Muscle Tissue Flashcards
a. This low-magnification photomicrograph shows skeletal muscle in longitudinal section. Muscle fibers (cells) are arranged in parallel fascicles; they are vertically oriented, and the length of each fiber extends beyond the upper and lower edge of the micrograph. The fascicles appear to be of different thicknesses. This is largely a reflection of the plane of section through the muscle. Note on the left the epimysium, the sheath of dense connective tissue surrounding the muscle. ×160. b. At higher magnification, cross-striations of the muscle fibers are readily seen. The nuclei of skeletal muscle fibers are located in the cytoplasm immediately beneath the plasma membrane. ×360.
Myofilament interaction is responsible for muscle cell contraction. Two types of myofilaments are associated with cell contraction.
Thin filaments and thick filaments
Thin filaments (6 to 8 nm in diameter, 1.0 μm long) are composed primarily of the protein ______.
Actin
Each thin filament of fibrous actin (F-actin) is a polymer primarily formed from ______actin molecules (G-actin).
Globular
Thick filaments (~15 nm in diameter, 1.5 μm long) are composed primarily of the protein _______.
Myosin II
Each thick filament consists of 200 to 300 ________ molecules. The long, rod-shaped tail portion of each molecule aggregates in a regular parallel but staggered array, whereas the head portions project out in a regular helical pattern.
Myosin II
The two types of myofilaments occupy the bulk of the cytoplasm, which in muscle cells is also called ___________
sarcoplasm
Muscle is classified according to the appearance of the contractile cells.
Two principal types of muscle are recognized:
Striated and smooth muscle
striated muscle, in which the cells exhibit ___________at the light microscope level
cross-striations
smooth muscle, in which the cells do not exhibit _______
cross-striations.
Skeletal muscle is attached to bone and is responsible for movement of the __________ and for maintenance of body position and posture. In addition, skeletal muscles of the eye (extraocular muscles) provide precise eye movement.
axial and appendicular
Visceral striated muscle is morphologically identical to skeletal muscle but is restricted to the _______, namely, the tongue, pharynx, lumbar part of the diaphragm, and upper part of the esophagus. These muscles play essential roles in speech, breathing, and swallowing.
Soft tissues
Cardiac muscle is a type of ______found in the wall of the heart and in the base of the large veins that empty into the heart.
striated muscle
The main differences between skeletal muscle cells and cardiac muscle cells are in their ____, shape, and organization relative to one another.
Size
Smooth muscle cells do not exhibit ________ because the myofilaments do not achieve the same degree of order in their arrangement.
cross-striations
the myosin-containing myofilaments in smooth muscle are highly _____
Labile
Smooth muscle is restricted to the viscera and vascular system, the ____________ muscles of the skin, and the intrinsic muscles of the eye.
arrector pili
A muscle fiber is formed during development by the fusion of small, individual muscle cells called _______
myoblasts
muscle fiber should not be confused with a connective tissue fiber; muscle fibers are skeletal muscle cells, whereas connective tissue fibers are ________ products of connective tissue cells
Extracellular
The nuclei of a skeletal muscle fiber are located in the cytoplasm immediately beneath the plasma membrane, also called the _________ which consists of the plasma membrane of the muscle cell, its external lamina, and the surrounding reticular lamina.
sarcolemma
The connective tissue that surrounds both individual muscle fibers and bundles of muscle fibers is essential for force transduction
Endomysium is the delicate layer of _____fibers that immediately surrounds individual muscle fibers.
Reticular
Perimysium is a thicker connective tissue layer that surrounds a group of fibers to form a bundle or _____.
Fasicle
Fascicles are functional units of muscle fibers that tend to work together to perform a specific function. Larger blood vessels and nerves travel in the _________
Perimysium
Epimysium is the sheath of dense connective tissue that surrounds a collection of _______ that constitutes the muscle. The major vascular and nerve supply of the muscle penetrates the epimysium.
