Lecture 8Lecture 8 General medicine_2nd General medicine_2nd semestersemester

Muscle tissueMuscle tissue

General characteristics of muscle tissue and itsGeneral characteristics of muscle tissue and itsclassificationclassification

Skeletal muscle tissue. Ultrastructure of theSkeletal muscle tissue. Ultrastructure of themyofibrils. Mechanism of the muscle myofibrils. Mechanism of the muscle

contractioncontraction

Cardiac muscle tissueCardiac muscle tissue

Smooth muscle tissue Smooth muscle tissue

function of muscle tissue is to move both the body as whole and itsfunction of muscle tissue is to move both the body as whole and its parts parts

with respect to one anotherwith respect to one another

mmorphological and functional characteristics common to all types of orphological and functional characteristics common to all types of muscle muscle

cells are as follows:cells are as follows: muscle cells retain and develop contractility to the highest extent of any muscle cells retain and develop contractility to the highest extent of any

cells of the bodycells of the body muscle muscle cells are elongated with well-developed nucleicells are elongated with well-developed nuclei the cytoplasm (termed as the cytoplasm (termed as sarcoplasmsarcoplasm) stains red with eosin and contains ) stains red with eosin and contains

contractile proteins - actin and myosin- arranged in the form of thin contractile proteins - actin and myosin- arranged in the form of thin fibers, called fibers, called myofibrilsmyofibrils that run parallel to the long axis of the cell; the that run parallel to the long axis of the cell; the plasma membrane is termed as plasma membrane is termed as sarcolemmasarcolemma

33 types of muscle tissue are to distinguish on both a structural and a types of muscle tissue are to distinguish on both a structural and afunctional basis:functional basis:

smooth musclesmooth muscle ((involuntary or visceralinvoluntary or visceral) ) - - occursoccurs chiefly in chiefly in walls of walls of hollowhollow

organsorgansskeletal muscleskeletal muscle ((oror striated voluntarystriated voluntary)) - is attached to bones and- is attached to bones and

fasciae, skinfasciae, skincardiac musclecardiac muscle ((or striated involuntaryor striated involuntary)) - form- formss the wall of the the wall of the heart CONTACTING/NONCONTRACTINGheart CONTACTING/NONCONTRACTING

Skeletal muscleSkeletal muscle

giant multinucleated cylindrical cellgiant multinucleated cylindrical cellss referred as referred as = = muscle fibers ormuscle fibers or rhabdomyocytes rhabdomyocytes ((gr. rhabdos = rod) rhabdos = rod)the length - mm to cm, the length - mm to cm, thickness varies from 10 to 100 thickness varies from 10 to 100 mm

nnuclei are numerous in each fiber, there being about 35 per uclei are numerous in each fiber, there being about 35 per 1 1 mm of mm of lengthlengththethey are ovoid, y are ovoid, localocated peripherally ted peripherally atat the sarcolemma the sarcolemma whosewhose surface is surface is coated withcoated with the the basal lamina basal lamina

thethe sarcoplasm contains numerous sarcoplasm contains numerous myofibrils myofibrils that are prominently cross that are prominently cross striated with light and dark bandsstriated with light and dark bands

cecell organelles are ll organelles are constitutedconstitutedwith with mitochondriamitochondria (termed (termed as sarcosomes), as sarcosomes), sarcoplasmic reticulumsarcoplasmic reticulum ((smooth ER), and smooth ER), and small small Golgi apparatusesGolgi apparatuses in juxtanuclear positionsin juxtanuclear positions

Cells,fibers, also contain cell Cells,fibers, also contain cell inclusions, mainly inclusions, mainly glycogen granules glycogen granules and and LipidLipid droplets droplets

sarkotubulessarkotubules

terminal cisternaeterminal cisternae

an an organelle organelle of great of great importance importance is the is the sarcoplasmic reticulumsarcoplasmic reticulum, a , a special type of sERspecial type of sER

iit consists of longitudinal t consists of longitudinal sarcotubulessarcotubules thatthat are are confluent with transversally confluent with transversally oriented channels called oriented channels called terminal cisternaeterminal cisternae

ppairs of parallel terminal airs of parallel terminal cisternae run transversally cisternae run transversally across the myofibrils in close across the myofibrils in close apposition to the T tubuleapposition to the T tubule

