stretching Actin & myosin are force generating proteins of the sarcomere and they work together during the muscle contraction cycle in order to produce movement Sarcoplasmic Reticulum (SR): smooth ER, surround the myofibril longitudinally (similar to how the sleeve of a loosely crocheted sweater surrounds your arm), communicate at the H zone, terminal cisterns are "end sacs", regulates intracellular levels of ionic calcium which provides final "go" sign for contraction T tubules: at each A band-I band junction, the sarcolemma of the muscle cell protrudes deep into the cell interior, forming an elongated tube, increase SA, its lumen (cavity) is continuous with the extracellular space, part of the "triad," ensures every myofibril in the muscle fiber contracts at the same time

Describe the entire process of excitation-contraction coupling, beginning with a nerve impulse in a motor neuron.

E-C coupling 1. The actin potential (AP) propagates along the sarcolemma and down the T tubules 2. Ca2+ are released -> transmission of the AP along the T tubules of the triad causes the voltage-sensitive tubule proteins to change shape -> opens the Ca2+ release channels in the terminal cisterns of the SR allowing Ca2+ to flow into the cytosol 3. Calcium binds to troponin and removes the blocking action of tropomyosin -> troponin changes shape, exposing binding sites for myosin on the thin filaments 4. Contraction begins -> myosin binding to actin forms cross bridges and contraction (cross bridge cycling) begins, E-C coupling is over

What type of tension is being produced when the overall length of the muscle is increasing while the muscle is functioning?

Eccentric: muscle generates force as it lengthens, more often cause delayed-onset muscle soreness, occur in calf muscle from walking up a hill

Describe the location of the sarcoplasmic reticulum (SR) with respect to the myofibrils.

Elaborate smooth endoplasmic reticulum- interconnecting tubules surround each myofibril the way the sleeve of a loosely crocheted sweater surrounds your arm Terminal cisternae: form larger, perpendicular cross channels at the A band-I band junctions and they always occur in pairs

What are the T tubules? Where are they located?

Elongated tube at each A band-I band junction, where the sarcolemma of the muscle cell protrudes deep into the cell interior (transverse tubules) Lumen of the T tubule is continuous with extracellular space Located between the paired terminal cisterns of the SR, forming "triads" As they pass from one myofibril to the next, the T tubules also encircle each sarcomere

Name the CT sheath that surrounds individual muscle fibers. Of what type of CT is this sheath composed?

Endomysium (consists of fine areolar CT)

What is the location of the blood vessels and nerves which supply the skeletal muscle tissue?

Enter or exit near the central part of the muscle and branch profusely through CT sheaths

In indirect attachments, what tissues are involved in transmitting pulling (i.e., tension forces generated during contraction) forces from the muscle fiber to the bone?

Epimysium Perimysium Endomysium All of these CT sheaths are continuous with one another as well as with the tendons that join muscles to bones

Name the CT sheath that holds multiple fascicles together to create a muscle. Of what type of CT is this sheath composed?

Epimysium (consists of dense irregular CT)

Name and describe the four functional characteristics of muscle.

Excitability (responsiveness)- Ability of a cell to receive and respond to a stimulus by generating an electric impulse that travels along the plasma membrane of the muscle cell and causes the cell to contract Stimulator is usually a chemical (for example, a neurotransmitter released by a nerve cell or a local change in pH) Contractility- Ability to shorten forcibly when adequately stimulated, setting muscle apart from all other tissue types Extensibility- Ability to extend or stretch Muscle cells shorten when contracting, but can stretch beyond resting length when relaxed Elasticity- Ability of muscle cell to recoil and resume its resting length after stretching

Which muscle property is most protective?

Extensibility and excitability

What is meant by the term excess post-exercise oxygen consumption (EPOC), which was formerly called "oxygen debt"? What accounts for EPOC?

Extra amount of oxygen that the body must take in for restorative processes such as replenishment of oxygen reserves in myoglobin, accumulated lactic acid reconverted to pyruvic acid, replacement of glycogen stores, and ATP and creatine phosphate reserves synthesized Represents the difference between the amount of oxygen needed for totally aerobic muscle activity and the amount actually used All anaerobic sources of ATP used during muscle activity contribute to EPOC

Which fiber type has the fastest contraction speed? the slowest?

