Anatomy Chapter 9

Characteristics of Muscle Tissue

Excitability/responsiveness, contractility(sets apart from tissue types), extensibility, elasticity

Tropomyosin

In thin filaments. polypeptide strands that are rod shaped protein, which spiral about the actin core and help stiffen and stabilize myosin. Either allow or don't allow myosin heads to bind to actin. They are arranged end to end along the actin filaments

Sarcomere

Inside myofibrils. region of a myofibril between two successive Z discs . Smallest contractile unit of striated muscle fiber(functional unit for skeletal muscle). millions of sarcomeres and when it shortens the muscle contracts. Not in smooth muscles. Each sarcomere has thick and thin bundles of proteins known as myofilaments. Actin and myosin are referred to as contractile proteins. Single I band stands two neighboring sarcomeres. Has thick filaments in the middle but they lack myosin heads.

3 phases of twitch myogram

Latent period, period of contraction, period of relaxation

Muscle response to changes in frequency

NS achieves greater muscular force by increasing the firing rate of motor neurons. if two identical stimuli are are delivered to a muscle in rapid succession, the second twitch will be stronger than the first one

Cross bridges

(binding of myosin head and actin) globular heads of myosin molecule projects from a myosin filament in muscle and in the sliding filament hypothesis of muscle contraction is held to attach temporarily to an adjacent actin filament and draw it to an A band of a sarcomere between myosin filaments, Basically, During contraction, they link the thick and thin filaments together. They swivel around their point of attachment, acting as motors to generate force. cross bridge formation requires Ca+

Origin skeletal muscle attachment

(less movable and proximal)

Insertion skeletal muscle attachment

(more movable and distal) when muscle contracts it moves towards immovable or less movable bone

Isometric contraction

(same length) tension may build to the muscle's peak tension-producing capacity, but the muscle neither shortens nor lengthens. they occur when a muscle attempts to move a load that is greater than the force (tension) the muscle is able to develop. Contract like this when trying to maintain posture or to hold joints stationary while movements occur at other joints

Isotonic contractions

(same tension) muscle length changes and moves a load. Once sufficient tension has developed to move the load, muscle shortens, and tension remains relatively constant through the rest of the contractile period. Isotonic contractions are either Concentric or Eccentric. Concentric contractions are those in which the muscle shortens and does work. these are more familiar. Ex: picking up book or kicking ball. Eccentric contractions are where the muscle generates force as it lengthens, are equally important for coordination and purposeful movements. These contractions are 50% more forceful and more often cause delayed onset soreness Ex: calf when walking up big hill. Both eccentric and concentric contractions are using in jumping, throwing activities, and bicep curls. In bicep curls, flexing the weight towards you is concentric and bringing eight away is eccentric. Eccentric contractions put the body in position to contract concentrically

Myofilaments/filament(extended macromolecular structure)

(striations along myofibrils) contractile myofilaments are of thick and thin. Sliding of thin filaments past thick filaments produces muscle shortening. muscle equivalents of actin or myosin. Play a role in mobility and shape. Reaches highest development in contractile muscle fibers. Elastic filaments maintain organization of A band and provide elastic recoil when tension is released

Events at a neuromuscular junction

1. action potential arrives at axon terminal of motor neuron 2. voltage gated Ca+ channels open and Ca+ enters the axon terminal moving down its electrochemical gradient 3. Ca+ entry causes ACh to be released by exocytosis 4. ACh diffuses across the synaptic cleft and binds to its receptors on the sarcolemma 5. ACh binding opens ion channels in the receptors that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions exit, which produces a local change in the membrane potential called an end plate potential 6. ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction

Events in excitation-contraction coupling of smooth muscle

1. calcium ions enter the cytosol from the extracellular fluid via voltage gate or nonvolatile gate Ca+ channels or from the sarcoplasmic reticulum 2. Ca+ binds to and activates calmodulin 3. Activated calmodulin activates the myosin light chain kinase enzymes 4. The activated kinase enzymes catalyze transfer of the phosphate to myosin, activating the myosin Atlases 5. Activated myosin forms cross bridges with actin of the thin filaments. Shortening begins

Events for a myoblast to fuse to form a multinucleate skeletal muscle fiber

1. embryonic mesoderm cells called myoblasts undergo cell division to increase number an enlarge 2. Several myoblasts fuse together to form a myotube 3. Myotube matures into skeletal muscle fiber