T tubule is an T tubule is an invagination of the invagination of the sarcolemmasarcolemma andand is not part of the is not part of the sarcoplasmic reticulumsarcoplasmic reticulum

ttwo terminal cisternae and T wo terminal cisternae and T tubule constitute tubule constitute structurestructure called called as as triad of skeletal triad of skeletal musclemuscle

ttriads usually occur in sites riads usually occur in sites where the dark and light bands where the dark and light bands meetmeet

sarcomere . . .

.

mmyofibrils yofibrils -- contractile fibrils contractile fibrils resolvable with the LMresolvable with the LM that are mostly randomly that are mostly randomly dispersed within the sarcoplasmdispersed within the sarcoplasmttheir diameter is about 1 -2 heir diameter is about 1 -2 mm (in (in cross sections, the myofibrils appear as dots, in cross sections, the myofibrils appear as dots, in longitudinal sections, they show a conspicuous cross bandinglongitudinal sections, they show a conspicuous cross banding))

tthe dark band is called the he dark band is called the anisotropicanisotropic, , or or A bandA band, because it is doubly refractive , because it is doubly refractive to polarized light and appers dark in the fresh stateto polarized light and appers dark in the fresh statetthe light he light isotropicisotropic, , oror I I band band,, is single refractive to polarized light and is pale in the is single refractive to polarized light and is pale in the freshfresheeach I band is intersected by a thin dark line - the ach I band is intersected by a thin dark line - the Z lineZ line, likewise, , likewise, the dark A bandthe dark A band is bisected by a thin, light-staining band, called is bisected by a thin, light-staining band, called H bandH band, within it , within it M-M-lineline is seen is seen

tthe segment of myofibril extending between two Z lines is he segment of myofibril extending between two Z lines is a a sarcomeresarcomere, its length , its length varies from 3.5 to 4 varies from 3.5 to 4 mm

Ultrastructure of myofibrilsUltrastructure of myofibrils

bby electron microscopy, the myofibrils are composed of more slender filamentous unitsy electron microscopy, the myofibrils are composed of more slender filamentous units called calledmyofilamentsmyofilaments

2 sets of filaments:2 sets of filaments:

tthe thick he thick filaments filaments contain contain myosinmyosin and are 10 and are 10 nm in diameter and nm in diameter and 1.5 1.5 m in lengthm in length

thethe thin thin filaments filaments contain contain protein actinprotein actin and are and are 5 nm in diameter 5 nm in diameter and 2 and 2 m in lengthm in length

Myosin filamentsMyosin filaments are are confined to the A band and confined to the A band and show slight thickening at show slight thickening at their centerstheir centers

ththese thickenings give rise to ese thickenings give rise to the transverse density at the the transverse density at the midpoint of A band midpoint of A band recognized as M linerecognized as M lineinin cross sections, they are cross sections, they are seen to be arranged in a seen to be arranged in a regular hexagonal patternregular hexagonal pattern

Actin filamentsActin filaments extend in extend in either direction from the Z either direction from the Z line and thus constitute the I line and thus constitute the I bandband

tthey are not limited to this hey are not limited to this band, but extend some band, but extend some distance into the adjacent A distance into the adjacent A bandsbands

where ends of filaments where ends of filaments overlap each other, the overlap each other, the cross bridges are cross bridges are developeddeveloped between between themthem

tthe part of the A band he part of the A band that that is is free of actin filaments is free of actin filaments is calledcalled the the H zoneH zone

Molecular components of filamentsMolecular components of filaments

Thick myofilament - 300 to 400 molecules of myosin (asymetric protein consisting of two identical heavy chains and two pairs of light chains

Thin myofilament - from F-actin + protein complex composed of troponin and tropomyosin

tropomyosin runs around F-actin in 40 nm distancestroponin: of 3 proteins Troponin I, C and T

