A&P 1 Ch 9

Myofilaments

The contractile proteins, actin and myosin, of muscle cells

sarcoplasm

cytoplasm of a muscle cell similar to other the cytoplasm of other cells but it contains unusually large amounts of glycosomes and myoglobin

velocity and duration of contraction

muscles vary in how fast they can contract and how long they can continue to contract before they fatigue. these characteristics are influenced by muscle fiber type, load, and recruitment.

muscle cells contain 3 specialized structures:

myofibrils, sarcoplasmic reticulum, and T tubules

which protein can act as an enzyme to hydrolyze (split) ATP?

myosin

activities that require a surge of power but only last a few seconds weight lifting, diving, and sprinting,

rely entirely on ATP and CP stores

when describing a muscle, what does "striated" mean?

striated means "with stripes"

unipennate:

fascicles attach only to one side of tendon (example: extensor digitorum longus)

sarcolemma

muscle cell plasma membrane

myasthenia gravis

(asthen = weakness, gravi = heavy) a disease characterized by drooping upper eyelids, difficulty speaking and swallowing, and generalized msucle weakness, involves a shortage of ACh receptors.an autoimmune disease in which the immune system destroys ACh receptors.

myo and mys are both prefixes meaning:

"muscles"

lever system

Consists of a lever (bone), effort (muscle action), load (weight of object to be moved), and fulcrum (joint).

skeletal muscle contraction step 4

Cross bridge cycling: the muscle contracts as a result of repeating cycle of steps that cause myofilaments to slide relative to each other

myoglobin

a red pigment that stores oxygen; similar to hemoglobin (transports oxygen in blood)

one type of electrical signal is called action potential (AP, sometimes called a nerve impulse)

an AP is a large change in membrane potential that spreads rapidly over long distances within a cell. generally AP's dont spread from cell to cell. for this reason, the signal has to be converted into a chemical signal-a chemical messenger called a neurotransmitter that diffuses across the small gap between excitable cells to start the signal again. the neurotransmitter that motor neurons use to "tell" skeletal muscle to contract is acetylcholine or ACh.

what is a motor unit?

an axon of a motor neuron and all the muscle fibers it innervates

mutlipennate:

appears as feathers inserting into one tendon (example: deltoid)

why are there no slow glycolytic fibers? why so they make no sense?

it makes sense that there are no slow glycolytic fibers because glycolysis is a fast but ineffective path for generating ATP. why use such an inefficient pathway if the contraction is going to be slow anyway (and so use up ATP slowly)?

muscle contraction depends on

the myosin and actin containing myofilaments

mechanical disadvantage

(Speed lever) load is far from fulcrum, with effort close to fulcrum - The load moved rapidly over large distance: offers wider range of motion

mechanical advantage

(power load) load is close to fulcrum, with effort far from fulcrum - small effort can move large load

same muscle may be:

- Prime mover of one movement - Antagonist for different movement - Synergist for third movement

the number of myosin cross bridges that are attached to actin is affected by 4 factors

- frequency of stimulation: when a muscle is stimulated more frequently, contractions are summed (temporal summation tetany) . the higher the frequency of muscle stimulation, the greater the force the muscle exerts. - number of muscle fibers recruited: the more motor units recruited, the greater the force. - size of muscle 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). - degree of muscle stretch: if a muscle is stretched to various lengths and maximally stimulated , the tension the muscle can generate varies with length. the amount of tension a muscle can generate with isometric contraction at various lengths- its length tension relationship- can be shown graphically.

contraction in smooth muscle is like contraction in skeletal muscle in the following ways:

- the final trigger for contraction is a rise in the intracellular calcium iron level - actin and myosin interact by the sliding filament mechanism - ATP energizes the sliding process

skeletal muscle attachments

-Most skeletal muscles span joints and are attached to bone in at least two places -When muscles contract, the movable bone; the muscle's insertion moves toward the immovable bone, the muscle's origin - in the muscles of the limbs, the origin usually lies proximal to the insertion

slow oxidative fibers

-contracts slowly because its myosin ATPase are slow -depends on oxygen delivery and aerobic pathways (its major pathways for forming ATP give it high oxidative capacity) - resists fatigue and has high endurance (typical of fibers depend on aerobic metabolism) - is thin (a large amount of cytoplasm impedes diffusion of O2 and nutrients from the blood) - has relatively little power ( a thin cell can only contain a limited number of myofibrils) - has many mitochondria (actual sites of oxygen use) - has rich capillary supply (the better to deliver bloodborne O2). - is red (its color stems from an abundant supply of myoglobin, muscle's oxygen-binding pigment) Add these features together and you have a muscle fiber best suited to endurance-binding pigment)

excitation-contraction (E-C) coupling

-is the sequence of events by which transmission of an action potential along the sarcolemma causes myofilaments to slide. The action potential is brief and ends before any signs of contraction are obvious. the electrical signal does not act directly on the myofilaments to slide. instead, it causes the rise in intracellular levels of calcium ions, which triggers a sequence of events that ultimately leads to sliding of the filaments

muscle contractions can be graded in two ways:

1. an increase in the frequency of stimulation causes temporal summation. the higher the frequency, the greater the strength or contraction of a given motor unit. 2. an increase in the strength of stimulation causes recruitment. the stronger the stimulation, the more motor units are activated, and the stronger the contraction. in the lab, it is easy to adjust the settings used to artificially stimulate a muscle . the the body, the brain determines the strength of a muscle's contraction by changing 1. the rate of firing of action potentials along the axon of its motor neuron (frequency) and 2. the number of its motor neurons that are activated (strength) . in this way, we automatically and continuously adjust the strength of our muscle contractions . we are only aware of this process when it doesn't work right. for example, if you life what you expect to be a heavy box that is in fact empty, you will use more muscle power than you should and the box will go flying!

two classes of ion channels are important for excitation and contraction of skeletal muscle:

1. chemically gated ion channels 2. voltage-gated ion channels

the difference between smooth and skeletal muscle fibers

1. smooth muscle fibers are small spindle -shaped cells : each cell have one centrally located nucleus . typically , smooth muscle fibers have a diameter of 5-10 um and are 30-200 um long. skeletal muscle fibers are up to 10 times wider and thousands of times longer 2. smooth muscle lacks the coarse connective tissue sheaths found in skeletal muscle: a small amount of fine connective tissue (endomysium) is secreted by the smooth muscles themselves . it is found between smooth muscle fibers and contains blood vessels and nerves. epimysium and perimysium are not present. 3. smooth muscle has varicosities instead of neuromuscular junctions: the innervating nerve fibers of the autonomic (involuntary) nervous system have numerous bulbous swellings called varicosities. unlike the highly structured neuromuscular junctions of the skeletal muscle, varicosities form diffuse junctions that have wide synaptic clefts. the varicosities simply "sprinkle" neurotransmitter in the general area of the smooth muscle cells. comparing the neural input to skeletal and smooth muscles, you could say that skeletal muscle gets high-speed internet while smooth muscle gets slow-speed dial up. 4. smooth muscle have less elaborate SR and no T tubules: the SR of smooth muscle fibers has no terminal cisterns and lacks a specific pattern relative to the myofilaments . although the SR does release some of the Ca2+ that triggers contraction, most Ca2+ enters through calcium channels directly from the extracellular space. the sarcolemma has multiple caveolae ("little caves"), pouch like infoldings containing large numbers of Ca2+ channels . these calcium channels are the major source of Ca2+ for smooth muscle contraction. this situation is quite different from what we see in skeletal muscle, which does not depend on extracellular Ca2+ for excitation-contraction coupling. - smooth muscle fibers are usually electrically connected by gap junctions: gap junctions are specialized cell connections that allow depolarization to spread from cell to cell. in contrast, skeletal muscle fibers are electrically isolated from one another.