Anaerobic pathway: glycolysis and lactic acid formation

As stored ATP and CP are exhausted, more ATP is generated by breaking down/catalyzing glucose obtained from the blood or glycogen stored in the muscle. This pathway occurs in both the presence and absence of oxygen, but is aerobic because it does not use oxygen. 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 muscles contract vigorously and reaches 70% activity of the maximum possible, the bulging muscle compress the blood vessels impairing blood flow and oxygen delivery. Under these conditions, most of the pyruvic acid acid is converted to lactic acid and called aerobic glycolysis. Lactic acid is the end product of cellular metabolism of glucose. Other organs pick up lactic acid and can use as energy source/storage. Aerobic pathway doesn't harvest as much ATP as the aerobic pathway does but it produces ATP faster. Together ATP and CP and the glycolysis-lactic acid pathway can support strenuous activity for a minute.

Perimysium and fascicles: CT sheath of skeletal muscle

Within each skeletal muscle the muscle fibers are grouped into fascicles that resemble bundles of sticks. Surrounding each fascicle is a layer of dense irregular CT called perimysium

Muscle twitch

a motor unit's response to a single action potential of its motor neuron. muscle fibers contract quickly and then relax

Generation of an action potential across the sarcolemma

a resting sarcolemma is polarized and there is a potential voltage difference across the membrane, and the inside is negative. An AP is the result of a predictable sequence of electrical changes. Once initiated, an action potential sweeps along the entire surface of the sarcolemma. 3 steps are involved in triggering and propagating an AP: Generation of an end plate potential, (Depolarization) generation and propagation of an AP, (Repolarization) restoring the sarcolemma to its initial polarized state

Myofibrils

a single muscle fiber that contains thousands of rod like these that run parallel in length. Densely packed in the fiber that mitochondria and other organelles appear to be squeezing between them. 80% of cellular volume. Contain sarcomeres which contain small rod like structures called myofilaments

Similarities of smooth and skeletal muscle contraction

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

Phase 1 leading to muscle fiber contraction: Motor neuron stimulates muscle fiber

action potential arrives at axon terminal at neuromuscular junction, acetylcholine released and it binds to receptors on sarcolemma, ion permeability of sarcolemma changes, local change in membrane voltage (demoralization) occurs, local depolarization(end plate potential) ignites action potential in sarcolemma

Phase 2 leading to muscle fiber contraction: Excitation-contraction coupling occurs

action potential travels across entire sarcolemma, action potential travels along T tubules, Sarcoplasmic reticulum releases Ca+; Ca+ binds to troponin; myosin-binding sites(active sites) on actin exposed, myosin heads bind to actin; contraction begins

Muscle contraction

activation of tension generating sites within muscle fibers and actin and myosin help this. Nerve initiated electrical impulses that travel along the sarcolemma. Because t tubules are continuations of the sarcolemma, the conduct impulses to the deepest regions of the muscle cell and every sarcomere. These impulses signal for the release of calcium from adjacent terminal cisterns

Acetylcholinesterase

after ACh binds to the ACh receptors, its effects are quickly terminated by this enzyme located in the synaptic cleft. It breaks down ACh to its building blocks. This removal of ACh prevents continued muscle cell contraction in the absence of additional NS stimulation

Myoblasts

all three types of muscle tissue develop from embryonic mesoderm cells

Gap junctions in smooth muscle

allow to transmit action potentials from fiber to fiber

Peristalsis

alternating contraction and relaxation of these layers mixes substances in the lumen and squeezes them through the organ's internal pathway

Providing energy for contraction

as muscle contracts, ATP supplies the energy to move and detach cross bridges, operate the calcium pump in the SR, and return Na+ and K+ to the cell exterior and interior respectively after extraction-contraction coupling. Muscles store limited amounts of ATP, 4-6 seconds worth at most, just to get you going. Because ATP is the only energy source used directly for contractile activities, it must be regenerated as fast as it is broken down if contraction continues. After ATP is hydrolyzed to ADP and inorganic phosphate in muscle fibers, it is regenerated within a fraction of a second by one or more of the three pathways

Direct Phosphorylation of ADP by Creatine Phosphate

as we begin to exercise rigorously, the demand for ATP soars and consumes the ATP stored in working muscles within a few twitches. Then, creatine phosphate, a high energy molecule stored in muscles, regenerates ATP while the metabolic pathways adjust to the suddenly higher demand for ATP. Creatine Phosphate and ADP combine to transfer angry and form creatine and ATP. Muscle cells store two or three times more CP than ATP. The CP-ADP reaction, catalyzed by the enzyme creatine phosphate, is so efficient that the ATP in muscle cells barely changes during the initial contraction. Together, ATP and CP provide for maximum muscle power for about 15 seconds. The coupled reaction is readily reversible and CP reserves are replenished during long periods of rest or inactivity.

