MUSCLE TISSUE Flashcards
The fourth basic tissue type with epithelia, connective tissues, and nervous tissue, is composed of cells that optimize the universal cell property of ______.
contractility
As in all cells, ______ and associated ______ generate the forces necessary for the muscle contraction, which drives movement within organ systems, of blood, and of the body as a whole.
actin microfilaments, proteins
Essentially all muscle cells are of ______ origin and differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins ______ and ______.
mesodermal, actin, myosin
Three types of muscle tissue can be distinguished on the basis of ______ and ______ characteristics, with the structure of each adapted to its ______ role.
morphologic, functional, physiologic
THREE TYPES OF MUSCLE TISSUE:
Skeletal muscle
Cardiac muscle
Smooth muscle
THREE TYPES OF MUSCLE TISSUE:
______ contains bundles of very long, multi- nucleated cells with cross-striations.
Skeletal muscle
THREE TYPES OF MUSCLE TISSUE:
______ - Their contraction is quick, forceful, and usually under voluntary control.
Skeletal muscle
THREE TYPES OF MUSCLE TISSUE:
______ also has cross-striations and is composed of elongated, often branched cells bound to one another at structures called intercalated discs which are unique to cardiac muscle. Contraction is involuntary, vigorous, and rhythmic.
Cardiac muscle
THREE TYPES OF MUSCLE TISSUE:
______ consists of collections of fusiform cells which lack striations and have slow, involuntary contractions.
Smooth muscle
In all types of muscle, contraction is caused by the sliding interaction of thick ______ along thin ______.
myosin filaments, actin filaments
The forces necessary for sliding are generated by other proteins affecting the weak interactions in the bridges between ______ and ______.
actin, myosin
As with neurons, muscle specialists refer to certain muscle cell ______ with special names.
organelles
The cytoplasm of muscle cells is often called ______, the smooth ER is the ______, and the muscle cell membrane and its external lamina are the ______.
sarcoplasm, sarcoplasmic reticulum, sarcolemma
The variation in ______ of muscle fibers depends on factors such as the specific muscle, age, gender, nutritional status, and physical training of the individual.
diameter
Exercise enlarges the skeletal musculature by stimulating formation of new ______ and growth in the diameter of individual ______.
This process, characterized by increased cell volume, is called ______.
myofibrils, muscle fibers, hypertrophy
Tissue growth by an increase in the number of cells is termed ______, which takes place very readily in smooth muscle, whose cells have not lost the capacity to divide by mitosis.
hyperplasia
______ consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of ______-______ μm.
Striated muscle, 10, 100
During ______ muscle development, ______ fuse, forming ______ with many nuclei.
embryonic, mesenchymal myoblasts, myotubes
______ then further differentiate to form striated muscle fibers.
Myotubes
Elongated nuclei are found peripherally just under the ______, a characteristic nuclear location unique to ______ muscle fibers/cells.
sarcolemma, skeletal
A small population of reserve progenitor cells called ______ remains adjacent to most fibers of differentiated skeletal muscle.
muscle satellite cells
Thin layers of connective tissue surround and organize the contractile fibers in all three types of muscle, and these layers are seen particularly well in ______ muscle.
skeletal
Thin layers of connective tissue surround and organize the contractile fibers in all three types of muscle, and these layers are seen particularly well in skeletal muscle.
The concentric organization given by these supportive layers resembles that in large peripheral nerves:
epimysium
perimysium
endomysium
The concentric organization given by these supportive layers resembles that in large peripheral nerves:
The ______, an external sheath of dense irregular connective tissue, surrounds the entire muscle. Septa of this tissue extend inward, carrying the larger nerves, blood vessels, and lymphatics of the muscle.
epimysium
The concentric organization given by these supportive layers resembles that in large peripheral nerves:
The ______ is a thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a ______. Each ______ of muscle fibers makes up a functional unit in which the fibers work together. Nerves, blood vessels, and lymphatics penetrate the perimysium to supply each fascicle.
perimysium, fascicle, fascicle
The concentric organization given by these supportive layers resembles that in large peripheral nerves:
Within fascicles a very thin, delicate layer of reticular fibers and scattered fibroblasts, the ______, surrounds the external lamina of individual muscle fibers.
endomysium
In addition to nerve fibers, capillaries form a rich network in the endomysium bringing ______ to the muscle fibers.