skeletal muscle fibers contain two sets of intracellular tubules that help regulate muscle contraction

1. the sarcoplasmic reticulum 2. T tubules

There are no striations in smooth muscle, as its name indicates , and therefore no sarcomeres. smooth muscle fibers do contain overlapping thick and thin filaments, but the myosin filaments are a lot shorter than the actin filaments and the type of myosin contained differs from skeletal muscle . the proportion and organization of smooth muscle myofilaments differ from skeletal muscles in the following ways:

1. thick filaments are fewer but have myosin heads along their entire length: the ratio of thick to thin filaments is much lower in smooth muscle than in skeletal (1:13 vs 1:2) . however, thick filaments of smooth muscle contain actin - gripping myosin heads along their entire length, a feature that makes smooth muscle as powerful as a skeletal muscle of the same size. also in smooth muscle the myosin heads are oriented in one direction on one side of the filament and in the opposite direction on the other side. - no troponin complex in thin filaments : as in skeletal muscle, tropomyosin mechanically stabilizes the thin filaments , but smooth muscle has no calcium - binding troponin complex. instead, a protein called calmodulin acts as the calcium - binding site. - intermediate filament-dense body network: smooth muscle fibers contain a lattice-like arrangement of noncontractile intermediate filaments that resist tension. they attach at regular intervals to cytoplasmic structures called dense bodies, which are also tethered to the sarcolemma, act as anchoring points for thin filaments and therefore correspond to Z discs of skeletal muscle. The intermediate filament-dense body network forms a strong , cable like intracellular cytoskeleton that harnesses the pull generated by the sliding of the thick and thin filaments. during contraction, areas of the sarcolemma between the dense bodies bulge outward, making the cell look puffy. dense bodies at the sarcolemma surface also bind the muscle cell to the connective tissue fibers outside the cell and to adjacent cells. this arrangement transmits the pulling force to the surrounding connective tissue and partly accounts for the synchronous contractions of most smooth muscle - thick and thin filaments arranged diagonally: bundles of contractile proteins crisscross within the smooth muscle cell so they spiral down the long axis of the cell like the stripes on a candy cane. bc of this diagonal arrangement , the smooth muscle cells contract in a twisting way so that they look like tiny corkscrews.

from the time an action potential reaches the axon terminal of a motor neuron until cross bridge cycling begins, several sets of ion channels are activated. list these channels in the order that they are activated and state what causes each to open

1. voltage gated calcium channels in the axon terminal of the motor neuron are opened by the action potential running down the axon. 2. chemically gated channels permeable to both Na and K are opened by ACh binding to ACh receptors on the junctional folds of the sarcolemma. 3. voltage gated Na and K channels are required for generating and propagating the action potential along the sarcolemma. the end plate potential intitates the action potential by opening the first set of voltage gated Na channels, and then the action potential itself opens more voltage gated channels as it moves along the sarcolemma. 4. calcium release channels in the terminal cisterns of the SR open to release calcium into the cytosol. these channels open because an AP running along the T tubules causes the voltage sensitive tubules proteins to change shape, and this shape change causes the calcium release channels of the SR to open.

Generation of an Action Potential Across the Sarcolemma

3 steps : generation of an end plate potential followed by action potential depolarization and repolarization

Fascicle arrangements

All skeletal muscles consist of fascicles (bundles of fibers) - fascicle arrangements vary, resulting in muscles with different shapes and functional capabilities

the most common form to muscular dystrophy

Duchenne muscular dystrophy (DMD)

nerve and blood supply

Each muscle receives a nerve, artery, and veins Consciously controlled skeletal muscle has nerves supplying every fiber to control activity Contracting muscle fibers require huge amounts of oxygen and nutrients Also need waste products removed quickly

skeletal muscle contraction step 1

Events at the neuromuscular junction: the motor neuron releases ACh that stimulates the skeletal muscle fiber, causing a local depolarization (decrease in membrane potential) called an end plate potential (EPP)

Dark A bands have a lighter midsection called

H zone

dystrophin

Links thin filaments to the integral proteins of the sarcolemma (which are in turn, anchored to the extracellular matrix)

Each H zone is bisected vertically by a dark line called:

M line (M for middle) formed my molecules of the protein myomesin

primer mover (agonist)

Major responsibility for producing specific movement

skeletal muscle contraction step 2

Muscle fiber excitation: the EPP triggers an action potential that travels across the entire sarcolemma

which region or organelle - cytosol, mitochondria, or SR- contains the highest concentration of calcium ions in a resting muscle fiber?

SR

Fusiform

Spindle-shaped muscles with parallel fibers (e.g., biceps brachii)

first class lever

The fulcrum is positioned between the effort and effort Example: seesaw, scissors

sarcomeres

The region of a myofibril between two successive Z discs averaging 2 um long, a sarcomere is the smallest contractile unit of a muscle fiber - the functional unit of skeletal muscle. it contains an A band flanked by half an I band at each end within each myofibril, the sarcomeres align end to end like boxcars in a train

synaptic vesicles

Within the moundlike axon terminal are synaptic vesicles, small membraneous sacs containing the neurotransmitter acetycholine. The trough-like part of the muscle fiber's sarcolemma that helps form the neuromuscular junction is highly folded. these junctional folds provide a large surface area for the thousands of ACh receptors located there.

Each light I band also has a midline interruption, a darker area called

Z disc (or Z line)

Striations

a repeating series of dark and light bands; evident along the length of each myofibril. in an intact muscle fiber, the dark A bands and light I bands are nearly perfectly aligned, giving the cell its striated appearance

myofibrils

a single muscle fiber contains hundreds of thousands of rod like myofibrils that run parallel to its length. the myofibrils 1-2 um in diameter, are so densely packed in the fiber that the mitochondria and other organelles appear to be squeezed between them. they account for about . they account for about 80% of cellular volume

which muscle fibers would be beneficial to help move houses?

fast glycolytic fibers would provide for short periods of intense strength needed to life and move furniture.

How do aerobic and resistance exercise differ in their effects on muscle size and function?

aerobic exercise increases muscle endurance, whereas anaerobic exercise (such as high intensity resistance exercise) increases muscle size and strength.

aerobic (endurance) exercise

aerobic or endurance exercise, such as swimming running fast walking, and biking results in several recognizable changes in skeletal muscles: - the number of capillaries surrounding the muscle fibers increases - the number of mitochondria within the muscle fibers increases - the fibers synthesize more myoglobin These changes occur in all fiber types, but are most dramatic in slow oxidative fibers, which depend primarily on aerobic pathways. the changes result in more efficient muscle metabolism and in greater endurance, strength, and resistance to fatigue. regular endurance exercise may convert fast glycolytic fibers into fast oxidative fibers.

chemically gated channels

are opened by chemical messengers (ex: neurotransmitters). this class of ion channel creates small local changes in the membrane potential. receptors for acetycholine are an example of this class. an ACh receptor is a single protein in the plasma membrane that is both a receptor and an ion channel.

systems operating under mechanical advantage (power levers)

are slower, but more stable - used where strength is a priority

concentric isotonic contraction

are those in which the muscle shortens and does work, such as picking up a book (more familiar)

which pathways predominate during exercise?

as long as a muscle cell has enough oxygen, it will form ATP by the aerobic pathway. when ATP demands are within the capacity of the aerobic pathway, light to moderate muscular activity can continue for several hours in well-conditioned individuals . however , when exercise demands begin to exceed the ability of the muscle cells to carry out the necessary reactions quickly enough, anaerobic pathways begin to contribute more and more of the total ATP generated. the length of time a muscle can continue to contract using aerobic pathways is called aerobic endurance , and the point at which muscle metabolism converts to anaerobic glycolysis is called anaerobic threshold.

