Chapter 10: Muscular Tissue (Chapter Questions)

What roles do contractile, regulatory, and structural proteins play in muscle contraction & relaxation?

-Contractile proteins - myosin heads attach to and "walk" along the thin filaments at both ends of the sarcomere, progressively pulling the thin filaments toward the M line. -Regulatory proteins - at onset of contraction, sarcoplasmic reticulum releases calcium ions into the sarcoplasm and they bind to troponin. Troponin moves tropomyosin away from the myosin-binding sites on actin. Once the binding sites are "free", the contraction cycle begins.

What are the four properties of muscular tissue?

-Electrical excitability - action potentials (muscle action potentials ---- electrical signals (in muscular tissue itself) ---- chemical stimuli - neurotransmitters, hormones, pH -Contractility - force of contractions -Extensibility - stretch without being damaged -Elasticity - return to original length & shape after contraction or extension

What types of fascia cover skeletal muscles?

-Epimysium - (dense irregular) outer layer encircling the ENTIRE muscle -Perimysium - (dense irregular) surround groups of 10 - 100 or more muscle fibers, separating them into bundles called FASCICLES Endomysium - (mostly reticular fibers) penetrates interior of each fascicle & separated individual muscle fibers from one another.

Length-tension relationship in a skeletal muscle

-Maximum tension during contraction occurs when the resting sarcomere length is 2.0 - 2.4 -A muscle develops its greatest tension when there is an optimal zone of overlap between thick and thin filaments

Muscle metabolism

-Muscle fibers have 3 ways to produce ATP 1. from creatine phosphate 2. by anaerobic glycolosis 3. by aerobic respiration

skeletal muscle abnormalities

-Myasthenia Gravis - AI (autoimmune) disease that causes chronic, progressive damage of neuromuscular junction -Muscular Dystrophy - group of inherited muscle-destroying diseases causing progressive degeneration of skeletal muscle fibers -Abnormal contractions of skeletal muscle ----spasm ----cramp ----tic ----tremor ----fasciculation ----fibrillation -Exercise-induced muscle damage

Contractile Proteins (generate FORCE)

-Myosin = ---- THICK filament; tail & 2 myosin heads -Actin = ---- main component of THIN filament; each actin molecule has myosin binding site where myosin head of thick filament binds during contraction

Oxygen consumption after exercise

-OXYGEN DEBT - refers to the added oxygen over and above the resting oxygen consumption that is taken into body after exercise ---- convert lactic acid back into glycogen stores in liver ---- resynthesize creatine phosphate and ATP in muscle fibers ---- replace oxygen removed from myoglobin -RECOVERY OXYGEN UPTAKE- elevated use of oxygen after exercise (better term than oxygen debt)

What are the general functions of muscular tissue?

-Producing body movements - walking/running -Stabilizing body positions -Storing & moving substances within body - sphincters (smooth muscle), blood flow, food through GI -Generating heat - thermogenesis, shivering

How are structures of thin and thick filaments different?

-Thin filaments - 8nm diameter/1-2 long/protein ACTIN -Thick filaments - 16 nm diameter/1-2 long/protein MYOSIN -Both involved in contractile process -2 thin for every 1 thick in overlap -Arranged in sarcomeres

Structural Proteins (keep thick & thin filaments of myofibrils in alignment; give myofibrils elasticity & extensibility; link myofibrils to sarcolemma & extracellular matrix)

-Titin ---- connects Z disc to M line of sarcomere (helping to stabilize thick filament position; accounts for extensibility and elasticity of myofibrils -a-Actinin ---- protein of Z discs that attaches to actin molecules of thin filaments & to titin molecules -Myomesin ---- forms M line of sarcomere; binds to titin molecules; connects adjacent thick filaments to one another -Nebulin ---- wraps around entire length of each thin filament; helps anchor thin to Z discs; regulates length of thin during development -Dystophin ---- links thin of sarcomere to integral membrane proteins in sarcolemma; help reinforce sarcolemma & help transmit tension by sarcomeres to tendons