O2
Collagens in these connective tissue layers of muscle serve to transmit the ______ forces generated by the contracting muscle cells/fibers; individual muscle fibers seldom extend from one end of a muscle to the other.
mechanical
All three layers plus the dense irregular connective tissue of the deep fascia, which overlies the epimysium, are continuous with the tough connective tissue of a tendon at ______ which join the muscle to bone, skin, or another muscle.
myotendinous junctions
Ultrastructural studies show that in these transitional regions, ______ fibers from the tendon insert themselves among muscle fibers and associate directly with complex infoldings of ______.
collagen, sarcolemma
Longitudinally sectioned skeletal muscle fibers show striations of alternating ______ and ______ bands.
light, dark
The sarcoplasm is highly organized, containing primarily long cylindrical filament bundles called ______ that run parallel to the long axis of the fiber.
myofibrils
The dark bands on the myofibrils are called ______ (anisotropic or birefringent in polarized light microscopy); the light bands are called ______ (isotropic, do not alter polarized light). In the TEM, each I band is seen to be bisected by a dark transverse line, the ______.
A bands, I bands, Z disc
The repetitive functional subunit of the contractile apparatus, the ______, extends from Z disc to Z disc and is about ______-μm long in resting muscle.
sarcomere, 2.5
Mitochondria and sarcoplasmic reticulum are found between the ______, which typically have diameters of ______-______ μm. ______ consist of an end-to-end repetitive arrangement of sarcomeres; the lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit a characteristic pattern of transverse striations.
myofibrils, 1, 2, Myofibrils
The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin ______, composed of ______ and ______, respectively, organized within each myofibril in a symmetric pattern containing thousands of each filament type.
myofilaments, myosin, F-actin
The thick myosin filaments are ______-μm long and ______-nm wide; they occupy the ______ at the middle region of the sarcomere.
1.6, 15, A band
______ is a large complex (~500 kDa) with two iden- tical heavy chains and two pairs of light chains. ______ heavy chains are thin, rodlike motor proteins (150-nm long and 2-3 nm thick) twisted together as ______.
Myosin, Myosin, myosin tails
______ containing the four myosin light chains form a head at one end of each heavy chain.
Globular projections
The myosin heads bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (______).
actomyosin ATPase activity
Several hundred ______ molecules are arranged within each thick filament with overlapping rodlike portions and the globular heads directed toward either end.
myosin
The thin, helical ______ are each ______-μm long and ______-nm wide and run between the thick filaments. Each ______ monomer contains a binding site for myosin.
actin filaments, 1.0, 8, G-actin
The thin filaments have two tightly associated regulatory proteins:
Tropomyosin
Troponin
The thin filaments have two tightly associated regulatory proteins:
______, a 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin strands
Tropomyosin
The thin filaments have two tightly associated regulatory proteins:
______, a complex of three subunits: TnT, which attaches to tropomyosin; TnC, which binds Ca2+; and TnI, which regulates the actin-myosin interaction
Troponin
______ complexes attach at specific sites regularly spaced along each tropomyosin molecule.
Troponin
______ consist of the portions of the thin filaments which do not overlap the thick filaments in the A bands, which is why ______ stain more lightly than A bands.
I bands, I bands
Actin filaments are anchored perpendicularly on the Z disc by the actin-binding protein ______ and exhibit opposite ______ on each side of this disc.
α-actinin, polarity
An important accessory protein in I bands is ______ (3700 kDa), the largest protein in the body, with scaffolding and elastic properties, which supports the thick myofilaments and connects them to the Z disc.
titin
Another large accessory protein, ______, binds each thin myofilament laterally, helps anchor them to α-actinin, and specifies the length of the actin polymers during myogenesis.
nebulin
The ______ contain both the thick filaments and the overlapping portions of thin filaments. Close observation of the A band shows the presence of a lighter zone in its center, the ______, corresponding to a region with only the rodlike portions of the myosin molecule and no thin filaments. Bisecting the H zone is the ______, containing a myosin-binding protein ______ that holds the thick filaments in place, and ______. This enzyme catalyzes transfer of phosphate groups from ______, a storage form of high-energy phosphate groups, to ______, helping to supply ATP for muscle contraction.