anaerobic pathway: glycolysis and lactic acid formation

as stored ATP and CP are exhausted, more ATP is generated by breaking down (catabolizing) glucose obtained from the blood or glycogen stored in the muscle. the initial phase of glucose breakdown is glycolysis. this pathway occurs in both the presence and the absence of oxygen, but because it does not use oxygen, it is an anaerobic pathway. during glycolysis, glucose is broken down to two pyruvic acid molecules, releasing enough energy to form small amounts of ATP (2 ATP per glucose) when sufficient oxygen is present, the pyruvic acid produced during glycolysis enters the mitochondria, producing still more ATP in the oxygen - using pathway called aerobic respiration, described shortly. when blood flow and oxygen delivery are impaired during vigorous muscle contraction, most of the pyruvic acid is converted into lactic acid, and the overall process is referred to as anaerobic glycolysis. oxygen delivery is impaired because bulging muscles compress the blood vessels within them. this happens when contractile actiivty reaches about 70% of the maximum possible (for example, when you run 600 meters with maximal effort.) lactic acid is the end product of glucose metabolism during anaerobic glycolysis. most of the lactic acid diffuses out of the muscles and into the blood stream. subsequently, the liver, heart, or kidney cells pick up the lactic acid and use it as an energy source. additionally, liver cells can reconvert it to pyruvic acid or glucose and release it back into the blood stream for muscle use or convert it to glycogen for storage. the anaerobic pathway is inefficient but fast. it harvests only about 5% as much ATP from each glucose molecule as the aerobic pathway, but it produces ATP about 2.5 times faster . for this reason, even when large amounts of ATP are needed for moderate periods (30-40 seconds) of strenuous muscle activity, glycolysis can provide most of this ATP. together, stored ATP and CP and the glycolysis -lactic acid pathway can support strenuous muscle activity for nearly a minute. although anaerobic glycolysis readily fuels spurts of vigorous exercise, it has short comings. huge amounts of glucose are used to produce relatively small harvests of ATP, and the accumulating lactic acid is partially responsible for muscle soreness during intense exercise.

fused or complete tetanus

as the stimulation frequency continues to increase, muscle tension increases until a maximal tension is reached; at this point all evidence of muscle relaxation dissapears and the contractions fuse into a smooth, sustained contraction plateau

direct phosphorylation of ADP by creatine phosphate

as we begin to exercise vigorously, the demand for ATP soars and the ATP stored in working muscles is consumed within a few twitches . then creatine phosphate (CP) a unique high energy molecule stored in muscles is tapped to regenerate ATP while other metabolic pathways adjust to the sudden high demand for ATP. coupling CP with ADP transfers energy and a phosphate group from CP to ADP to form ATP almost instantly. muscle cells store 2 to 3 times more CP than ATP. the CP reaction with ADP, catalyzed by the enzyme creatine kinase, is so efficient that the amount of ATP in muscle cells changes very little during the initial period of contraction. together, stored ATP and CP provide for maximum muscle power for about 15 seconds - long enough to energize a 100 meter dash. CP is replenished during periods of rest or inactivity

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 called the T tubule. the lumen (cavity) of the T tubule is continuous with the extracellular space. as a result, T tubules tremendously increase the muscle fiber's surface area. this allows changes in the membrane potential to rapidly penetrate deep into the muscle fiber. along its length, each t tubule runs between the paired terminal cisterns of the SR, forming triads.

load and recruitment

because muscles are attached to bones, they are always pitted against some resistance, or load, whemn they contract. as you might expect, they contract fastest when there is no asses load on them. a greater load results in a longer latent period, slower shortening, and a briefer duration of shortening. in the same way that many hands on a project can get a job done more quickly and can keep working together longer, the more motor units that are contracting, the faster and more prolonged the contraction.

example of using both concentric and eccentric

bicep curls . flexing is concentric , extending is eccentric

compare the structures of skeletal and smooth muscle fibers:

both are elongates, but smooth are tapered at the ends, and skeletal are long cylindrical shaped. smooth: uninucleate , non striated skeletal: multinucleate , striated

the greater the load, the ___ the duration of muscle shortening

briefer

convergent

broad origin; fascicles converge toward single tendon insertion. EX: pectoralis major

muscle fiber contraction: cross bridge cycling

ca2+ needed!!!! when intracellular calcium levels are low, the muscle cell is relaxed bc tropomysosin molecules physically block the myosin binding sites on actin. as calcium levels rise, the ions bind to regulatory site on troponin. 2 calcium ions must bind to a troponin, causing it to change shape and roll tropomyosin into the groove of the actin helix, away from the myosin binding site. in short, the tropomyosin blockade is removed when sufficient calcium is present. once binding sites on actin are exposed, the events of the cross bridge cycle occur in rapid succession. the cycle repeats and with each cycle, the myosin head takes another "step" by attaching to an actin site further along the thin filament. the thin filaments continue to slide as long as calcium and adequate ATP are present. myosin walks along the adjacent thin filaments during muscle shortening like a centipede. the thin filaments cannot slide backwards as the cycle repeats again and again because some myosin heads ("the legs") are always in contact with actin ("the ground"). contracting muscles routinely shorten by 30-35% of their total resting length, so each myosin cross bridge attaches and detaches many times during a single contraction. it is likely that only half of the myosin heads of a thick filament are pulling at the same instant. the others are randomly seeking their next binding site. as soon as calcium is released from the SR, the calcium pumps of the SR begin to reclaim it from the cytosol. as calcium levels drop, calcium comes off of troponin, which again changes shape and pulls tropomyosin up to block actin's myosin binding sites. the contraction ends, and the muscle fiber relaxes. when cross bridge cycling ends, the myosin heads remain in their upright high energy configuration, ready to bind actin when the muscle is stimulated to contract again. except for the brief period following muscle cell excitation, calcium ion concentrations in the cytosol are kept almost undetectably low. when nerve impulses arrive in quick succession, intracellular calcium levels soar due to successive "puffs" or bursts of calcium released from the SR. in such cases, the muscle cells do not completely relax between successive stimuli and contraction is stronger and more sustained (within limits) until nervous stimulation ceases.

calcium is the trigger for contraction of all muscle types. how does its binding site differ in skeletal and smooth muscle fibers?

calcium binds to troponin on the thin filaments in skeletal muscles calcium binds to a cytoplasmic proteins called calmodulin in smooth muscle cells

what causes DMD

caused by a defective gene for dystrophin, the cytoplasmic protein described above. dytrophin links the cytoskeleton to the extracellular matrix and like a girder, helps stabilize the sarcolemma. the fragile sarcolemma of DMD patients tears during muscle contraction, allowing entry of excess Ca2+ , which damages the contractile fibers. inflammatory cells (macrophages and lymphocytes) accumulate in the surrounding connective tissue. as the regenerative capacity of the muscles is lost, damaged cells undergo apoptosis, and muscle mass drops. there is still no cure for DMD. current treatments are aimed at reducing or preventing spine and joint deformities and helping those with DMD remain mobile as long as possible. two newly approved drugs may broaden the treatment options for certain patients

chris just ran with her cross country team, she is breathing heavy, why?

