WK5_MUSCLE TISSUE.pdf

HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 The blood vessels supplying skeletal muscle ramify in STRIATED...

HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 The blood vessels supplying skeletal muscle ramify in STRIATED MUSCLE the epimysium and penetrate via the septa of the The units of organization of skeletal muscles are the perimysium to form, in the endomysium, a rich Muscle Fibers, capillary network around the individual muscle Long cylindrical multinucleated cells unlike fibers. in smooth that is uninucleated Much larger than smooth muscle fibers, ranging from 10-30 cm in length and from H VB0.1-0.5 mm in diameter Parallel Fibers aggregated in bundles of fascicles large enough that they are visible to the naked eye Note: The picture above shows that the dark purple areas found at the periphery are the nucleus and the alterna ng light pink and dark pink lines are the stria ons of the muscle fibers. Connec ve ssue surrounding the muscles: Note: A muscle unit has a lot of mitochondria since it requires Epimysium - dense connec ve ssue lots of energy for muscle contrac on (ATP genera on); surrounding the whole muscle; outermost Myosin has head and is thicker than ac n; Sarcoplasmic layer Re culum is where Calcium goes out (needed for contrac on) Perimysium - thin septa that extend inward from it to surround each of the fascicles Arrangement of Fascicles Endomysium - the delicate re culum Varies from muscle to muscle inves ng the individual fibers; surrounds In short muscles, fascicles are oriented parallel to the smallest unit and has rich capillary the direc on of the pull blood supply In very long muscles, fibers are usually shorter than the muscles as a whole and are connected to one or more transverse bands of connec ve ssue Unipinnate, the fasciculi are oriented obliquely with respect to a longitudinal band of connec ve ssue along one side of the muscle Bipinnate, having oblique fascicles that radiate from a connec ve ssue core in the muscle; pa ern resembles that of a barbs extending obliquely from the axial core of a feather Mul pennate, with oblique fascicles radia ng from several longitudinal connec ve ssue strands and converge on the tendon. It is generally found in HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 muscles where considerable power and a short protec on against stress development during muscular range of mo on are required contrac on. - Apparent at the age of 5, progressive replacement of Development and Repair damaged muscle fibers by connec ve and adipose In the embryo, uninucleate precursors called ssue myoblasts proliferate and fuse to form long - At age of 12, unable to walk properly mul nuclear cells called myotubes - Gower’s Sign - uses hands to push themselves up. These elongated syncy a then develop, in their They cannot overcome stress on muscle and are cytoplasm, myofibrils, the contrac le elements of unable to stand on their own. differen ated muscle fibers. During postnatal growth, the muscle fibers increase All of the common cell organelles are represented in in length and thickness, reaching a diameter of 10-70 the sarcoplasm. um, depending on the muscle and the species. Fibers A small Golgi complex is associated with in the same muscle may vary considerably in their one pole of thickness. most of the nuclei. Atrophy – thinning or decrease in Mitochondria (sarcosomes) congregate in size/diameter of muscle the juxtanuclear sarcoplasm and are also Hypertrophy - further increase in deployed in longitudinal rows between the size/diameter of muscle myofibrils where they provide energy for Satellite cells, situated within the external lamina, muscle contrac on (ATP) (Well supported on participate in muscle repair after injury. However, the sides where it is located) skeletal muscle has only limited capacity for repair in Lipid droplets are found in small numbers adults. between the myofibrils or among the After minor injury, satellite cells within an intact mitochondria at the poles of the nuclei. endomysium may divide and fuse to restore Glycogen can be demonstrated throughout damaged fibers. (The satellite cells become fibers) the sarcoplasm If the injury is more extensive, disrupting the Myoglobin is present in low concentra on. endomysium, regeneration of muscle fibers does not As