Fascicles
histochemical reactions based on oxidative enzyme activity, specifically the ___________ and nicotinamide adenine dinucleotide–tetrazolium (NADH-TR) reactions, confirm the observations seen in fresh tissue and reveal several types of skeletal muscle fibers
succinic dehydrogenase
This cross-section of muscle fibers stained with the NADH-TR reaction demonstrates two fiber types. The deeply stained, smaller muscle fibers exhibit strong oxidative enzyme activity and correspond to the type I slow oxidative fibers. The lighter staining, larger fibers correspond to the type IIb fast glycolytic fibers. ×280. Inset. Portions of the two fiber types at higher magnification. The reaction also reveals the mitochondria that contain the oxidative enzymes. The contractile components, the myofibrils, are unstained. ×550.
The current classification of skeletal muscle fibers is based on contractile speed, __________of the fiber’s myosin ATPase reaction, and metabolic profile.
Enzymatic velocity
Fibers characterized by oxidative metabolism contain large amounts of myoglobin and an increased number of _________, with their constituent cytochrome electron transport complexes
Mitochondria
Myoglobin is a small, globular, 17.8 kDa oxygen-binding protein that contains a ferrous form of _______
iron (Fe+2).
_________resembles hemoglobin in the erythrocytes and is found in various amounts in muscle fibers. ________ functions primarily to store oxygen in muscle fibers and provides a ready source of oxygen for muscle metabolism.
Myoglobin
Traumatic injuries to skeletal muscles (e.g., crash injuries) cause breakdown (rhabdomyolysis) and release of ______ from the injured muscle cells into the circulation.
myoglobin
The _________ is removed from the bloodstream by kidneys; however, large amounts of ________ are toxic to the renal tubular epithelium, causing acute renal failure. Detection of ________ in the blood is a sensitive but nonspecific test for muscle injury.
Myoglobin
The three types of skeletal muscle fibers:
type I (slow oxidative), type IIa (fast oxidative glycolytic), and type IIb (fast glycolytic) fibers.
Type I fibers or slow oxidative fibers are small fibers that appear red in fresh specimens and contain many _______ and large amounts of myoglobin and cytochrome complexes
Mitochondria
Type I fibers are slow-twitch, fatigue-resistant motor units (a twitch is a single, brief _________ of the muscle).
Contraction
Type I fibers
These fibers have great resistance to fatigue but generate less tension than other fibers. Their myosin ATPase reaction velocity is the slowest of all of the fiber types. Type I fibers are typically found in the limb muscles of mammals and in the breast muscle of migrating birds. More importantly, they are the principal fibers of the long erector spinae muscles of the back in humans, where they are particularly adapted to the long, slow contraction needed to maintain erect posture. A high percentage of these fibers make up the muscles of high-endurance athletes such as marathon runners.
Type IIa fibers or fast oxidative glycolytic fibers are the intermediate fibers seen in fresh tissue. They are of medium size with many ________ and high myoglobin content.
Mitchondria
In contrast to type I fibers, type IIa fibers contain large amounts of _______ and are capable of anaerobic glycolysis.
Glycogen
Athletes who have a high percentage of these fast oxidative glycolytic fibers include 400- and 800-m sprinters, middle-distance swimmers, and hockey players.
Type IIa fibers or fast oxidative glycolytic fibers
Type IIb fibers or fast glycolytic fibers are large fibers that appear light pink in fresh specimens and contain _____ myoglobin and fewer mitochondria than type I and type IIa fibers.
Less
__________ have a low level of oxidative enzymes but exhibit high anaerobic enzyme activity and store a considerable amount of glycogen.
Type IIb fibers or fast glycolytic fibers
Type IIb fibers or fast glycolytic fibers are fast-twitch, ________ motor units and generate high peak muscle tension.
Fatigue-prone
Type IIb fibers or fast glycolytic fibers myosin ATPase velocity is the _______ of all the fiber types. They also fatigue rapidly as a result of production of lactic acid. Thus, type IIb fibers are adapted for rapid contraction and precise, fine movements.
Fastest
Type IIb fibers or fast glycolytic fibers constitute most fibers of the extraocular muscles and the muscles that control the movements of the digits. These muscles have a greater number of neuromuscular junctions than do type _______ thus allowing more precise neuronal control of movements in these muscles. Short-distance sprinters, weight lifters, and other field athletes have a high percentage of type IIb fibers.