chris is breathing heavy because it takes some time for her heart rate and overall metabolism to return to the resting state after exercise. moreover, she has likely incurred an oxygen debt that requires her to take in extra oxygen , called EPOC, for the restorative process. although running can be an aerobic exercise , there is generally some anaerobic respiration that occurs as well - the amount depends on exercise intensity . the weakness she feels is in part due to muscle fatigue. muscle fatigue is likely due to a combination of factors in her muscles including increased inorganic phosphate and magnesium , decreased ATP and glycogen , and other ionic imbalances .

the most common patterns of arrangement

circular, convergent, parallel, pennate

elastic filament

composed of the giant protien titin; titin extends from the Z disc to the thick filament, and then runs within the thick filament (forming its core) to attach to M line; holds thick filaments in place, and maintains organization of A band, and helps muscle cell to spring back into shape after being streched (the part of the titin that spans the I band is extensible, unfolding when the muscle stretches and recoiling when the tension is released.) titin does not resist stretching in the ordinary range of extension, but it stiffens as it uncoils, helping the muscle resist excessive stretching, which might pull the sarcomeres apart.

the more LATERAL thin filaments

containing actin (blue) extend across the I band and partway into the A band. The Z disc, a protein sheet, anchors the thin filaments.

the CENTRAL thick filaments

containing myosin (red) extend the entire length of the A band they are connected at the middle of the sarcomere at the M line

fast glycolytic fibers

contracts rapidly due to the activity of fast myosin ATPases - uses little oxygen - depends on plentiful glycogen reserves for fuel rather than blood - delivered nutrients - tires quickly bc glycogen reserves are short-lived , making it a fatigable fiber - has a relatively large diameter, indicating both the plentiful myofilaments that allow it to contract powerfully before it "tires out" and its lack of dependence on continuous oxygen and nutrient diffusion from the blood - has few mitochondria , little myoglobin , and few capillaries (making it white) For these reasons, a fast glycolytic fiber is best suited for short - term, rapid, intense movements (moving furniture across a room).

recruitment or multiple motor unit summation

controls the force of contraction more precisely. it is achieved in the lab by delivering stimuli of increasing voltage, calling more and more muscle fibers into play.

period of contraction

cross bridges are active, from the onset to the peak of tension development, and the myogram tracing rises to a peak. this period lasts 10-100 ms.

prolonged activities such as marathon running where endurance rather than power is the goal

depend mainly on aerobic respiration using both glucose and fatty acids as fuels

repolarization

during repolarization stage, a muscle fiber is said to be in a refractory period, because the cell cannot be stimulated again until repolarization is complete. note that repolarization restores only the electrical conditions of the resting (polarized) state. The ATP dependent Na+ -K+ pump restores the ionic conditions of the resting state, but thousands of actin potentials can occur before ionic imbalances interfere with contractile ability. once initiated, the action potential is unstoppable. it ultimately results in contraction of the muscle fiber. although the action potential potential itself lasts for only a few milliseconds, the contraction phase of a muscle fiber may persist for 100 milliseconds or more and far outlasts the electrical event that triggers it.

aerobic respiration

during rest and light to moderate exercise, even if prolonged, 95% of the ATP used for muscle activity comes from aerobic respiration . aerobic respiration requires oxygen and mitochondria and involves a sequence of chemical reactions that break the bonds of fuel molecules and release energy to make ATP. aerobic respiration begins with glycolysis and is followed by reactions that take place in the mitochondria . it breaks down glucose entirely to water and carbon dioxide, and generates large amounts of ATP. the carbon dioxide released diffuses out of the muscle tissue into the blood stream, to be removed from the body by the lungs. as exercise begins, muscle glycogen provides most of the fuel. shortly thereafter, blood borne glucose , pyruvic acid from glycolysis, and free fatty acids are the major source of fuels. after ab 30 min, fatty acids become the major energy fuels. aerobic respiration provides a high yield of ATP (about 32 atp per glucose) , but it is slow bc of its many steps and it requires continuous delivery of oxygen and nutrient fuels to keep it going.

the motor unit

each muscle is served by at least one motor unit, and each motor nerve contains axons (fibrous extensions) of up to hundred of motor neurons. as an axon enters a muscle, it branches into a number of endings, each of which forms a neuromuscular junction with a single muscular fiber. A motor unit consists of one motor neuron and all the muscle fibers it innervates, or supplies. when a motor neuron fires (transmits an action potential), all the muscle fibers it innervates contract. The number of muscle fibers per unit may be as high as several hundred, or as few as four. muscles that exert fine control (such as those controlling the fingers and eyes) have small motor units. by contrast, large, weight bearing muscles, whose movements are less precise (such as the hip muscles) have small motor units. the muscle fibers in a single motor unit are not clustered together but are spread throughout the muscle. as a result, stimulation of a single motor unit causes a weak but uniform contraction of the muscle.

the following regulatory proteins help form the structure of the myofibrils

elastic filament and dystrophin

multi uit smooth muscle

examples of multi unit smooth muscle are: the smooth muscles in the large airways to the lungs and in large arteries , the arrector pili muscles attached to hair follicles, and the internal eye muscles that adjust pupil size and allow the eyes to focus. in contrast to unitary muscle, gap junctions and spontaneous depolarizations are absent . like skeletal muscle, multi unit smooth muscle: - consists of muscle fibers that are structurally independent of one another - is richly supplied with nerve endings, each of which forms a motor unit with a number of muscle fibers - responds to neural stimulation with graded contractions that involve recruitment however , skeletal muscle is served by a somatic (voluntary) division of the nervous system. multi unit smooth muscle , like unitary muscle , is innervated by the automomic (involuntary) division and also responds to hormones

four key characteristics are muscle tissue:

excitability and responsiveness: the ability for a cell to receive and respond to stimulus by changing it's membrane potential. in the case of muscle, the stimulus is usually a chemical - for example, a neurotransmitter released by a nerve cell. contractibility: is the ability to shorten forcibly when adequately stimulated. this ability set muscles apart from all other tissues extensibility: the ability to extend or stretch. muscle cells shorten when contracting, but they can be stretched, even beyond their resting length, when relaxed elasticity: the ability for a muscle cell to recoil and resume its resting length after stretching

skeletal muscle contraction step 3

excitation-contraction coupling: the AP in the sarcolemma propagates along the T tubules and causes release of Ca2+ from the terminal cisterns of the SR. Ca2+ is the final trigger for contraction. it is the internal messenger that links the AP to contraction. Ca2+ binds to troponin and this causes the myosin-binding sites on actin to be exposed so that myosin heads can bind to actin.

2 factors that influence force and 2 that influence velocity of muscle contraction

factors that influence muscle contractile force include the frequency of stimulation of muscle fibers, muscle fiber size, the number of muscle fibers stimulated, and the degree of muscle stretch. factors that influence velocity of contraction include muscle muscle fiber type, load, and the number of motor units contracting.

circular

fascicles arranged in concentric rings Ex. Orbicularis oris

bipennate:

fascicles insert from opposite sides of tendon (ex: rectus femoris)

parallel

fascicles parallel to long axis of straplike muscle EX: sartorius

period of relaxation

final phase, lasting 10-100ms, is initiated by reentry of Ca2+ into the SR; because the number of active cross bridges is declining, contractile force is declining. muscle tension decreases to zero and the tracing returns to baseline. notice that a muscle contracts faster than it relaxes.

