There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle. Skeletal muscle is composed of large multinucleated fibers that contract quickly and voluntarily. Cardiac muscle is composed of branched cells connected by intercalated disks that contract involuntarily and rhythmically. Smooth muscle is composed of grouped fusiform cells that contract slowly and involuntarily. Each muscle type has a structure adapted to its function and shows regeneration capacity.

1. Muscle Tissue
Department Of General Histology

2. Introduction
• Muscle tissue is composed of cells differentiated for optimal use of the
universal cell property termed contractility. Microfilaments and associated
proteins together generate the forces necessary for cellular contraction,
which drives movement within certain organs and the body as a whole.
Nearly all muscle cells are of mesodermal origin and they differentiate
mainly by a gradual process of cell lengthening with simultaneous synthesis
of myofibrillar proteins.
• Three types of muscle tissue can be distinguished on the basis of
morphologic and functional characteristics (Figure 10–1) and the structure
of each type is adapted to its physiologic role. Skeletal muscle is
composed of bundles of very long, cylindrical, multinucleated cells that
show cross-striations. Their contraction is quick, forceful, and usually under
voluntary control. It is caused by the interaction of thin actin filaments and
thick myosin filaments whose molecular configuration allows them to slide
upon one another. The forces necessary for sliding are generated by weak
interactions in the bridges between actin and myosin. Cardiac muscle
also has cross-striations and is composed of elongated, branched individual
cells that lie parallel to each other. At sites of end-to-end contact are the
intercalated disks, structures found only in cardiac muscle. Contraction
of cardiac muscle is involuntary, vigorous, and rhythmic. Smooth muscle
consists of collections of fusiform cells that do not show striations. Their
contraction process is slow and not subject to voluntary control.

3. Types of muscle
Skeletal muscle is composed of large,
elongated, multinucleated fibers that show
strong, quick, voluntary contractions.

4. Cardiac muscle is composed of irregular
branched cells bound together
longitudinally by intercalated disks and
shows strong, involuntary contractions.

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6. Smooth muscle is composed of grouped,
fusiform cells with weak, involuntary
contractions. The density of intercellular
packing seen reflects the small amount of
extracellular connective tissue present.

7. Development of skeletal muscle

8. Organization of skeletal muscle
(a): An entire skeletal muscle is
enclosed within a dense connective
tissue layer called the epimysium
continuous with the tendon binding it to
bone.
(b): Each fascicle of muscle fibers is
wrapped in another connective tissue
layer called the perimysium.
(c): Individual muscle fibers (elongated
multinuclear cells) is surrounded by a
very delicate layer called the
endomysium, which includes an external
lamina produced by the muscle fiber
(and enclosing the satellite cells) and
ECM produced by fibroblasts.

9. Skeletal muscle
Micrograph shows a cross section of striated
muscle demonstrating connective tissue and
cell nuclei. The endomysium around individual
muscle fibers is indicated by arrowheads. At left
is a portion of the epimysium. All three of these
tissues contain collagen types I and III
(reticulin).
Adjacent section immunohistochemically stained
for laminin, which specifically stains the external
lamina part of the endomysium produced by the
muscle fibers themselves.

10. Capillaries of skeletal muscle
The blood vessels were injected with plastic polymer before
the muscle was collected and sectioned longitudinally. A rich
network of capillaries in endomysium surrounding muscle
fibers is revealed by this method.

11. Myotendinous junction
Tendons develop together with skeletal muscles and join muscles to the periosteum
of bones. The collagen fibers of tendons are continuous with those in the connective
tissue layers in the muscle, forming a strong unit that allows muscle contraction to
move the skeleton. The longitudinal section shows part of a tendon (T) inserted into
the endomysium and perimysium of a muscle.

12. Striated skeletal muscle in
longitudinal section
Parts of three muscle fibers separated by very small amounts of
endomysium. One fibroblast nucleus (F) is shown. Muscle
nuclei (N) are found against the sarcolemma. Along each fiber
thousands of dark-staining A bands alternate with lighter I
bands.

13. At higher magnification, each fiber can be seen to have three or four
myofibrils, with their striations slightly out-of-alignment with one
another. Myofibrils are cylindrical bundles of thick and thin
myofilaments which fill most of each muscle fiber. The middle of
each I band can be seen to have a darker Z line (or disk). X500.
Giemsa.

14. TEM showing the more electron-dense A bands bisected by a
narrow, less electron-dense region called the H zone and in
the I bands the presence of sarcoplasm with mitochondria
(M), glycogen granules, and small cisternae of SER around
the Z line.

15. Structure of a myofibril: a series of
sarcomeres

16. (a): Diagram indicates that each muscle fiber contains several parallel bundles called myofibrils. (b): Each myofibril consists of a long
series of sarcomeres which contain thick and thin filaments and are separated from one another by Z discs. (c): Thin filaments are
actin filaments with one end bound to -actinin, the major protein of the Z disc. Thick filaments are bundles of myosin, which span
the entire A band and are bound to proteins of the M line and to the Z disc across the I bands by a very large protein called titin,
which has spring-like domains. (d): The molecular organization of the sarcomeres has bands of greater and lesser protein density,
resulting in staining differences that produce the dark and light-staining bands seen by light microscopy and TEM. (e): TEM cross-
sections through different regions of the sarcomere, as shown here, were useful in determining the relationships between thin and
thick myofilaments and other proteins, as shown in part b of this figure. Thin and thick filaments are arranged so that each myosin
bundle contacts six actin filaments.