Regulatory Proteins (switch contraction process ON & OFF)

-Tropomyosin = ---- THIN filament; when muscle fiber relaxed, tropomyosin covers myosin-binding sites on actin molecules -Troponin = ---- THIN filament; when calcium ions bind to troponin, it changes shape; this moves tropomyosin away from myosin-binding sites on actin molecules

smooth muscle tissue

-VISCERAL (single-unit) ----found in skin and in tubular arrangements that form part of the walls of small arteries and veins and hollow organs such as stomach, intestines, uterus, and urinary bladder ----fibers connect by gap junctions -MULTIUNIT ----lack gap junctions and contract independently ----found in walls of large arteries, airways to lungs, arrector pili muscles, muscles of iris that adjust pupil diameter and ciliary body that adjust focus of lens in eye

Components of a Sarcomere

-Z discs -A Band -I Band -H Zone -M Line

development of muscle

-all muscles of the body are derived from mesoderm (EXCEPT for iris of eyes and arrector pili muscles attached to hairs) -SOMITE: columns of mesoderm that undergo segmentation into series of cube-shaped structures (1st pair appears on 20th day of embryonic development) Cells of SOMITE: ----myotome - forms skeletal muscles of head, neck & limbs ----dermatome - forms connective tissues including dermis of skin ----sclerotome - gives rise to vertebrae

aging and muscular tissue

-between ages 30-50, progressive loss of skeletal muscle mass replaced by fibrous connective tissue and adipose tissue -10% of muscle mass lost during these years -another 40% lost during ages 50 - 80 -exercise can slow this process of muscle mass loss

microscopic anatomy of smooth muscle

-contain intermediate filaments with no regular pattern of overlap and therefore NO STRIATIONS -lack T tubules -contain caveolae (small pouch-like invaginations of plasma membrane that contain extracellular Ca++ that can be used for muscular contraction) -thin filaments attach to structures called dense bodies (functionally similar to Z discs in striated muscle fibers) -when smooth muscle contracts, it rotates as a corkscrew turns

physiology of smooth muscle

-contraction in smooth muscle fiber starts more slowly and lasts much longer than skeletal muscle fiber contraction -it can shorten and stretch to a greater extent than the other muscle types

ATP Produced by Creatine Phosphate

-duration of energy provided is 15 seconds -formed from ATP while the muscle is relaxed -transfers a high-energy phosphate group to ADP, forming ATP during muscle contraction

ATP produced by Anaerobic glycolysis

-duration of energy provided is 2 minutes -breakdown of muscle glycogen into glucose -production of pyruvic acid from glucose via glycolysis produce both ATP and lactic acid -no oxygen is needed

ATP produced by Aerobic respiration (marathon race)

-duration of energy provided is several minutes to hours -within mytochondria, pyruvic acid, fatty acids, and amino acids are used to produce ATP -requires oxygen

muscle fatigue

-inability of a muscle to maintain force of contraction after prolonged activity Possible causes: -inadequate release of calcium ions from the SR, resulting in decline of Ca++ concentration is sarcoplasm -depletion of creatine phosphate -insufficient oxygen -depletion of glycogen -buildup of lactic acid and ADP -failure of action potentials in motor neuron to release enough ACh

contraction period (2nd phase of twitch contraction)

-lasts 10-100 msec -Ca++ binds to troponin -myosin-binding sites on actin are exposed -cross-bridges form -peak tension develops in the muscle fiber

relaxation period (3rd phase of twitch contraction)

-lasts 10-100 msec -Ca++ is actively transported back into sarcoplasmic reticulum -myosin-binding sites are covered by tropomyosin -myosin heads detach from actin -tension in muscle fiber decreases

What features distinguish the 3 types of muscular tissue?