Prefix sarco refers to

flesh (muscles)

in mechanical disadvantage (speed levers)

force is lost, but speed and range of movement are gained

smooth muscle tissue

found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages. it's role is to force fluids and other substances through internal body channels. forms the valves to regulate the passage of substances through internal body openings, dilates and and constricts the pupils of your eyes, and forms the arrector pili muscles attached to hair follicles. consists of elongated cells like skeletal muscles, but are not striated they are involuntary like cardiac muscles contracts are slow and sustained VISCERAL, NON STRIATED, AND INVOLUNTARY

slightly longer bursts of activity like tennis soo=ccer and 100 meter dash

fueled almost entirely by anaerobic glycolysis

glycosomes

granules of stored glycogen that provide glucose during muscle cell activity for ATP production

muscular dystrophy

group of inherited muscle destroying diseases that generally appear during childhood. the affected muscles initially enlarge due to deposits of fat and connective tissue, but the muscle fibers atrophy and degenerate

Muscle makes up approximately what percentage of the body's weight?

half; 50%

Both troponin and tropomyosin

help control the myosin-actin interactions involved in contraction

synergist

helps primer movers - adds extra force to same movement - reduces undesirable or unnecessary movement

Myofibril arrangement

hexagonal arrangement: 6 thin filaments surround each thick filament, and three thick filaments enclose each thin filament. the H zone of the A band appears less dense bc the thin filaments do not extend into this region the M line in the center of the H zone is slightly darker because of the fine protein strands there that hold adjacent thick filaments together the myofilaments are held together in alignment at the Z discs and the M lines, and are anchored to the sarcolemma at the Z discs.

Events at the Neuromuscular Junction

how does a neuron stimulate a skeletal muscle fiber? The result of the events at the neuromuscular junction is a transient change in membrane potential that causes the interior of the sarcolemma to become less negative (a depolarization). thid local depolarization is called an end plate potential (EPP). the EPP spreads to the adjacent sarcolemma and triggers an AP there. After ACh binds to the ACh receptors, its effects are quickly terminated by acetylcholinesternase, an enzyme located in the synaptic cleft. acetycholinesternase breaks down ACh to its building blocks, acetic acid and choline. removing ACh prevents continued muscle fiber contraction in the absence of additional nervous system stimulation.

isotonic

if the muscle tension developed overcomes the load and muscle shortening occurs, the contraction is an isotonic contraction, as when you life a 5 lb bag of sugar. once sufficient tension has developed to move the load, the tension remains relatively constant through the rest of the contractile period. isotonic comes in two different "flavors" - concentric and eccentric

rigor mortis

illustrates the fact that cross bridge detachment is ATP driven. Most muscles begin to stiffen 3 to 4 hours after death. peak rigidity occurs at 12 hours and then gradually dissipates over the next 48-60 hours. dying cells are unable to exclude calcium (which is higher concentration in the extracellular fluid), and the calcium influx into muscle cells promotes formation of myosin cross bridges. shortly after breathing stops, ATP synthesis ceases, but ATP continues to be consumed and cross bridge detachment is impossible once all of the ATP is gone. Actin and myosin become irreversibly cross-linked, producing the stiffness of rigor mortis, which gradually disappears as muscle proteins break down after death.

the recruitment process is not random. instead it is dictated by the size principle :

in any muscle: - the motor units with the smallest muscle fibers are activated first because they are controlled by the smallest, most highly excitable motor neurons. - as motor units with larger and larger muscle fibers begin to be excited , contractile strength increases - the largest motor units , containing large, coarse muscle fibers, are controlled be the largest , least excitable (highest threshold) neurons and are activated only when the most powerful contraction is necessary

mechanism of contraction

in most cases, adjacent smooth muscle fibers exhibit slow , synchronized contractions, the whole sheet responding to a stimulus in unison. this synchronization reflects electrical coupling of smooth muscle cells by gap junctions that transmit depolarization from fiber to fiber. some smooth muscle fibers in the stomach and small intestines are pacemaker cells: once excited, they act as "drummers" to set the pace of contraction for the entire muscle sheet. these pace makers depolarize spontaneoulsy in the absence of external stimuli. however , neural and chemical stimuli can modify both the rate and intensity of smooth muscle contraction.

actin is composed of

kidney shaped polypeptide subunits called globular actin or G actin. each G actin has a myosin - binding site (or active site) to which the myosin heads attach during contraction. G actin subunits polymerize into long actin filaments called filamentous or F, actin. two intertwined actin filaments, resembling a twisted double strand of pearls, form the backbone of each thin filament.

Neural regulation

in some cases, the activation of smooth muscle by a neural stimulus is identical to that in skeletal muscle : neurotrasmitter binding generates an action potential, which is coupled to a rise in calcium ions in the cytosol. however, some types of smooth muscle respond to neural stimulation with graded potentials (local electrical signals) only. Recall that all somatic nerve endings , that is, nerve endings that excite skeletal muscle , release the nerotransmitter ACh. however, different autonomic nerves serving the smooth muscle of visceral organs release different neurotransmitters, each of which may excite of inhibit a particular group of smooth muscle cells. the effect of a specific neurotransmitter on a smooth muscle cells depends on the type of receptor molecules on the cell's sarcolemma. for example, when ACh binds to ACH receptors on smooth muscle in the bronchioles (small air passageways of the lungs), the response is strong contraction that narrows the bronchioles. when norepinephrine, released by a different type of autonomic nerve fiber, binds to norepinephrine receptors on the same smooth muscle cells, the effect is inhibitory - the muscle relaxes, which dilates the bronchioles. however , when norepinephrine binds to smooth muscle in the walls of most blood vessels , it stimulates the smooth muscle cells to contract and constrict the vessel.

eccentric isotonic contraction

in which the muscle generates force as it lengthens for example: walking down a steep hill causes microtrauma in the muscles that causes soreness

sarcoplasmic reticulum

is an elaborate smooth endoplasmic reticulum . the SR regulates intracellular levels of ionic calcium . it stores calcium and releases it on demand when the muscle fiber is stimulated to contract. as you will see, calcium provides the final "go" signal for contraction interconnecting tubules of SR surround each myofibril the way the sleeve of a loosely knitted sweater surrounds your arm. most SR tubules run longitudinally along the myofibril, communicating with each other at the H zone. others called terminal cisterns ("end sacs") form larger, perpendicular cross channels at the A and I band junctions, as they always occur in pairs. closely associated with the SR are large numbers of mitochondria and glycogen granules, both involved in production of energy during contraction.

two main categories of contractions

isotonic and isometric (depends on whether the muscle changes length or not)

Intracellular calcium performs other important roles in the body in addition to triggering muscle contraction. give one example:

it acts as a second messenger. it also is involved in exocytosis

Why is the size principle important?

it allows the increases in force during weak contractions (slow movements) to occur in small steps, whereas gradations in muscle force are progressively greater when large amounts of force are needed for vigorous activities such as jumping or running. the size principle explains how the same hand that lightly pats ur cheek can deliver a stinging slap at the volleyball during a match

temporal or wave summation

it occurs because the second contraction begins before the muscle has completely relaxed. the second contraction is greater than the first because the muscle is already partially contracted and because even more calcium is squirted into the cytosol. in other words, the contractions are added together. (however, the refractory period is always honored.) so if a second stimulus arrives before repolarization is complete, no wave summation occurs. if the muscle is stimulated at an increasingly faster rate: - the relaxation time between twitches becomes shorter and shorter - the concentration of calcium in the cytosol rises higher and higher - the degree of wave summation becomes greater and greater, progressing to a sustained but quivering contraction referred to as unfused, or incomplete tetanus

3 phases of muscle twitch

latent period, period of contraction, period of relaxation

Effort nearer than load to fulcrum

lever operates at a mechanical disadvantage

effort farther from load

lever operates at mechanical advantage

second class lever

load is between fulcrum and effort example: wheelbarrow, standing on toes

which structure provides the ATP needed for muscle activity?