required, oxygen dissociates from occur. Instead, the damaged area is invaded by myoglobin and becomes available for fibroblasts that deposit collagen, resulting in a oxida ons. fibrous scar. In longitudinal sec ons, stained with iron Restoration of muscle function after such an injury hematoxylin, the cross-stria ons that are depends mainly on hypertrophy of the remaining barely detectable in fresh muscle are greatly undamaged fibers. accentuated, with heavily stained bands alterna ng with rela vely pale bands. Structure of Muscle Fibers Plasma membrane of muscle fibers is called sarcolemma and the cytoplasm, the sarcoplasm. Sarcolemma of the long cylindrical fibers is reinforced by an external lamina (External lamina is like the basement membrane in the epithelial cells) Myofibrils occupy the bulk of the sarcoplasm (The cytoplasm of a muscle cell is full of ac n and myosin) Peripheral loca on of the nuclei (In cardiac muscle, nuclei are central) 400 kD protein dystrophin in the cytoplasm ○ provides mechanical reinforcement to the membrane Note: Mitochondria - on the side; Blue - Sarcoplasmic ○ thereby protec ng it against stresses re culum is blue due to the calcium and penetrates into the T developed during muscular contrac on tubule ○ X-linked Duchenne muscular dystrophy The bands that appear dark in stained prepara ons Note: are anisotropic (birefringent) when examined with DMD - results from lack of dystrophin. No Dystrophin to the polarizing microscope and are, therefore, provide mechanical reinforcement to the membrane and in HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 designated A-bands, whereas the lighter-staining Lipid droplets are common in the bands are isotropic and are called the I-bands. sarcoplasm of these fibers, owing to the The rela ve lengths of the bands depend on the abundance of their mitochondria, they stain state of contrac on of the muscle. intensely with the cytochemical reac on for The I-bands are very short during contrac on, longer the enzyme succinic dehydrogenase during relaxa on, and longest in muscle that has The Z bands tend to be wider compared been passively stretched. with other fiber types The length of the A-bands remains constant in all Innervated by small axons with small and phases of the cycle of contrac on and relaxa on. rela vely simple motor end plates Each I-band is bisected by a transverse line, the Motor units consis ng of red fibers contract Z-line or Z-disc. more slowly than other types The segments of myofibrils between successive Very resistant to fa gue due to their ability Z-lines are called sarcomeres and all of the for oxida ve regenera on of ATP, a high morphological changes during the contrac le cycle energy compound needed to recycle the are described with reference to this structural unit. contrac le process. These proper es make The sarcomere includes an A-band and half of the them effec ve in postural maintenance two con guous I-bands. Spherical subsarcolemmal mitochondria are large and abundant Slender, paired mitochondria are transversely oriented on either side of the Z-lines. In interfibrillar spaces, mitochondria, which is one sarcomere in length, form longitudinal columns. These may be thick with mul ple mitochondria or thin consis ng of rows of single mitochondria B. White Fibers (Fast-Twitch Fibers): Largest of all the fiber types. Subplasmalemmal mitochondria are smaller and numerous compared to red fibers and interfibrillar mitochondria are rela vely few Cytochemical reac ons for succinic dehydrogenase is correspondingly weak Genera on of ATP depends on anaerobic glycolysis of glucose derived from abundant Note: Sarcomere - from Z to Z, one whole A, 2 halves of I (half deposits of glycogen in the sarcoplasm per side) Innervated by larger axons that form motor end plates about twice the size of those red Heterogeneity of Skeletal Muscle Fibers fibers ❖ 2 Types of Fiber: Contract rapidly and generate large forces 1. Twitch Fibers (fast fibers) - propagate an but they fa gue rapidly. ac on poten al and respond with Best suited for brief bursts of intense all-or-none contrac on. muscle ac vity 2. Tonic Fibers (slow fibers)- unable to Subsarcolemmal mitochondria are fewer propagate an ac on poten al and require a and longitudinal columns are rare and o en series of nerve impulses. Their