A bands, H zone, M line, myomesin, creatine kinase, phosphocreatine, ADP
Despite the many proteins present in sarcomeres, ______ and ______ together represent over half of the total protein in striated muscle. The overlapping arrangement of thin and thick filaments within sarcomeres produces in TEM cross sections hexagonal patterns of structures which were important in determining the functions of the filaments and other proteins in the myofibril.
myosin, actin
In skeletal muscle fibers the membranous smooth ER, called here ______, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils.
sarcoplasmic reticulum
Calcium release from cisternae of the sarcoplasmic reticulum through voltage-gated Ca2+ chanels is triggered by membrane ______ produced by a ______.
depolarization, motor nerve
To trigger Ca2+ release from sarcoplasmic reticulum through- out the muscle fiber simultaneously and produce uniform contraction of all myofibrils, the sarcolemma has tubular infoldings called ______ or ______.
These long fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm and encircle each myofibril near the aligned ______ and ______.
transverse, T-tubules, A-, I-band
Adjacent to each T-tubule are expanded ______ of sarcoplasmic reticulum.
terminal cisternae
In longitudinal TEM sections, this complex of a T-tubule with two terminal cisternae is called a ______.
triad
The triad complex allows depolarization of the sarcolemma in a ______ to affect the sarcoplasmic reticulum and trigger release of ______ ions into cytoplasm around the thick and thin filaments, which initiates contraction of sarcomeres.
T-tubule, Ca2+
______
- During this process neither the thick nor the thin filaments change their length.
MECHANISM OF CONTRACTION
______ occurs as the overlapping thin and thick filaments of each sarcomere slide past one another.
Contraction
Contraction is induced when an action potential arrives at a synapse, the ______, and is transmitted along the T-tubules to terminal cisternae of the sarcoplasmic reticulum to trigger Ca2+ release. In a resting muscle, the myosin heads cannot bind actin because the binding sites are blocked by the ______ complex on the ______ filaments.
neuromuscular junction (NMJ), troponin-tropomyosin, F-actin
Calcium ions released upon neural stimulation bind ______, changing its shape and moving ______ on the F-actin to expose the myosin-binding active sites and allow crossbridges to form.
troponin, tropomyosin
Binding actin produces a conformational change or pivot in the ______, which pulls the thin filaments farther into the ______, toward the ______.
myosins, A band, Z disc
Energy for the myosin head pivot that pulls actin is provided by hydrolysis of ______ bound to the myosin heads, after which myosin binds another ATP and detaches from actin. In the continued presence of ______ and ______, these attach- pivot-detach events occur in a repeating cycle, each lasting about ______, which rapidly shorten the sarcomere and contract the muscle.
A single muscle contraction results from ______ of these cycles.
ATP, Ca2+, ATP, 50 milliseconds, hundreds
When the neural impulse stops and levels of free ______ ions diminish, ______ again covers the myosin-binding sites on actin and the filaments passively slide back and sarcomeres return to their relaxed length.
Ca2+, tropomyosin
In the absence of ATP, the actin-myosin crossbridges become stable, which accounts for the rigidity of skeletal muscles (______) that occurs as mitochondrial activity stops after death.
rigor mortis
______ nerves branch out within the perimysium, where each nerve gives rise to several ______ that pass through endomysium and form synapses with individual muscle fibers.
Myelinated motor, unmyelinated terminal twigs
______ enclose the small axon branches and cover their points of contact with the muscle cells.
Schwann cells
The ______ of the Schwann cell fuses with that of the sarcolemma.
external lamina
Each axonal branch forms a dilated termination situated within a trough on the muscle cell surface, which are part of the synapses termed the ______, or ______.
neuromuscular junctions, motor end plates (MEPs)
As in all synapses the axon terminal contains mitochondria and numerous synaptic vesicles; here the vesicles contain the neurotransmitter ______.
acetylcholine
Between the axon and the muscle is the ______.
synaptic cleft
Adjacent to the synaptic cleft, the sarcolemma is thrown into numerous deep ______, which provide for greater postsynaptic surface area and more transmembrane acetylcholine receptors.
junctional folds
When a ______ reaches the MEP, acetylcholine is liberated from the axon terminal, diffuses across the cleft, and binds to its receptors in the folded ______.
nerve action potential, sarcolemma
The ______ contains a nonselective cation channel that opens upon neurotransmitter binding, allowing influx of cations, depolarizing the sarcolemma, and producing the ______.
acetylcholine receptor, muscle action potential
Acetylcholine quickly dissociates from its receptors, and free neurotransmitter is removed from the synaptic cleft by the extracellular enzyme ______, preventing prolonged contact of the transmitter with its receptors.
acetylcholines- terase
The muscle action potential moves along the ______ and along ______ which penetrate deeply into sarcoplasm.
sarcolemma, T-tubules
At triads the depolarization signal triggers the release of ______ from terminal cisterns of the sarcoplasmic reticulum, initiating the ______.