Fastest -> fast glycolytic fiber, fast oxidative fiber, contracts rapidly due to the activity of fast myosin ATPases Slowest -> slow oxidative fiber because its myosin ATPase are slow

What is the latent period and what is taking place during the latent period?

First few milliseconds following simulation when E-C coupling is occuring, cross bridges begin to cycle but muscle tension is not measurable so the myogram (recording of contractile activity consisting a line of tracings) does not show a response

When does a muscle begin to develop (or exert) external tension?

Forces generated by the cross bridges (internal tension) stretches the CT sheaths -> turn taut and transfer their tension (external tension) to the load (muscle insertion)

What is the role of gap junctions and pacemaker cells in single-unit smooth muscle?

Gap junctions: synchronized contractions; allow smooth muscles to transmit action potentials to each other Pacemaker cells: (stomach and small intestines) "drummers" setting pace of contraction for entire muscle sheet, depolarize spontaneously in absence of external stimuli

Which myofibril structures or regions change length during the contraction process?

I band, H zone, and Z discs

What is the function of myoglobin?

In the sarcoplasm, protein that stores oxygen within the muscle, dark red pigment

What is the function of the T tubules?

Increase the muscle fiber's SA Conduct impulses to the deepest regions of the muscle cell and every sarcomere Signal for release of calcium from adjacent terminal cisterns Rapid communication system that ensures that every myofibril in the muscle fiber contracts at virtually the same time

Why is the triad of functional significance?

Integral proteins protrude into the inter-membrane spaces from the T tubule and SR and act as the voltage sensors Regulate the release of Ca2+ throughout muscles

What is the role of myosin light chain kinase (MLCK)?

Interacts with calmodulin and phosphorylates the myosin to activate it

What type of tension is being produced when internal tension is being developed, but no movement occurs?

Isometric: tension may build to the muscle's peak tension-producing capacity, but the muscle neither shortens or lengthens, occurs when a muscle tries to move a load that is greater than the force (tension) the muscle is able to develop

What specializations of the muscle fiber (at the motor end plate) enable it to respond to signals from the motor neuron?

Junction folds of sarcolemma provide a large surface area for millions of ACh receptors

Describe the structure of an actin molecule. How are individual actin monomers arranged to make up a thin filament?

Kidney-shaped polypeptide subunits called G actin which bear the active sites to which myosin heads attach during contraction In thin filaments, G actin subunits are polymerized into long actin filaments called F actin 2 intertwined actin filaments -> form backbone of each thin filament

What factors contribute to the onset of muscle fatigue?

Lack of ATP, lactic acid are not a factor Ionic imbalances As AP are transmitted, potassium is lost from the muscle cells, and accumulate in the fluids of the T tubules -> disturbs the membrane potential of the muscle cell and halts Ca2+ release from the SR

What is taking place during the relaxation period?

Lasting 10-100 ms due to pumping of Ca2+ back into the SR Due to number of active cross bridges declining --> contractile force is declining

What type of tension is developed most efficiently (i.e., requires less ATP expenditure per unit of work accomplished)? least efficiently?

Less ATP -> eccentric More ATP -> concentric

Describe the process of signal transmission at the neuromuscular junction (i.e., communication of nerve AP to the muscle fiber).

Nerve impulse (AP) reaches the end of an axon Voltage-gated Ca channels open and the Ca enters axon terminal by moving down electrical gradient Axon terminal releases ACh into the synpatic cleft by exocytosis ACh diffuses across the cleft and attaches to the ACh receptors on the sarcolemma of the muscle fiber ACh binding opens ion channels that allow passage of Na+ into fiber and K+ out of the fiber (More Na+ enters than K+ exits -> generating a membrane potential) ACh generates electrical events that generate action potential Effects terminated by breakdown in synaptic cleft by acetylcholineterase and diffusion away from the junction

What is the advantage of the parallel arrangement of fibers? of the pennate arrangement? the convergent?