contrac on interrupted. is more prolonged than that of twitch fibers. C. Intermediate Fibers (Contrac on takes me but it is more Mitochondrial disposi on in intermediate sustained) fibers is similar to that of red fibers, but ❖ 3 Types of Fiber in Cytochemical Staining Methods: thick mitochondrial columns are rarely A. Red Fibers (Slow Twitch, Oxida ve Fibers): rela vely found (Fawce , 1994) small in diameter, darker color, greater myoglobin content, rich supply of capillaries, numerous large mitochondria HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 Ultrastructure of Striated Muscle Fibers The structure of the common organelles is not fundamentally different from that in other cells The mitochondria have numerous closely spaced cristae and their distribu on, in in mate rela on to the myofibrils, brings the source of energy (ATP) close to its site of u liza on. The endoplasmic re culum: ○ consists of membrane-bounded tubules Between each pair of terminal cisternae, there is a occupying the narrow spaces between the slender transverse tubule (T-tubule) extending myofibrils inward from the sarcolemma ○ largely devoid of associated ribosomes. T-tubules are slender invagina ons of the ○ Specialized func on: Site of calcium sarcolemma that penetrate deep into the interior of sequestra on (the process of binding or the muscle fiber crossing many myofibrils. confining calcium ions such that they are The two parallel terminal cisternae and the separated from other components of a intervening T-tubule form a complex referred to as biological system) during muscle relaxa on. It is the triad. also where free calcium ions are released into the sarcoplasm to trigger contrac ons in response to nerve impulses (axons). Excitation-Contraction Coupling 1. It begins at the myoneural junc on with the genera on of an ac on poten al that spreads over the sarcolemma and along the membrane of the T-tubules into the interior of the muscle fiber. 2. This ac vates events at the interface between the T-tubules and the terminal cisternae of the triads that result in the rapid release of calcium from the sarcoplasmic re culum. 3. The calcium released in the sarcoplasm binds to the myofibrils and triggers shortening. 4. When depolariza on of the sarcolemma by nerve impulses ceases, calcium is ac vely transported back into the lumen of the terminal cisternae. 5. The consequent lowering of the calcium concentra on around the myofibrils brings about Sarcotubules are longitudinal anastomosing lateral muscle relaxa on. branches form a denser at the H-zone of each A-band. Substructure of the Myofibrils At the junc on of the A-band with the I-band, the Myofibrils are composed of even smaller longitudinal sarcotubules are confluent with a myofilaments (myosin and ac n). transversely oriented tubule of larger caliber, called Myosin filaments, 1.5 pm in length and 15 the terminal cisterna. nm in diameter, oriented longitudinally in a The longitudinal tubules that span successive I-bands parallel array, spaced about 45 nm apart, a and the intervening Z-line also terminate in a principal cons tuent of the A-bands. separate terminal cisterna at the A-I junc on. ○ They are held in the lateral register Thus, a pair of parallel terminal cisterna is associated by slender cross-links in the middle with each A-I junc on along the length of the of the A-band. The transverse myofibril, two pairs to each sarcomere. alignment of these links creates a Note: Sacrotubules meet to form the terminal cisternae. thin linear density called the M-line that bisects the paler H-band at the middle of each A-band. ○ Enzyma c proteolysis of the myosin molecule cleaves it into two fragments, a straight por on HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 called light meromyosin (LMM), In very thin sec ons of muscle from a species with a represen ng the greater part of rela vely simple disc, the ac n filaments appear to the tail, and a shorter segment terminate at the edge of the disc. There, each called heavy meromyosin (HMM), filament is a ached to four thin diverging Z-filaments which includes the heads and a In longitudinal sec ons, the connec ng Z-filaments short por on of the backbone of form a zigzag pa ern across the myofibril. the molecule. Filamin, an ac n-binding protein, is also found in the Ac n filaments are