Chapter 10 - Muscle Tissue

Myoglobin

This protein, found only in muscle, binds oxygen molecules that diffuse into muscle fibers from interstitial fluid.

Narrow, zigzag-shaped regions of dense protein material called

*Z discs* separate one sarcomere from the next. +Thus, a sarcomere extends from one Z disc to the next Z disc.

Inside each synaptic vesicle are thousands of molecules of ____, the neurotransmitter released at the NMJ.

*acetylcholine (ACh)*

Within each motor end plate are ____ that bind specifically to ACh (acetylcholine).

*acetylcholine receptors*

Individual ____ molecules join to form a thin filament that is twisted into a helix.

*actin*.

Thin filaments are anchored to Z discs. Their main component is the contractile protein

*actin*.

Myofibrils contain ____ which generate force during contraction.

*contractile proteins*

Within myofibrils are smaller protein structures, called

*filaments* or *myofilaments*.

The action of smooth muscle is usually

*involuntary*, and, like cardiac muscle, some smooth muscle tissue, such as the muscles that propel food through your gastrointestinal tract, has autorhythmicity.

Acetylcholine receptors are abundant in ____, deep grooves in the motor end plate that provide a large surface area for ACh.

*junctional folds*

Each of your skeletal muscles is a separate organ composed of hundreds to thousands of cells called

*muscle fibers* because of their elongated shapes.

At high magnification the sarcoplasm appears stuffed with little parallel threads. These small structures are the

*myofibrils*, the contractile organelles of skeletal muscle.

The two projections of each myosin molecule (golf club heads) are called

*myosin heads*. +The heads project outward from the thick filament in a spiraling fashion, each head extending toward one of the six thin filaments that surround each thick filament.

The ____ (twisted golf club handles) points toward the M line in the center of the sarcomere. Tails of neighboring myosin molecules lie parallel to one another, forming the shaft of the thick filament.

*myosin tail*

Each ____ molecule is shaped like two golf clubs twisted together.

*myosin*

In skeletal muscle, about 300 molecules of ____ form a single thick filament.

*myosin*

The two contractile proteins in muscle are

*myosin* and *actin*.

On each actin molecule is a ____ where a myosin head can attach.

*myosin-binding site*

A muscle fiber contracts in response to muscle action potential that arise at the ____, the synapse between a motor neuron and a skeletal muscle fiber.

*neuromuscular junction (NMJ)*

Because skeletal muscle fibers often are very long cells, the ___ usually is located near the midpoint of a skeletal muscle fiber. Muscle action potentials arise at the ___ and then propagate toward both ends of the muscle fiber. This arrangement permits nearly simultaneous activation (and thus contraction) of all parts of the muscle fiber.

*neuromuscular junction (NMJ)*

Because the cells do not physically touch, the action potential from the motor neuron cannot "jump the gap" to directly excite the muscle fiber. Instead, the motor neuron communicates with the muscle fiber indirectly, by releasing a chemical called a ____.

*neurotransmitter*

Skeletal muscle fibers with a high myoglobin content have a darker appearance and are called

*red muscle fibers*. Red muscle fibers are in the "dark meat" in chicken legs and thighs.

The filaments inside a myofibril do not extend the entire length of a muscle fiber. Instead, they are arranged in compartments called

*sarcomeres*, which are the basic functional units of a myofibril.

The sarcolemma surrounds the

*sarcoplasm*.

A fluid-filled system of membranous sacs called the ____ encircles each myofibril. This elaborate system is similar to smooth endoplasmic reticulum in nonmuscular cells.

*sarcoplasmic reticulum (SR)*

Besides, contractile and regulatory proteins, msucle contains about a dozen ____ that contribute to the alignment, stability, elasticity, and extensibility of myofibrils.

*structural proteins*

A small gap, called the ____, separates the two cells.

*synaptic cleft*

Dilated end sacs of the sarcoplasmic reticulum called ____ butt against the T tubule from both sides.

*terminal cisterns*

Thousands of tiny tunnel-like invaginations of the sarcolemma, called ____, extend in toward the center of each muscle fiber.

*transverse (T) tubules*

One T tubule and the two terminal cisterns on either side of it form a ____.

*triad*

In relaxed muscle, myosin is blocked from binding to actin because strands of ____ cover the myosin-binding sites on actin.