Z line

attaches neighboring sarcomeres. Thin filaments are attached to these on each side of sarcomere

Neuromuscular junction/motor end plate

axon of motor neuron divides profusely as it enters the muscle. Each axon gives off several short curling branches that form this with a single muscle fiber. Each muscle fiber has only one neuromuscular junction located midway in length. Includes: axon terminals, synaptic cleft, and junctional folds

Aerobic respiration

because creatine phosphate is limited, muscles must metabolize nutrients to transfer energy from foodstuffs to ATP. During rest and light exercise, 95% of the ATP used for muscle activity comes from aerobic respiration. This occurs in the mitochondria, requires oxygen, chemical reactions that break bond of fuel molecules and release energy to make ATP. Includes glycolysis and oxygen combined to form carbon dioxide , water, and ATP. As exercise begins, muscle glycogen provides most of the fuel. Then, blood born glucose, pyruvic acid from glycolysis, and free fatty acids become the major energy fuels. Provides high yield of ATP but it is slow because of the many steps required for constant oxygen and nutrient fuels

1. Generation of an end plate potential

binding of ACh molecules to ACh receptors at the neuromuscular junction opens chemically ligand gated ion channels that allow Na+ and K+ to pass. More Na+ diffuses in than K+ diffuses out because the driving force of Na+ is greater. A transient change in membrane potential occurs as the interior of the sarcolemma becomes less negative(depolarization). Initially depolarization is a local event called an end plate potential

Calmodulin in contraction of smooth muscle

calcium activates myosin by interacting with a regularatory cytoplasmic calcium binding protein

Myosin kinase or myosin light chain kinase

calmodulin interacts with a kinase enzyme which phosphorylates the myosin and activating it

Caveolae

T-tubules are absent but the sarcolemma has these that are punchbag infolding containing large numbers of Ca+ channels

Dense bodies

in smooth muscle and they attached to sarcolemma and act as anchoring points for thin filaments and therefore correspond to Z discs of skeletal muscle

Muscle fatigue

is a state of physiological inability to contract even through the muscle still may be receiving stimuli

Sarcoplasmic reticulum

is an elaborate smooth endoplasmic reticulum. it's interconnecting tubules surround each myofibril. Most SR tubules run longitudinally along the myofibril communicating with each other at the H zone. Large numbers of mitochondria and glycogen granules, both involved in producing the energy during contraction. Regulates intracellular levels of ionic calcium. It stores calcium and releases it on demand when the muscle fiber is stimulated to contract. calcium provides final go signal for contraction

Why is size principle important?

know how a hand can slap a volleyball hard and then pat someones back. It allows the increase in force during weak contractions to occur in small steps. Although all motor units of muscle may be recruited simultaneously to produce a strong contraction, motor units are more commonly activated asynchonously. some are in tetanus while others are resting and recovering. this technique helps prolong a strong contraction by preventing or delaying fatigue. Explains how weak contractions promoted by infrequent stimuli can remain smooth

Thin filaments

lateral and contain actin(blue) extend across the I band and partway into the A band. It also has troponin/tropomyosin which are regulatory proteins

Aerobic endurance

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

Troponin

major protein in thin filaments, globular three polypeptide

Muscle Functions

movement(facial expression), stabilize joint, maintain posture, and generate heat(80-85% comes from skeletal)

Muscle(organ)

muscle consist of hundreds to thousands of muscle cells plus CT wrappings, blood vessels, nerve fibers. It is covered externally by the epimysium

Myosin/ Thick filament

muscle contraction depends on myosin and actin containing myofilaments. Thick filaments are composed of this and head protrude at opposite ends of each filament. Each myosin molecule consists of two heavy and four light polypeptide chains and has a rod like tail that is attached by a flexible hinge to two globular heads. Tail has two helical polypeptide chains. Head cocks when in high energy position and want to reach and bind to actin(attached to z-disc). The heads are active binding sites and ATP binding site and myosin heads are only present in myosin-actin overlap. Contain hundreds of myosin molecules bundled together with their tails forming the central part of the thick filament and heads facing outward.