the dominant Z-discs of skeletal muscle. component of the I-band. They interdigitate with myosin filament in the neighboring A-bands to varying degrees depending on the state of muscle contrac on. Notes(from Ayura): When a heavy meromyosin frac on is added to isolated ac n filaments, large numbers of these HMM fragments a ach to the filaments decora ng them in a dis nc ve arrow-head pa ern. This demonstrates, in vitro, the ac n-binding property of the heads of the myosin molecules that is essen al for muscle contrac on. Each myofibril is enclosed in a delicate sleeve of 10 nm filaments and the collars around the Z-lines of neighboring myofibrils are interconnected by short transverse filaments. Beneath the sarcolemma, there are inconspicuous rib-like bands, called costameres, seen in longitudinal sec ons as small densi es that repeat along the fiber in register with the Z-lines of the peripheral myofibrils. In contracted or swollen muscle fibers, indenta ons occur along the length of the fiber at these subsarcolemmal densi es which can be shown to Muscle exhibits a resistance to stretch that is contain the anchoring protein, vinculin. independent of the interac on of ac n and myosin filaments and a third type of filament, responsible SMOOTH MUSCLE for this passive elas city, was postulated. Composed of fusiform (spindle-shapedI uninucleate These extremely thin strands (~ 4 nm) consist of n, nucleus that respond to the autonomic nervous an excep onally large protein having a molecular system weight of about 3 x 10 D. The molecules are Elongated nucleus situated in the wider central approximately 1 um long, spanning the distance from por on the M-line of the A-band to the Z-disc. Nucleus contains one or two nucleoli Ti n is the main structural basis for Chroma n distributed in small clumps myofibrillar elas city. It is believed to Peripheral cytoplasm contains the contrac le maintain the central posi on of thick components filaments in the sarcomere. ○ Note: Not all types of muscle have the same contrac le mechanism Forms the contrac le por on of the wall of the diges ve tract from the middle of the esophagus to the internal sphincter of the anus The myofilaments of successive sarcomeres are linked end-to-end by Z-disc. HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 1. ESOPHAGUS (Note: 1st portion of esophagus has striated muscle) 5. SMALL INTESTINE 2. STOMACH 3. PYLORUS (Note: Smooth muscle (tunica muscularis) on pyloric sphincter thickens to timing the flow of food and prevent acid reflux) 6. LARGE INTESTINE (Note: Note: Shortened villi, has inner circular and outer longitudinal that moves in peristalsis motion) 4. JEJUNUM (Note: Has presence of villi which are fusiform cells, the smooth muscle on submucosa is present as single or double) 7. LUNGS HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 Found in the walls of respiratory passages Clinical importance: In atherosclerosis, there from the trachea to the alveolar ducts. is migration of smooth muscle cells from Note: No more smooth muscle on alveoli. the tunica media to the tunica intima, which During opera on when a person is under will then be part of formation of anesthesia, the smooth muscles will be atherosclerotic plaques. immobile, making respira on difficult. Hence, proper intuba on is important. Organizational Diversity Are organized in different pa erns to meet local requirements. May occur: ○ Singly in loose connec ve ssue ○ Gathered together in slender fascicles as in the ny arrector pili muscle associated with each hair bulb ○ Annular band as in the constrictor pupillae 8. BLOOD VESSELS muscle of the iris Blood vessels also have smooth muscle in their wall that controls their caliber. (Note: Veins have presence of sphincter and artery has smaller muscle) Tunica media regulates the internal diameter of the blood vessel and is important in regula ng blood pressure. It is thicker in arteries than in veins. (Note: If the blood vessel is vasoconstricted, blood Note: Arrector pili muscle contracts, resulting in goosebumps pressure increases) that occur in certain instances (e.g. seeing a ghost). It is located beside the root of each hair follicle. HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 In precapillary arterioles, single circumferen ally oriented fibers are spaced at intervals along