Type I
The structural and functional subunit of the muscle fiber is the ________
myofibril
skeletal muscle consists of bundles of muscle fibers called fascicles. In turn, each fascicle consists of a bundle of elongate muscle fibers (cells). The muscle fiber represents a collection of longitudinal units, the myofibrils, which in turn are composed of myofilaments of two types: thick (myosin) filaments and thin (actin) filaments. The myofilaments are organized in a specific manner that imparts a cross-striated appearance to the myofibril and to the fiber. The functional unit of the myofibril is the sarcomere; it extends in both directions from one Z line to the next Z line. The A band marks the extent of the myosin filaments. Actin filaments extend from the Z line into the region of the A band, where they interdigitate with the myosin filaments as shown.
Myofilaments are the individual filamentous polymers of ________(thick filaments) and actin and its associated proteins (thin filaments).
myosin II
Myofilaments are the actual contractile elements of striated muscle. The bundles of myofilaments that make up the myofibril are surrounded by a well-developed, __________ also called the sarcoplasmic reticulum.
smooth-surfaced endoplasmic reticulum (sER)
Cross-striations are the principal histologic feature of ________muscle.
striated muscle.
Cross-striations are evident in H&E–stained preparations of longitudinal sections of muscle fibers. They may also be seen in unstained preparations of living muscle fibers examined with a phase contrast or polarizing microscope, in which they appear as alternating light and dark bands. These bands are termed the A band and the _______
I band
Cross-striations are evident in _________ preparations of longitudinal sections of muscle fibers. They may also be seen in unstained preparations of living muscle fibers examined with a phase contrast or polarizing microscope, in which they appear as alternating light and dark bands. These bands are termed the A band and the I band
H&E–stained
In polarizing microscopy, the dark bands are __________ (i.e., they alter the polarized light in two planes)
birefringent
the dark bands, being doubly refractive, are _________ and are given the name A band.
anisotropic
The light bands are monorefringent (i.e., they do not alter the plane of polarized light). Therefore, they are _________ and are given the name I band.
isotropic
The light I band is bisected by a dense line, the Z line, also called the ______
Z disc
The dark A band is bisected by a less dense, or light, region called the _______
H band
bisecting the light H band is a narrow dense line called the ______
bisecting the light H band is a narrow dense line called the M line
The M line is best demonstrated in electron micrographs (Fig. 11.5), although in ideal H&E preparations, it can be detected in the light microscope.
This low-magnification electron micrograph shows the general organization of skeletal muscle fibers. Small portions of three muscle fibers in longitudinal profile are included in this micrograph. The muscle fiber on the right reveals a nucleus at its periphery. Two fibers—one in the middle and another on the left—exhibit regular profiles of myofibrils separated by a thin layer of surrounding sarcoplasm (Sr). Each repeating part of the myofibril between adjacent Z lines is a sarcomere (S). The cross-banded pattern visible on this micrograph reflects the arrangement, in register, of the individual myofibrils (M); a similar pattern found in the myofibril reflects the arrangement of myofilaments. The detailed features of a sarcomere are shown at higher magnification in Figure 11.10a. The presence of the connective tissue in the extracellular space between the fibers constitutes the endomysium of the muscle. ×6,500.
The sarcomere is the basic contractile unit of striated muscle. It is the portion of a myofibril between two adjacent Z lines. A sarcomere measures 2 to 3 μm in relaxed mammalian muscle. It may be stretched to more than 4 μm and, during extreme contraction, may be reduced to as little as 1 μm (Fig. 11.6). The entire muscle cell exhibits cross-striations because sarcomeres in adjacent myofibrils are in register.
A typical thin filament is 5 to 6 nm in diameter and consists of a double-stranded helix of polymerized actin monomers (Fig. 11.7). Each thin filament is fine-tuned to approximately 1.0 to 1.3 μm in length, depending on muscle type. The two important regulatory proteins in striated muscles, tropomyosin and troponin, are entwined with two actin strands. Other thin filament–associated proteins include tropomodulin and nebulin.
a. Thin filament is primarily composed of the two helically twisted strands of actin filaments (F-actin). Each actin molecule contains binding sites for myosin, which is physically blocked by tropomyosin to prevent muscle contraction. Troponin complex is a key regulatory protein; its TnC component binds calcium. This initiates a conformational shift in the troponin complex resulting in the repositioning of tropomyosin and troponin off the myosin binding sites on actin molecules. b. This three-dimensional reconstruction of a 10-actin–long stretch segment of the thin filament is based on the crystal structures of actin, tropomyosin, and troponin filtered to 25Å resolution. Note the asymmetric shape of the troponin molecule with its extended IT arm and elongated, rod-shaped tropomyosin.