Parallel arrangment -> shorten the most, achieve faster contractile speeds, not very powerful Pennate arrangement -> shorten little but very powerful, the greater the number of muscle fibers, the greater the power Convergent -> origin is wider than point of insertion, allows for maximum force production

Name the CT sheath that surrounds a fascicle. Of what type of CT is this sheath composed?

Perimysium (consists of dense irregular CT)

What benefit is provided by a lever that operates at a mechanical advantage? at a mechanical disadvantage?

Power lever operates at a mechanical advantage: the load is close to the fulcrum (point on which a lever is rested) and the effort is applied far from the fulcrum, hence a small effort can move a large load Speed lever operates at a mechanical disadvantage: if load is far from the fulcrum and the effort is applied near the fulcrum, greater force is required to move the load Benefit -> longer load arm is moved at a greater distance and speed In summary: Effort farther than load from fulcrum= lever operates at a mechanical advantage Effort nearer than load to fulcrum= lever operates at a mechanical disadvantage

Describe how the following organelles are modified or adapted for function in skeletal muscle tissue: nucleus; mitochondria; smooth endoplasmic reticulum.

**Have no clue

Classify each muscle fiber type with respect to its predominant pathway for ATP synthesis, the amount of myoglobin present, and the activity of its myosin ATPase.

1. Speed of contraction Slow fibers and fast fibers with fiber shortening Reflects how fast their myosin ATPases split ATP, and the pattern of electrical activity of their motor neurons Contraction duration also varies with fiber type and depends on how quickly Ca 2+ moves from the cytosol into the SR 2. Major pathways for forming ATP Cells that rely mostly on oxygen using aerobic pathways for ATP generation -> oxidative fibers Those that rely more on anaerobic glycolysis and creatine phosphate -> glycolytic fibers Based on these 2 criterias, we have: Slow oxidative fibers Fast oxidative fibers Fast glycolytic fibers

Name and describe the muscle fiber's three avenues of ATP production.

1. direct phosphorylation of ADP by creatine phosphate, catalyzed by the enzyme creatine kinase Product: 1 ATP per CP 2. anaerobic pathway: glycolysis and lactic acid formation (glucose -> glycolysis in cytosol, release ATP -> pyruvic acid -> lactic acid released into blood) Products: 2 ATP per glucose, lactic acid 3. aerobic pathway: cellular respiration (glucose -> pyruvic acid -> fatty acid and amino acid supply mitochondria -> CO2 & H2O & ATP) Products: 32 ATP per glucose, CO2, H2O

What is acetylcholine? What role does it play at the neuromuscular junction?

A neurotransmitter that triggers electrical events that ultimately generate an action potential (open Na+ channels, releasing Na+)

At what point (#45) does the power stroke occur?

ADP and Pi are released and the myosin head pivots and bends to low energy state -> pulls the actin filament toward the M line

What changes are induced in skeletal muscle in response to a program of endurance (i.e., "cardio") exercise training? Which muscle fiber type is most responsive to endurance training?

Aerobic changes- Number of capillaries surrounding the muscle fiber increases Number of mitochondria within muscle fibers increases Fibers synthesize more myoglobin Changes are most dramatic in slow oxidative fibers, result in more efficient muscle metabolism and in greater endurance, strength, and resistance to fatigue, may convert fast glycolytic fibers -> fast oxidative fibers Slow-twitch (type 1) fibers are especially responsive

Describe the stress-relaxation response.

Allows hollow organ to fill or expand slowly to accommodate greater volume without causing strong contractions that would expel its contents; important for stomach and intestines to absorb nutrients

Which sources (#78) of ATP are produced anaerobically? Which are produced aerobically?

Anaerobically -> creatine pathway and glycolysis Aerobically -> aerobic respiration

What happens to the A band, I band, H zone, and Z discs during contraction? to sarcomere length?