Mechanism of contractioncontraction

by the by the sliding filament theorysliding filament theory of Huxley of Huxley

this concept this concept sayssays: : as a muscle fiber is stimulated to contract, the actin and myosin as a muscle fiber is stimulated to contract, the actin and myosin filaments react by filaments react by sliding past each othersliding past each other but with but with no changeno changess in in length of either myofilamentslength of either myofilaments

the thick the thick myosin filaments in the A band are relatively myosin filaments in the A band are relatively stationarystationary, whereas the , whereas the thin actin filaments, attached to the Z thin actin filaments, attached to the Z disks, extend further into the A band disks, extend further into the A band and may eventually and may eventually obliterate the H zoneobliterate the H zone

bbecause the thin filaments are attached to Z disks, the disk are drawn ecause the thin filaments are attached to Z disks, the disk are drawn toward each other, the sarcomeres are compressed, the myofibril is toward each other, the sarcomeres are compressed, the myofibril is shortened, and contraction of the muscle occursshortened, and contraction of the muscle occurs

NoteNote:: contraction is not due to a shortening of the actin and myosin filaments contraction is not due to a shortening of the actin and myosin filaments but to an increase in the overlap between the filamentsbut to an increase in the overlap between the filaments

eenergy for the contraction process is produced by the breakdown of nergy for the contraction process is produced by the breakdown of ATP by ATPase localized in the cross bridges that interconnect the ATP by ATPase localized in the cross bridges that interconnect the actin and myosin filamentsactin and myosin filaments

Muscles - structure

CardiacCardiac muscle muscle

cardiac muscle cells, termedcardiac muscle cells, termed a a cardiomyocytescardiomyocytes

ccardiac muscle occurs in the myocardium (muscle layer

of the heart) and in the walls of large vessels joining the

heart

ňň

cardiomyocytes: cardiomyocytes: contracting and non contracting and non contractingcontracting

Contracting (working) cardiomyocytes

cells are of cylindrical form, 20 cells are of cylindrical form, 20 m in diameter and 50 to 200 m in diameter and 50 to 200 m in m in lengthlength atat specialized junctional zones called specialized junctional zones called intercalated discsintercalated discs, cardiac muscle , cardiac muscle cells arecells are joined end to end and form a linear unit - joined end to end and form a linear unit - cardiac muscle fibercardiac muscle fiber

intercalated discsintercalated discs - sites of intercellular contacts

thethe cardiomyocyte is enveloped by a thin sarcolemma and contains cardiomyocyte is enveloped by a thin sarcolemma and contains one elongated nucleus one elongated nucleus situated centrally between diverging myofibrilssituated centrally between diverging myofibrils, in, in its vicinity a small Golgi apparatus, lipid its vicinity a small Golgi apparatus, lipid droplets and deposits of lipofuscin pigment droplets and deposits of lipofuscin pigment areare found foundmyofibrils are oriented parallel and run from one end to anothermyofibrils are oriented parallel and run from one end to another and and show the cross show the cross banding, the sarcomeres are composed of the same parts as in skeletal muscle banding, the sarcomeres are composed of the same parts as in skeletal muscle (A, I (A, I bands, Z, M lines, H zone)bands, Z, M lines, H zone)nnumerous mitochondria arranged in rows and sarcoplasmic reticulum are seen among the umerous mitochondria arranged in rows and sarcoplasmic reticulum are seen among the myofibrilsmyofibrils

TThehe intercalated discs are areas of extensive cell contact intercalated discs are areas of extensive cell contactaat the sites of intercalated discs, special adhering and communicating t the sites of intercalated discs, special adhering and communicating

junctions are junctions are fascia adherens (corresponds to the zonula adherens,belt fascia adherens (corresponds to the zonula adherens,belt desmozone.), macula adherens (spotdesmozone.), macula adherens (spot desmosome) and nexus (gap desmosome) and nexus (gap junction)junction)