Troponin consists of a complex of three globular subunits. Each tropomyosin molecule contains one troponin complex. Troponin-C (TnC) is the smallest subunit of the troponin complex (18 kDa). It binds Ca2+, an essential step in the initiation of contraction. Troponin-T (TnT), a 30 kDa subunit, binds to tropomyosin, anchoring the troponin complex. Troponin-I (TnI), also a 30 kDa subunit, binds to actin, thus inhibiting actin–myosin interaction. Both the TnT and TnI subunits join together to form an asymmetrical IT arm, which is visible on a three-dimensional reconstruction of the troponin complex (see Fig. 11.7).
Myosin has two globular heads (S1 region) that are connected via lever arms (S2 region) with a long tail (Fig. 11.8). Each myosin monomer contains one 18 kDa essential light chain (ELC) and one 22 kDa regulatory light chain (RLC) that are wrapped around the lever arm region just below the myosin head (see Fig. 11.8)
Each globular head of the S1 region represents a heavy chain motor domain that projects at an approximate right angle at one end of the myosin molecule. The myosin head has two specific binding sites, one for ______ with ATPase activity and one for actin.
ATP
Enzymatic digestion of myosin produces two fragments, a heavy meromyosin (HMM) and light meromyosin (LMM). The HMM consists of the heads, lever arms, and both pairs of light chains, whereas the LMM consists of the tail
Myosin molecules in striated muscle aggregate tail to tail to form bipolar thick myosin filaments; the tail segments overlap so that the globular heads project from the thick filament
a. Thick filament assembly is initiated by the two tails of myosin molecules that bind together in an antiparallel fashion. b. Diagram showing further assembly of myosin molecules into a thick bipolar filament. The myosin heads point away from the bare zone, which is free of myosin heads. Note that myosin tails in the bare zone have both antiparallel and parallel arrangements, but in the distal portion of the filament, they overlap only in the parallel fashion. c. Diagram of a section of myosin bipolar thick filament. Note spiral arrangement of myosin heads. d. Three-dimensional reconstruction of the frozen–hydrated tarantula thick filament, filtered to 2-nm resolution. It shows several myosin heads (one illustrated in yellow) and tails of myosin molecules in parallel arrangement.
Proteins known as _________ are essential in regulating the spacing, attachment, and alignment of the myofilaments. These structural protein components of skeletal muscle fibrils constitute less than 25% of the total protein of the muscle fiber.
Accessory protiens
This high-magnification electron micrograph shows a longitudinal section of the myofibrils. The I band, which is bisected by the Z line, is composed of barely visible, thin (actin) filaments. They are attached to the Z line and extend across the I band into the A band. The thick filaments, composed of myosin, account for the full width of the A band. Note that in the A band, there are additional bands and lines. One of these, the M line, is seen at the middle of the A band; another, the less electron-dense H band, consists only of thick filaments. The lateral parts of the A band are more electron dense and represent areas where the thin filaments interdigitate with the thick filaments. ×35,000. b. Diagram illustrating the distribution of myofilaments and accessory proteins within a sarcomere. The accessory proteins are titin, a large elastic molecule that anchors the thick (myosin) filaments to the Z line; α-actinin, which bundles thin (actin) filaments into parallel arrays and anchors them at the Z line; nebulin, an elongated inelastic protein attached to the Z lines that wraps around the thin filaments and assists α-actinin in anchoring the thin filament to Z lines; tropomodulin, an actin-capping protein that maintains and regulates the length of the thin filaments; tropomyosin, which stabilizes thin filaments and, in association with troponin, regulates binding of calcium ions; M line proteins (myomesin, M-protein, obscurin), which hold thick filaments in register at the M line; myosin-binding protein C, which contributes to normal assembly of thick filaments and interacts with titan; and two proteins (desmin and dystrophin) that anchor sarcomeres into the plasma membrane. The interactions of these various proteins maintain the precise alignment of the thin and thick filaments in the sarcomere and the alignment of sarcomeres within the cell.