Ca2+, contraction cycle
An axon from a single motor neuron can form ______ with one or many muscle fibers.
MEPs
Innervation of single muscle fibers by single motor neurons provides precise ______ of muscle activity and occurs, for example, in the ______ for eye movements.
control, extraocular muscles
Larger muscles with coarser movements have motor axons that typically branch profusely and innervate ______ or more muscle fibers. In this case the single axon and all the muscle fibers in contact with its branches make up a ______.
100, motor unit
Individual striated muscle fibers do not show ______—they contract either ______ or ______.
graded contraction, all the way, not at all
To vary the force of contraction, the fibers within a muscle fascicle do not all ______ at the same time.
contract
With large muscles composed of many motor units, the firing of a single motor axon will generate tension proportional to the number of ______ it innervates.
muscle fibers
Thus, the number of motor units and their variable size control the ______ and ______ of a muscle contraction.
intensity, precision
______
- is an autoimmune disorder that involves circulating antibodies against proteins of acetylcholine receptors. Antibody binding to the antigenic sites interferes with acetylcholine activation of their receptors, leading to intermittent periods of skeletal muscle weakness. As the body attempts to correct the condition, junctional folds
of sarcolemma with affected receptors are internalized, digested by lysosomes, and replaced by newly formed recep- tors. These receptors, however, are again made unresponsive to acetylcholine by similar antibodies, and the disease follows a progressive course. The extraocular muscles of the eyes are commonly the first affected.
MYASTHENIA GRAVIS
______ and ______ contain sensory receptors acting as proprioceptors (L. proprius, one’s own + capio, to take), providing the central nervous system (CNS) with data from the musculoskeletal system.
Striated muscles, myotendinous junctions
Among the muscle fascicles are stretch detectors known as ______, approximately 2-mm long and 0.1-mm wide.
muscle spindles
A muscle spindle is encapsulated by modified perimysium, with concentric layers of flattened cells, containing interstitial fluid and a few thin muscle fibers filled with nuclei and called ______.
intrafusal fibers
Several sensory nerve axons penetrate each muscle spindle and wrap around individual ______.
intrafusal fibers
Changes in length (______) of the surrounding (______) muscle fibers caused by body movements are detected by the muscle spindles and the sensory nerves relay this information to the spinal cord.
distension, extrafusal
Different types of ______ and ______ mediate reflexes of varying complexity to help maintain posture and to regulate the activity of opposing muscle groups involved in motor activi- ties such as walking.
sensory, intrafusal fibers
A similar role is played by ______, much smaller encapsulated structures that enclose sensory axons penetrating among the collagen bundles at the myotendinous junction.
Golgi tendon organs
Tendon organs detect changes in tension within tendons produced by muscle contraction and act to ______ motor nerve activity if tension becomes excessive.
Because both of these proprioceptors detect increases in tension, they help regulate the amount of effort required to perform movements that call for variable amounts of muscular force.
inhibit
______ is a large actin-binding protein located just inside the sarcolemma of skeletal muscle fibers, which is involved in the functional organization of myofibrils.
Dystrophin
Research on ______ revealed that mutations of the dystrophin gene can lead to defective link- ages between the cytoskeleton and the extracellular matrix (ECM). Muscle contractions can disrupt these weak linkages, causing the atrophy of muscle fibers typical of this disease.
Duchenne muscular dystrophy
Skeletal muscles such as those that move the eyes and eyelids need to ______ rapidly, while others such as those for bodily posture must maintain ______ for longer periods while resisting fatigue.
These metabolic differences are possible because of varied expression in muscle fibers of contractile or regulatory protein isoforms and other factors affecting oxygen delivery and use.
contract, tension
Different types of fibers can be identified on the basis of:
(1) their maximal rate of contraction (fast or slow fibers) and
(2) their major pathway for ATP synthesis (oxidative phosphorylation or glycolysis). Fast versus slow rates of fiber contraction are due largely to myosin isoforms with different maximal rates of ATP hydrolysis.