As thin filaments slide centrally, the Z discs are pulled toward the M line -> overall, I bands shorten, the distance between successive Z discs shorten, the H zones disappear, the A bands move closer together but their length does not change

What causes myosin to detach from actin after the power stroke is completed?

Attachment of ATP

Describe the structure of the neuromuscular junction.

Axon terminals, synaptic cleft, and junctional folds of the sarcolemma ACh is released at the neuromuscular junction

What is the role of Ca2+ and the regulatory proteins in the sliding filament mechanism?

Bind to tropomyosin, changing its shape and moving it away from myosin binding sites

How is smooth muscle contraction regulated?

By nerves, hormones, or local chemical changes Neural regulation: neurotransmitter binding generates action potential => rise in Ca ions Effect differs depending on receptors: ACh => strong contraction that narrows bronchioles Norepinephrine => inhibitory, muscle relaxes dilating bronchioles Smooth muscle in walls of most blood vessels => contraction and contraction of vessel Hormones: some smooth muscles don't have nerve supply; instead they depolarize spontaneously or in response to chemical stimuli that bind to G protein-linked receptors Ex: histamine, excess carbon dioxide, low pH, lack of oxygen Alter according to local tissue needs Chemical Factors: causes smooth muscle to contract or relax without an action potential by enhancing or inhibiting Ca ion entry into the sarcoplasm

What two factors permit relaxation?

Ca2+ levels fall Myosin-actin interaction is inhibited by ATP

What type of tension does a muscle develop when it is acting as an agonist?

Concentric: muscle shortens and does work, such as picking up a book

What is a motor unit?

Consists of one motor neuron and all the muscle fibers it innervates or supplies -> simulation of it causes a weak contraction of the entire muscle

Describe the structure of a myosin molecule. How are myosin molecules arranged to make up a thick filament?

Consists of two heavy and four light polypeptide chains and has a rodlike tail attached by a flexible hinge to two globular heads "business end" of myosin, tail consists of two intertwined helical polypeptide heavy chains About 300 myosin molecules form a thick filament, with their tails forming the central part of the thick filament and their heads facing outwards at the end of each thick filament -> central portion is smooth, but its ends are studded with a staggered array of myosin heads Head bears actin, ATP-binding sites, and intrinsic ATPase activity that splits ATP to generate energy for muscle contraction

What is the effect of load and recruitment on the duration of a muscle contraction?

Contract fastest when there is no added load on them; greater load -> a longer latent period -> a slower contraction -> shorter duration of contraction The more motor units contracting -> the faster and more prolonged the contraction

Which property is the muscle's most distinguishing, or unique, characteristic?

Contractility

What happens to the strength of contraction when two or more identical stimuli are delivered to the muscle in rapid succession (i.e., before complete relaxation occurs)? What is this phenomenon called?

Contractions are added together with more Ca in the cytosol; wave summation or tetanus

What are graded muscle responses? What are the two general ways of producing a graded muscle response?

Contractions of varying intensities- Frequency (speed of stimulation) Temporal/wave summation -> produced smooth, continuous muscle contractions by rapidly stimulating a specific number of muscle cells Tetanus (complete + incomplete) Intensity Recruitment Muscle force can increase until reaches maximal stimulus

What happens to pyruvic acid when there is not enough O2 present to completely hydrolyze it?

Converted to lactic acid

Identify the ways in which ATP is used to directly power muscle contraction events.

Cross Bridge Cycle 1. Cross bridge formation -> energized myosin head attaches to an actin myofilament, forming a cross bridge 2. The power stroke -> ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state -> pulls on actin filament toward the M line 3. Cross bridge detachment -> after ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches 4. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy or "cocked positon" Goes back to Step 1

Relative to muscle function, what is the significance of Sharpey's fibers?

Def: matrix of connective tissue consisting of bundles of strong collagenous fibers connecting periosteum to bone The tendons of muscles intertwine with these strong and anchored fibers in order to create a strong bond to the bone -> beneficial when muscles pull on bones so the muscles do not detach

What structures anchor smooth muscle fibers and transmit the tension they develop?

Dense bodies

What is the difference between unfused tetanus and complete tetanus? How is complete tetanus achieved in a muscle fiber?