the vessel. Their contrac on narrows the lumen, decreasing blood flow through the capillary bed. In the wall of larger arteries flow is controlled by a con nuous layer of circular smooth muscle. From Ayura: ❖ Speakers’ note (PPT): The force generated by contrac on of the fibers is transmi ed through their investment of re cular fibers to the inters al connec ve ssue, and thus spreads throughout the muscle mass, diminishing the size of the organ void its contents. ❖ Ques on by Doc: You wanted to pee and you denied yourself from peeing, what could happen to the Note: Arterioles are smaller arteries. The arterioles are not signal from the brain? Like, the brain sent a signal for en rely covered by muscles, in contrast to arteries that are you to pee, and then this signal was ignored and en rely covered. As seen in the picture above, there are only then later on faded. A er a few minutes, the brain muscle bands in an arteriole, while the artery is covered by sends another signal for you to pee. (Doc did not the tunica media. answer). In the wall of the intes ne, smooth muscle fibers ❖ Ques on by Doc in rela on to the previous form two concentric layers with the fibers of the ques on: In geriatric pa ents, would this be a inner layer oriented circumferen ally and those of problem with smooth or skeletal muscle? Answer: the outer layer longitudinally. Remember, there are voluntary and involuntary Contrac on of the inner layer constricts, and the sphincters. (Doc implies that it could be a problem of outer layer tends to shorten the intes ne. smooth or skeletal muscle). Their coordinated ac on produces peristal c waves of contrac on that are propagated along the intes ne to propel its contents. Variations in Form Fibers in the urinary bladder and ductus deferens are o en branched. Myoepithelial cells have myofibril-like bundles of ac n and myosin filaments. The cell processes embrace the acini of the gland and hormonally induced contrac on serves to expel the milk. Pericytes found in capillaries and postcapillary venules; assumed to be capable of contrac ng to influence blood flow. In hollow organs such as the urinary bladder, smooth muscle is not organized in dis nct layers but interwoven coarse bundles oriented in various direc ons. HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 Speakers’ note (Ayura): Filaments occur in longitudinal or oblique bundles Doc: The myoepithelial cells here are considered as the that converge on, and a ach to, the cytoplasmic smooth muscle because it has a contractile property, same dense bodies and subplasmalemmal dense plaques with the pericytes. to form robust cytoskeletal networks. Doc: The myoepithelial cells are smooth muscles that are not typically elongated. Rarely, spindle-like structures of these cells are captured in cuttings. But most of the time, these muscles are captured/ illustrated as round. Doc: In pathology, related to cancer cells, myoepithelial cells are always observed. Once it is breached, the myoepithelial cells may go out of the epithelial cells. This will be a criteria for the diagnosis of cancer. However, in physiology, Speakers’ note (Ayura): The diagram is how smooth muscle is myoepithelial cells are for contraction and help in secretion of organized. Skeletal muscles are much more organized ( H, M, milk. Z band) than smooth muscles (oblique bundles). The actin and Doc: Pericytes found in capillaries and postcapillary venules myosin in smooth muscles are attached to cytoplasmic dense are assumed to have contractile properties. So yung bodies (the brown circles in the diagram). The brown circles in pinapakita ko sa inyo palagi na fusiform, sa may mga the diagram have the same function with the H band, Z band, capillaries, meron din dun siya contractile properties. of the skeletal muscles. Ultrastructure of Smooth Muscle Short myosin filaments seem to be intercalated Cytoplasm of smooth muscle fibers contains a small within bundles of rela vely long ac n filaments golgi complex, a cluster of elongate mitochondria, a The ra o of ac n to myosin filaments is 12:1 to 14:1 few short profiles of endoplasmic re culum and in smooth muscle. (Compared to 2:1 or 4:1 in many free ribosomes. skeletal muscle). Three kinds of filaments can be resolved: The orienta on of bundles of filaments is ○ Thin ac n filaments 4-8 nm in