A&P Chapter 10

Stress-relaxation response

Allows smooth muscle to maintain contractile function after considerable stretching.

Contraction cycle

At onset of contraction, SR releases calcium ions into sarcoplasm. They bind to troponin there. Troponin moves tropomyosin away from myosin-binding sites on actin. When binding sites are "free", contraction cycle begins.

NMJ continued

Axon terminal divides into cluster of synaptic end bulbs, neural part of NMJ. Suspended in cytosol within each synaptic synaptic end build are synaptic vesicles. Each vesicles have thousands of molecules of acetylcholine (ACh), which is neurotransmitter released.

Cardiac muscle tissue details

Between layers of cardiac muscle fibers are sheets of connective tissue that contain blood vessels, nerves, and conduction system of the heart. Have same arrangement of actin and myosin and same bands, zones, and Z discs as skeletal muscle fibers. Different because intercalated discs are unique. These are irregular transverse thickenings of sarcolemma that connect ends of cardiac muscle fibers to one another. Discs contain desmosomes which hold fibers together, and gap junctions that allow muscle action potentials to spread from one cardiac muscle fiber to another. Lacks epimysium.

Twitch contraction

Brief contraction of all muscle fibers in motor unit in response to single action potential in motor neuron. Lasts from 20-200 msec.

Extensibility

Connective tissue in muscle limited range of extensibility. Smooth muscle subject to greatest extensibility.

Multiunit smooth muscle tissue

Consists of individual fibers, each with its own motor neuron terminals and with few gap junctions between neighboring fibers. Stimulation of one multiunit fiber causes contraction of that fiber only. Found in walls of large arteries, airways, arrestor pili muscle, and muscles of iris that adjust pupil diameter.

Motor unit

Consists of somatic motor neuron and all skeletal muscle fibers it stimulates. Single somatic motor neuron makes contact with an average of 150 skeletal muscle fibers, and all of muscle fibers in one motor unit contract in unison. Dispersed throughout muscle. Can have many muscle fibers per unit, or a little.

3 types of myofibril proteins

Contractile proteins, regulatory proteins, and structural proteins.

Sarcoplasm

Cytoplasm of muscle fiber. Includes glycogen (lots of glucose), which is large molecule composed of many glucose molecules. Can be used for synthesis of ATP. Also contains myoglobin which are found only in muscle, and binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. Released oxygen when needed by mitochondria for ATP production.

A band

Darker middle part of sarcomere. Extends entire length of thick filaments. Zone of overlap is where thick and thin filaments lie side by side

Latent period

Delay between stimulus and beginning of contraction. About 2 msec. During latent period, AP sweeps over sarcolemma and calcium ions are released from sarcoplasmic reticulum.

Relaxation in smooth muscle

Delayed. Prolonged presence of Ca2+ in cytosol provides for smooth muscle tone, state of continued partial contraction. Contracts or relaxes in response to autonomic nervous system.

Microscopic anatomy of skeletal muscle fiber

Develops from fusion of myoblasts in embryo, so have many nuclei. Once fusion has occurred, muscle fiber loses ability to undergo cell division.

3 layers of connective tissue that extent from fascia.

Epimysium (outer most layer), perimysium (also dense irregular connective tissue, but in the middle.), and endomysium (inner, penetrates each fascicle, mostly reticular fibers).

Nerve and blood supply

Generally, an arty and one or two veins accompany each nerve that penetrates skeletal muscle. Neurons that stimulate skeletal muscle to contract are somatic motor neurons. Each has axon that extend from brain or spinal cord to group of skeletal muscle fibers. Axons of somatic motor neuron branches many times. Capillaries plentiful in muscular tissue. Bring in O2 and nutrients and remove heat and waste products. Helps bring things needed for contraction.

Contractility

Generates tension. Sometimes shortens and sometimes doesn't.

Relaxation period

Lasting 10-100 msec. Ca2+ actively transported back into SR, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in muscle fiber decreases. Duration depends on type of skeletal muscle fiber.

Contraction period

Lasts about 10-100 msecs. Ca2+ binds to troponin, myosin-binding sites on actin are exposed, and cross-bridges form. Peak tension develops in muscle fiber.

I band

Lighter, less dense area that contains rest of thin filaments but no thick filaments, and a Z disc passes through center of each I band.