Dystrophin is a rod-shaped cytoskeletal protein with a short head and a long tail that is located just beneath the skeletal muscle cell membrane. F-actin is bound at the end portion of the tail. Two groups of transmembrane proteins—α- and β-dystroglycans and α-, β-, γ-, and δ-sarcoglycans—participate in a dystrophin–glycoprotein complex that links dystrophin to the extracellular matrix proteins laminin and agrin.
Several forms of muscular dystrophy are attributed to mutations of single genes encoding several proteins of the dystrophin–glycoprotein complex. Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are associated with mutations that affect dystrophin expression (Fig. F11.2.2); different forms of limb girdle muscular dystrophy (LGMD) are caused by mutations in the genes found on the short arm of the X chromosome encoding the four different sarcoglycans, and another form of congenital
In resting muscle, myosin heads are prevented from binding with actin molecules by tropomyosin, which covers myosin-binding sites on actin molecules
Following nerve stimulation, Ca2+ is released into the sarcoplasm and binds to troponin, which then acts on the tropomyosin to expose the myosin-binding sites on actin molecules (Fig. 11.11b). Once the binding sites are exposed, the myosin heads are able to interact with actin molecules and form cross-bridges, and the two filaments slide over one another.
Attachment is the _____ stage of the cross-bridge cycle; the myosin head is tightly bound to the actin molecule of the thin filament.
At the beginning of the cross-bridge cycle, the myosin head is strongly bound to the actin molecule of the thin filament, and ATP is absent (Fig. 11.11c). Position of the myosin head in this stage is referred as an original or unbent confirmation. This very short-lived arrangement is known as the rigor configuration. The muscular stiffening and rigidity that begins at the moment of death is caused by lack of ATP and is known as rigor mortis. In an actively contracting muscle, this step ends with the binding of ATP to the myosin head.
Release is the _______stage of the cross-bridge cycle; the myosin head is uncoupled from the thin filament.
Second
In this stage of the cross-bridge cycle, ATP binds to the myosin head and induces conformational changes of the actin-binding site. This change reduces the affinity of the myosin head for the actin molecule of the thin filament, causing the myosin head to uncouple from the thin filament (Fig. 11.11d).
Bending is the ______stage of the cross-bridge cycle and “resets” the motor of the myosin; the myosin head, as a result of hydrolysis of ATP, assumes its pre–power stroke position.
Third
The ATP-binding site on the myosin head undergoes further conformational changes, causing the myosin head to bend by rotating the lever arm of myosin to assume its pre–power stroke position. This movement is initiated by the breakdown of ATP into adenosine diphosphate (ADP) and inorganic phosphate (Pi); both products, however, remain bound to the myosin head (Fig. 11.11e). In this stage of the cycle, the linear displacement of the myosin head relative to the thin filament is approximately 5 nm. This stage is sometimes referred to as a “recovery stroke.”
Force generation is the stage of the cross-bridge cycle; the myosin head releases inorganic phosphate and the power stroke occurs.
Fourth
The myosin head binds weakly to its new binding site on the actin molecule of the thin filament (Fig. 11.11f), causing the release of inorganic phosphate (Fig. 11.11g). This release has two effects. First, the binding affinity between the myosin head and its new attachment site increases. Second, the myosin head generates a force as it returns to its original unbent position. Thus, as the myosin head straightens, it forces movement of the thin filament along the thick filament. This is the “power stroke” of the cycle. During this stage, ADP is lost from the myosin head (Fig. 11.11h).
Reattachment is the _____ and last stage of the cross-bridge cycle; the myosin head binds tightly to a new actin molecule.
Fifth
The myosin head is again tightly bound to a new actin molecule of the thin filament (rigor configuration), and the cycle can repeat (see Fig. 11.11c).