-skeletal = striated/voluntary -cardiac = heart only/striated/involuntary/autorythmycity -smooth = nonstriated/involuntary/walls of hollow organs and skin (attached to hair follicles)

effective stretching

-stretching cold muscles does NOT increase flexibility and may cause injury -tissues stretch best when slow, gentle force is applied at elevated tissue temps

Control of Muscle Tension

-the total force or tension that a single muscle fiber can produce depends mainly on the rate at which nerve impulses arrive at the neuromuscular junction -frequency of stimulation (number of impulses per second) -maximum tension is also affected by amount of stretch before contraction and by nutrient and oxygen availability -TOTAL tension a whole muscle can produce depends on the number of muscle fibers that are contracting in unison

Myofibril

-threadlike contractile elements within sarcoplasm of muscle fiber that extend length of fiber; composed of filaments (thick and thin)

frequency of stimulation

-wave summation (stimuli arrive at different times causing larger contractions) ---- due to wave summation, the tension produced during a sustained contraction is greater than that produced by a single twitch -unfused tetanus (sustained, but wavering contraction) -fused tetanus (sustained contraction in which individual twitches cannot be detected)

Neuromuscular Junction (NMJ)

-where muscle action potentials arise -the synapse between a somatic motor neuron and a skeletal muscle fiber

triad

1 T tubule and 2 terminal cisterns on either side of it

Nerve impulse (nerve action potential)

1. Release of acetylcholine (ACh is released from synaptic vesicle) 2. Activation of ACh receptors (ACh binds to ACh receptor) 3. Production of muscle action potential 4. Termination of ACh activity (ACh is broken down)

order of recruitment for muscle fibers

1. SO 2. FOG 3. FG

Levels of Organization within a Skeletal Muscle

1. Skeletal Muscle 2. Fascicle 3. Muscle Fiber (cell) 4. Myofibril 5. Filaments (myofilaments)

Types of Skeletal Muscle Fibers

1. Slow oxidative fibers (SO) 2. fast oxidative-glycolytic fibers (FOG) 3. fast glycolytic (FG) fibers

Microscopic organization of skeletal muscle

1. during embryonic development, many myoblasts fuse to form on skeletal muscle fiber; once fusion has occurred, a skeletal muscle fiber loses the ability to undergo cell division, but satellite cells retain this ability. 2. sarcolemma of fiber encloses sarcoplasm & myofibrils, which are striated. 3. sarcoplasmic reticulum wraps around each myofibril 4. thousands of T tubules invaginate from sarcolemma toward center of muscle fiber

Organization of skeletal muscle and its connective tissue coverings

A skeletal muscle consists of individual muscle fibers (cells) bundled into fascicles and surrounded by three connective tissue layers that are extension of the fascia.

Role of calcium ions in regulation of contraction by troponin & tropomyosin

An increase in the Ca++ level in the sarcoplasm starts the sliding of thin filaments. When the level of Ca++ in the sarcoplasm declines, sliding stops.

Why is a rich blood supply important for muscle contraction?

Because the blood capillaries bring in oxygen and nutrients and remove heat and the waste products of muscle metabolism. Especially during contraction, a muscle fiber synthesizes and uses considerable ATP (adenosine triphosphate) and these reactions require oxygen, glucose, fatty acids & other substances that are delivered to muscle fiber in blood.

Skeletal Muscle Proteins

Contractile ---- Myosin ---- Actin Regulatory ---- Tropomyosin ---- Troponin Structural ---- Titin ---- a-Actinin ---- Myomesin ---- Nebulin ---- Dystrophin

regeneration of muscular tissue

HYPERTROPHY - the enlargement of existing cells ----due to skeletal muscle fibers losing ability to undergo cell division, growth of skeletal muscle after birth is due to hypertrophy ----skeletal and cardiac muscle tissue HYPERPLASIA - an increase in number of fibers ----smooth muscle tissue (such as uterus) -skeletal muscle tissue can regenerate ONLY to a limited extent -smooth muscle tissue has considerably greater powers of regeneration when compared with the other 2 muscle tissues

