Download Smooth muscle

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 Ttubule into fiber
in the sarcoplasm are numerous myofibrils that
are prominently cross striated with light and dark
bands
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
sarcomere
....
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
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
sarcotubules
terminal cisternae
Ultrastructure of myofibrils
by electron microscopy, the myofibrils consist of more slender filamentous units called
myofilaments
2 sets of filaments:
actin
myosin
 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
 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
myosin
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
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)
ň
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:
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 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