*tropomyosin*

Thin filaments also contain smaller amounts of two *regulatory proteins*

*tropomyosin* and *troponin* which help switch the contraction process on and off.

The tropomyosin strand, in turn, is held in place by

*troponin*

Skeletal muscle fibers (muscle cells) vary in composition and function. For example, muscle fibers (muscle cells) vary in their content of myoglobin, the red-colored protein that binds oxygen in muscle fibers (muscle cells). Those with a low myoglobin content appear pale and are called

*white muscle fibers*. White muscle fibers are prevalent in the "wheat meat" in chicken breasts.

*Asynchronously*

+(At different times). +Normally, the various motor units of a whole muscle are stimulated to contract asynchronously.

*Latent period*

+A brief delay that occurs between application of the stimulus and the beginning of a contraction. +During the latent period, the muscle action potential sweeps over the sarcolemma, and calcium ions are released from the SR.

*Calsequestrin*

+A calcium-binding protein. +Bind to the Ca2+, enabling even more Ca2+ to be stored (sequestered) within the SR.

*Concentric isotonic contraction*

+A muscle shortens and pulls on another structure, such as a tendon, to produce movement and to reduce the angle at a joint such as picking up a book off a table involves concentric isotonic contractions of the biceps brachii muscle in the arm.

*Wave summation*

+A phenomenon in which stimuli arriving in close sequence cause larger contractions.

*Unfused tetanus*

+A sustained but wavering contraction. +When a skeletal muscle is stimulated at a rate of 20 to 30 times per second, it can only partially relax between stimuli.

*Fused tetanus*

+A sustained contraction in which individual twitches cannot be detected when a skeletal muscle fiber is stimulated at a higher rate of 80 to 100 times per second, it does not relax at all.

*Rigor mortis*

+After death, cellular membranes become leaky. +Calcium ions leak out of the sarcoplasmic reticulum into the sarcoplasm and allow myosin heads to bind to actin. +ATP synthesis ceases shortly after breathing stops, however, so the myosin heads cannot detach from actin. +This is the resulting condition, in which muscles are in a state of rigidity (cannot contract or stretch).

*Creatine phosphate*

+An energy-rich molecule that is found only in muscle fibers (muscle cells). +Excess ATP is used to synthesize creatine phosphate.

1) Release of acetylcholine

+An impulse travels from the brain or spinal cord along a motor neuron to the muscle fiber. +Arrival of the impulse at the synaptic end bulb stimulates voltage-gated channels to open. +Because calcium ions (Ca2+) are more concentrated in the extracellular fluid, Ca2+ flows inward through the open channels. +The entering Ca2+ stimulates the synaptic vesicles to undergo exocytosis. +During exocytosis, the synaptic vesicles fuse with the motor neuron's plasma membrane, releasing ACh into the synaptic cleft. +The ACh then diffuses across the synaptic cleft between the motor neuron and the motor end plate.

4) Myosin detaches from actin

+At the end of the power stroke, the myosin head remains firmly attached to actin until it binds another molecule of ATP. +As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin. +The contraction cycle repeats for as long as ATP and Ca2+ are available in the sarcoplasm. +Splitting of ATP by the myosin ATPase again reorients the myosin head and transfers energy from ATP to the myosin head. +Once reoriented and energized, the myosin head combines with the next myosin-binding site farther along the thin filament. The myosin heads keep rotating back and forth with each power stroke.

3) Power stroke occurs

+Binding of the myosin head to actin triggers the power stroke of contraction. +During the power stroke, the myosin head rotates or swivels and releases the ADP. +The myosin head generates force as it rotates toward the center of the sarcomere, sliding the thin filament past the thick filament toward the M line.

2) Activation of ACh receptors

+Binding of two molecules of ACh to a receptor on the motor end plate opens an ion channel in the ACh receptor. +Once the channel is open, small cations, most importantly Na+, can flow across the membrane.

*Intercalated discs*

+Cardiac muscle fibers interconnect with one another by irregular transverse thickenings of the sarcolemma. +The discs contain desmosomes, which hold the fibers together, and gap junctions, which allow muscle action potentials to spread from one cardiac muscle fiber to another.