Curare

Poison used by South American Indians on arrows cases muscle paralysis by bind to and bluing ACh receptors.

Aerobic cellular respiration.

Pyruvic acid enters mitochondria where it is completely oxidized in reactions that generate ATP, carbon dioxide, water, and heat. One molecule of glucose yields 36 molecules of ATP. Longterm.

Types of skeletal muscle fibers

Red muscle fibers have higher myoglobin content than white muscle fibers. Red muscle fibers also have more mitochondria and blood capillaries.

Cardiac response to AP

Remains contracted 10-15x longer than skeletal muscle tissue. Due to prolonged delivery of Ca2+ into sarcoplasm. Contracts when stimulated by its own auto rhythmic muscle fibers. Contracts and relaxes about 75x per minute. Mitochondria larger and more numerous. Depends largely on aerobic cellular respiration to generate ATP, and requires constant supply of oxygen. Can undergo hypertrophy like physiological enlarged heart in athletes or pathological enlarged heart in CAD.

Myofibrils

Small threads in sarcoplasm. Contractile organelles of skeletal muscle Extend entire length of muscle fiber. Prominent striations make skeletal muscle fiber appear striated.

Titin

third most plentiful protein in skeletal muscle (after actin and myosin). Huge molecule. 50x larger than average protein. Spans half a sarcomere, from Z disc to M line. Each titian molecule connects Z disc to M line of sarcomere, helping to stabilize position of thick filament. Very elastic, accounts for much of elasticity and extensibility of myofibrils.

Structural proteins

titin, alpha-actinin, myomesin, nebulin, and dystrophin.

Isotonic contraction

Tension developed in muscle remains almost constant while muscle changes its length. Isotonic contractions are used for body movements and for moving objects. Two types of isotonic contractions are concentric and eccentric.

Isometric contraction

Tension generated is not enough to exceed resistance of object to be moved, and muscle does not change in length. Example would be holding book steady using outstretched arm. Important for maintaining posture. Energy is still expended. Stabilize some joints as others are moved. Most activities include both isometric and isotonic contractions.

Two sources of Oxygen

1) O2 that diffuses into muscle fibers from blood and 2) O2 released by myoglobin within muscle fibers. Both myoglobin and hemoglobin are oxygen-binding proteins. Bind oxygen when it is plentiful and release oxygen when it is scarce. Supplies enough ATP for prolonged activity provided sufficient Oxygen and nutrients are available. Include pyretic acid obtained from glycolysis of glucose,fatty acids from breakdown of triglycerides in adipose cells, and amino acids from breakdown of proteins.

4 functions of muscular tissue

1) Producing body movements. 2) Stabilizing body positions. 3) Storing and moving substances within the body through sphincters. 4) Generating heat. Thermogenesis.

3 types of skeletal muscle fibers

1) slow oxidative fibers, 2) fast oxidative-glycolytic fibers, and 3) fast glycolytic fibers.

Elasticity

Ability to return to original length and shape after contraction or extension.

Contraction cycle 4 steps

1) ATP hydrolysis: Myosin heads hydrolyze ATP and become reprinted and energized. 2) Attachment of myosin to actin to form cross-bridges 3) Power stroke: myosin cross-bridges rotate toward center of sarcomere, sliding thin filament past thick filament towards the M line. 4) Detachment of myosin from actin: As myosin heads bind ATP, the cross-bridges detach from actin.

Nerve impulse muscle action potential

1) Release of acetylcholine: ACh released from synaptic vesicle. Triggered by Ca2+ ions. 2) 2 ACh molecules binds to ACh receptor to open it. Small cations like Na+ can now flow through. 3) Muscle action potential is produced: Na+makes inside of muscle fiber more positively charged. Action potential then propagates from sarcolemma into T-tubules. 4) ACh is broken down: ACHe breaks down ACh into acetyl and choline.

4 properties that enable muscular tissue function

1) electrical excitability. 2) contractility 3) extensibility 4) elasticity

Production of ATP in muscle fibers

3 ways of making more ATP: 1) creatine phosphate, 2) anaerobic cellular respiration, and 3) aerobic cellular respiration. Use of creatine phosphate for ATP production unique to muscle fibers.

