Download NAME: NIMIBOFA ESENDU MATRIC NUMBER: 14/MHS05/005

NAME: NIMIBOFA ESENDU
MATRIC NUMBER: 14/MHS05/005
NURSING SCIENCE
ANA 203
HISTOLOGY OF MUSCLE AS A TISSUE AND ITS TYPES
Muscle tissue is categorized on the basis of a functional property: the ability of its cells to
contract. In muscle tissue, the bulk of the cytoplasm volume consists of the contractile protein
fibrils actin and myosin. Muscle is responsible for movement of the body and changes in the size
and shape of internal organs. Muscle cells are generally referred to as muscles fibers. The term
fibers is used for both for muscle cells and for the extracellular elements e.g. collagen produced
by connective tissue cells. Muscle fibers are typically arraigned in parallel arrays allowing them
to work together effectively.
TYPES OF TISSUE
Three types of muscle tissue can be identified histologically: skeletal, cardiac and smooth
muscles. The fibers in skeletal muscle and cardiac muscle exhibit cross striations at the light
microscope level and they are both referred to as STRIATED muscles.
SKELETAL MUSCLE
This constitutes the muscle that is attached to the skeleton and control motor movements and
posture. There are a few instances where this type of muscle is restricted to soft tissue: the
tongue, pharynx, diaphragm and upper part esophagus.
Skeletal muscle fibers (cells) are actually a multinucleated syncytium formed by the fusion of
individual small muscle cells or myoblasts, during development. They are filled with
longitudinally arrayed subunits called myofibrils. The myofibrils are made up of the
myofilaments myosin (thick filaments) and actin (thin filaments). The striations reflect the
arrangement of actin and myosin filaments and support structures. The individual contractile
units are called sarcomeres. A myofibril consists of many sarcomeres arranged end to end. The
entire muscle exhibits cross-striations because sarcomeres in adjacent myofibrils and muscle
fibers are in register. The most obvious feature in longitudinal sections of skeletal muscle is the
alternating pattern of dark and light bands, called respectively the A (anisotropic) and I
(isotropic) band. The I band is bisected by a dense zone called the Z line, to which the thin
filaments of the I band are attached.
The nuclei are located peripherally, immediately under the plasma membrane (sarcolemma). The
thickness of individual muscle fibers varies (depending for example on location in the body and
exercise) but each fiber is of uniform thickness throughout its length. Skeletal muscle fibers do
not branch.
Connective tissue elements surround muscle fibers. Individual muscle fibers are surrounded by a
delicate layer of reticular fibers called the endomysium. Groups of fibers are bundled into
fascicles by a thicker CT layer called the perimysium. The collection of fascicles that constitutes
one muscle is surrounded by a sheath of dense CT called the epimysium, which continues into
the tendon. Blood vessels and nerves are found in the CT associated with muscle. The
endomysium contains only capillaries and the finest neuronal branches.
Cardiac muscle
Cardiac muscle is the type of muscle found in the heart, and at the base of the venae cavae as
they enter into the heart. Cardiac muscle is intrinsically contractile but is regulated by autonomic
and hormonal stimuli.
Cardiac muscle exhibits striations because it also has actin and myosin filaments arranged into
sarcomeres. Generally these striations do not appear as well-defined as in skeletal muscle. (At
the ultrastructural level, some differences in the arrangement of the sarcoplasmic reticulum and T
tubules can be seen. Cardiac muscle also has a much greater number of mitochondria in its
cytoplasm. More details on the anatomy and physiology of muscle will be discussed in H&D and
Cardiovascular Blocks.)
At the light microscope level, a number of features distinguish cardiac from skeletal muscle.
Cardiac muscle cells have only one or two nuclei, which are centrally located. The myofibrils
separate to pass around the nucleus, leaving a perinuclear clear area (not always evident in
standard preparations). This clear area is occupied by organelles, especially mitochondria (which
are of course not visible in LM). As in skeletal muscle, individual muscle fibers are surrounded
by delicate connective tissue. Numerous capillaries are found in the connective tissue around
cardiac muscle fibers.