Sliding filament mechanism of muscle contraction

In two adjacent sarcomeres, thin filaments move toward the M line of each sarcomere

Contraction cycle

Sarcomeres exert force & shorten through repeated cycles during which the myosin heads attach to actin (cross-bridges), rotate, and detach. 1. ATP hydrolysis - Myosin heads hydrolyze ATP and become reoriented and energized 2. Attachment of myosin to actin to from cross-bridges - Myosin heads bind to actin, forming cross-bridges 3. Power stroke - myosin cross-bridges rotate toward center or sarcomere 4. Detachment of myosin from actin - as myosin heads bind ATP, the cross-bridges detach from actin ----CONTRACTION CYCLE CONTINUES IF ATP is AVAILABLE and calcium ion level in sarcoplasm is high

I band

a lighter, less dense area that contains the rest of the thin filaments but NO thick filaments; Z disc passes through center of each I band.

muscle tone

a small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units. ---- flaccid = a state of limpness in which muscle tone is lost;

synaptic cleft

a small gap that separates the two cells

smooth muscle tone

a state of continual partial contraction due to the prolonged presence of Ca++ in the cytosol provides this ----this is important in the GI tract where walls maintain steady pressure

stress-relaxation response

allows smooth muscle to undergo great changes in length while retaining the ability to contract effectively ----i.e. after organ empties, the smooth muscle in the wall rebounds and the wall retains its firmness

slow oxidative (SO) fibers

appear dark red because they contain large amounts of myoglobin and many blood capillaries; because of many large mitochondria, SO fibers generate ATP mainly by aerobic respiration (hence oxidative) ----about HALF of ALL skeletal muscle fibers are SO fibers

sarcomeres

basic functional units of a myofibril

twitch contraction

brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron ---- twitches last anywhere from 20 to 200 msec (this is very long compared to the brief 1-2 msec that a muscle action potential lasts)

latent period (1st phase of twitch contraction)

brief delay between application of the stimulus and the beginning of the contraction (last about 2 msec) ---- muscle action potential sweeps over the sarcolemma & calcium ions are released from sarcoplasmic reticulum

Fascicle

bundle of muscle fibers wrapped in PERIMYSIUM

muscle contraction

calcium ions released from terminal cisterns of sarcoplasmic reticulum

motor unit

consists of a somatic motor neuron plus all of the skeletal muscle fibers it stimulates ---- the total strength of a contraction depends on the size of the motor units and the number that are activated at a given time

Filaments (myofilaments)

contractile proteins within myofibrils that are of the two types: thick = myosin/thin = actin, tropomyosin, & troponin; sliding of thin past thick produces muscle shortening (which equals contraction)

sarcoplasm

cytoplasm of a muscle fiber (this is within the sarcolemma)

A band

darker middle part of sarcomere which extends the ENTIRE LENTH of thick filaments

junctional folds

deep grooves in the motor end plate that provide a large surface area for ACh.

terminal cisterns

dilated end sacs of the sarcoplasmic reticulum which butt against the T tubule from both sides.

sarcoplasmic reticulum

fluid-filled system of membranous sacs which encircles each myofibril

synaptic vesicles

hundreds of membrane-enclosed sacs within each synaptic end bulb

acetylcholine receptors

integral transmembrane proteins to which ACh specifically binds.

synapse

is a region where communication occurs between two neurons, or between a neuron and a target cell (i.e between somatic motor neuron and muscle fiber)

myofibrils

little threads stuffed within the sarcoplasm; these are the contractile organelles of skeletal muscle

Muscle Fiber (cell)

long cylindrical cell covered by ENDOMYSIUM and sarcolemma; contains sarcoplasm, myofibrils, may peripherally located nuclei, mitochondria, T tubules, sarcoplasmic reticulum, & terminal cisterns. Fiber has a striated appearance.