Smooth muscle tissue details

Activated involuntarily. Visceral smooth muscle tissue most common. Found in skin and tubular arrangements that form part of walls of small arteries and veins and hollow organs like stomach, intestines, uterus, and urinary bladder. Autrorhythmic.

Connective tissue components

Connective tissue surrounds and protects muscular tissue. Subcutaneous layer or hypodermis, which separates muscle from skin, is composed of areolar connective tissue and adipose tissue. Provides pathway for nerves, blood vessels, and lymphatic vessels to enter and exit muscles. Adipose stores fat and protects muscle from trauma. Fascia is dense sheet of irregular connective tissue that lines body wall and limbs and supports and surrounds muscles and other organs of body. Fascia allows free movement of muscles, carries nerves, blood vessels, and lymphatic vessels, and fills spaces between muscles.

More on contraction

Continues as long as ATP and Ca2+ level near thin filament is sufficiently high. Cross-bridges attach and detach 5x every second. During maximal contraction, distance between two Z discs can decrease to half resting length. Z discs then pul on neighboring sarcomeres, and whole muscle fiber shortens. Some components elastic. They stretch slightly before they transfer tension generated by sliding filaments. Elastic component includes titian molecules, connective tissue round muscle fibers, and tendons.

Physiology of smooth muscle

Contraction starts more slowly and lasts much longer. Can both shorten and stretch more than skeletal. Ca2+ flow in from little supply of SR and interstitial fluid. Takes longer for Ca2+ to reach filaments in center of fiber and trigger contractile process. Slow onset of contraction of smooth muscle.

Control of muscle tension

Depends on frequency of stimulation. Total tension a whole muscle can produce depends on number of muscle fibers contracting in unison.

Fast glycolytic fibers

FG fibers largest in diameter and contain most myofibrils. Can generate most powerful contractions. Low myoglobin content and few blood capillaries. Contain large amounts of glycogen and generate ATP through glycolysis. FG fibers contract strongly and quickly. Adapted for intense anaerobic movements of short duration like weight lifting or throwing a ball. Fatigue quickly. FG fibers of weight liger might be 50% larger than those of sedentary person or endurance athlete because of increased synthesis of muscle proteins.

Fast oxidative-glycolytic fibers

FOG fibers intermediate in diameter between other two types of fibers. Contain large amounts of myoglobin and many blood capillaries. Also have dark red appearance. Generate ATP by aerobic cellular respiration, which gives them moderately high resistance to fatigue. Also generate ATP by anaerobic glycolysis. "Fast". Can reach peak tension more quickly than SO but briefer in duration, less than 100 msec. Contribute to walking and sprinting.

Tendon

All 3 connective tissues may extend beyond muscle to form tendon that attaches muscle to bone. When connective tissue elements extend as broad, flat sheet, it is called aponeurosis.

Development of muscle

All muscles of body derived from mesoderm. Mesoderm > somites > myotome (skeletal muscles of head, neck,and limbs, dermatome (connective tissues), and sclerotome (give rise to vertebrae). Cardiac muscle develops from mesodermal cells Smooth muscle develops from mesodermal cells.

Myosin

Component of thick filaments and functions as motor protein in all three types of muscle tissue. Motor proteins pul various cellular structures to achieve movement by converting the chemical energy in ATP to mechanical energy of motion, that is production of force. Each myosin molecule is shaped like two golf clubs twisted together. Myosin tail points toward M line in center of sarcomere. Tails of neighboring myosin molecules lie parallel to one another, forming shaft of thick filaments. Myosin heads project outward from shaft in spiraling fashion, extending toward one of 6 thin filaments that surround each thick filament.

Motor unit recruitment

Different motor units of entire muscle are not stimulated to contract in unison. While some motor units are contraction, others are relaxed. Delays muscle fatigue and allows contraction of whole muscle to be sustained for long periods. Weakest motor units recruited first. Produces smooth instead of series of jerks.

Distribution and recruitment of different types of fibers

Skeletal muscles mixture of all three fibers. Half are SO. Within a particular motor unit, all fibers are of the same type. Activated in order of need. Activation of various motor units controlled by brain and spinal cord.

Anticholinesterase agents

Slow enzymatic activity of acetylcholinesterase, slowing removal of ACh from synaptic cleft. Can strengthen weak muscle contractions. Neostigme is example for myasthenia gravis.

