Lecture 8
General med_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 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 and
functional basis:
smooth muscle (involuntary or visceral) - occurs chiefly in wall of hollow
organs
skeletal muscle (or striated voluntary) - is attached to bones and
fasciae
cardiac muscle (or striated involuntary) - forms the wall of the heart
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 mm
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 lipid
droplets
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
sarkotubules
terminal cisternae
myofibrils - contractile fibrils resolvable with the LM that are mostly randomly dispersed
within the sarcoplasm
their diameter is about 1 -2 mm (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 mm
sarcomere
....
Ultrastructure of myofibrils
by electron microscopy, the myofibrils are composed of more slender filamentous units called
myofilaments
2 sets of filaments:
 thick filaments
contain myosin and are
10 nm in diameter and
1.5 mm in length
 thin filaments
contain protein actin
and are 5 nm in
diameter and 2 mm 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 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
Cardiac muscle
cardiac muscle cells, termed a cardiomyocytes
cardiac muscle occurs in the myocardium (muscle layer
of the heart) and in the walls of large vessels joining the
heart
ň
cardiomyocytes: contracting and non contracting
Contracting (working) cardiomyocytes
cells are of cylindrical form, 20 mm in diameter and 50 to 200 mm 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 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 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 mm (in
the walls of small ducts and blood vessels)
to 500 mm (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:
nucleus
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
thick filament
dense body
thin filaments
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 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
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