*Motor unit*

+Consists of one motor neuron plus all the skeletal muscle fibers (muscle cells) it stimulates. +A single motor neuron makes contact with an average of 150 muscle fibers (muscle cells) within a muscle, and all of the muscle fibers (muscle cells) in one motor unit contract in unison. +Typically, the muscle fibers (muscle cells) of a motor unit are dispersed throughout a muscle rather than clustered together.

*Contraction period*

+During the second phase. +(Upward tracing). +Calcium ions bind to troponin, myosin-binding sites on actin are exposed, myosin heads attach to actin, and the resulting power strokes generate the force of contraction.

*Relaxation period*

+During the third phase. +Calcium ions are actively transported back into the SR; the level of Ca2+ in the sarcoplasm decreases, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, power strokes cease, and the muscle fiber relaxes.

*Muscle tone*

+Even at rest, a muscle exhibits this, a small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units. +To sustain muscle tone, small groups of motor units are activated in a constantly shifting pattern, alternating from one group of motor units to the next. +Muscle tone keeps skeletal muscles firm, but it does not result in a contraction strong enough to produce movement. +A skeletal muscle contracts only after it is stimulated by motor neurons. +Hence, muscle tone is established by the brain and spinal cord, which activate the muscle's motor neurons.

*Sliding filament mechanism*

+Once an action potential is propagated along the sarcolemma and into the T tubules, muscle contraction begins. +Muscle contraction occurs because myosin heads attach to and "walk" along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line. +As a result, the thin filaments *slide* inward and meet at the center of a sarcomere.

*Aerobic cellular respiration*

+Oxygen-required reactions. +Aerobic cellular respiration provides enough ATP for prolonged activity as long as sufficient oxygen and nutrients are available.

4) Termination of ACh activity

+The effect of ACh binding lasts only briefly because ACh is rapidly broken down by an enzyme in the synaptic cleft called *acetylcholinesterase (AChE)*.

2) Myosin attaches to actin

+The energized myosin head attaches to the myosin-binding site on actin and releases the phosphate group.

*Muscle fatigue*

+The inability of a muscle to contract forcefully after prolonged activity. +Even before actual muscle fatigue occurs, a person may experience tiredness and the desire to cease activity. +This response, termed *central fatigue*, is caused by changes in the central nervous system (brain and spinal cord) and may be a protective mechanism to stop a person from exercising before muscle becomes damaged.

3) Generation of muscle action potential

+The inflow of Na+ (down its electrochemical gradient) makes the inside of the muscle fiber more positively charged. +This change in the membrane potential triggers a muscle action potential. +The muscle action potential then travels along the sarcolemma and into the T tubules, stimulating the contraction process.

1) ATP splits

+The myosin head includes an ATP-binding site and ATPase, an enzyme that splits ATP into ADP (adenosine diphosphate) and P (a phosphate group). +This splitting reaction reorients and energizes the myosin head. +Notice that ADP and a phosphate group are still attached to the myosin head.

*Motor unit recruitment*

+The process by which the number of contracting motor units is increased. +The force of a muscle contraction becomes greater as more motor units are activated.

*Myogram*

+The record of a muscle contraction.

*Ca2+ active transport pumps*

+The sarcoplasmic reticulum membrane also contains this and uses ATP to constantly move Ca2+ from the sarcoplasm into the SR.

*Isotonic contraction*

+The tension (force of contraction) developed in the muscle remains constant while the muscle changes its length. +Isotonic contractions are used for body movements and for moving objects.

*Isometric contraction*

+The tension generated is not enough to exceed the resistance of the object to be moved and the muscle does not shorten. +Example: holding a book steady using an outstretched arm. +As the weight of the book pulls the arm downward, stretching the shoulder and arm muscle, isometric contraction of the shoulder and arm muscles counteracts the stretch.

Both *thin* and *thick filaments* are directly involved in the contractile process.

+The thick and thin filaments overlap one another to a greater or lesser extent, depending on whether the muscle is contracted, relaxed, or stretched. +The pattern of their overlap, consisting of a viariety of zones and bands, creates the striations that can be seen both in single myofibrils and in whole muscle fibers.

*Flaccid*

+This happens to muscle when the motor neurons serving a skeletal muscle are damaged. +A state of limpness in which muscle tone is lost.

*Eccentric isotonic contraction*

+When the length of a muscle increases during a contraction. +Example: lowering the book to place it back on the table.