Sarcolemma

plasma membrane of muscle fiber. Transverse t tubules are invaginations of sacrolemma, and open to outside of fiber so are filled with interstitial fluid. Muscle action potential travels along sarcolemma and through T tubules. This allows all parts of muscle fiber to get excited at same instant.

Z discs

separate one sarcomere from next. Sarcomere extends from one Z disc to the next. Rest of zones and bands overlap create striations.

Length-tension relationship

How forcefulness of muscle contraction depends on length of sarcomeres within muscle before contraction begins. Maximum tension occurs when zone of overlap between a thick and thin filament extends from edge of H zone to one end of a thick filament. As sarcomeres of muscle are stretched to longer length, zone of overlap shortens, and fewer myosin heads can make contact with thin filaments. Tension the fiber can produce decreases. When muscle is stretched to 170% of its optimal length, there is no overlap between thick and thin filaments. Muscle cannot contract, and tension is 0. As sarcomere lengths become increasingly shorter than optimum, tension that can develop again decreases. This is because thick filaments crumple as they are being compressed by Z discs. Resting muscle fiber length is held very close to optimum by firm attachments of skeletal muscle to bones and to other inelastic tissues.

Concentric isotonic contraction

If tension generated is great enough to overcome resistance of object to be moved, muscle shortens and pulls on another structure like a tendon, to produce movement. Picking book up off table involves this. Lowering book on table causes shortened biceps to lengthen in a controlled manner while it continues to contract.

Refractory period

If two stimuli applied immediately after the other, muscle will respond to first stimulus but not to second. When muscle fiber received enough stimulation to contract, it temporarily loses excitability and cannot respond for a time. Period of lost excitability, called refractory period, is characteristic of all muscles and nerve cells. Duration of refractory period caries with muscle involved. Skeletal muscle refractory period about 5 msec. Cardiac muscle has longer refractory period of about 300 msec.

Alpha-actinin, myomesin, and nebulin

In dense material of Z discs. Bind to actin molecules of thin filament and to titin. Myomesin form M line. M line proteins bind to titian and connect adjacent thick filaments to each other. Nebulin is long, nonelastic protein wrapped around entire length of each thin filament. Helps anchor thin filaments to Z discs and regulates length of thin filaments during development.

Tropomyosin and troponin

In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover myosin-binding sites on actin. Tropomyosin strands held inlace by troponin molecules. Ca2+ bind to troponin, making it undergo change in shape. This moves tropomyosin away from myosin-binding sites on actin and muscle contraction subsequently begins as myosin binds to actin.

Muscle fatigue

Inability of muscle to maintain force of contraction after prolonged activity. Results mainly from changes within muscle fibers. Central fatigue is when person has desire to cease activity. One factor may be inadequate release of calcium ions from SR. Also might be depletion of creatine phosphate.

Excitation-Contraction coupling

Increase in Ca2+ in sarcoplasm starts muscle contraction,a and decrease stops it. When relaxed, concentration of Ca2+ is low. As muscle action potential propagates along sarcolemma and into T tubules, causes Ca2+ release channels in SR membrane to open. When these open, Ca2+ flows out of ST into sarcoplasm around thick and thin filaments. Released calcium ions combine with troponin, causing them to change shape. This moves tropomyosin away from myosin-binding sites on actin. This lets myosin heads form cross-bridges and starts contraction.

Dystrophin

Links thin filaments of sarcomere to integral membrane proteins of sarcolemma, which are attached in turn to proteins in cognitive tissue extracellular matrix that surround muscle fibers. Thought to reinforce sarcolemma and help transmit tension generated by sarcomeres to tendons.

Smooth muscle tissue

Located in walls of hollow internal structures like blood vessels, airways, and most organs in abdominopelvic cavity. Also found in skin. Non-striated so smooth. Involuntary.

H zone

Narrow center of each A band that contains thick but no thin filaments.

Neuromuscular junction

Neurons that stimulate skeletal muscle fibers to contract are called somatic motor neurons. Each has axon that extends from brain or spinal cord to group of skeletal muscle fibers. Muscle fiber contracts in response to one or more action potentials propagating along its sarcolemma and through system of T tubules. Muscle action potentials arise at neuromuscular junction (NMJ), synapse between a somatic motor neuron and skeletal muscle fiber.