Unfused tetanus: sustained but osciliating contraction Complete tetanus: muscle tension reaches maximum tension, all evidence of muscle relaxation disappears and the contraction fuse into a smooth sustained contraction plateau

Where is each type of smooth muscle located?

Unitary smooth muscle (visceral muscle)-> in walls of all hollow organs except heart Multi Unit-> airways of lungs of large arteries, arrector pili muscles attached to hair follicles, internal eye muscles

Smooth muscle can be categorized as single-unit smooth muscle (which Marieb calls "unitary smooth muscle) or multi-unit smooth muscle. Describe the ways in which these two types of smooth muscle differ.

Unitary smooth muscle (visceral muscle): in walls of all hollow organs except heart Arranged in opposing (longitudinal and circular) sheets Innervated by varicosities of autonomic nerve fibers and exhibit rhythmic apontaneous action potentials Electrically coupled by gap junctions thus contract as a unit Respond to chemical stimuli Multi Unit: airways of lungs of large arteries, arrector pili muscles attached to hair follicles, internal eye muscles; no gap junctions and spontaneous depolarizations Similar to skeletal muscle in that: Muscle fibers structurally independent of each other Richly supplied with nerve endings, each forming a motor unit with a number of muscle fibers Responds to neural stimulation with graded contractions involving recruitment Also innervated by autonomic division and responds to hormones

Which type of smooth muscle exhibits rhythmicity and self-excitation?

Unitary/visceral

What is the most prevalent chemical compound (by weight) in muscle?

Water (**double check)

At what point in cross bridge cycling (#45) does ATP become hydrolyzed, and provide the energy for activation of the myosin head?

When the myosin head detaches from the cross bridge and ATP is hydrolyzed, the myosin head returns to the "cocked" position

What is the significance of the Ca2+ release channels and the Ca2+ active transport pumps in the SR?

While Ca2+ is in the sarcoplasm, tropomyosin will not cover the myosin-binding site on the G actin; muscles will continue contracting. When Ca2+ pumps the ion back into the cisternae, tropomyosin covers the myosin binding site and contraction stops

What are the four general functions of the muscular system?

Produce movement- responsible for all locomotion (moving from place to place) and manipulation Ex. blood courses through body because of beating cardiac muscle Maintain posture and body position- function almost continuously, constantly making adjustments to counteract the pull of gravity Stabilize joints- even when they pull on bones to cause movement, they strengthen and stabilize joints of skeleton Generate heat- skeletal muscles generate heat as they contract -> maintain normal body temperature Additional functions- skeletal muscles protect the fragile internal organs; smooth muscles form valves to regulate the passage of substances through internal body openings, dilates and constricts the pupils of eyes, and forms the arrector pili muscles attached to hair follicles

What is a sarcomere?

Region of a myofibril between 2 successive Z discs Smallest contractile unit of a muscle fiber (functional unit of a skeletal muscle) Contains an A band flanked by half an I band at each end Within a myofibril, sarcomeres align end to end like boxcars in a train

What is the function of the SR and terminal cisternae?

Regulates levels of ionic calcium, stores calcium and releases it on demand when the muscle fiber is stimulated to contract

What gives skeletal muscle fibers a striated appearance?

Repeating series of dark (A bands) and light bands (I bands) that are nearly perfectly aligned

What changes are induced in skeletal muscle in response to a program of resistance exercise training? Which muscle fiber types seem to be especially responsive to resistance training

Resistance changes- Muscle hypertrophy -> additional muscle bulk because of increased number of muscle fibers More mitochondria, myofilaments and myofibrils Store more glycogen Develop more CT between muscle cells Collectively, these changes promote significant increases in muscle strength and size Convert fast oxidative fibers to fast glycolytic fibers Fast-twitch (type 2) fibers are especially responsive

What is a muscle twitch?

Response of a muscle to a single simulation -> the muscle fibers contract quickly and then relax

Describe the major structural and functional differences between smooth and skeletal muscle.