Electrical excitability

+a property of both muscle cells and neurons, is the ability to respond to certain stimuli by producing electrical signals called *action potentials (impulses)*.

Perimysium

+also a layer of dense, irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called *fascicles*.

Muscular hyperplasia

+an increase in the number of fibers.

Generates heat

+as muscle tissue contracts, it also produces heat. +much of the heat generated by muscle tissue is used to maintain normal body temperature. +*involuntary* contractions of skeletal muscles, known as *shivering*, can dramatically increase the rate of heat production.

Axon

+each motor neuron has this threadlike process that extends from the brain or spinal cord to a group of skeletal muscle fibers.

Muscle tissue has four special properties that enable it to function and contribute to homeostasis:

+electrical excitability +contractility +extensibility +elasticity

Three layers of connective tissue extend from the fascia to further protect and strengthen skeletal muscle:

+epimysium +perimysium +endomysium

Cardiac muscle tissue

+is found only in the heart, where it forms most of the heart wall. +like skeletal muscle, cardiac muscle is *striated*, but its action is *involuntary* -- its alternating contraction and relaxation cannot be consciously controlled. +rather, the heart beats because it has a pacemaker that initiates each contraction; this built-in (intrinsic) rhythm is called *autorhythmicity*.

Smooth muscle tissue

+is located in the walls of hollow internal structures, such as blood vessels, airways, and most organs in the abdominopelvic cavity. +it is also attached to hair follicles in the skin. +smooth muscle tissue gets its name because, under a microscope, this tissue lacks striations; hence, it appears *non-striated* or *smooth*.

Produces body movements

+movements of the whole body, such as walking and running, and localized movements, such as grasping a pencil, keyboarding, or nodding the head, rely on the integrated functioning of skeletal muscles, bones, and joints.

Endomysium

+penetrates the interior of each fascicle and separates individual muscle fibers from one another.

Through sustained contraction or alternating contraction and relaxation, muscle tissue has four key functions:

+produces body movements +stabilizes body positions +moves substances within the body +generates heat

There are three types of muscle tissue

+skeletal +cardiac +smooth

Stabilizes body positions

+skeletal muscle contractions stabilize joints and help maintain body positions, such as standing or sitting. +postural muscles contract continuously when you are awake; for example, sustained contractions of your neck muscles hold your head upright when you are listening intently to your anatomy and physiology lecture.

Muscle tissue

+stabilizes the body's position +regulates organ volume +generates heat to maintain normal body temperature +propels fluids and foods through various body systems.

Moves substances within the body

+sustained contractions of ringlike bands or smooth muscles called *sphincters* prevent outflow of the contents of a hollow organ. +temporary storage of food in the stomach or urine in the urinary bladder is possible because smooth muscle sphincters close off the outlets of these organs. +cardiac muscle contractions of the heart pump blood through blood vessels. +contraction and relaxation of smooth muscle in the walls of blood vessels help adjust their diameter and thus regulate the rate of blood flow.

Muscular hypertrophy

+the dramatic muscle growth thar occurs after birth occurs by enlargement of existing muscle fibere called this (rather than by muscular hyperplasia).

Skeletal muscle tissue

+the function of most skeletal muscles is to move the bones of the skeleton. +a few skeletal muscles attach to and move the skin or other skeletal muscles. +skeletal muscle is referred to as *striated* because alternating light and dark protein bands (*striations*) are visible when the tissue is examined under a microscope.

*Myosin*

+the main component of thick filaments, is a contractile protein that pushes or pulls various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion, that is, the production of force.

Epimysium

+the outermost layer of dense connective tissue, encircles the entire muscle.

Aponeurosis

+when the connective tissue layers extend as a broad, flat sheet.

The typical length of a mature skeletal muscle fiber is about

10 cm (4 inches) although some are as long as 30 cm (12 inches).

The diameter of the *thick filaments* is about

16 nm.

The diameter of the *thin filaments* is about

8 nm.

Release of ____ from the terminal cisterns of the sarcoplasmic reticulum triggers muscle contraction.

Ca2+ (calcium ions)

____ tissue surrounds and protects muscle tissue.

Connective

Myoblasts

During embryonic development, each skeletal muscle fiber arises from the fusion of a hundred or more small mesodermal cells called this.