Non-contracting cardiomyocytes (subendocardial layer, develops from cardiac jelly)

in addition to the cardiomyocytes whose primary function is contraction, n addition to the cardiomyocytes whose primary function is contraction, there are specialized cells whose function is to generate the stimuli for the there are specialized cells whose function is to generate the stimuli for the heart beatheart beat cells cells are located just beneath the endocardium,subendocardial layer.) and are located just beneath the endocardium,subendocardial layer.) and in a whole represent impuls conducting system of the heart (sinoatrial in a whole represent impuls conducting system of the heart (sinoatrial node - node of Keith-Flack, atrio- ventricular node - node of Tawara, node - node of Keith-Flack, atrio- ventricular node - node of Tawara, atrioventricular bundle - bundle of Hiss)atrioventricular bundle - bundle of Hiss)

bby light microscopy, cells of y light microscopy, cells of the the conducting systemconducting system are larger, thicker and are larger, thicker and more palely stained, with abundant central sarcoplasm and relatively few more palely stained, with abundant central sarcoplasm and relatively few myofibrils, located usually in a peripheral positionmyofibrils, located usually in a peripheral position

tthe cells contain also large quantities of glycogenhe cells contain also large quantities of glycogen

noncontracting noncontracting cells form similar to contractile cells form similar to contractile cardiac muscle cells fibers cardiac muscle cells fibers called called Purkinje fibersPurkinje fibers

Smooth muscleSmooth musclessmooth muscle tissue is structurally the simplest of the muscle typesmooth muscle tissue is structurally the simplest of the muscle types, i, it is called smooth t is called smooth because it has no visible crossbecause it has no visible cross striation, involuntary because it is not under conscious control, striation, involuntary because it is not under conscious control, and visceral because it is predominantly found in organsand visceral because it is predominantly found in organs

is composed of spindle shaped cells with one nucleus is composed of spindle shaped cells with one nucleus 3–10 µm thick and 20 - 500 µm long - leiomyocytes (gr. leios = smooth)

ccell nuclei have a cigar shaped form ell nuclei have a cigar shaped form in dilated cells, but are of corkscrew in dilated cells, but are of corkscrew appearance in cells which are appearance in cells which are contractedcontracted

cells vary greatly in length, from 20 cells vary greatly in length, from 20 m (in the walls of small ducts and m (in the walls of small ducts and blood vessels) to 500 blood vessels) to 500 m (in the m (in the pregnant human uterus)pregnant human uterus)

tthe cytoplasm - called he cytoplasm - called sarcoplasmsarcoplasm - - appears rather homogeneous and is appears rather homogeneous and is filled with fine contractile filaments filled with fine contractile filaments (myofilaments) (myofilaments) that that are hardly seenare hardly seen

RRememberemember:: the spindle-shaped form is the spindle-shaped form is imperceptible only in the imperceptible only in the longitudinal sections of smooth longitudinal sections of smooth musclemuscle

iin the transverse section the smooth n the transverse section the smooth muscle cells appear as homogenous muscle cells appear as homogenous discs of various diametersdiscs of various diameters - t - the he largest of the discs are cut through largest of the discs are cut through the middle of the cells and usually the middle of the cells and usually containcontain the nucleus the nucleus

The fine structural parameters of smooth The fine structural parameters of smooth muscle muscle cells are as follows:cells are as follows:

•- nucleus is elongated or cigar- shaped and - nucleus is elongated or cigar- shaped and smoothly contouredsmoothly contoured

•- juxtanuclear sarcoplasm contains long - juxtanuclear sarcoplasm contains long slender mitochondria, a few tubules of smooth slender mitochondria, a few tubules of smooth endoplasmic reticulum, free ribosomes and endoplasmic reticulum, free ribosomes and small Golgi apparatussmall Golgi apparatus

•- the plasma membrane is characteristically - the plasma membrane is characteristically studded with small vesicular inpocketings or studded with small vesicular inpocketings or caveoli (micropinocytosis)caveoli (micropinocytosis)

•- the surface of each smooth muscle cell is - the surface of each smooth muscle cell is invested by a thick extracellular coating that invested by a thick extracellular coating that corresponds to the basal lamina of epithelial corresponds to the basal lamina of epithelial cellscells