diameter longitudinal or oblique. This is as opposed to the ○ Thicker myosin filaments 15 nm in diameter lateral myosin filaments responsible for the ○ Intermediate filaments 10 nm cross-banding of skeletal muscle. Desmin – a cytoskeletal protein Myofibrils terminate in the cytoplasmic dense bodies characteris c of muscle and the ac n filaments on the two sides of a dense Vimen n body are of opposite polarity. (equivalent to z-lines of skeletal muscle). In isolated thick filaments, the projec ng heads of the myosin molecules, which form cross-bridges to the surrounding ac n filaments, are found along their en re length. (Thick filaments of skeletal muscle have a bare central segment that is devoid of cross bridges ) Speakers’ note (Ayura): Ques on by Doc: Does the smooth muscle have H band, M band, and Z band? Answer: None. Follow up ques on: So how does it contract? Answer: Sliding also but it does not have an organiza on like H band, M band, A dis nc ve feature of the ultrastructure of smooth and Z band. These organiza ons only exist in striated muscles. muscle fibers is the presence of a great number of Ques on by Doc: What is the func on of the calcium here? vesicular invagina ons of the plasmalemma Answer: It is for contrac on. (caveolae). HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 Contrac ons is slow but it can be sustained for long periods Speaker’s Note(Ayura): Caveolae - where Calcium enters. Close relationship between the caveolae and smooth It can shorten to one-quarter of its res ng length and endoplasmic reticulum is comparable to the can generate a force, per cross sec on area, interaction between T-tubules and sarcoplasmic comparable to that of striated muscle, while it is reticulum consuming less energy. The rela vely slow speed and broad range of Speaker’s Note(Ayura): Skeletal muscles have T-tubules shortening may be a ributable to the fact that the instead of caveolae but their function is the same. All about ac n filaments are considerably longer (4.6 um) than calcium. those of skeletal muscle (1.6 um). The force generated is believed to be transmi ed to the cell surface via the a achment of the contrac le units to the cytoplasmic dense bodies at nodal points in the cytoskeleton. The tropomyosin complex that is ac vated by calcium ions to ini ate contrac on of skeletal muscle is absent in smooth muscle. Instead, calcium diffusing into the cell with depolariza on of the membrane or calcium released from the subplasmalemmal re culum, binds to a protein calmodulin. The complex, so formed, ac vates ATPase in the heads of the myosin molecules and cleavage of ATP induces a conforma onal change in the myosin heads that results in the transloca on of the adjacent ac n filaments. Visceral smooth muscle is also called unitary smooth muscle. ○ Exhibit autorhythmicity. Intrinsically generated s muli are conducted via gap junc ons, from cell to cell throughout a large area of muscle that contracts in unison. This creates a wave of peristalsis in Speaker’s Note(Ayura): Sarcoplasmic reticulum also has the intes ne.Same mechanism occurs in the Calcium ureters and bile duct. Smooth muscles in the arteries, ciliary body and iris Physiology of Smooth Muscle of the eye and wall of the ductus deferens, described Shortening is produced by a similar sliding of ac n as mul unit smooth muscle. filaments with respect to the myosin filaments. ○ Each fiber is innervated and contrac on is rela vely rapid. HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024 HISTOLOGY HISTOLOGY OF MUSCLE TISSUE DR. JOANNA MELISSA F. MARQUEZ, MD, MPH, DPSP Date: September 6, 2024 ○ Post ganglionic autonomic nerve fibers pass over the muscle surface or penetrate into it, synapsing with fibers en passant. URETER Smooth muscles in the walls of blood vessels normally maintain a state of par al contrac on called muscle tonus, but this is subject to modula on by blood-borne hormones. ○ Norepinephrine, angiotensin and vasopressin result in vasoconstric on. ○ Bradykinin and prostaglandin result in loss of muscle tone or vasodila on. Response to hormones is not confined to vascular smooth muscle. ○ In the uterus, estrogen levels change with the menstrual cycle, together with ac n and myosin content of the uterine smooth muscle. HISTOLOGY Histology of Muscle Tissue Date: September 06, 2024