Wave summation

Occurs along with both tetanus when Ca2+ is released from SR by subsequent stimuli while levels of Ca2+ in sarcoplasm are still elevated from first stimulus., peak tension generated during fused tetanus is 5-10 times larger than peak tension produced during single twitch. Stretch of elastic opponents affects wave summation. During wave summation, elastic components are not given much time to spring back between contractions, and remain taut. This helps next muscle contraction be stronger than one before.

Oxygen consumption after exercise

Oxygen debt refers to added oxygen, over and above resting oxygen consumption, that is taken into body after exercise. Pay back. 3 ways: 1) to convert lactic acid back into glycogen stores in the liver, 2) to resynthesize creatine phosphate and ATP in muscle fibers, and 3) to replace O2 removed from myoglobin. Recovery oxygen uptake is butternut term than oxygen debt.

Synapse

Synapse is region where communication occurs between two neurons, or neuron and target cell. Synaptic cleft separates two cells. Cells do not touch, so first cell communicates with 2nd by releasing neurotransmitter.

Actin

Thin filaments anchored to Z discs. Actin molecules join to form an actin filament that is twisted into helix. On each actin molecule is myosin-binding site, where myosin head can attach.

Dense bodies

Thin filaments that attach to structures which are similar to Z discs in striated muscle fibers. Bundles of intermediate filaments attach to dense bodes and stretch from one dense body to another. During contraction, sliding filament mechanism involving thick and thin filaments generates tension that is transmitted to intermediate filaments. These pull on dense bodies attached to sarcolemma, causing lengthwise shortening of muscle fiber.

Electrical excitability

Through muscle action potentials. Stimulate by electrical sigmas arising in muscular tissue itself (like heart's pacemaker), or chemical stimuli, like neurotransmitters released by neurons, hormones, or changes in pH.

Calmodulin

binds to Ca2+ in cytosol. Replaces troponin. After binding to Ca2+, calmodulin activates enzyme called myosin light chain kinase. Enzyme uses ATP to add phosphate group to portion of myosin head. Myosin light chain kinase works slowly.

Botulinum toxin

blocks exocytosis of synaptic vesicles at NMJ. Bacteria proliferates in improperly canned foods, and toxin is extremely lethal. This is put in Botox as a muscle relaxant.

Regeneration of muscular tissue

hypertrophy causes growth of skeletal muscle after birth (increase of growth of existing cells) rather than hyperplasia which is increase in number of fibers. Satellite cells are slow. Regeneration is limited. Smooth muscle can grow via hyperplasia. New smooth muscle can arise from pericytes, stem cells found in associated with blood capillaries and small veins.

Eccentric isotonic contraction

When length of muscle increases during contraction, it is eccentric. During eccentric contraction, tension exerted by myosin cross-bridges resists movement of load and slows lengthening process. Repeated eccentric isotonic contractions produce more damage and soreness than do concentric isotonic contractions.

Frequency of stimulation

When second stimulus occurs after refractory period of first stimulus is over, but before skeletal muscle fiber has relaxed, second contraction will be stronger than first. Called wave summation. When skeletal muscle fiber is stimulated at rate of 20-30 times per second, it can only partially relax between stimuli. Result is wavering contraction called unfused (incomplete) tetanus. When skeletal muscle fiber is stimulated at higher rate of 80-100 times per sec, does not relax at all. Result is fused (complete) tetanus, sustained contraction in which individual twitches cannot be detected.

Filaments

Within myofibrils are smaller protein structures called filaments. Thin filaments composed mostly of actin, while thick filaments composed mostly of myosin. Both directly involved in contractile process. Two thin filaments for every thick filament in regions of filament overlap. Arranged in compartments called sarcomeres, which are basic functional units of myofibrils.

Motor end plate

Muscle part of NMJ. Within each motor end plate are millions of acetylcholine receptors. These have junctional folds that provide large surface area for ACh. ACh receptors are ligand-gated ion channels.