Myogram

muscle contraction is investigated in the lab using an isolated muscle. The muscle is attached to the apparatus that produces this, a recording of contractile activity. Line recording activity is called tracing

Structure and Organization levels of Skeletal muscle from outside in

muscle, fascicle, muscle fiber, myofibril, sarcomere, myofilament

Velocity and Duration of Contraction

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

Smooth muscle contraction

no sarcomeres, t tubules, or troponin. Calcium comes into cytosol from extracellular space and binds to calmodulin, Calmodulin activates kinase enzymes which then activate myosin and CB starts. it takes 30 times longer to contract than skeletal muscle, but matin the same contractile tension for a long time with less energy cost

Smooth muscle

no troponin in thin filaments, thick and thin filaments arranged diagonally, intermediate filament and dense bodies of smooth muscle fibers harness the pull germinated by myosin cross bridges and attach my the sarcoplasm

Wave/temporal summation

occurs because the second contraction occurs before the muscle has completely relaxed. The muscle is already partly contracted and more calcium is being squirted in the cytosol to replace that being reclaimed by the SR, muscle tension produced during the second contraction causes more shortening than the first. Contractions are added together. Thus if a second stimulus arrives before repolarization is complete, no wave summation occurs

Striations

repeating series of dark and light bands and are effident along length of each myofibril. Dark A bands and light I bands are perfectly aligned giving cell striation. Dark A band has lighter region in midsection called H zone. Each H zone is bisected vertically by a dark line called M line formed by molecule of the protein myomesin. Each light 1 band also has a midline interrupt, a darker area called Z disc

Muscle hypertrophy

results mainly from high intensity resistance exercise such as weight lifting or isometric exercise, which pits muscle against high resistance or immovable forces

Myofibril/fibril (complex organelle composed of bundles of myofilaments)

rodlike contractile elements that occupy most of muscle cell volume. Composed of sarcomeres arranged end to end, they appear branded, and bands of adjacent myofibrils are aligned

Excitation-Contraction(E-C) coupling

sequence of events by which transmission of an AP along the sarcolemma causes myofilaments to slide. AP is brief and ends before any signs of contraction. Electrical signal does not act directly on the myofilaments. Instead, it causes the rise in intracellular levels of calcium ions, which allows filaments to slide

Cross Bridge Cycle

series of events during which myosin heads pull thin filaments toward the center of the sarcomere: 1. cross bridge formation; energized myosin head attaches to an actin myofilament, forming a cross bridge 2. The power(working) stroke; ADP and P are released and the myosin head pivots and bends, changing to its bent low energy state. As a result it pulls the actin filament toward the M line 3. Cross bridge detachment; after ATP attached to myosin, the link between myosin and actin weakens, and the myosin head detaches 4. Cocking of the myosin head; as ATP is hydrolyzed to ADP and P,the myosin head returns to its prestroke high -energy, or cocked position

Sarcoplasmic reticulum and T tubules

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

Varicosities

smooth muscle lacks the highly structured neuromuscular junctions of skeletal muscle. Instead, the innervating nerve fibers, which are are of the autonomic (involuntary) nervous system, have numerous bulbous swellings

Special features of smooth muscle contraction

smooth muscle tone, slow prolonged contractions,low energy requirements. Also responds differently to stretch and can lengthen and shorten more than other muscle types

Synaptic cleft

the end of the axon (axon terminal) and the muscle fiber are close but remain separated by the synaptic cleft, which is filled with gel-like extracellular substance rich in glycoproteins and collagen fibers

2. Depolarization: Generation and propagation of an action potential

the end plate potential ignites an AP by spreading to adjacent membrane areas and opening voltage gated sodium ion channels. Na+ enters, following its electro chemical gradient, and once a certain membrane voltage (threshold) is reached an AP is initiated. The AP (propagates) moves along length of the sarcolemma in all directions from the neuromuscular junction. As it propagates, the local depolarization wave of AP spreads to adjacent areas of the sarcolemma and opens voltage gated sodium channels there. Na+ , normally restricted from entering, diffuses into the cell following its electrochemical gradient