Cardiac muscle cells are joined to one another in a linear array. The boundary between two cells
abutting one another is called an intercalated disc. Intercalated discs consist of several types of
cells junctions whose purpose is to facilitate the passage of an electrical impulse from cell to cell
and to keep the cells bound together during constant contractile activity. Unlike skeletal muscle
fibers, cardiac muscle fibers branch and anastomose with one another. Although made up of
individual fibers, heart muscle acts as a functional syncytium during contraction for the efficient
pumping of blood.
Specialized fibers, called Purkinje fibers, arise from the atrioventricular node and travel along
the interventricular septum toward the apex of the heart, sending branches into the ventricular
tissue. Purkinje fibers are of larger diameter than ordinary cardiac fibers, with fewer myofibrils
and an extensive, well-defined clear area around the nucleus. They conduct impulses at a rate
about four times faster than that of ordinary cardiac fibers and serve to coordinate the contraction
of the atria and ventri
Smooth muscle
Smooth muscle is the intrinsic muscle of the internal organs and blood vessels. It is also found in
the iris and ciliary body of the eye and associated with hair follicles (arrector pili). No striations
are present in smooth muscle due to the different arrangement of actin and myosin filaments.
Like cardiac muscle, smooth muscle fibers are intrinsically contractile but responsive to
autonomic and hormonal stimuli. They are specialized for slow, prolonged contraction.
Smooth muscle fibers are generally arranged in bundles or sheets. Each fiber is fusiform in shape
with a thicker central portion and tapered at both ends. The single nucleus is located in the
central part of the fiber. Fibres do not branch. They range enormously in size, from 20 (in wall of
small blood vessels) to 500 (in wall of uterus during pregnancy) micrometers. Smooth muscle
fibers lie over one another in a staggered fashion (tapered part of one fiber over thicker part of
another). In longitudinal sections, it is often not possible to distinguish the fiber boundaries, and
smooth muscle may closely resemble connective tissue (bundles of collagen). Where smooth
muscle bundles are interlaced with bundles of connective tissue (e.g. in the uterus), one can
distinguish the smooth muscle by the orientation of the nuclei (all oriented in the same direction),
and the greater abundance of nuclei per unit area (every smooth muscle cell has a nucleus,
fibroblast nuclei are more scattered in bundles of CT). Also, smooth muscle nuclei often have a
corkscrew shape in longitudinal section due to contraction of the muscle fiber during fixation. In
cross section, smooth muscle appears as profiles of various sizes, depending on whether the cut
went through the thick central part or tapered end of any individual fiber. Nuclei are seen only in
the thicker profiles.
One distinguishing physiological feature of smooth muscle is its ability to secrete connective
tissue matrix. In the walls of blood vessels and the uterus in particular, smooth muscle fibers
secrete large amounts of collagen and elastin
LINES AND BANDS OF MUSCLE TISSUE
Light microscopic studies show that, each myofibril consists of a number of two alternating
bands which are also called the sections, segments or disks. the two bands are:
1. Light band or 'I' band.
2. Dark band or 'A' band.
LIGHT BAND OR 'I' BAND
The light band is isotropic in nature. When the polarized light is passed through the muscle fiber
at this area the light rays are refracted at the same angle. So, this band is called 'I' (isotropic)
band.
DARK BAND OR 'A' BAND
The dark band is anisotropic in nature. When the polarized light is passed through the muscle
fiber at this area, the light rays are refracted at different directions (An=not; iso=it;
trops=turning). So, this band is otherwise called 'A' (anisotropic) band. Dark band is also called
Q disk(Querscheibe=cross disk).
In an intact muscle fiber, 'I' band and 'A' band of the adjacent myofibrils are placed side-by-side.
It gives the appearance of characteristics cross striations in the muscle fiber.
I band is divided into two portions by means of a narrow and dark line called 'Z' line or 'Z' disk
(in German zwischenscheibe=between disks). The 'Z' line is formed by a protein disk which does
not permit passage of light. The portion of myofibril in between two 'Z' lines is called sarcomere.
Within the A-band is a paler region called the H-zone (from the German "heller", brighter).
Named for their lighter appearance under a polarization microscope. H-band is the zone of the
thick filaments that is not superimposed by the thin filaments.
Inside the H-zone is a thin M-line (from the German "Mittelscheibe", the disc in the middle of
the sarcomere) formed of cross-connecting elements of the cytoskeleton.