The two heads of the myosin molecule work together in a productive and coordinated manner. Although an individual myosin head may detach from the thin filament during the cycle, heads of other myosins in the same thick filament will attach to actin molecules, thereby resulting in movement. Because the myosin heads are arranged as mirror images on either side of the H band (antiparallel arrangement), this action pulls the thin filaments into the A band, thus shortening the sarcomere.
Cardiac muscle has the same types and arrangement of contractile filaments as skeletal muscle. Therefore, cardiac muscle cells and the fibers they form exhibit cross-striations evident in routine histologic sections. In addition, cardiac muscle fibers exhibit densely staining cross-bands, called intercalated discs, that cross the fibers in a linear fashion or frequently in a way that resembles the risers of a stairway
cardiac muscle fibers consist of numerous cylindrical cells arranged end to end. Furthermore, some cardiac muscle cells in a fiber may join with two or more cells through intercalated discs, thus creating a branched fiber.
Numerous large mitochondria and glycogen stores are adjacent to each myofibril.
Numerous large mitochondria and glycogen stores are adjacent to each myofibril.
In addition to the juxtanuclear mitochondria, cardiac muscle cells are characterized by large mitochondria that are densely packed between the myofibrils. These large mitochondria often extend the full length of a sarcomere and contain numerous, closely packed cristae
Fascia adherens (adhering junction) is the major constituent of the transverse component of the intercalated disc and is responsible for its staining in routine H&E preparations. It holds the cardiac muscle cells at their ends to form the functional cardiac muscle fiber (see Fig. 5.20, page 129). It always appears as a transverse boundary between the cardiac muscle cells.
Electron micrograph showing the end-to-end apposition of two cardiac muscle cells. The intercellular space appears as a clear undulating area. On the cytoplasmic side of the plasma membrane of each cell, there is a dense material similar to that seen in a zonula adherens containing actin filaments. Because the attachment site here involves a portion of the end face of the two cells, it is called a fascia adherens. ×38,000.
Small terminal cisternae of the _____ are in close proximity to the T tubules to form a diad at the level of the Z line
SER
In contrast to skeletal muscle, long-lasting depolarization in cardiac muscle activates DHSRs and prompts their slow conformation change into functional Ca2+ channels
in the first stage of the cardiac muscle contraction cycle, Ca2+ from the lumen of the T tubule is transported to the sarcoplasm of cardiac muscle cell, which then opens gated Ca2+-release channels in adjacent terminal sacs of the sarcoplasmic reticulum.
Gated Ca2+-release channels in cardiac muscle sarcoplasmic reticulum are composed of RyR2 isoform of ryanodine receptors, which is the primary isoform in the cardiac muscle. This calcium-triggered calcium release mechanism causes a rapid release of additional Ca2+ that initiates subsequent steps of the contraction cycle, which are identical to those in skeletal muscle.
The differences between initiation of cardiac and skeletal muscle contractions—the longer lasting membrane depolarization and activation of voltage-sensitive Ca2+ channels in the wall of the T tubule—account for an approximately 200-millisecond delay from the start of depolarization in a cardiac muscle twitch (see Fig. 11.22). In addition, contrary to skeletal muscle, release of Ca2+ from the sarcoplasmic reticulum alone is insufficient to initiate cardiac muscle contraction.
Smooth muscle generally occurs as bundles or sheets of elongated fusiform cells with finely tapered ends (Fig. 11.23 and Plate 26). The smooth muscle cells, also called fibers, lack the striated pattern found in skeletal and cardiac muscle.
Smooth muscle cells are interconnected by gap junctions, the specialized communication junctions between the cells (Fig. 11.24). Small molecules or ions can pass from cell to cell via these junctions and provide communication links that regulate contraction of the entire bundle or sheet of smooth muscle.
Smooth muscle cells possess a contractile apparatus of thin and thick filaments and a cytoskeleton of desmin and vimentin intermediate filaments.
The thin filaments in a smooth muscle cell are attached to cytoplasmic densities or dense bodies that are visible among the filaments (Fig. 11.25). These structures are distributed throughout the sarcoplasm in a network of intermediate filaments containing the protein desmin. Intermediate filaments are part of the cytoskeleton of the cell. Note that vascular smooth muscle contains vimentin filaments in addition to desmin filaments.