17. Molecules composing thin and thick
filaments
Each thin filament is composed of F-actin,
tropomyosin, and troponin complexes.

18. Each thick filament consists of many
myosin heavy chain molecules bundled
together along their rod-like tails, with
their heads exposed and directed toward
neighboring thin filaments.

19. Besides interacting with the neighboring thin filaments, thick
myofilament bundles are held in place by less well-
characterized myosin-binding proteins within the M line.

20. Transverse tubule system
TEM shows portions of
two fibers in cross-section
and the intercellular space,
and includes several
transverse or T tubules cut
lengthwise (arrows).

21. TEM of a longitudinal section of skeletal muscle shows T tubules cut transversely
(arrowheads) near the A-I interface, the most common location of T tubules in muscles
of primates. Between the three myofibrils seen shown here is sarcoplasm containing
mitochondria (M) and sarcoplasmic reticulum. Cisternae of this reticulum usually lie
on each side of the transverse tubules, forming the triad of structures responsible for
the cyclic release of Ca2+ from the cisternae and its sequestration again which occurs
during muscle contraction and relaxation. The association between SR cisternae and T
tubules is shown diagrammatically in the next figure.

22. Events of muscle contraction.

24. Sliding filaments and sarcomere
shortening in contraction
In their relaxed state the sarcomere, I band and H zone are at their expanded
length. The spring-like action of titin molecules, which span the I band, help pull
thin and thick filaments past one another in relaxed muscle.

25. The Z discs at the sarcomere boundaries are drawn closer together during
contraction as they move toward the ends of thick filaments in the A band.
Titin molecules are compressed during contraction.

26. At maximal contraction, the H zone and I bands narrow and may disappear
altogether.

27. The neuromuscular junction (NMJ).
Silver staining can reveal
the nerve bundle (NB),
the terminal axonal twigs,
and the motor end plates
(MEP) on striated muscle
fibers (S).

28. A SEM shows the branching ends of a
motor axon, each covered by an extension
of the last Schwann cell and expanded
terminally as a motor end plate embedded
in a groove in the external lamina of the
muscle fiber.

29. Diagram indicating key features
of a typical neuromuscular
junction: synaptic vesicles of
acetylcholine (ACh), a synaptic
cleft, and a postsynaptic
membrane. This membrane, the
sarcolemma, is highly folded to
increase the number of Ach
receptors at the NMJ. Receptor
binding initiates muscle fiber
depolarization, which is carried to
the deeper myofibrils by the T
tubules.

30. Sensory receptors associated with
skeletal muscle
Diagram shows both a muscle spindle
and a tendon organ. Muscle spindles
have afferent sensory and efferent motor
nerve fibers associated with the intrafusal
fibers, which are modified muscle fibers.
The size of the spindle is exaggerated
relative to the extrafusal fibers to show
better the nuclei in the intrafusal fibers.

31. TEM cross-section near the end of a muscle spindle shows the capsule (C), sensory myelinated axons (MA), and the intrafusal muscle
fibers (MF). These thin fibers differ from the ordinary skeletal muscle fibers in having essentially no myofibrils. Their many nuclei can
either be closely aligned (nuclear chain fibers) or piled in a central dilatation (nuclear bag fibers). Satellite cells (SC) are also present
within the external lamina of intrafusal fibers. Muscle spindles detect contraction of neighboring (extrafusal) muscle fibers during body
movement and participate in the nervous control of body posture and the coordinate action of opposing muscles. The tendon organ
collects information about the degree of tension among tendons and relays this data to the CNS, where the information is processed
with that from muscle spindles to protect myotendinous junctions and help coordinate fine muscular contractions.

32. Skeletal muscle fiber types

33. Cardiac muscle

34. Cardiac muscle fibers
Longitudinal sections of
cardiac muscle at the light
microscope level show nuclei
(N) in the center of the
muscle fibers and widely
spaced intercalated discs (I)
that cross the fibers. The
occasional intercalated discs
should not be confused with
the repetitive, much more
closely spaced striations (S),
which are similar to those of
skeletal muscle but less well-
organized. Nuclei of
fibroblasts in the
endomysium are also present.

35. TEM of an intercalated disc
(arrows) shows a steplike
structure representing the short
interdigitating processes of the
adjacent muscle cells.
Transverse regions of the disc
have many desmosomes (D) and
adherent junctions called fascia
adherentes (F), somewhat
similar to the macula adherentes
of epithelial cells. Fascia
adherentes serve as anchoring
sites for actin filaments of the
terminal sarcomeres. Less
electron-dense regions of the
disc have abundant gap
junctions. The sarcoplasm has
numerous mitochondria (M)
and myofibrillar structures
similar to those of skeletal
muscle but slightly less
organized.