mitchondria

anatomy of motor neurons and the neuromuscular junction

motor neurons that activate skeletal muscle fibers are called somatic motor neurons, or motor neurons of the somatic (voluntary) nervous system. these neurons reside in the spinal cord(except for those that supply the muscles of the head and neck). each neuron has a long threadlike extension called an axon that extends from the cell body in the spinal cord to the muscle fiber it serves. these axons exit the spinal cord and pass throughout the body bundled together as nerves. each axon of each motor neuron branches profusely as it enters the muscle so that it can innervate multiple muscle fibers. when it reaches the muscle fiber, each axon divides again, giving off several short, curling branches that collectively form an oval neuromuscular junction, or motor end plate, with a single muscle fiber. Each muscle fiber has only one neuromuscular junction, located approximately midway along its length. the end of the axon, called the axon terminal, and the muscle fiber are exceedingly close (50-80 nm apart) but remain separated by a space, the synaptic cleft, which is filled with a gel-like extracellular substance rich in glycoproteins and collagen fibers.

muscle twitch

muscle contraction is investigated in the lab using an isolated muscle. the muscle is attached to an apparatus that produces a myogram (a recording of contractile activity consisting of one or more recorded lines called tracings). remember that muscles can contract without shortening (an isometric contraction). in the lab, a muscle twitch is the response of a muscle to a single stimulation. the muscle fibers contract quickly and then relax. every twitch myogram has three distinct phases.

skeletal and smooth muscle cells (but not cardiac muscle cells) are elongated and are called:

muscle fibers

what prevents the filaments from sliding back to their original position each time a myosin cross bridge detaches from actin?

there are always some myosin cross bridges bound to the actin myofilament during the contraction phase. this prevents the backward sliding of the actin filaments.

naming skeletal muscles

muscle location: bone or body region with which muscle associated . EX: temporalis (over temporal bone) muscle shape: distinctive shapes . EX: deltoid muscle (deltoid = triangle) muscle size: EX maximus (largest), minimus (smallest), longus (long) direction of muscle fibers or fascicles : EX: rectus (fibers run straight), transversus (fibers run at right angles), and oblique (fibers run at angles to imaginary defined axis) number of orgins: EX: biceps (2 origins) and triceps (three origins) location of attachments: names according to point of origin and insertion (origin named first) EX: sternocleidomastoid attaches to sternum and clavivle with insersion on mastoid process muscle action: named for action they produce . EX: flexor or extensor several criteria can be combined : EX: extensor carpi radialis longus

Muscle actions and interactions

muscle tissue consists of all contractile tissues

Other proteins that bind filaments or sarcomeres together and maintain their alignment are

nebulin, myomesin, and C proteins. intermediate (desmin) filaments extend from the Z disc connect each myofibril to the next throughout the width of the muscle cell

Voluntary movements require

neurons in your brain, but the contraction of a skeletal muscle comes down to activating a few motor neurons in the spinal cord. motor neurons are the way that the nervous system connects with skeletal muscles and "tells" them to contract

smooth muscle is...

nonstriated, involuntary muscle except for the heart, which is made of cardiac muscle, the muscle in the walls of all the body's hollow organs is almost entirely smooth muscle. most smooth muscle is organized into sheets of tightly packed fibers . these sheets are found in the walls of all but the smallest blood vessels and in the walls of hollow organs of the respiratory, digestive, urinary, and reproductive tracts. in most cases, there are two sheets of smooth muscle with their fibers oriented at right angles to each other, as in the intestine. - in the longitudinal layer , the muscle fibers run parallel to the long axis of the organ. consequently, when these fibers contract, the organ shortens - in the circular layer , the fibers run around the circumference of the organ. contraction of this layer constricts the lumen (cavity inside) of the organ.

cardiac muscle tissue

occurs only in the heart, where it constitutes the bulk of the heart walls; striated; not voluntary cardiac muscle usually contracts at a fairly steady rate set by the hearts pacemaker, but neural controls allow the heart to speed up for brief periods, as when you race across the tennis court to make an overhead smash

voltage gated ion channels

open or close in response to changes in membrane potential. they underlie all action potentials. in skeletal muscle fibers, the initial change in membrane potential is created by chemically gated channels. in other words, chemically gated ion channels cause a small local depolarization ( a decrease in the membrane potential) that then triggers the voltage gated ions to create an action potential.

Antagonist

opposes or reverses particular movement - primer mover and antagonist are located on opposite sides of joint across which they act

Tropomyosin

polypeptide strands; a rod-shaped protien, spiral about the actin core and help stiffen and stabalize it; arrranged end to end along the actin filaments and in relaxed muscle fiber, they block myosin-binding sites on actin so that myosin heads on the thick filaments cannot bind to the thin filaments

muscle functions

produce movement: skeletal muscles are responsible for all locomotion and manipulation and manipulation. they enable you to jump out of a car quickly, move your eyes, and smile or frown. cardiac and smooth muscles pump blood through the blood vessels and smooth muscle propels substances through the digestive, urinary, and reproductive tracts. Maintain posture and body position: these muscles function continuously yet we rarely even notice them, making one tiny adjustment after another to counteract the never ending downward pull of gravity Stabilize joints: even as they bull on bones to cause movement, they strengthen and stabilize the joints of the skeleton. Generate heat: muscles generate heat as they contract, which plays a role in maintaining normal body temp

muscles only ___ and never ___

pull, push

ion channels

rapidly changing the membrane potential in neurons and muscle cells requires the opening and closing of membrane channel proteins that allow certain ions to pass across the membrane. the movement of ions through these ion channels changes the membrane voltage.

plasma membrane of muscle cells

sarcolemma

Myofibrils are made up of

sarcomeres

cytoplasm of a muscle cell

sarcoplasm

pennate

short fascicles attach obliquely to a central tendon running length of muscle (ex: rectus femoris)

each myosin consists of

six polypeptide chains: 2 heavy (high molecular weight) and 4 light chains. the heavy chains twist together to form myosin's rodlike tail, and each chain ends in a globular head that is attached to the tail via a flexible hinge. the globular heads, each associated with 2 light chains, are the "business end" of myosin. during contraction, they link the thick and thin filaments together, forming cross bridges, and swivel around their point of attachment, acting as motors to generate force. myosin itself splits ATP (acts as an ATPase) and uses the release of energy to drive movement. each thick filament contains about 300 myosin molecules bundled together, with their tails forming the central part of the thick filament and their heads facing outward at the end of each end of the molecule. as a result, the central portion of the thick filament (H zone) is smooth, but its ends are studded with a staggered array of myosin heads.

muscle tone

skeletal muscles are voluntary, but even relaxed muscles are almost always slightly contracted muscle tone is due to spinal reflexes that activate first one group of motor units and then another in response to activated stretch receptors in the muscles. muscle tone does not produce active movements, but it keeps the muscles firm, healthy, and ready to respond to stimulation, skeletal muscle tone also helps stabilize joints and maintain posture.