Histochemical staining is used to identify fibers with differing amounts of “______” and “______” ATPases.
fast, slow
Other histological features reflecting metabolic differences among muscle fibers include the density of surrounding ______, the number of ______, and levels of ______ and ______, a globular sarcoplasmic protein similar to hemoglobin which contains iron atoms and allows for O2 storage.
capillaries, mitochondria, glycogen, myoglobin
Each of these features exists as a continuum in skeletal muscle fibers, but fiber diversity is divided into three major types:
Slow oxidative
Fast glycolytic
Fast oxidative-glycolytic
Each of these features exists as a continuum in skeletal muscle fibers, but fiber diversity is divided into three major types:
______ muscle fibers are adapted for slow con- tractions over long periods without fatigue, having many mitochondria, many surrounding capillaries, and much myoglobin, all features that make fresh tissue rich in these fibers dark or red in color.
Slow oxidative
Each of these features exists as a continuum in skeletal muscle fibers, but fiber diversity is divided into three major types:
______ fibers are specialized for rapid, short- term contraction, having few mitochondria or capillar- ies and depending largely on anaerobic metabolism of glucose derived from stored glycogen, features which make such fibers appear white. Rapid contractions lead to rapid fatigue as lactic acid produced by glycolysis accumulates.
Fast glycolytic
Each of these features exists as a continuum in skeletal muscle fibers, but fiber diversity is divided into three major types:
______ fibers have physiological and histological features intermediate between those of the other two types.
Fast oxidative-glycolytic
The metabolic type of each fiber is determined by the rate of ______ along its motor nerve supply, so that all fibers of a motor unit are similar.
impulse conduction
Most skeletal muscles receive ______ input from multiple nerves and contain a mixture of fiber types.
motor
Determining the fiber types in needle biopsies of skeletal muscle helps in the diagnosis of specific ______, motor neuron diseases, and other causes of muscle atrophy. Different fiber types also exist in cardiac muscle at various locations within the heart and in smooth muscle of dif- ferent organs.
myopathies
During embryonic development ______ around the primitive heart tube align into chainlike arrays.
mesenchymal cells
Rather than fusing into multinucleated cells/fibers as in developing skeletal muscle fibers, ______ cells form complex junctions between interdigitating processes.
Cells within one fiber often branch and join with cells in adjacent fibers.
cardiac muscle
Consequently, the ______ consists of tightly knit bundles of cells, interwoven in spiraling layers that provide for a characteristic wave of contraction that resembles wringing out of the heart ventricles.
heart
Mature cardiac muscle cells are ______-______ μm in diameter and ______-______ μm long, with a striated banding pattern comparable to that of skeletal muscle.
15, 30, 85, 120
Unlike skeletal muscle, however, each cardiac muscle cell usually has only one ______ and is ______ located.
nucleus, centrally
Surrounding the muscle cells is a delicate sheath of ______ with a rich capillary network.
endomysium
A thicker ______ separates bundles and layers of muscle fibers and in specific areas forms larger masses of fibrous connective tissue comprising the “______.”
perimysium, cardiac skeleton
A unique characteristic of cardiac muscle is the presence of ______ that cross the fibers at irregular intervals where the myocardial cells join.
These ______ represent the interfaces between adjacent cells and consist of many junctional complexes.
transverse lines, intercalated discs
Transverse regions of these irregular, steplike discs are composed of many ______ and ______ junctions, which together provide strong intercellular adhesion during the cells’ constant contractile activity.
desmosomes, fascia adherens
The less abundant, longitudinally oriented regions of each intercalated disc run parallel to the myofibrils and are filled with ______ which provide ionic continuity between the cells.
These regions serve as “______,” promoting rapid impulse conduction through many cardiac muscle cells simultaneously and contraction of many adjacent cells as a unit.
gap junctions, electrical synapses
The structure and function of the contractile apparatus in cardiac muscle cells are essentially the same as in ______.
skeletal muscle
Mitochondria occupy up to ______% of the cell volume, higher than in slow oxidative skeletal muscle fibers.
40
______, the major fuel of the heart, are stored as ______ in small lipid droplets. ______ granules as well as perinuclear ______ pigment granules may also be present.