fast glycolytic fibers (FG)

low myoglobin content, relatively few blood capillaries and few mitochondria and appear white in color; large amounts of glycogen and generate ATP mainly by glycolysis. ----these fast-twitch fibers are adapted for intense anaerobic movements of short duration such as weight lifting

The scientific study of muscles

myology

H zone

narrow, in the center of each A band; contains thick but NO thin filaments I = THIN H = THICK

Z discs

narrow, plate-shaped regions of dense protein which separate one sarcomere from the next

Skeletal muscle

organ made up of fascicles that contains muscle fibers (cells), blood vessels, & nerves; wrapped in EPIMYSIUM

cardiac muscle tissue

principle tissue in the heart ----same arrangement of actin and myosin and same bands, zones, and Z discs as skeletal muscle fibers ----intercalated discs ----desmosomes ----gap junctions -remains contracted 10 - 15 times longer than skeletal muscle tissue due to prolonged delivery of Ca++ into the sarcoplasm -continuous rhythmic activity -requires constant supply of oxygen ----physiological enlarged heart - EXERCISE ----pathological enlarged heart - DISEASE

myogram

record of a muscle contraction

myoglobin

red-colored protein found ONLY in muscle, binds oxygen molecules that diffuse into muscle fibers from interstitial fluid.

calmodulin

regulatory protein in smooth muscle that binds to Ca++ in the cytosol

relaxed muscle fiber

sarcoplasmic reticulum stores calcium ions

Three types of muscular tissue

skeletal cardiac smooth

M line

supporting proteins that hold the thick filaments together at the center of the H zone (M = middle of the sarcomere)

axon terminal

the end of the motor neuron at the neuromuscular junction

myoblasts

the fusion of a hundred or more small mesodermal cells

synaptic end bulbs

the neural part of the NMJ

somatic motor neurons

the neurons that stimulate skeletal muscle fibers to contract

acetylcholine (ACh)

the neurotransmitter released at the NMJ inside each synaptic vesicle

sarcolemma

the plasma membrane of a muscle cell

motor unit recruitment

the process in which the number of active motor units increases ----recruitment is one factor responsible for producing smooth movements rather than a series of jerks

strength training

the process of exercising with progressively heavier resistance for the purpose of strengthening the musculoskeletal system ----helps to increase bone strength (by increasing deposition of bone minerals) ----muscle mass raises resting metabolic rate

motor end plate

the region of the sarcolemma opposite the synaptic end bulbs; it is the muscle fiber part of the NMJ

exercise and skeletal muscle tissue

the relative ratio of FG and SO fibers in each muscle is genetically determined and helps account for individual difference in physical performance ----i.e. people with higher proportion of FG fibers often excel in activities that require periods of intense activity, such as weight lifting or sprinting; people with higher percentages of SO fibers are better at activities that require endurance, such as long distance running

Excitation-contraction coupling

the steps that connect excitation (a muscle action potential propagating along the sarcolemma and into the T tubules) to contraction (sliding of the filaments)

isotonic contractions

the tension (force of contraction) developed in the muscle remains almost constant while the muscle changes its length. -CONCENTRIC & ECCENTRIC ----these contractions are used for body movements and for moving objects.

isometric contractions

the tension generated is not enough to exceed the resistance of the object moved, and the muscle does not change its length. ----holding a book steady using an outstretched arm ----these contractions are important for maintaining posture and for supporting objects in a fixed position ----these do not results in body movement but energy is still expended ----they are important because they stabilize some joints as others are moved

transverse (T) tubules

thousands of tiny invaginations of the sarcolemma tunnel in from the surface toward the center of each muscle fiber

fast oxidative-glycolytic fibers (FOG)

typically the largest fibers; contain large amounts of myoglobin and many blood capillaries; dark red appearance; can generate ATP by aerobic respiration & anaerobic glycolosis