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: number of muscle fibers recruited, size of muscle fibers, frequency of stimulation, degree of muscle stretch

What happens if the muscle is stimulated at an increasingly faster rate?

the relaxation time between twitches becomes shorter and shorter, the concentration of Ca+ in cytosol rises higher, the wave summation becomes greater progressing to a sustained but quivering contraction and referred to as unfused or incomplete tetanus, as stimulation frequency increases, muscle tension increases until hits maximum. At this point evidence of muscle relaxation disappears and contractions fuse into a smooth sustained contraction plateau called fused/complete tetanus

3. Repolarization: restoring the sarcolemma to its initial polarized state

the repolarization wave, like the other wave, is a consequence of changes in membrane permeability. in this case, Na+ channels close and voltage gated K+ channels open. Since the K ion concentration is higher inside the cell than in the extracellular fluid, K diffuses rapidly out of the muscle fiber, restoring negative charges conditions inside

Motor unit

consists of one motor neuron and all the muscle fibers it intervals, or supplies. When a motor neuron fires(transmits AP) all the muscle fibers it innervates contract. The number of muscle fibers per motor unit may be as high as 100 or few as 4. Muscles that exert fine control(fingers and eyes) have small motor units. Large weight bearing muscles, whose movements are less precise(hips) have large motor units. muscle fibers in single motor unit are not clustered together but spread throughout muscle. As a result, stimulation of a single motor unit causes a weak contraction of the entire muscle

Actin molecule/Thin filament

consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins( troponin and tropomyosin). Actin subunits have active sites for myosin attachment. Firmly attached to Z-disc

A band

contains an area of overlap between thick and thin filaments

Muscle cell 3 structures highly modified

contains myyofibrils, saroplasmic reticulum, and T tubules

CT sheaths function

continuous with one another as well as with the tendons that join muscles to bone. When muscle fibers contract, they pull on these sheaths, which transmit the pulling force to the bone to be moved. Contribute some to elasticity of muscle tissue and provide routs of entry for blood vessels and nerve fibers

Principles of muscle mechanics

contraction of single muscle fiber and of a skeletal muscle consisting of a large number of of fibers are the same, force exerted by a contracting muscle on an object is called muscle tension, the opposing force exerted on a muscle by the weight of an object to be removed is called a load, a contracting muscle does not always have to shorten an move the load. Isometric and isotonic contractions. Increasing muscle tension is measured for isometric contractions, whereas amount of muscle shortening is measuring for isotonic contractions. A skeletal muscle contracts with varying force and for different periods of time in response to our need at the time

Period of contraction

cross bridges are active, from the onset to the peak of tension development, and the myogram tracing rises to a peak. Lasts for 10-100 ms. If the tension becomes great enough to overcome the resistance of the load, the muscle shortens

Sarcoplasma

cytoplasm of a muscle cell. similar to cytoplasm of other cells, but contains unusually large amounts of gylcosomes and myoglobin

Recruitment process

dictated by size principle: 1. the motor units with the smallest muscle fibers are activated first because they are controlled by the smallest most highly excitable motor neurons 2. as motor units with larger muscle fibers begin to be excited, contractile strength increase 3. the largest motor units, contain large course muscle fibers and are controlled by the largest, least excitable(highest threshold) neurons and are activated when the most powerful contraction is necessary

Fascicle(portion of muscle)

discrete bundle of muscle cells, segregated from the rest of the muscle by a CT sheath. Surrounded by perimysium

Skeletal muscle

discrete organ made up of several kinds of tissue. Skeletal muscle fibers, blood vessels, nerves, and CT.

Sliding filament theory of contraction

during contraction, the thin filaments slide past the thick ones so that actin and myosin filaments overlap to a greater degree. The thick and thin filaments don't change length. when NS simulates muscle cells, the myosin heads on the thick filaments latch onto myosin-binding sites on actin in the thin filaments, and the sliding begins. These cross bridges attachments form and break several times during a contraction, generating tension and propel the thin filaments toward the center of the sarcomere. Occurs simultaneously through out the cells and the muscle cell shortens. As thin filaments slide centrally, the z discs to which they attach are pulled toward the m line. overall as the muscle shortens, this occurs: I bands shorten, distance between Z discs shorten, H zone disappears, contiguous A bands move closer together, but their length does not change