Fatty acids, triglycerides, Glycogen, lipofuscin
Muscle of the heart ______ is much thicker than that of the atria, reflecting its role in pumping blood through the cardiovascular system.
ventricles
______ in ventricular muscle fibers are well-developed, with large lumens and penetrate the sarco- plasm in the vicinity of the myofibrils’ Z discs.
T-tubules
In atrial muscle T-tubules are much ______ or entirely ______.
smaller, absent
______ is less well-organized in cardiac compared to skel- etal muscle fibers.
Sarcoplasmic reticulum
CARDIAC MUSCLE
The junctions between its terminal cisterns and T-tubules typically involve only one structure of each type, forming profiles called ______ rather than triads in TEM sections.
dyads
Components of this cardiac muscle ______ system have the same basic functions as their counterparts in skeletal muscle fibers.
transverse tubule
Cardiac muscle fiber contraction is ______ and ______, as evidenced by the continued contraction of the cells in tissue culture. Impulses for the ______ (or ______) are initiated, regulated, and coordinated locally by nodes of unique myocardial fibers specialized for impulse generation and conduction.
intrinsic, spontaneous, rhythmic contraction, heartbeat
CARDIAC MUSCLE
As with skeletal muscle fibers, contraction of individual myocardial fibers is ______. The rate of contraction is mod- ified by autonomic innervation at the nodes of conducting cells, with the ______ nerve supply accelerating and the ______ supply decreasing the frequency of the impulses.
all-or-none, sympathetic, parasympathetic
______ about 0.2-0.3 μm in diameter are found near atrial muscle nuclei and are associated with small Golgi complexes.
Secretory granules
Secretory Granules
These granules release the peptide hormone ______ which acts on target cells in the kidney to affect ______ and ______. The contractile cells of the heart’s atria thus also serve an endocrine function.
atrial natriuretic factor (ANF), Na+ excretion, water balance
Most common injury sustained by cardiac muscle is that due to ______, or tissue damage due to lack of oxygen when coronary arteries are occluded by heart disease.
ischemia
Lacking muscle satellite cells, adult mammalian cardiac muscle has little potential to ______ after injury.
regenerate
However, certain fish and amphibians, as well as newborn mice, do form new muscle when the heart is partially removed, despite the lack of ______. Research on the possibility of mammalian heart muscle regeneration builds on work with the animal models, focusing primarily on the potential of mesenchymal stem cells to form new, site-specific muscle.
satellite cells
______ is specialized for slow, steady contraction under the influence of autonomic nerves and various hormones.
Smooth muscle
______
- This type of muscle is a major component of blood vessels and of the digestive, respiratory, urinary, and reproductive tracts and their associated organs.
SMOOTH MUSCLE
Fibers of smooth muscle (also called ______) are elongated, tapering, and unstriated cells, each of which is enclosed by an ______ and a network of type ______ and type ______ collagen fibers comprising the endomysium.
visceral muscle, external lamina, I, III
Smooth muscle cells range in length from ______ μm in small blood vessels to ______ μm in the pregnant uterus.
At each cell’s central, broadest part, where its diameter is ______-______ μm, is a single elongated ______.
20, 500, 5, 10, nucleus
SMOOTH MUSCLE
The cells stain uniformly along their lengths, and close packing is achieved with the narrow ends of each cell adjacent to the broad parts of neighboring cells.
With this arrangement cross sections of smooth muscle show a range of cell diameters, with only the largest profiles containing a ______.
nucleus
SMOOTH MUSCLE
All cells are linked by numerous ______. The borders of the cell become scalloped when smooth muscle contracts and the nucleus becomes distorted.
gap junctions
SMOOTH MUSCLE
Concentrated near the nucleus are ______, ______, ______, and ______ of a Golgi apparatus.
mitochondria, polyribosomes, RER, vesicles
At the smooth muscle cell surface are numerous small plasmalemma invaginations resembling ______, which in these cells compartmentalize various signaling components.
caveolae
SMOOTH MUSCLE
The fibers have rudimentary ______, but lack ______; their function is unnecessary in these smaller, tapering cells with many gap junctions.
sarcoplasmic reticulum, T-tubules
______ of smooth muscle cells contain the major ion channels that control Ca2+ release from sarcoplasmic cisternae at myofibrils that initiates contraction.