The *contraction cycle* -- the repeating sequence of events that causes the filaments to slide -- consists of four steps:

1) ATP splits 2) Myosin attaches to actin 3) Power stroke occurs 4) Myosin detaches from actin

An impulse (nerve action potential) excites a skeletal muscle fiber in the following way:

1) Release of acetylcholine 2) Activation of ACh receptors. 3) Generation of muscle action potential 4) Termination of ACh activity.

The ATP present inside muscle fibers (muscle cells) is enough to power contraction for only a few seconds. If muscle contractions continue past that time, the muscle fibers (muscle cells) must make more ATP. Fortunately, muscle fibers have THREE ways to produce ATP:

1) from creatine phosphate 2) by anaerobic cellular respiration 3) by aerobic cellular respiration +All body cells make ATP through anaerobic and aerobic cellular respiration.

This extra oxygen is used to "pay back" or restore metabolic conditions to the resting level in three ways:

1) to convert lactic acid back onto glycogen stores in the liver. 2) to resynthesize creatine phophate and ATP in muscle fibers (muscle cells), and 3) to replace the oxygen removed from myoglobin.

In addition, the sarcoplasm contains

a red-colored protein called *myoglobin*.

Motion results from the

alternating contraction and relaxation of muscles, which make up 40 -50 percent of total adult body weight.

During childhood, human growth hormone and other hormones (testosterone) stimulate

an increase in the size of skeletal muscle fibers.

Myofibrils

are about 2 um in diameter and extend the entire length of a muscle fiber. Their prominent striations make the entire muscle fiber appear striated.

*Fast oxidative -glycolytic (FOG) fibers*

are intermediate in diameter between the other two types of fibers. +Also contain large amounts of myoglobin and many blood capillaries. +Generate ATP by aerobic cellular respiration, gives moderately high resistance to fatigue.

*Fast glycolytic (FG) fibers*

are largest in diameter and contain the most myofibrils. +Hence, they can generate the most powerful contractions.

*Synaptic vesicles*

are membrane-enclosed sacs suspended in the cytosol within each synaptic end bulb.

Motor neurons

are neurons that stimulate muscle fibers to contract.

T tubules

are open to the outside of the fiber and thus are filled with interstitial fluid.

*Slow oxidative (SO) fibers*

are smallest in diameter and thus are the least powerful type of skeletal muscle fibers. +They appear dark red because they contain large amounts of myoglobin and many blood capillaries. +Generate ATP mainly by aerobic cellular respiration.

Skeletal muscles

are well supplied with nerves and blood vessels.

*Acetylcholinesterase (AChE)*

breaks down ACh into acetyl and choline, products that cannot activate the ACh receptor.

An example of a tendon is the

calcaneal (Achilles) tendon of the gastrocnemius (calf), which attaches the muscle to the calcaneus.

In a relaxed muscle fiber, the sarcoplasmic reticulum stores

calcium ions (Ca2+).

Microscopic blood vessels called ____ are plentiful in muscle tissue; each muscle fiber is in close contact with one or more ____.

capillaries

The blood ____ bring in oxygen and nutrients and remove heat and the waste products of muscle metabolism.

capillaries

*Multiunit smooth muscle tissue*

consists of individual fibers, each with its own motor neuron terminals and with few gap junctions between neighboring fibers. +Found in the walls of large arteries, in airways to the lungs, in the arrector pili muscles that attach to hair follicles, and in the internal eye muscles.

The axon of a motor neuron typically branches many times,

each branch extending to a different skeletal muscle fiber.

Generally, an artery and one or two veins accompany

each nerve that penetrates a skeletal muscle.

The stimuli that trigger action potentials in muscle cells may be

electrical signals arising in the muscle tissue itself, such as occurs in the heart's pacemaker, or chemical stimuli, such as neurotransmitters released by neurons, hormones distributed by the blood, or even local changes in pH.

An example of an aponeurosis is the

epicranial aponeurosis on top of the skull between the frontal and occipital bellies of the occipitofrontalis muscle.

Skeletal muscle tissue works primarily

in a *voluntary* manner; that is, its activity can be consciously (voluntarily) controlled by the somatic (*voluntary*) division of the nervous system.

Sarcoplasm

includes a substantial amount of glycogen, which is a large molecule composed of many glucose molecules. Glycogen can be used for synthesis at ATP.