Smooth Structure: spindle-shaped cells of variable size each with one centrally located nucleus, 10 times shorter and narrower compared to skeletal muscle fibers, lacks coarse CT sheaths seen in skeletal muscles but contains small amount of endomysium with blood vessels and nerves Organized in sheets of closely apposed fibers in all but smallest blood vessels and walls of hollow organs of the respiratory, digestive, urinary, and reproductive tracts Longitudinal layer- parallel to long axis of the organ and shorten during contraction Circular layer- fiber runs around the circumference of the organ and contraction constricts the lumen (cavity inside organ) and elongates the organ Lacks highly structured neuromuscular junctions of skeletal muscle Varicosities- numerous bulbous swellings that release neurotransmitter into wide synaptic cleft (called diffuse junctions) in smooth muscle cells (instead of NMJ) Sarcoplasmic reticulum much less developed and lacks specific pattern relative to myofilaments No T tubules but has multiple caveolae, pouchlike infoldings secrete bits of extracellular fluid containing a high concentration of Ca2+ close to the membrane -> when Ca channels open, Ca2+ influx occurs rapidly, most Ca2+ enters through Ca channels directly from extracellular space No striations and thus no sarcomeres; myosin filaments (different from skeletal muscle type) are a lot shorter than the actin filaments Function: contraction in rectum, urinary bladder, and uterus helps organs expel contents; accounts for constricted breathing of asthma and stomach cramps Fewer thick filaments but have myosin heads along the entire length; ratio of thick to thin much lower in smooth than skeletal and heads are oriented in one direction on one side of the filament and oppositely on the other side of the filament No troponin complex in thin filaments, has calmodulin instead Thick and thin filaments arranged diagonally thus contract like tiny corkscrews Intermediate filament-dense body network: lattice-like arrangement of non-contractile intermediate filaments that resist tension and attach to dense bodies, cable-like intracellular cytoskeleton that harnesses pull generated by the sliding of thick and thin filaments; accounts for synchronous contractions Contraction mechanism by gap junctions allowing for slow, synchronized contractions Skeletal Fibers up to 10 times wider and has coarse connective tissue sheaths, highly structured neuromuscular junctions Doesn't depend on extracellular Ca for excitation- contraction coupling, coupling ends when cytoplasmic calcium is actively transported into the SR and out of cell Has troponin in thin filaments Contraction mechanism- electrically isolated from each other and stimulated by own neuromuscular junction

Name and compare the two types of indirect muscle attachments.

Tendon: ropelike Mostly tough collagen fibers that can withstand abrasion of rough bony projections that would tear apart more delicate muscle tissues Conserve space Aponeurosis: sheetlike

What is the relationship between the initial length of the sarcomere (or muscle fiber) and the amount of tension it can produce?

Tension that the muscle can generate varies with length; ideal when muscle is slightly stretched and the thin and thick filaments overlap optimally, because this permits sliding along nearly the entire length of the thin filaments Problems: If muscle is stretched so much that filaments don't overlap, myosin heads have nothing to attach to and cannot generate tension Too compressed so that thin filaments interfere with one another, little or no shortening occurs

Identify the two primary types of myofilaments found in myofibrils.

Thick: contain myosin, extend the entire length of the A band, connected in the middle of the sarcomere at the M line Thin: contain actin across the I band and partway into the A band

Muscle contraction is currently best described by the sliding filament theory. Why is this name appropriate for the contraction process?

Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree: When the NS stimulates muscle fibers, the myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments, and the sliding begins Attachment forms and breaks several times during contraction -> creating tension and propel thin filaments toward the center of the sarcomere Simultaneously, the muscle shortens As thin filaments slide centrally, the Z discs are pulled toward the M line -> overall, I bands shorten, the distance between successive Z discs shorten, the H zones disappear, the A bands move closer together but their length does not change

What is the most common lever system in the body? Where is the fulcrum with respect to the load and effort in this lever system?

