1. Muscle is composed of excitable cells called muscle fibers that contain contractile filaments of actin and myosin which allow the cell to change shape. 2. Muscle fibers are classified as skeletal, smooth, or cardiac muscle depending on their structure and location in the body. 3. Skeletal muscle fibers are long, cylindrical, and multinucleated, containing repetitive contractile units called sarcomeres made up of bands of actin and myosin proteins.

1. Muscle
Microanatomy
Dr. Komal Parmar

2. Introduction
• Excitable tissue. Forms the major component of soft tissue in
body.
• Muscle cells, like neurons, can be excited chemically,
electrically, and mechanically to produce an action potential
that is transmitted along their cell membranes.
• Unlike neurons, they respond to stimuli by activating a
contractile mechanism.
• Possess cytoskeletal elements that are capable of lengthening
or shortening and so enable the cell to change its shape.
• The cytoplasm of muscle cells is called sarcoplasm and the
surrounding cell membrane or plasmalemma is called
sarcolemma
• Each muscle fiber sarcoplasm contains numerous myofibrils,
which contain two types of contractile protein filaments, actin
and myosin.

3. Terminology Muscle fibre:
Individual Celll
Myofibril: Long chains
of contractile proteins

4. Classification Of Muscle

5. Skeletal muscle found throughout
the body is derived from the
paraxial mesenchyme, which is
formed from ingression at the
streak and subsequently segmented
into somites.
Skeletal muscle originates from a
pool of premyoblastic cells which
arise in the dermatomyotome of
the maturing somite and begin to
differentiate into myoblasts at 4–5
weeks of gestation.
Smooth muscle cells developed in
situ from the splanchnopleuric
mesenchyme in the walls of the
viscera.
Cardiac myocytes differentiate from
the splanchnic coelomic cells of
the pericardium initially subjacent
to the endoderm.

7. Skeletal Muscle
• Skeletal muscle fibers are long, cylindrical,multinucleated
cells,with peripheral nuclei.
• Organization:
Epimysium: dense,
irregular connective
tissue
Perimysium: dense,
irregular connective
tissue
Endomysium:
reticular connective
tissue

9. LONGITUDINAL
SECTION

13. Striations and Sarcomere
• Differences in the refractive indexes of the various parts of
the muscle fiber are responsible for the characteristic cross-
striations seen in skeletal muscle when viewed under the
microscope.
• The parts of the cross-striations are frequently identified by
letters. The light I band is divided by the dark Z line, and the
dark A band has the lighter H band in its center.
• A transverse M line is seen in the middle of the H band, and
this line plus the narrow light areas on either side of it are
sometimes called the pseudo-H zone.
• The area between two adjacent Z lines is called a sarcomere.
• The orderly arrangement of actin, myosin, and related
proteins that produces this pattern as shown

14. Motor and Cytoskeletal Proteins
• Motor proteins are class of molecular motors that are able to move
along the surface of a suitable substrate. They convert chemical
energy into mechanical work by the hydrolysis of ATP.
• Actin filaments (microfilaments): Cytoskeletal
• Actin filaments are flexible fi laments with a width of 8 nm and a
solid cross-section
• The filaments are formed by the ATP-dependent polymerization of
actin monomer
• The polymerized form is termed F-actin (fibrillar actin) and the
unpolymerized form is G-actin (globular actin).

15. • A wide variety of Actin-binding Proteins are capable of
modulating the form of actin within the cell.
• Types:
• Bundling proteins: Tie actin filaments together in longitudinal
arrays to form cables or core structures. E.g. fimbrin, villin,
myosin
• Gel-forming proteins: Interconnect adjacent actin filaments to
produce loose filamentous meshworks (gels) composed of randomly
orientated F-actin.
• Filament severing proteins: Severing proteins, such as gelsolin and
severin, bind to F-actin fi laments and sever them, which produces
profound changes within the actin cytoskeleton and in its coupling to the
cell surface.
• Membrane-associated proteins

16. Myosins – The Motor Proteins
• Myosin proteins have a globular head region consisting of a
heavy and a light chain. The heavy chain bears an α-helical tail
of varying length. The head has an ATPase activity and can
bind to and move along actin fi laments – the basis for myosin
function as a motor protein.
• The best-known class is myosin II

18. Sarcotubular System
• The muscle fibrils are surrounded by structures made up of
membranes
• These structures form the sarcotubular system, which is made
up of a T system and a sarcoplasmic reticulum.
• The T system of transverse tubules, which is continuous with
the sarcolemma of the muscle fiber, forms a grid perforated by
the individual muscle fibrils.
The sarcoplasmic
reticulum, which forms an
irregular curtain around
each of the fibrils, has
enlarged terminal cisterns
in close contact with the T
system

20. Motor End Plate and Muscle
Spindle

21. Skeletal Muscle-Tendon Linkage
The epimysial, perimysial and
endomysial sheaths coalesce where
the muscles connect to adjacent
structures at tendons,
aponeuroses, and fasciae.
There are no desmosomal
attachments at these
myotendinous junctions,
Actin filaments from the adjacent
sarcomeres, which would normally
insert into a Z-disc at this point,
instead penetrate a dense,
subsarcolemmal filamentous matrix
that provides attachment
to the plasma membrane.
At the extracellular surface of the junctional
sarcolemma, integrins provide contact
with the basal lamina which in turn adheres
closely to collagen and reticular fibres (type
III collagen) of the adjacent tendon or other
connective tissue structure.

22. Circumferential Myofibrils
Striated muscle fibers may in some
places be encircled by thin spiral
muscle fibers. These peculiar
structures are called
circumferential myofibrils
(Ringbinden).
They are thought to be signs of
degradation.

23. Smooth Muscles
• In smooth muscle tissue the contractile proteins actin and
myosin are not organized into regular sarcomeres, visible as
transverse striations.
• Involuntary
• Much smaller in size
• The nucleus is single, located at the midpoint, and often twisted
into a corkscrew shape by the contraction of the cell.
• Typically found in the walls of tubular structures and hollow
viscera
• Smooth muscle has no attachment structures equivalent to the
fasciae, tendons and aponeuroses associated with skeletal
muscle.
• Each cell is covered almost entirely by a prominent basal lamina
which merges with a reticular layer consisting of a network of
fine elastin, reticular fibres (collagen type III) and type I collagen
fibres.

24. The cell attaches to components
of this extracellular matrix at
dense plaques
cell–cell attachment occurs at
intermediate junctions or
desmosomes, formed of two
adjacent dense plaques.

26. Microstructure

32. Cardiac Muscle Cell
• In cardiac muscle, as in skeletal muscle, the contractile
proteins are organized structurally into sarcomeres.
• fine cross-striations that are visible in the light microscope.

35. Intercalated disc