•- in certain limited areas of the surface of - in certain limited areas of the surface of smooth muscle cells, the basal lamina is smooth muscle cells, the basal lamina is lacking and the membranes of neighboring lacking and the membranes of neighboring cells come into very close association; such cells come into very close association; such sites are termed as sites are termed as nexuses or gap nexuses or gap junctionsjunctions •they constitute low resistance pathways, they constitute low resistance pathways, permitting a spread of excitation from one permitting a spread of excitation from one cellular unit to anothercellular unit to another

the bulk of the cytoplasm is occupied the bulk of the cytoplasm is occupied by two sets of myofilamentsby two sets of myofilaments::

actin filaments are dominantactin filaments are dominantmyosin filaments myosin filaments occur occur only in a only in a small amountsmall amount

myofilaments do not exhibit the myofilaments do not exhibit the paracrystalline organizationparacrystalline organization thatthat is seen in the skeletal or cardiac is seen in the skeletal or cardiac musclemuscle

Physiology:Physiology: smooth mucle cells contract smooth mucle cells contract slowly and have low force of slowly and have low force of contractioncontraction

tthey are innervated with hey are innervated with adrenergic and cholinergic nerve adrenergic and cholinergic nerve fibers that act antagonisticallyfibers that act antagonistically

nucleusnucleus

thick thick filamentfilament

thin filamentsthin filaments

dense bodydense body

intermediate intermediate filamentfilament

Regeneration of muscle tissueRegeneration of muscle tissue

ccardiac muscleardiac muscle cells cells hahaveve no regenerative capacity no regenerative capacity(d(defects or damage of heart muscle are generally replaced by proliferation of efects or damage of heart muscle are generally replaced by proliferation of

connective tissue forming myocardial scarsconnective tissue forming myocardial scars))

rhabdomyocytesrhabdomyocytes undergoes regeneration in a limited extent undergoes regeneration in a limited extent

as a as a source of regenerating cells source of regenerating cells serveserve the the satellite cellssatellite cells, , mononucleated spindle cells located beneth the lasal lamina mononucleated spindle cells located beneth the lasal lamina surrounding mature muscle fibersurrounding mature muscle fiber(s(satellite cells are inactive myoblasts that persist after muscle differentiationatellite cells are inactive myoblasts that persist after muscle differentiation, cells, cells are are activated following injury, they proliferate and fuse to form new skeletal muscle fibersactivated following injury, they proliferate and fuse to form new skeletal muscle fibers))

ssmooth muscle mooth muscle cellscells are are capable of a modest regenerative capable of a modest regenerative responseresponse

(f(following injury, viable mononucleated smooth muscle cells undergo ollowing injury, viable mononucleated smooth muscle cells undergo mitosis and provide for the replacement of the damaged tissuemitosis and provide for the replacement of the damaged tissue))

Histogenesis of muscle tissueHistogenesis of muscle tissue

inin vertebrates, muscles develop from vertebrates, muscles develop from mesodermmesoderm, except some , except some intrinsic muscles of the eye and skinintrinsic muscles of the eye and skin that that are ectodermal in origin are ectodermal in origin

ssmooth musclemooth muscle, the most primitive of the three forms of muscle, , the most primitive of the three forms of muscle, derivesderives from from thethe(spnachic?)(spnachic?) embryonic connective tissue - embryonic connective tissue - mesenchymemesenchyme

mmost ost skeletal muscleskeletal muscless develop from the paired mesodermal develop from the paired mesodermal somitessomites

tthe middle portion of the somite, the myotome (muscle plate) he middle portion of the somite, the myotome (muscle plate) thickens and differentiates into myoblasts thickens and differentiates into myoblasts thatthat give rise to give rise to multinucleated cylindrical cells, called muscle fibersmultinucleated cylindrical cells, called muscle fibers

ccardiac muscle cellsardiac muscle cells develop from splanchnic mesoderm that develop from splanchnic mesoderm that surrounds the primitive heart tubessurrounds the primitive heart tubes (MYOEPICARDIAL MANTLE- (MYOEPICARDIAL MANTLE-MYOBLASTS.)MYOBLASTS.)