Third-class lever: effort is applied between the load and the fulcrum, operate at a mechanical disadvantage, example is the biceps of the arm lifting the distal forearm and anything carried int he hand, permits rapid, extensive movements (e.g. throwing) thicker and more powerful muscles involved *non-irrelevant First-class: effort applied at one of the lever and the load is at the other with the fulcrum somewhere between, mix of mechanical advantage and disadvantage (seesaws and scissors) Second-class: effort is applied at one end of the lever and the fulcrum is located at the other, with the load between them, levers for strength (stand on your toes)

What 4 factors influence the amount of force that a muscle generates during contraction?

1. number of muscle fibers recruited The more motor units -> the greater the muscle force 2. relative size of fibers The bulkier the muscle and the greater the cross sectional area, the more tension it can develop The large fibers of large motor units produce the most powerful movements Regular resistance exercise increases muscle force by causing muscle cells to hypertrophy (increase in size) 3. frequency of stimulation Contractions are added together -> more vigorous and produce tetanus Higher frequency -> the greater the force 4. degree of muscle stretch Tensions generated varies with length Ideal relationship-> muscle is slightly stretched and thick and thin filaments overlap optimally

Describe the chemical composition of muscle fibers.

75% water, 20% protein, 1-10% fat, and 1% glycogen (**double check)

Where is most of the cell's ATP produced?

95% of the ATP used for muscle activity comes from aerobic respiration

What causes each of the bands, zones, and lines in a myofibril?

A band: thick filaments extends the entire length, and thin filaments partway = overlapping I band: only thin filaments H zone: only thick filaments M line: fine proteins that hold adjacent thick filaments together

What is the role of ATP and the myosin head in the sliding filament mechanism?

ATP necessary for myosin heads to detach from myosin cross bridge and its hydrolyzation returns myosin head to its pre-stroke high energy "cocked" position Myosin head binds to cross bridge and breaks and reattaches taking another step by attaching to an actin site further along the thin filament, shortening the muscles

What prevents prolonged stimulation of the muscle fiber by acetylcholine (ACh)?

Acetylcholinesterase, an enzyme located in the synaptic cleft -> breaks down ACh to its building blocks, acetic acid and choline

What is recruitment (i.e., multiple motor unit summation)?

Achieved by delivering shocks of increasing voltage to the muscle, increase the stimulus intensity, controls the force of contraction more precisely, rapidly stimulates a specific number of muscle cells Subthreshold sitmuli: stimuli that produce no observable contractions Threshold stimulus: where first observable contraction occurs, beyond this point, the muscle contracts more and more vigorously as the stimulus strength increases Maximal stimulus: strongest stimulus that increases contractile force, all the muscle's motor units are recruited Follows a size principle -> smallest muscle fiber activated first; as motor units with larger muscle fibers begin to be excited, contractile strength also increases; the largest motor units have 50x the contractile force as small ones Significance? allows the increases in force during weak contractions to occur in small steps

Describe the relative positions of actin and myosin when the muscle is at its optimal length for strength production.

Actin is above the myosin heads, length at which is generates maximum source when muscle is slightly stretched; actin and myosin slightly overlap

Identify the proteins which make up thin filaments.

Actin, nebulin, tropomyosin, troponin

Identify the regulatory protein in smooth muscle which binds to Ca2+ to activate the contraction process.

Calmodulin

What happens to force production in a muscle that is in extreme stretch or contraction?

Cannot generate tension

What are the sources of the sarcoplasmic Ca2+ which triggers smooth muscle contraction? Which of these is the main source?

Caveolae in the SR and calcium channels from the extracellular space (main source)

What causes Ca2+ to be released from the terminal cisternae?

Change in shape of voltage sensitive tubule proteins, opening Ca2+ release channels

Name and describe the four arrangements of fascicles/fibers within skeletal muscles.