____ is due to increased production of myofibrils and other organelles resulting from forceful, repetitive muscular activity, such as strength training. Because hypertrophied muscles contain more myofibrils, they are capable of more forceful contractions.

Muscular hypertrophy

*Recovery oxygen uptake*

is a better term than oxygen debt for the elevated use of oxygen after exercise.

Fibromyalgia

is a chronic, painful, nonarticular rheumatic disorder that affects the fibrous connective tissue components of muscles, tendons, and ligaments.

Muscle atrophy

is a decrease in the size and, therefore, strength of a muscle.

*Titin*

is a key structural protein and is the third most plentiful protein in skeletal muscle (after actin and myosin). +Each titin filament connects a Z disc to an M line, thereby helping stabilize the position of the thick filament. +The part of the titin molecule that extends from the Z disc is very elastic. +Because it can stretch to at least four times its resting length and then spring back unharmed, titin accounts for much of the elasticity and extensibility of myofibrils.

The *I band*

is a lighter, less dense area that contains the rest of the thin filaments but no thick filaments.

*H zone*

is a narrow zone in the center of each A band that contains thick filaments but no thin filaments.

*Motor neuron*

is a neuron that stimulates skeletal muscle fibers to contract. +Each *motor neuron* has a threadlike axon that extends from the brain or spinal cord to a group of skeletal muscle fibers.

*Synapse*

is a region where communication occurs between a neuron and another cell -- in this case, between a motor neuron and a skeletal muscle fiber.

*Anaerobic cellular respiration*

is a series of ATP-producing reactions that do not require oxygen when muscle activity continues pas the 15-second mark and the supply of creatine phosphate within the muscle fibers is depleted, glucose is catabolized to generate ATP.

Fascia

is a sheet or broad band of dense connective tissue that supports and surrounds muscles and other organs of the body.

*Dystrophin*

is a structural protein that links thin filaments of the sarcomere to integral membrane proteins of the sarcolemma, which are attached in turn to proteins in the connective tissue extracellular matrix that surrounds muscle fibers.

The *endomysium*

is a thin sheath of aerolar connective tissue.

*Creatine*

is synthesized in the liver, kidneys, and pancreas and then transported in the blood stream to muscle fibers (muscle cells).

Elasticity

is the ability of muscle tissue to return to its original length and shape after contraction or extension.

Contractility

is the ability of muscle tissue to shorten forcefully when stimulated by an action potential.

Extensibility

is the ability of muscle tissue to stretch within limits, without being damaged.

*Twitch contraction*

is the brief contraction of all the muscle fibers (muscle cells) in a motor unit in response to a single impulse in its motor neuron. +In the laboratory, a twitch can be produced by direct electrical stimulation if a motor neuron or its muscle fibers (muscle cells).

Sarcoplasm

is the cytoplasm of a muscle fiber.

*A band*

is the darker middle part of the sarcomere which extends the entire length of the thick filaments.

*Axon terminal*

is the end of a motor neuron.

*Visceral (single-unit) smooth muscle tissue*

is the more common of the two types of smooth muscle tissue. +It is found in the skin and in part of the walls of small arteries and veins and of hollow organs such as the stomach, intestines, uterus, and urinary bladder. +Like cardiac muscle tissue, visceral smooth muscle tissue is *autorhythmic*. +The fibers connect to one another by gap junctions, forming a network through which muscle action potentials can spread.

*Synaptic end bulbs*

is the neural part of the NMJ (neuromuscular junction).

Sarcolemma

is the plasma membrane of a muscle fiber.

*Cardiac muscle tissue*

is the principal tissue in the heart wall. +Compared with skeletal muscle fibers, cardiac muscle fibers are shorter in length and less circular in transverse section. +They also exhibit "branching."

The *motor end plate*

is the region of the sarcolemma of the muscle fiber that is opposite the synaptic end bulbs and is the muscle fiber part of the NMJ.

Fibrosis

is the replacement of muscle fibers by fibrous scar tissue. For this reason, regeneration of skeletal muscle tissue is limited.

When a skeletal muscle contracts

it generates *tension* (force of contraction) while pulling on its attachment points. +if the tension generated is great enough to overcome the resistance of the object to be moved, the muscle shortens and movement occurs. +an example is lifting a book off a table.