Anaerobic cellular respiration

Does not need oxygen. When supply of creatine phosphate is depleted, glucose is catabolized to generate ATP. Glucose easily passes into contraction muscle fibers through facilitated diffusion, and is also produced by breakdown of glycogen within muscle fibers. Glycolysis quickly breaks down glucose molecule into two molecules of pyretic acid. Glycolysis occurs in cytosol and produces net gain of two molecules of ATP. During periods of heavy exercise, not enough O2 is available to skeletal muscle fibers. This makes most of pyruvic acid to lactic acid in cytosol. Liver cells can produce some of lactic acid into glucose. Can provide about 30 to 40 seconds of maximal muscle activity.

Creatine phosphate

Most of excess ATP used to synthesize creatine phosphate. Creatine kinase catalyzes transfer of one of high-energy phosphate groups from ATP to creatine, forming creatine phosphate and ADP. Creatine is small, amino acid-like molecule synthesized in liver, kidneys, and pancreas, and transported to muscle fibers. 3-6x more plentiful than ATP in sarcoplasm of relaxed muscle fiber. When contraction begins, CK catalyzed transfer of phosphate grow from creatine phosphate back to ADP. This makes ATP. Creatine phosphate is first source of energy when muscle contraction begins. Creatine phosphate and ATP provide enough energy for muscles to contract maximally for 15 seconds.

Skeletal muscle tissue

Most skeletal muscles move bones of skeleton. Striated. Voluntary. Some controlled subconsciously like diaphragm.

Muscle fibers

Muscle cells.

Slow oxidative fibers

SO are smallest in diameter and are least powerful type of muscle fibers. Appear dark red because contain large amounts of myoglobin and many blood capillaries. Generated ATP by aerobic cellular respiration. Fibers are slow because ATPase in myosin heads hydrolyzes ATP slowly and contraction cycle proceeds at slower pace. Twitch contractions last from 100-200 msec, take longer to reach peak tension. Capable of prolonged contractions for many hours. Adapted for maintaining posture and enduracne type activities.

Sarcplasmic Reticulum

SR encircles each myofibril. Terminal cisterns of this butt against T tubule from both sides. Transverse tubule and two terminal cisterns on either side of it form triad. Stores calcium ions. Release of Ca2+ from terminal cisterns of sarcoplasmic reticulum triggers muscle contraction.

Ca2+ Active transport pumps

Sarcoplasmi reticulum membrane uses these which use ATP to move Ca2+ constantly from sarcoplasm into SR. While muscle action potential continues, Ca2+ release channels stay open. Calcium ions flow into SR more rapidly than they are transported back by pumps. After last action potential, Ca2+ release channels close. Concentration in sarcoplasm quickly decreases. Inside SR, calsequestrin binds to Ca2+, enabling more Ca2+ to be sequestered or stored within ST. Concentration of Ca2+ in SR is 10,000 times higher than in cytosol of relaxed muscle fiber. Tropomyosin then covers myosin-binding sites, and muscle fiber relaxes.

Microscopic anatomy of smooth muscle

Single nucleus within each fiber. Thick and thin filaments not arranged in orderly sarcomeres like striated muscle. Contain intermediate filaments. Lack T-tubules and only have small amount of SR for storage of Ca2+.Have suchlike invaginations of PM called caveolae that contain extracellular Ca2+ that can be used for muscular contraction.

Cardiac muscle tissue

Striated. Involuntary. Autorhythmicity is built-in rhythm/pacemaker. Several hormones and neurotransmitters can adjust heart rate by speeding or slowing pacemaker.

M line

Supporting proteins that hold thick filaments together at center of at center of H zone. middle of sarcomere.

Two regulatory proteins

Tropomyosin and troponin. Part of thin filaments.

Two contractile proteins

Myosin and actin.

Sliding filament mechanism

Myosin heads attach to and walk along thin filaments at both ends of sarcomere, progressively pulling thin filaments toward M line. Thin filaments slide inward, and meet at center of sarcomere. Z discs come closer together, and sarcomere shortens. Lengths of individual thick and thin filaments do not change. Shortening of sarcomeres causes shortening of whole muscle fiber, which leads to shortening of entire muscle.

Muscle tone

Small amount of fatness or tension in muscle due to weak, involuntary contractions of its motor units. When motor neurons serving a skeletal muscle are damaged or cut, muscle becomes flaccid. Muscle tone keeps skeletal muscles firm, but does not result in force strong enough to produce movement.