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CHRISTIAN PECULIAR
14/MHS02/020
NURSING SCIENCE
HISTOLOGY OF MUSCLE AS A TISSUE
Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin
and myosin that slide past one another, producing a contraction that changes both the
length and the shape of the cell. Muscles function to produce force and motion. They are
primarily responsible for maintaining and changing posture, locomotion, as well as
movement of internal organs, such as the contraction of the heart and the movement of
food through the digestive system via peristalsis.
Muscle tissues are derived from the mesodermal layer of embryonic germ cells in a process
known as myogenesis. There are three types of muscle, skeletal or striated, cardiac, and
smooth. Muscle action can be classified as being either voluntary or involuntary. Cardiac
and smooth muscles contract without conscious thought and are termed involuntary,
whereas the skeletal muscles contract upon command.[1] Skeletal muscles in turn can be
divided into fast and slow twitch fibers.
Muscles are predominantly powered by the oxidation of fats and carbohydrates, but
anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical
reactions produce adenosine triphosphate (ATP) molecules that are used to power the
movement of the myosin heads.[2]
The term muscle is derived from the Latin musculus meaning "little mouse" perhaps
because of the shape of certain muscles or because contracting muscles look like mice
moving under the skin.
TYPES OF MUSCLE TISSUE
The body contains three types of muscle tissue: (a) skeletal muscle, (b) smooth muscle, and
(c) cardiac muscle.
Muscle tissue is a soft tissue, and is one of the four fundamental types of tissue present in
animals. There are three types of muscle tissue recognized in vertebrates:
Skeletal muscle or "voluntary muscle" is anchored by tendons (or by aponeuroses at a few
places) to bone and is used to effect skeletal movement such as locomotion and in
maintaining posture. Though this postural control is generally maintained as an unconscious
reflex, the muscles responsible react to conscious control like non-postural muscles. An
average adult male is made up of 42% of skeletal muscle and an average adult female is
made up of 36% (as a percentage of body mass).[5]
Smooth muscle or "involuntary muscle" is found within the walls of organs and structures
such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood
vessels, and the arrector pili in the skin (in which it controls erection of body hair). Unlike
skeletal muscle, smooth muscle is not under conscious control.
Cardiac muscle (myocardium), is also an "involuntary muscle" but is more akin in structure
to skeletal muscle, and is found only in the heart.
Cardiac and skeletal muscles are "striated" in that they contain sarcomeres that are packed
into highly regular arrangements of bundles; the myofibrils of smooth muscle cells are not
arranged in sarcomeres and so are not striated. While the sarcomeres in skeletal muscles
are arranged in regular, parallel bundles, cardiac muscle sarcomeres connect at branching,
irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short,
intense bursts, whereas smooth muscle sustains longer or even near-permanent
contractions.
Skeletal (voluntary) muscle is further divided into two broad types: slow twitch and fast
twitch:
Type I, slow twitch, or "red" muscle, is dense with capillaries and is rich in mitochondria and
myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and
sustain aerobic activity using fats or carbohydrates as fuel.[6] Slow twitch fibers contract for
long periods of time but with little force.
Type II, fast twitch muscle, has three major subtypes (IIa, IIx, and IIb) that vary in both
contractile speed [7] and force generated.[6] Fast twitch fibers contract quickly and
powerfully but fatigue very rapidly, sustaining only short, anaerobic bursts of activity before
muscle contraction becomes painful. They contribute most to muscle strength and have
greater potential for increase in mass. Type IIb is anaerobic, glycolytic, "white" muscle that
is least dense in mitochondria and myoglobin. In small animals (e.g., rodents) this is the
major fast muscle type, explaining the pale color of their flesh.
The density of mammalian skeletal muscle tissue is about 1.06 kg/liter.[8] This can be
contrasted with the density of adipose tissue (fat), which is 0.9196 kg/liter.[9] This makes
muscle tissue approximately 15% denser than fat tissue.
LINES AND BANDS OF MUSCLE TISSUE
SKELETAL MUSCLE
Skeletal muscle cells are long cylindrical myocytes that are referred to as fibers or
myofibers. Each fibre is surrounded by an external (=basal) lamina
Origin and syncytial development. Developmentally, myogenic stem cells give rise to
uninucleate myoblasts, which fuse to form multinucleateskeletal muscle cells (myofibers).
Thus, the adult phenotype of skeletal muscle cells is a true morphologic syncytium [Gr. syn
=with, together +cyt- =cell].
i. Dark bands are called A bands
because they are anisotropic (birefringent) in polarized light (i.e., they rotate plane
polarized light). The parallel array of thick filaments are mainly responsible for the LM
appearance of the A band. In the center of this band a paler region, called the H band
[formerly hensen disk], can be seen in relaxed muscle. It represents an area of thick
filaments that are not overlapped by thin filaments (see below). At the middle of the A band
(and the H band) is a middle line called the M line
ii. Light bands are called I bands
because they are isotropic in polarized light (i.e., they do not rotate plane polarized light). A
dark transverse line, called the Z line (Z disk) [Ger. zwischen =between + scheibe =disks],
bisects each I band.
Arrangement of filaments in sacromeres Thick filaments occupy central portions of the
sarcomere; thin filaments attach at each end to the Z lines and run parallel to, and between,
the thick filaments.
I I bands are composed of thin filaments. Each sarcomere has ½ of an I band at its ends.
Thus, a whole I band is shared between adjacent sarcomeres.
ii. A bands are composed mostly of thick filaments and the thin filaments between them.
iii. H bands are composed of only thick filaments, and in relaxed muscle represent the area
between the ends of thin filaments that are attached to each Z line at the other ends. In
contracted muscle, when the thin filaments merge or overlap, the H band disappears.
CARDIAC MUSCLE
Cardiac muscle has cross striations formed by rotating segments of thick and thin protein filaments.
Like skeletal muscle, the primary structural proteins of cardiac muscle are myosin and actin. The actin
filaments are thin, causing the lighter appearance of the I bands in striated muscle, whereas the
myosin filament is thicker, lending a darker appearance to the alternating A bands as observed with
electron microscopy. However, in contrast to skeletal muscle, cardiac muscle cells are typically
branch-like instead of linear.
Another histological difference between cardiac muscle and skeletal muscle is that the T-tubules in
the cardiac muscle are bigger and wider and track laterally to the Z-discs. There are fewer T-tubules
in comparison with skeletal muscle. The diad is a structure in the cardiac myocyte located at the
sarcomere Z-line. It is composed of a single T-tubule paired with a terminal cisterna of the
sarcoplasmic reticulum. The diad plays an important role in excitation-contraction coupling by
juxtaposing an inlet for the action potential near a source of Ca2+ ions.
Under light microscopy, intercalated discs appear as thin, typically dark-staining lines dividing
adjacent cardiac muscle cells. The intercalated discs run perpendicular to the direction of muscle
fibers. Under electron microscopy, an intercalated disc's path appears more complex. At low
magnification, this may appear as a convoluted electron dense structure overlying the location of the
obscured Z-line. At high magnification, the intercalated disc's path appears even more convoluted,
with both longitudinal and transverse areas appearing in longitudinal section.[9]