Three types of muscle tissue

skeletal, cardiac, smooth

skeletal muscle tissue

skeletal, striated, and voluntary skeletal muscle fibers are the longest muscle cells responsible for overall body mobility it can contract rapidly, but tires easily and must rest after short periods of activity VERY adaptable; can exert force to pick up a paper clip, or a 20 lb weight!

the greater the load, the ____ the muscle shortening

slower

types of smooth muscle

smooth muscle in the body differ in its 1. fiber arrangement and organization 2. innervation and 3 . responsiveness to various stimuli for simplicity, it is normally categorized in 2 major types: unitary and multi unitary

special features of smooth muscle contraction

smooth muscle is intimately involved in the functioning of most hollow organs and has a number of unique characteristic. we have already considered some of these - smooth muscle tone, slow prolonged contractions, and low energy requirments. but smooth muscle also responds differently to stretch and can lengthen and shorten more than other muscle types. lets take a look.

length and tension changes

smooth muscle stretches much more and generates more tension than skeletal muscles stretched to a comparable extent. the irregular , overlapping arrangement of smooth muscle filaments and the lack of sarcomeres allow them to generate considerable force, even when they are substantially stretched. the total length change that skeletal muscles can undergo and still functionefficiently is about 60% (from 30% shorter and 30% longer than resting length) , but smooth muscle can contract when it is anywhere from half to twice its resting length- a total ranger fo 150%. this allows hollow organs to tolerate tremendous changes in volume without becoming flabby when they empty.

energy efficiency of smooth muscle contraction

smooth muscle takes 30 times longer to contract and relax than does skeletal muscle, but it can maintain the same contractile tension for prolonged periods at less than 1% of the energy cost. if skeletal muscle is like a speedy windup car that quickly runs down, then smooth muscle is like a steady locomotive engine that lumbers along tirelessly. Part of the striking energy economy of smooth muscle is the sluggishness of its ATPases compared to those in skeletal muscle. moreover, smooth muscle myofilaments may latch together during prolonged contractions . this latching also saves energy. the smooth muscle contractions in arterioles (small arteries) and other visceral organs routinely maintains a moderate degree of contraction, called smooth muscle tone, day in and day out without fatiguing . smooth muscle has low energy requirements , and as a rule, it makes enough ATP via aerobic pathways to keep up with the demand.

hormones and local chemical factors

some smooth muscle layers have no nerve supply at all. instead , they depolarize spontaneusly or in response to to chemical stimuli that bind to G protein linked receptors. other smooth muscle cells respond to both neural and chemical stimuli. several chemical factors cause smooth muscle to contract or relax without and action potential by enhancing or inhibiting Ca2+ entry into the sarcoplasm . they include certain hormones, histamine, excess carbon dioxide, low pH, and lack of oxygen. this direct response to these chemical stimuli alters smooth muscle activity according to local tissue needs and probably is most responsible for smooth muscle tone. . for example, the hormone gastrin stimulates stomach smooth muscle to contract so it can churn foodstuffs more efficiently. we will consider activation of smooth muscle in specific organs as we discuss each organ in subsequent chapters.

Unitary smooth muscle

sometimes called visceral muscle bc it is in the walls of all hollow organs except the heart, is far more common. all the smooth muscle characteristics described so far pertain to unitary smooth muscle . for example, the cells of unitary muscle: - are arranged in opposing (longitudinal and circular) sheets - are innervated by varocosities of autonomic nerve fibers and often exhibit rhythmic spontaneous action potentials - are electrically coupled by gap junctions and so contract as a unit (for this reason recruitment does not occur in unitary smooth muscle) - respond to various chemical stimuli

subthreshold stimulus

stimuli that produce no observable contractions

maximal stimulus

strongest stimulus that increases contractile force. it represents the point at which all the muscle's motor units are recruited . in the lab, increasing the stimulus intensity beyond the maximal stimulus does not produce a stronger contraction

triads

successive groupings of the three membraneous structures (terminal cistern, t tubule, and terminal cistern). as they pass from one myofibril to the next, the t tubules also encircle each sarcomere.

fixator

synergist that immobilizes bone or muscle's origin - gives prime mover stable base on which to act

isometric contraction

tension develops but the load is not moved tension may build up to the muscles peak tension producing capacity, but the muscle neither shortens nor lengthens because the load is greater than the force (tension) the muscle is able to develop - think of trying to lift a piano with one hand. muscles contract isometrically when they act primarily to maintain upright posture or to hold joints stationary while movements occur at other joints.

length-tension relationship

the amount of tension a muscle can generate with isometric contraction at various lengths- its length tension relationship- can be shown graphically.

how does skeletal muscle respond to exercise?

the amount of work a muscle does is reflected in changes in the muscle itself. when used actively or strenuously, muscles may become larger or stronger, or more efficient and fatigue resistant. exercise gains are based on the overload principle. forcing a muscle to work hard increases its strength and endurance. as muscles adapt to greater demand, they must be overloaded to produce further gains. Inactively, on the other hand, always leads to muscle weakness and atrophy.

regulation of contraction

the contraction of smooth muscle can be regulated by nerves , hormones, or local chemical changes. lets briefly consider each of these methods ....

third class lever:

the effort is applied between the load and the fulcrum ex: tweezers, forceps, most skeletal muscles

direct, or fleshy attachments

the epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage

what is the final trigger for contraction? what is the initial trigger?

the final trigger is a certain amount of calcium ions in the cytosol. the initial trigger is depolarization of the sarcolemma.

latent period

the first few milliseconds following stimulation when excitation-contraction coupling is occuring; during this period, cross bridges begin to to cycle but muscle tension is not yet measurable so the myogram does not show a response.

muscle tension

the force exerted by a contracting muscle on an object

force of muscle contraction

the force of muscle contraction depends on the number of myosin cross bridges that are attached to actin. this in turn is affected by four factors , the first two of which we have already discussed.

aerobic endurance

the length of time a muscle can continue to contract using aerobic pathways

resistance exercise

the moderately weak but sustained muscle activity required for endurance exercise does not promote significant skeletal muscle hypertrophy, even though the exercise may go on for hours. muscle hypertrophy - think of the bulging biceps of a professional weight lifter - results mainly from high intensity resistance exercise (typically under anaerobic conditions) such as weight lifting or isometric exercise, which pits muscles against high resistance or immovable forces. Here strength, not stamina, is important, and a few minutes every other day is sufficient to allow a proverbial weakling to put on 50% more muscle within a year. the additional muscle bulk largely reflects in increased size of individual muscle fibers (particularly the fast glycolytic variety) rather than an increased number of muscle fibers. (However , some of the bulk may result from longitudinal splitting of the fibers and subsequent growth of these "split" cells, or from the proliferation and fusion of satellite cells. vigorously stressed muscle fibers also contain more mitochondria , form more myofilaments and myofibrils , store more glycogen, and develop more connective tissue between muscle cells. Collectively these changes promote significant increases in muscle strength and size. resistance activities can also convert fast oxidative fibers to fast glycolytic fibers . however, if the specific exercise routine is discontinued , the converted fibers revert back to their original metabolic properties

indirect attachments

the muscles connective tissue wrapping extend beyond the muscle either as a ropelike tendon or a a sheet like aponeurosis; much more common. the tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element (bone or cartilage) or the the fascia of other muscles

muscle response to changes in stimulus frequency

the nervous system can achieve greater muscular force by increasing the firing rate of motor neurons. for example , if two identical stimuli (electrical shocks or nerve impulses) are delivered to a muscle in rapid succession, the second twitch will be stronger than the first. on a myogram the second twitch will appear to ride on the shoulders of the first. this phenomenon is called temporal, or wave summation. j

load

the opposing force exerted on the muscle by the weight of the object is called the load

Troponin

the other major protein in thin filaments, is a globular protein with three polypeptide subunits. one subunit attaches troponin to actin. another subunit binds tropomyosin and and helps position it on actin. the third subunit binds calcium ions.

anaerobic threshold

the point at which muscle metabolism converts to anaerobic glycolysis

Triad Relationships

the roles of the t tubules and the SR in providing signals for contraction are tightly linked. at the triads, membrane spanning proteins form the t tubules and the SR link together the gap between the two membranes. - the protruding integral proteins of the t tubules act as voltage centers - the integral proteins of the SR form gated channels through which the terminal cisterns release Ca2+

threshold stimulus

the stimulus at which the first observable contraction occurs is called the threshold stimulus . beyond this point , the muscle contracts more vigorously as the stimulus strength increases.

muscle fiber type

there are seevral ways of classifying muscle fibers, but learning about these classes will be easier if you pay attention to just two functional characteristics: - speed of contraction: on the basis of speed (velocity) of fiber shortening, there are slow fibers and fast fibers . the difference 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 Ca2+ moves from the cytosol into the SR. - major pathways for forming ATP: the cells that rely mostly on the oxygen - using aerobic pathways for ATP generation are oxidative fibers. those that rely more on anaerobic glycolysis and creatine phosphate are glycolytic fibers. Using these two criteria , we can classify skeletal muscle cells as : slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers.

connective tissue sheaths cont.

they are all continuous of one another as well as the tendons that attach muscles to bones. when muscle fibers contract, they pull on these sheaths, which transmit the pulling force to the bone to be moved. the sheaths . they contribute somewhat to the natural elasticity of the muscle tissue, and also provide routes for the entry and exit of the blood vessels and nerve fibers that serve the muscle.