Circular- concentric rings Muscles with this arrangement surround external body openings, which they close by contacting sphincters Convergent- converge toward a single tendon/ attachment Broad origin Triangular or fan shaped like the pectoralis major muscle Parallel- fascicles run parallel to the long axis Muscles either straplike like the sartorius muscle of the thigh or spindle shaped like the biceps brachii muscle of the arm (fusiform muscles) Pennate- short and attach obliquely (uni/multi/bi) Uni: fascicles insert into only one side of the tendon (extensor digitorum longus muscle) Bi: in which fascicles insert into the tendon from opposite sides so the muscle looks like a feather (rectus femoris) Multi: looks like many feathers side by side with all their quills inserted into one large tendon (deltoid muscle)

Describe the difference between indirect and direct muscle attachments.

Direct: epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage Indirect: muscle's connective tissue wrappings extend beyond the muscle either as a ropelike tendon or as a sheetlike aponeurosis Tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element or to the fascia of other muscles Much more common because of durability and small size

When is the (ADP + Pi ) which fueled the power stroke (#45) released from the myosin head?

During the power stroke

What is the function of the Z disc?

Midline interruption of the I band, coin-shaped sheet composed largely of the protein alpha-actinin Anchors thin filaments Intermediate (desmin) filaments extend from the Z disc and connect each myofibril to the next throughout the width of the muscle cell

What role does complete tetanus play in voluntary movements?

Shows superhuman strength by lifting a fallen tree limb off a companion However, vigorous muscle activity cannot continue indefinitely -> inevitably leads to muscle fatigue, muscle cannot contract and its tension drops to zero

Explain the all-or-none law as it relates to skeletal muscle fibers and motor units.

Single excitation event will elicit a single contraction due to the motor neurons (will definitely occur) Functional relationship- fired excitation signal will trigger contraction in all fiber parts of the motor unit

Classify the three types of muscle tissue (according to location, appearance, and type of nervous system control).

Skeletal tissue- Skeletal, striated, and voluntary Longest muscle cells Responsible for overall body mobility Exert tremendous power Highly adaptable Cardiac Muscle- Cardiac, striated, and involuntary Occurs only in the heart, where it constitutes the bulk of the heart walls Usually contracts at a fairly steady rate but neural controls allows the heart to be sped up for brief periods Smooth Muscle- Visceral, non-striated, and involuntary Found in the walls of hollow visceral organs such as stomach, urinary bladder, respiratory passages Force fluids and other substances through internal body channels Consists of elongated cells like skeletal muscle

Which contracts more quickly -smooth or skeletal muscle? Which contracts more efficiently? Which is most fatigue-resistant?

Skeletal- quicker; Smooth- more efficient and fatigue resistant; muscle myofilaments can latch together during prolonged contractions to save energy; low energy requirements and makes enough ATPase via aerobic pathways to keep up with demand

Which fiber type receives the richest blood supply?

Slow oxidative

What is the synaptic cleft?

Space filled with gel-like extracellular substance rich in glycoproteins and collagen fibers (between axon terminal and muscle fiber (motor end plate))

What is muscle tone, and why is it important?

Spinal reflexes that activate first one group of motor units and then another in response to activated stretch receptors in the muscles -> even relaxed skeletal muscles are always slightly contracted No new active movements, keeps the muscles firm, healthy, and ready to respond to stimulation Helps to stabilize joins and maintain posture

What is muscle fatigue?

State of physiological inability to contract even though the muscle still may be receiving stimuli

Describe the relationship between resting length and tension development in smooth muscle.

Stretching provokes contraction but muscle will adapt to increased tension, adapting to new length

How does the size of a motor unit influence the strength of contraction and the control of a movement?

The greater cross sectional area the more tension it can develop and the greater its strength; larger less refined movements

What is the effect of load on the velocity of contraction?

The greater the load, the slower the muscle shortening If the load exceeds the muscle's maximum tension, the speed of shortening is zero and the contraction is isometric

Which protein covers the active sites on the actin filament in a resting muscle fiber?

Tropomyosin

Which of the actin is/are regulatory proteins?

Tropomyosin, troponin

Specifically, how do troponin and tropomyosin help regulate the contraction process?

Troponin changes shape when bound by calcium and rolls tropomyosin into groove of actin helix away from myosin binding sites Tropomyosin blocks active binding sites during low calcium levels Troponin activates movement of tropomyosin inactivating its "blockade"