Lecture 6 Generalmedicine_2nd semester

Muscle tissue

General characteristics of muscle tissue and its

classification

Skeletal muscle tissue. Ultrastructure of the

myofibrils. Mechanism of the muscle contraction

Cardiac 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 develop and retain contractility to the highest level of any cells of the body

muscle cells have elongated shape and 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 and functional basis:

skeletal muscle (or striated voluntary)

- is attached to bones and fasciae skeletal muscle- rhabdomyocyte

smooth muscle (involuntary or visceral)

- in walls of hollow organs smooth muscle cell - leiomyocyte

cardiac muscle (or striated involuntary)

- forms the wall of the heart cardiomycyte

Skeletal 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 um

each fiber contains numerous nuclei (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 sarcolemma projects long, finger-like processes called transverse tubules or T- tubule into fiber

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

sarcomere . . . .

myofibrils - contractile fibrils resolvable with the LM - mostly randomly dispersed within the sarcoplasm their diameter is about 1 -2 um (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 state the light isotropic, or I band, is single refractive to polarized light and is pale in the fresh each 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

cell organelles are constituted by mitochondria (termed as sarcosomes),

sarcoplasmic reticulum (smooth ER), and small Golgi apparatuses

in juxtanuclear positions

muscle fibers contain also cell inclusions, mainly glycogen granules and

lipid droplets

an organelle of great importance is the sarcoplasmic reticulum that store calcium

sarcotubules terminal cisternae

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 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

Ultrastructure of myofibrils

by electron microscopy, the myofibrils consist of more slender filamentous units called myofilaments

2 sets of filaments:

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

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

actin myosin

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 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

actin myosin

Molecular 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 distances troponin: of 3 proteins troponin I, C and T

Mechanism of contraction

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

Innervation of rhabodomyocytes = motor end plates (40–60 um)

acetylcholine

Cardiac muscle

cardiac muscle cells = cardiomyocytes

cardiac muscle forms the myocardium (muscle layer

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

heart (large veins)

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cardiomyocytes: contracting and non contracting

Contracting (working) cardiomyocytes cells are of cylindrical form, 20 um in diameter and 50 to 200 um in length cardiac muscle cells are joined end to end and form a linear unit - cardiac muscle fiber, fiber is transsected with zones called 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 found myofibrils 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

the intercalated discs are areas of extensive cell contact at sites of intercalated discs, special adhering and communicating junctions are found: fascia adherens (corresponds to the zonula adherens), macula adherens (spot desmosome) and nexus (gap junction)

Non-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 fibers

Smooth muscle smooth 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 tubular organs is composed of spindle shaped cells with one nucleus 3–10 um thick and 20 - 50 um 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 um (in the walls of small ducts and blood vessels) to 500 um (in the pregnant human uterus) the cytoplasm - called sarcoplasm - appears 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 on transverse sections 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 characteristics 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 dominant myosin 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

nucleus

thick filament

thin filaments

dense body

intermediate filament

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 basal 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 the 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 somites

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