Lecture 8Lecture 8 General med_2nd General med_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 its parts

with respect to one another

morphological and functional characteristics common to all types of muscle

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

any cells of the body muscle cells are elongated with well-developed nuclei the cytoplasm (termed as sarcoplasm) stains red with eosin and

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

3 types of muscle tissue are to distinguish on both structural andfunctional basis:

smooth muscle (involuntary or visceral) - occurs chiefly in wall of hollow

organsskeletal muscle (or striated voluntary) - is attached to bones and

fasciaecardiac muscle (or striated involuntary) - forms the wall of the heart

Skeletal muscleSkeletal muscle

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

numerous nuclei are in each fiber (about 35 per 1 mm of its length)they are ovoid, located peripherally at the sarcolemma whose surface is coated with the basal lamina

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

cell organelles are constituted with mitochondria (termed as sarcosomes), sarcoplasmic reticulum (smooth ER), and small Golgi apparatuses in juxtanuclear positions

fibers contain also cell inclusions, mainly glycogen granules and lipiddroplets

sarkotubulessarkotubules

terminal cisternaeterminal cisternae

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

it consists of longitudinal sarcotubules that are confluent with transversally oriented channels called terminal cisternae

pairs of parallel terminal cisternae run transversally across the myofibrils in close apposition to the T tubule

T- tubule is an invagination of the sarcolemma and is not part of the sarcoplasmic reticulum

two terminal cisternae and T tubule constitute structure called as triad of skeletal muscle

triads usually occur in sites where the dark and light bands meet

sarcomere . . .

.

myofibrils - contractile fibrils resolvable with the LM that are mostly randomly dispersed within the sarcoplasmtheir diameter is about 1 -2 m (in cross sections, the myofibrils appear as dots, in longitudinal sections, they show a conspicuous cross banding)

the dark band is called the anisotropic, or A band, because it is doubly refractive to polarized light and appears dark in the fresh statethe light isotropic, or I band, is single refractive to polarized light and is pale in the fresheach I band is intersected by a thin dark line - the Z line, likewise, the dark A band is bisected by a thin, light-staining band, called H band, within it M-line is seen

the segment of myofibril extending between two Z lines is a sarcomere, its length varies from 3.5 to 4 m

Ultrastructure of myofibrilsUltrastructure of myofibrils

by electron microscopy, the myofibrils are composed of more slender filamentous units calledmyofilaments

2 sets of filaments:

thick filaments contain myosin and are 10 nm in diameter and 1.5 m in length

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

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

these thickenings give rise to the transverse density at the midpoint of A band recognized as M line

in cross sections, they are seen to be arranged in a regular hexagonal pattern

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

they are not limited to this band, but extend some distance into the adjacent A bands

where ends of filaments overlap each other, the cross bridges are developed between them

the part of the A band that is free of actin filaments is called the H zone

Molecular components of filamentsMolecular components of filaments

Thick myofilament - 300 to 400 molecules of myosin (asymetric protein consisting of a double heads and tail)

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 sliding filament theory of Huxley

the theory says: as a muscle fiber is stimulated to contract, the actin and myosin filaments react by sliding past each other but with no changes in length of either myofilaments

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

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

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

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

Muscles - structure

CardiacCardiac muscle muscle

cardiac muscle cells, termed a cardiomyocytescardiomyocytes

cardiac muscle occurs in the myocardium (muscle layer

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

heart

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cardiomyocytes: cardiomyocytes: contracting contracting and non non contractingcontracting

Contracting (working) cardiomyocytesContracting (working) cardiomyocytes

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

intercalated discs - sites of intercellular contacts

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

TThe intercalated discs are areas of extensive cell contacthe intercalated discs are areas of extensive cell contactaat sites of intercalated discs, special adhering and communicating junctions aret sites of intercalated discs, special adhering and communicating junctions are

found:found: fascia adherens (corresponds to the zonula adherens), macula adherens fascia adherens (corresponds to the zonula adherens), macula adherens (spot (spot desmosome) desmosome) and nexus (gap junction)and nexus (gap junction)

Non-contracting cardiomyocytesNon-contracting cardiomyocytes

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

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

the cells contain also large quantities of glycogen

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

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

is composed of spindle shaped cells with one nucleus 3–10 µm thick and 20 - 50 µm long - leiomyocytes (gr. leios = smooth)

cell nuclei have a cigar shaped form in dilated cells, but are of corkscrew appearance in cells which are contracted

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

the cytoplasm - called sarcoplasm - appears rather homogeneous and is filled with fine contractile filaments (myofilaments) that are hardly seen

Remember: the spindle-shaped form is imperceptible only in the longitudinal sections of smooth muscle

in the transverse section the smooth muscle cells appear as homogenous discs of various diameters - the largest of the discs are cut through the middle of the cells and usually contain the nucleus

Fine structural parameters of smooth muscle cells:

- nucleus is elongated or cigar- shaped and smoothly contoured

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

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

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

- in certain limited areas of the surface of smooth muscle cells, the basal lamina is lacking and the membranes of neighboring cells come into very close association; such sites are termed as nexuses or gap junctions

they constitute low resistance pathways, permitting a spread of excitation from one cellular unit to another

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

actin filaments are dominantmyosin filaments occur only in a small amount

myofilaments do not exhibit the paracrystalline organization that is seen in the skeletal or cardiac muscle

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

they are innervated with adrenergic and cholinergic nerve fibers that act antagonistically

nucleusnucleus

thick thick filamentfilament

thin filamentsthin filaments

dense bodydense body

intermediate intermediate filamentfilament

Regeneration of muscle tissue

cardiac muscle cells have no regenerative capacity

(defects or damage of heart muscle are generally replaced by proliferation of connective tissue forming myocardial scars)

skeletal muscle fibers (rhabdomyocytes) undergoe regeneration in a limited extent

as a source for regeneration serve so called satellite cells, mononucleated spindle cells located beneth the lasal lamina surrounding mature muscle fiber

(satellite cells are inactive myoblasts that persist after muscle differentiation, cells are activated following injury, they proliferate and fuse to form new skeletal muscle fibers)

smooth muscle cells are capable of a modest regenerative response(following injury, viable mononucleated smooth muscle cells undergo

mitosis and provide for the replacement of the damaged tissue)

Histogenesis of muscle tissue

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

smooth muscle, the most primitive of the three forms of muscle, derives from the embryonic connective tissue - mesenchyme

most skeletal muscles develop from paired mesodermal condensations called ssomitesomites

the middle portion of each somite, the myotome (muscle plate) thickens and differentiates into myoblasts that give rise to

multinucleated cylindrical cells, called muscle fibers

cardiac muscle cells develop from splanchnic mesoderm that surrounds the primitive heart tubes