Thick filaments containing smooth muscle myosin differ slightly from those found in skeletal muscle. They, too, are composed of two polypeptide heavy chains and four light chains. However, the structure of thick filaments in smooth muscle is different than in skeletal muscle. Rather than a bipolar arrangement, SMM molecules are oriented in one direction on one side of the filament and in an opposite direction on the other side of the filament. In this arrangement, myosin molecules are staggered in parallel between two immediate neighbors and are also bound to an antiparallel partner via a short overlap at the very tip of their tails (Fig. 11.26). The polarity of the myosin heads is the same along the entire length of one side of the filament and the opposite on the opposite side. This side-polar myosin filament also has no central “bare zone” but instead has asymmetrically tapered bare ends. This organization maximizes the interaction between thick and thin filaments, allowing the overlapped thin filaments to be pulled over the entire length of the thick filaments.
Dense bodies contain a variety of attachment plaque proteins, including α-actinin, which anchors both thin filaments and intermediate filaments either directly or indirectly to the sarcolemma. They play an important role in transmitting ___________generated inside the cell to the cell surface, altering the cell’s shape
Contractile forces
Contraction in smooth muscles is initiated by a variety of impulses, including mechanical, electrical, and chemical stimuli.
Mechanical impulses, such as passive stretching of vascular smooth muscle, activate mechanosensitive ion channels, leading to initiation of spontaneous muscle contraction (myogenic reflex).
Electrical depolarizations can occur, such as those during neural stimulation of smooth muscle. The release of the neurotransmitters acetylcholine and norepinephrine from their synaptic nerve endings stimulates receptors located in the neuronal plasma membrane and changes the membrane potential. This causes opening of voltage-sensitive Ca2+ channels
Chemical stimuli, such as those elicited by angiotensin II, vasopressin, or thromboxane A2, act on specific cell membrane receptors, leading to muscle contraction. These substances use second-messenger pathways that do not require the generation of an action potential and cell depolarization to trigger contraction. The most common second-messenger pathways used by smooth muscle are inositol 1,4,5-trisphosphate (IP3), G-protein-coupled, and nitric oxide (NO)-cGMP pathways.
Smooth muscle cells lack a T system. A characteristic feature of smooth muscle cells is the presence of large numbers of invaginations of the cell membrane that resemble caveolae
Contraction of smooth muscle is initiated by a Ca2+-mediated change in thick filaments utilizing calmodulin–myosin light chain kinase system.
As in striated muscle, contraction is initiated by an increase in the Ca2+ concentration in the cytosol, but the contraction does not act through a troponin–tropomyosin complex on the thin filament. Rather, in smooth muscle, an increase in Ca2+ concentration stimulates a myosin light chain kinase (MLCK) to phosphorylate one of the two regulatory light chains of the smooth muscle myosin molecule. The Ca2+ binds to calmodulin to form the Ca2+–calmodulin complex, which in turn binds to MLCK to activate the phosphorylation reaction of the regulatory light chain of myosin
When the light chain is phosphorylated, SMM changes its confirmation from inactive (folded) to active (unfolded) configuration that can assemble into side-polar myosin filaments. Phosphorylation also activates the actin-binding site of the myosin head, allowing for attachment to actin filament. In the presence of ATP, the myosin head bends, producing contraction. When it is dephosphorylated, the myosin head dissociates from actin. This phosphorylation occurs slowly, with maximum contraction often taking up to a second to achieve. In addition, dephosphorylation promotes disassembly of myosin filaments and return of myosin to its folded inactive state
Although Ca2+ enters the cytoplasm during depolarization by voltage-gated Ca2+ channels, some Ca2+ channels, called ligand-gated Ca2+ channels, are activated by hormones via their second-messenger pathways
These springy titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work. One end of the titin molecule is elastic and is attached to the Z disk, acting as a spring and changing length as the sarcomere contracts and relaxes. The other part of the titin molecule tethers it to the myosin thick filament. The titin molecule may also act as a template for the initial formation of portions of the contractile filaments of the sarcomere, especially the myosin filaments.