Refractory period

during repolarization, a muscle fiber is said to be this because the cell can't be stimulated again until repolarization is complete. repolarization restores only the electrical conditions of the resting polarized state. The ATP dependent Na+ and K+ pump restores the ionic conditions of the resting state, but thousands of APs can occur before ionic imbalance interfers with contractile activity. once initiated AP is unstoppable and ultimately results in contraction of the muscle fiber

Isotonic and Isometric contractions

electrochemical and mechanical events occurring in a muscle are identical in both isotonic and isometric contractions. However, the results are different. In isotonic contractions, thin filaments slide. In isometric contractions, the cross bridges generate force but do not move thin filaments, so no change in banding pattern from resting state

Muscle fiber(cell)

elongated multinucleate cell; it has a banded(striated) appearance and it is surrounded by endomysium

Atapase

enzyme breaks down and sometimes slow or fast twitch

Direct attachment

epimysium fused to periosteum(bone) or perichondrium(cartilage). No tendon

Muscle tone

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

Smooth muscle varies on

fiber arrangement and organization, innervation, responsiveness to various stimuli. For simplicity: unitary or multi unit

What induces a muscle fiber to contract?

fiber must be activated/stimulated by a nerve ending so that a change in membrane potential occurs, it must generate an electrical current, called an action potential in its sarcolemma, action potential is automatically propagated along the sarcolemma, then intracellular ion level must rise briefly, providing the final trigger for contraction

Period of relaxation

final phase in twitch myogram, lasting 10-100 ms, is initiated by reentry of Ca+ into the SR. The number of cross bridges is declining, so the contractile force is declining. Muscle tension decreases to zero and the tracing returns to the baseline. If the muscle shortened during contraction, it now turns to normal length. A muscle contracts faster than it relaxes, as shown by asymmetric nature of the myogram tracing

Excess postexercise oxygen consumption(EPOC)

firmly called the oxygen debt, 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

Latent period

first few milliseconds following stimulation when excitation-contraction coupling is occurring. cross bridges begin to cycle but muscle tension is not yet measurable and the myogram does not show a response

What else does smooth muscle do

forms valves to regulate passage of of substances through internal body openings, dilates pupils, forms arrestor pili muscles attached to hair follicles

Smooth Muscle tissue

found on the walls of hollow visceral organs, such as stomach, urinary bladder, and respiratory packages. it forces fluid and other substances through internal body channels. Smooth muscle consist of elongated cells and has no striations. It is subject to involuntary control. Contractions are slow and sustained

2 ways of graded muscle contraction

frequency of stimulation and changing the strength of stimulation

Glycosomes

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

Contraction

happens here in the activation of myosin cross bridges, which are force generating sites. Shortening occurs if and when the cross bridges generate enough tension of the thin filaments to exceed the forces oppose shortening(such as lifting a bowling ball). Contraction ends when the cross bridges become inactive, the tension declines, and the muscle fiber relaxes.

Graded muscle responses

healthy muscle contractions are usually smooth and vary in strength as different demands are placed on them. These variations needed for proper control of skeletal movement, are referred to as graded muscle responses

Skeletal muscle fiber

huge cells and contain calcium-regulated molecular motors. Alternating dark bands and light bands from proteins inside muscle cells

T tubule

in SR and at each A and I band unction, the sarcolemma of the muscle cell protrudes deep into the cell inferior, forming an elongated tube. they increase the muscle fiber's surface area. Encircle each sarcomere as passing the myofibrils.(rapid communication that ensures every myofibril in the muscle fiber contracts at the same time)

Terminal cisterns

in SR and form larger perpendicular cross channels at the A-I band junctions and occur in pairs

Diffuse junctions

the varicosities release neurotransmitter into a wide synaptic cleft in the general area of the smooth muscle cells

Sliding Filament theory of fully relaxed sarcomere of muscle fiber

thin and thick filaments overlap only at the ends of the A band

Acetylcholine (ACh)

tough like part of muscle fiber's sarcolemma that help form the neuromuscular junction is highly folded

Muscle response to changes in stimulus strength

wave summation contributes to contractile force, but its primary function is to produce smooth, continuous muscle contractions by rapidly stimulating a specific number of muscle cells. Recruitment/multiple motor unit summation, controls force of contraction precisely. In lab, recruitment is achieved by delivering shocks of increasing voltage to the muscle, calling more and more muscle fibers into play. Stimuli that produce no observable contractions are sub threshold stimuli. Stimulus where first observable contraction occurs is called the threshold stimulus. The muscle contracts more virtuously past this point. Maximal stimulus is the strongest stimulus that increases contractile force. The point in which all the muscle's motor units are recruited. In lab, increasing the stimulus intensity beyond maximal stimulus does not produce a stronger contraction.