LESS COMMON fast oxidative fibers

they have many characteristics intermediate between the other two types (glycogen stores and power, for example) . like fast glycolytic fibers, they contract quickly, but like slow oxidative fibers, they are oxygen dependent and have a rich supply of myoglobin and capillaries. some muscles have a predominance of one fiber type, but most contain a mixture of fiber types, which gives them a range of contractile speeds and fatigue resistance . but as might be expected, all muscle fibers in a particular motor unit are of the same type. Although everyone's muscles contain mixtures of the three fiber types, some people have relatively more of one kind. these differences are genetically initiated , but can be modified by exercise and no doubt determine athletic capabilities, such as endurance vs strength, to a large extent. for example, muscles of marathon runners have a high percentage of slow oxidative fibers (about 80%) , while those of sprinters contain a higher percentage (about 60%) of fast oxidative fibers and glycolytic fibers. interconversion between the "fast" fiber types occurs as a result of specific exercise regimes, as we'll describe below.

how does the stress-relaxation response suit the role of smooth muscle in hollow organs?

they have to temporarily store the organs contents

connective tissue sheaths

together these sheaths support each cell and reinforce and hold together the muscle, preventing the bulging muscles from bursting during exceptionally strong contractions Epimysium (outside the muscle) is an "overcoat" of dense irregular connective tissue that surrounds the whole muscle. sometimes it blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin. Perimysium and Fascicles: within each skeletal muscle, the muscle fibers are grouped into fascicles that resemble a bundle of sticks. surrounding each fascicle is a layer of dense irregular connective tissue called perimysium Endomysium ("within the muscle") is a wispy sheath of connective tissue that surrounds each individual individual muscle fiber. it consists of fine areolar connective tissue.

which must get out of the way in order for cross bridges to form?

tropomyosin

which binds to Ca2+?

troponin

Both neurons and muscle cells have excitable membranes. True or false?

true

response to stretch

unlike skeletal muscle, smooth muscle spontaneously contracts when it is stretched. this is useful because it can move substances along an internal tract. however, the increased tension persists only briefly, and soon the muscle adapts to its new length and relaxes, while still retaining the ability to contract on demand. this stress-relaxation response allows a hollow organ to fill or expand slowly to accommodate a greater volume without causing strong contractions that would expel its contents. this is an important attribute , bc organs such as the stomach and intestines must store their contents long enough to digest and absorb the nutrients . likewise , your urinary bladder must be able to store the continuously made urine until it is convenient to empty your bladder, or you would just spend all your time in the bathroom.

muscle fatigue

vigorous muscle activity cannot continue indefinitely . muscle fatigue is a state of physiological inability to contract even though the muscle is still receiving stimuli. you might think that running out fo ATP is the critical event that causes muscle fatigue. in fact, ATP levels inside muscle cell do drop, but muscle fatigue serves to prevent complete depletion of ATP in muscle , which would result in death of muscle cells and rigor mortis (not good!). the mechanism of muscle fatigue is complex. although it is not fully understood , it involves alterations in exictation - concentration coupling . the following chemical changes may be involved :

sliding filament model

we almost always think of "shortening" when we hear the word contraction, but the physiologists contraction refers only to the activation of myosin's cross bridges, which are the force generating sites. shortening only occurs if the cross bridges generate enough tension on the thin filaments to exceed the forced that oppose shortening, such as when you lift a bowling ball. contraction ends when the cross bridges become inactive, the tension declines, and the muscle fiber relaxes. in a relaxed muscle fiber, the thin and thick filaments overlap only at the ends of the A band. the sliding filament model of contraction states that during contraction, the thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree. neither the thick nor the thin filaments change length during contraction. heres how ti works: - when the nervous system stimulates muscle fibers, the myosin heads on thick filaments latch onto myosin binding heads on actin in the thin filaments, and the sliding begins - these cross bridge attachments form and break several times during contraction, acting like tiny rachets to generate tension and propel the thin filaments toward the center of the sarcomere. - as this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens. -at the microscope level, the following things occur as a muscle cell shortens: --- the I bands shorten --- the distance between successive Z discs shortens . as the thin filaments slide centrally, the z discs to which they attach are pulled toward the M line. --- the H zones disappear --- the contagious A bands move closer together, but their length does not change.

excessive postexercise oxygen consumption (EPOC)

whether or not fatigue occurs , vigorous exercise alters a muscle's chemistry dramatically . for a muscle to return to its preexercise state, the following must occur : - its oxygen reserves (stored in myoglobin) must be replenished - the accumulated lactic acid must be reconverted to pyruvic acid - glycogen stores must be replaced - ATP and creatine phosphate reserves must be resynthesized the use of these muscle stores during anaerobic exercise simply defers when the oxygen is consumed, because replacing them requires oxygen uptake and aerobic metabolism after exercise ends. additionally, the liver must convert any lactic acid persisting in blood to glucose or glycogen . once exercise stops, the repayment process begins . the extra amount of oxygen that the body must take in for these restorative processes is called the excess postexercise oxygen consumption (EPOC) , formerly called the oxygen debt. EPOC 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.

Duchenne muscular dystrophy (DMD)

which is inherited as a sex-linked recessive disease. it is expressed almost exclusively in males (one in every 3600 male births) and is diagnosed when a boy is between 2 and 7 years old. active, normal appearing boys become clumsy and fall frequently as their muscles weaken. the disease progresses relentlessly from the extremities upward, finally affecting the head and chest muscles, and cardiac muscles. the weakness continues to progress, but with supportive care, DMD patients are living into their 30s and beyond.

developmental aspects of muscles

with rare exceptions, all three types of muscle tissue develop from cells called myoblasts that arise from embryonic mesoderm. in forming skeletal muscle tissue, several myoblasts fuse to form multinuclear myotubes. soon functional sarcomeres appear, and skeletal muscle fibers are contracting by week 7 when the embryo is only about 2.5 cm long. intitially, ACh receptors "sprout" over the entire surface of the developing myoblasts. as spinal nerves invade the muscle masses, the nerve endings target individual myoblasts and release a growth factor that stimulates clustering of ACh receptors at the newly forming neuromusclular junction in each muscle fiber. then, the nerve endings release a different chemical that eliminates the receptor sites not innervated or stabilizaed by the growth factor. as the somatic nervous system assumes control of muscle fibers, the number of fast and slow contractile fiber types is determined by the nerves that innervate them

what would happen if a muscle fiber suddenly ran out of ATP when sarcomeres had only partially contracted?

without ATP, rigor would occur because the myosin heads could not detach.