Smooth muscle contractions also

move food and substances such as bile and enzymes through the gastrointestinal tract, push gametes (sperm and oocytes) through the passageways of the reproductive systems, and propel urine through the urinary system.

Stimulation of one visceral (single-unit) muscle fiber causes contraction of many adjacent fibers, but stimulation of one ____ fiber causes contraction of that fiber only.

multiunit

Fascia holds together

muscles with similar functions; allows free movement of muscles; carries nerves, blood vessels, and lymphatic vessels; and fills spaces between muscles.

Hence, each mature skeletal muscle fiber has a hundred or more

nuclei.

The generation of ATP requires:

oxygen, glucose, fatty acids, and other substances that are delivered to the muscle fiber in the blood.

*Oxygen debt*

refers to the added oxygen, over and above the oxygen consumed at rest, that is taken into the body after exercise.

*Excitatioon-contraction coupling*

refers to the steps that connect excitation (a muscle action potential propagating along the sarcolemma and into the T tubules) to contraction (sliding of the filament). +An increase in Ca2+ concentration in the sarcoplasm starts muscle contraction; a decrease stops it. +However, a huge amount of Ca2+ is stored inside the sarcoplasmic reticulum.

Both cardiac muscle and smooth muscle are

regulated by the *autonomic (involuntary)* division of the nervous system and by hormones released by endocrine glands.

Dystrophin is thought to help

reinforce the sarcolemma and transmit the tension generated by the sarcomeres to muscle tendons.

Myoglobin

releases oxygen when mitochondria need it for ATP production. +The mitochondria lie in rows throughout the muscle fiber, strategically close to the muscle proteins that use ATP during contraction so that ATP can be produced quickly as needed.

Satellite cells

retain the capacity to fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers. +However, the number of new skeletal muscle fibers formed is not enough to compensate for significant skeletal damage or degeneration. In such cases, skeletal muscle tissue undergoes *fibrosis*.

The multiple nuclei of a skeletal muscle fiber are located just beneath the ____.

sarcolemma

A few myoblasts do persist in mature skeletal muscle as

satellite cells.

The *hypodermis*

separates muscle from skin. +it is composed of areolar connective tissue and adipose tissue, provides a pathway for nerves and blood and lymphatic vessels to enter and exit muscles, serves as an insulating layer that reduces heat loss, and protects muscles from physical trauma.

Smooth muscle fibers are considerably smaller in length and diameter than ____ and are tapered at both ends.

skeletal muscle fibers

Most skeletal muscles also are controlled

subconsciously to some extent. +for example, your diaphragm continues to alternately contract and relax while you are asleep so that you don't stop breathing. +also, you do not need to consciously think about contracting the skeletal muscles that maintain your posture or stabilize body positions.

The epimysium, perimysium, and endomysium may extend together beyond the muscle fibers to form a ropelike ____ that attaches a muscle to the periosteum of a bone.

tendon

Skeletal muscle contractions promote

the flow of lymph and aid the return of blood to the heart.

Normally, smooth muscle is subject to

the greatest amount of stretching. +for example, each time your stomach fills with food, the muscle in the wall is stretched.

A mnemonic for the I and H bands:

the letter I is thin (contains thin filaments), while the letter H is thick (contains thick filaments).

Your muscular strength reflects the primary function of muscle --

the transformation of chemical energy into mechanical energy to produce body movements.

Muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading

throughout the muscle fiber. +This arrangement ensures that an action potential excites all parts of the muscle fiber at essentially the same instant.

Once fusion has occurred, the muscle fiber loses its ability to undergo cell division...

thus, the number of skeletal muscle fibers is set before you are born, and most of these cells last a lifetime.

Toward each end of the A band is a *zone of overlap*,

where the thick and thin filaments lie side by side.

Many *fascicles* are large enough to be seen

with the naked eye. They give a cut of meat its characteristic "grain"; if you tear a piece of meat, it rips apart along the fascicles.

When a muscle action potential propagates along the sarcolemma and into the T tubules, it causes ____ in the SR membrane to open.

*Ca2+ release channels*

A Z disc passes through the center of each

*I band*.

Supporting proteins that hold the thick filaments together at the center of the H zone form the ____, so named because it is at the middle of the sarcomere.

*M line*

Especially during contraction, a muscle fiber synthesizes and uses considerable ___.

ATP (adenosine triphosphate)