Synaptic vesicles

within mound like axon terminal, which are small membranous sacs containing the neurotransmitter acetylcholine (ACh).

Endomysium: CT sheath of skeletal muscle

within muscle and is a wispy sheath of fine areolar CT that surrounds each individual muscle fiber

Actin

protein that is kidney shaped and has is a polypeptide unit which bears to active sites to which myosin heads attach during contraction

Junctional folds

provide large surface area for millions of ACh receptors located here.

Myoglobin

red pigment, that stores oxygen

Steps in E-C Coupling

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

How does a motor neuron stimulate a skeletal muscle fiber?

1. when a nerve impulse reaches the end of an axon, the axon terminal releases ACh into the synaptic cleft 2. ACh diffuses across the cleft and attaches to ACh receptors on the sarcolemma of the muscle fiber 3. ACh binding triggers electrical events that ultimately generate an action potential

Muscle Fiber contraction: Cross bridge cycling

Calcium ions promote muscle cell contraction. When intracellular calcium levels are low, the muscle cell is relaxed, and tropomyosin molecules physically block the active(myosin blinding) sites on actin. As Ca+ levels rise, the ions bind to regulatory sites on troponin. Two calcium ions must bind to troponin, causing it to change shape and then roll tropomyosin into the groove of the actin helix, away from the myosin binding sites. Basically, tropomyosin"blockade" is removed when sufficient calcium is present. Once binding sites are on actin are exposed, the events of the cross bridge cycle occur in rapid succession. cycle repeats and thin filaments continue to slide as long as the calcium signal adequate ATP are present. With each cycle the myosin head takes another step by attaching to an actin site further along the thin filament. When nerve impulses arrive in quick succession,intracellular Ca+ levels soar due to successive "puff" or rounds of Ca+released from the SR. As the Ca+ pumps of the SR reclaim calcium ions from cytosol and troponin again changes shape, tropomyosin again blocks actin's myosin-binding sites. The contraction ends, and the muscle fiber relaxes. when cross bridge cycling ends, the myosin head remains upright in high energy configuration ready to bind actin when muscle is stimulated to contract again.

3 pathways for regenerating ATP during muscle activity

Direct phosphorylation of ADP by creatine phosphate, Anaerobic glycolysis which converts glucose to lactic acid, Aerobic respiration. All body cells use glycolysis and aerobic respiration to produce ATP

Indirect attachment

Muscle's CT wrappings extend beyond the muscle as a replica tendon or an aponeurosis. This anchors the muscle to CT covering of a skeletal element or to the fascia of other muscles. More common because of durability and small size

Thick filaments

central and contain myosin(red) extend the entire length of the A band. They are connected in the middle of the sarcomere at the M line

H zone

central portion of thick filament is smooth with a staggered array of myosin heads

Muscle Fiber type

classify these by speed of contraction and major pathways for forming ATP

Somatic motor neurons/motor nuerons of somatic NS

nerve cells that activate skeletal muscle cells. These motor neurons in the brain or spinal cord and extensions of axons (bundled within nerves) extend to the muscle cells they serve.

Contraction of smooth muscle can be regulated by

nerves and hormones and local chemical factors

Epimysium: CT sheath of skeletal muscle

outside the muscle is an overcoat of Dense irregular CT that surrounds the whole muscle. Sometimes bends

I band

part of sarcomere that has thin filaments

Triads

part of t tubules. in SR and each tubule runs between paired terminal cisterns of the SR forming these. Roles of T tubules and SR in providing signals for contraction are linked. At the triads, integral proteins protrude into inter membrane spaces from the t tubules and SR. These proteins of the T tubules act as voltage sensors. These proteins of SR form gated channels through which the terminal cisterns release Ca

Sarcolemma

plasma membrane wraps around outside(flesh like)

Anaerobic threshold

point at which muscle metabolism converts to anaerobic glycolysis