Caveolae
The characteristic contractile activity of smooth muscle is generated by ______ of ______ and ______ organized somewhat differently from those of striated muscle.
myofibrillar arrays, actin, myosin
In smooth muscle cells bundles of thin and thick myofilaments crisscross the sarcoplasm ______.
obliquely
SMOOTH MUSCLE
The ______ have a less regular arrangement among the thin filaments and fewer crossbridges than in striated muscle.
myosin filaments
Moreover smooth muscle actin filaments are not associated with troponin and tropomyosin, using instead ______ and Ca2+-sensitive ______ to produce contraction.
calmodulin, myosin light-chain kinase (MLCK)
SMOOTH MUSCLE
The contraction mechanism, however, is basically similar to that in ______.
striated muscle
SMOOTH MUSCLE
The actin myofilaments insert into anchoring cytoplasmic and plasmalemma- associated ______ which contain ______ and are functionally similar to the Z discs of striated and cardiac muscle.
dense bodies, α-actinin
Smooth muscle cells also have an elaborate array of 10-nm intermediate filaments, composed of ______, which also attach to the dense bodies.
desmin
The submembranous dense bodies include ______ of ______ linking adjacent smooth muscle cells.
cadherins, desmosomes
Dense bodies in smooth muscle cells thus serve as points for transmitting the ______ force not only within the cells, but also between adjacent cells
contractile
The ______ and other ______ layers help combine the force generated by the smooth muscle fibers into a concerted action, for example, peristalsis in the intestine.
endomysium, connective tissue
______ muscle is not under voluntary motor control and its fibers typically lack well-defined neuromuscular junctions.
Smooth
SMOOTH MUSCLE
Contraction is most commonly stimulated by ______ nerves, but in the gastrointestinal tract smooth muscle is also controlled by various paracrine secretions and in the uterus by oxytocin from the pituitary gland.
autonomic
Axons of autonomic nerves passing through smooth muscle have periodic ______ or ______ that lie in close contact with muscle fibers.
swellings, varicosities
SMOOTH MUSCLE
Synaptic vesicles in the varicosities release a neurotransmitter, usually ______ or ______, which diffuses and binds receptors in the sarcolemmae of numerous muscle cells.
There is little or no specialized structure to such junctions.
acetylcholine, norepinephrine
As in cardiac muscle, stimulation is propagated to more ______ fibers via ______ that allow all the smooth muscle cells to contract synchronously or in a coordinated manner.
distant, gap junctions
In addition to contractile activity, smooth muscle cells also supplement ______ activity, synthesizing ______, ______, and ______, with a major influence on the ECM in tissues where these contractile cells are abundant.
fibroblast, collagen, elastin, proteoglycans
Active synthesis of ______ by the small cells/fibers of smooth muscle may reflect less specialization for strong contractions than in skeletal and cardiac muscle and is similar to this synthetic function in other contractile cells, such as ______ and ______.
ECM, myofibroblasts, pericytes
Benign tumors called ______ commonly develop from smooth muscle fibers but are seldom problematic.
They most frequently occur in the wall of the uterus, where they are more commonly called ______ and where they can become sufficiently large to produce painful pressure and unexpected bleeding
leiomyomas, fibroids
In skeletal muscle, although the multinucleated cells cannot undergo ______, the tissue can still display limited ______.
mitosis, regeneration
The source of regenerating cells is the sparse population of mesenchymal ______ lying inside the external lamina of each muscle fiber.
satellite cells
______ cells are inactive, reserve myoblasts which persist after muscle differentiation.
Satellite
After injury the normally ______ cells become activated, proliferating, and fusing to form new skeletal muscle fibers.
quiescent satellite
A similar activity of satellite cells has been implicated in muscle growth after extensive exercise, a process in which they fuse with existing fibers to increase muscle mass beyond that which occurs by ______.
cell hypertrophy
Following major traumatic injuries, scarring and excessive connective tissue growth interferes with ______ muscle regeneration.
skeletal
Cardiac muscle lacks ______ and shows very little ______ capacity beyond early childhood.
satellite cells, regenerative
Defects or damage (eg, infarcts) to heart muscle are generally replaced by proliferating ______ and growth of ______, forming only ______.
fibroblasts, connective tissue, myocardial scars
Smooth muscle, composed of simpler, smaller, mononucleated cells, is capable of a more active ______ response. After injury, viable smooth muscle cells undergo ______ and replace the damaged tissue.
regenerative, mitosis
Contractile ______ from the walls of small blood vessels participate in the repair of vascular smooth muscle.
pericytes