36. Cardiac muscle ultrastructure
TEM of cardiac muscle shows an abundance of mitochondria (M) and rather sparse
sarcoplasmic reticulum (SR) in the areas between myofibrils. T tubules are less well-
organized and are usually associated with one expanded terminal cisterna of SR,
forming dyads (D) rather than the triads of skeletal muscle. Functionally these
structures are similar in these two muscle types.

37. Muscle cell from the cardiac atrium
shows the presence of membrane-
bound granules aggregated at the
nuclear poles. These granules are most
abundant in muscle cells of the right
atrium (~600 per cell), but smaller
quantities are also found in the left
atrium and the ventricles. The atrial
granules contain the precursor of a
polypeptide hormone, atrial
natriuretic factor (ANF). ANF
targets cells of the kidneys to bring
about sodium and water loss
(natriuresis and diuresis). This
hormone thus opposes the actions of
aldosterone and antidiuretic hormone,
whose effects on kidneys result in
sodium and water conservation.

38. Smooth muscle
In a cross-section of smooth
muscle in the wall of the small
intestine, cells of the inner
circular (IC) layer are cut
lengthwise and cells of the
outer longitudinal layer (OL)
cross transversely. Only some
nuclei (arrows) of the latter
cells are in the plane of section,
so that many cells appear to be
devoid of nuclei.

39. Section of smooth muscle
in bladder, shows fibers in
cross-section (XS) and
longitudinal section (LS)
with the same fascicle.
There is much collagen in
the branching
perimysium (P), but very
little evidence of
endomysium is apparent.
X140. Mallory trichrome.

40. Section stained only for
reticulin reveals a thin
endomysium around each
fiber, with more reticulin in
the connective tissue of small
arteries (A). Reticulin fibers
in the basal laminae of
smooth muscle cells help hold
the cells together as a
functional unit during the
slow, rhythmic contractions
of this tissue.

41. Smooth muscle ultrastructure
TEM of a transverse section of
smooth muscle showing six or
seven cells sectioned at various
points along their lengths, yielding
profiles of various diameters with
only the largest containing a
nucleus. Thick and thin filaments
are not organized into myofibril
bundles and there are few
mitochondria (M). There is
evidence of a sparse external
lamina around each cell and
reticular fibers are abundant in the
ECM. A small unmyelinated nerve
(N) is also seen between the cells.

42. Longitudinal section showing several dense bodies in the cytoplasm (arrows) and at
the cell membrane. Thin filaments and intermediate filaments both attach to the
dense bodies. In the cytoplasm near the nucleus (N) are mitochondria, glycogen
particles, and Golgi complexes. In the area shown at the lower right, the cell
membrane shows invaginations called caveoli (C) (L. caveoli, little cavities), which
in many cells are indicative of endocytosis, but in smooth muscle cells, where they
are particularly numerous, may also function as the T tubules of skeletal muscle
fibers and regulate release of Ca2+ from sarcoplasmic reticulum.

43. Smooth muscle contraction
The diagram shows thin filaments
attach to dense bodies located in the
cell membrane and deep in the
cytoplasm. Dense bodies contain -
actinin for thin filament attachment.
Dense bodies at the membrane are
also attachment sites for
intermediate filaments and for
adhesive junctions between cells.
This arrangement of both the
cytoskeleton and contractile
apparatus allows the multicellular
tissue to contract as a unit, providing
better efficiency and force.

44. Contraction decreases the
length of the cell, deforming
the nucleus and promoting
contraction of the whole
muscle. The micrograph
shows a region of contracted
tissue in the wall of a urinary
bladder. The long nuclei of
individual fibers assume a
cork-screw shape when the
fibers contract, reflecting the
reduced cell length at this
time.

45. Regeneration of Muscle Tissue
• The three types of adult muscle have different potentials for regeneration after
injury.
• In skeletal muscle, although the nuclei are incapable of undergoing mitosis,
the tissue can undergo limited regeneration. The source of regenerating cells
is the sparse population of mesenchymal satellite cells that lies within the
external lamina of each mature muscle fiber. Satellite cells are inactive,
reserve myoblasts that persist after muscle differentiation. After injury or
certain other stimuli, the normally quiescent satellite cells become activated,
proliferating and fusing to form new skeletal muscle fibers. A similar activity
of satellite cells has been implicated in muscle growth after extensive exercise,
a process in which they fuse with their parent fibers to increase muscle mass
beyond that occurring by cell hypertrophy. The regenerative capacity of
skeletal muscle is limited, however, after major muscle trauma or
degeneration.
• Cardiac muscle lacks satellite cells and has virtually no regenerative capacity
beyond early childhood. Defects or damage (eg, infarcts) in heart muscle are
generally replaced by fibroblast proliferation and growth of connective tissue,
forming myocardial scars. Smooth muscle, composed of simpler,
mononucleated cells, is capable of a more active regenerative response. After
injury, viable smooth muscle cells undergo mitosis and replace the damaged
tissue. Contractile pericytes from the walls of small blood vessels participate
in the repair of vascular smooth muscle.

46. Thank you for attention!