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The muscle tissue is one of the four types of tissues.
Structure of the muscle tissue
The muscle tissue which is also known as the muscle mass is made up of a large number of
individual muscle cells or myocytes. The muscle cells are commonly called muscle fibers because
these cells are long and slender in appearance. Skeletal muscle fibers are multinucleated and are
arranged parallel to one another with some connective tissue in between. Muscle mass is
separated from the neighboring tissues by a thick fibrous tissue layer known as fascia. Beneath
the fascia, muscle is covered by a connective tissue sheath called epimysium. In the muscle, the
muscle fibers are arranged in various groups called bundles or fasciculi. Connective tissue sheath
that covers each fasciculus is called perimysium. Each muscle fiber is covered by a connective
tissue layer called the endomysium. Embryologically, the muscle tissue emanates from
mesoderm and the mesenchyme.
Function of muscle tissue
1. Contraction for locomotion and skeletal movement: Muscle tissue is the tissue that is
responsible for motion and movement and is a characteristic of multicellular organisms.
In living multicellular organisms the basis for motion mediated by muscle cells is the
conversion of chemical energy (ATP) into mechanical energy by the contractile apparatus
of muscle cells. The contractile apparatuses include the proteins, actin and myosin
[filaments]. It is the interaction of these two proteins that initiates the contraction of
muscle cells, which results in movement, eventually.
2. Contraction for propulsion
3. Contraction for pressure regulation.
The muscle tissue can be divided into three based on their structure and function namely:
1. Skeletal muscle
2. Smooth muscle
3. Cardiac muscle
Also, based on the morphology, muscle tissue can be divided into two namely:
1. Striated muscle
2. Non-striated or smooth muscle
Based on function, we have:
1. Voluntary
2. Involuntary
A. Skeletal muscle
The skeletal muscle tissue is a type of muscle that is protected by the layer of dense connective
tissue called Epimysium. It sends septa, called Perimysium into the muscle tissue dividing the
tissue into fascicles (bundles). Within the fascicles, we have muscle fibres which are surrounded
by a layer of connective tissue, endomysium, rich in reticular fibres and blood vessels.
Embryologically, the skeletal muscle originates from the mesoderm. The muscle-forming cells
called Myoblasts undergo frequent divisions and coalesce with the formation of a
multinucleated, syncytial muscle fibres. The nuclei of the myotube are still located centrally in
the muscle fiber. In the course of the synthesis of the myofilaments and myofibrils, the nuclei are
gradually displaced to the periphery of the cell
Features of the skeletal muscle includes:
1. Skeletal muscle consists of muscle fibers, which are long, cylindrical multinucleated cells
that show cross-striations, hence also called striated Muscle.
2. Skeletal muscle cells nuclei are long, oval and usually found at the periphery of the cell
under the cell membrane This characteristic nuclear location is helpful in discriminating
skeletal muscle from cardiac and smooth muscle, both of which have centrally located
3. Their contraction is quick, forceful, and usually under voluntary control.
4. Skeletal muscles have a well-defined neuromuscular junction.
5. The skeletal muscle is innervated by somatic nerves.
Skeletal muscle type.
Skeletal muscle has three types based on their physiological, biochemical, and
histochemical characteristics namely:
a. Type I or slow, red oxidative fibers contain many mitochondria and abundant
myoglobin, a protein with iron groups that bind O2 and produce a dark red color.
They are adapted for slow, continuous contractions over prolonged periods, as
required for example in the postural muscles of the back.
b. Type IIa or fast, intermediate oxidative-glycolytic fibers have many mitochondria
and much myoglobin, but also have considerable glycogen. They utilize both
oxidative metabolism and anaerobic glycolysis and are intermediate between the
other fiber types both in color and in energy metabolism. They are adapted for
rapid contractions and short bursts of activity, such as those required for
c. Type IIb or fast, white glycolytic fibers have fewer mitochondria and less
myoglobin, but abundant glycogen, making them very pale in color. They depend
largely on glycolysis for energy and are adapted for rapid contractions, but tire
quickly. They are typically small, such as the muscles that move the eyes and
B. Smooth muscle: They are also known as spindle-shaped cells with centrally placed
nuclei. Each of these cells is enclosed by a thin basal lamina and a fine network of
reticular fibers. The connective tissues serve to combine the forces generated by each
smooth muscle fiber into a concerted action, e.g., peristalsis in the intestine.
Note: The largest smooth muscle cells occur in the uterus during pregnancy (12x600 µm). The
smallest are found around small arterioles (1x10 µm).
Smooth muscle contraction process is slow and not subject to voluntary control, hence called the
visceral or involuntary muscle.
Embryologically, Smooth muscle cells arise from undifferentiated mesenchymal cells. These cells
differentiate first into mitotically active cells, myoblasts, which contain a few myofilaments.
Myoblasts give rise to the cells which will differentiate into mature smooth muscle cells.
Features of smooth muscle includes:
Non striated fibers
Smooth muscle fibers are elongated, tapering, fusiform cells
Lack of well-defined neuromuscular junctions.
In addition to contractile activity, smooth muscle cells also synthesize collagen, elastin,
and proteoglycans, extracellular matrix (ECM) components normally synthesized by
C. Cardiac muscle: Cardiac muscle, also known as the myocardium, consists of muscle
cells, the cardiomyocytes, with one centrally placed oval nucleus. Cardiac muscle is
also called involuntary striated muscle.
Although equal in ultrastructure to skeletal muscle, the cross-striations in cardiac
muscle are less distinct, in part because rows of mitochondria (accounting for about
40% of its cytoplasm) and many lipid and glycogen droplets are found between the
myofibrils. Cardiac muscle does not contain cells equivalent to the satellite cells of
skeletal muscle. Therefore cardiac muscle cannot regenerate.
Features of cardiac muscles:
Cardiac muscle has cross-striations and is composed of elongated, branched
individual cells that lie parallel to each other.
Contraction of cardiac muscle is involuntary, vigorous, and rhythmic.
At sites of end-to-end contact are the intercalated disks (which connect the
individual cells and permit the conduction of electrical impulses). These structures
are found only in cardiac muscle.
The cardiac muscle cells are not as long as the skeletal muscle cells.
They may be mononucleated or binucleated. In either cases, the nuclei are located
centrally in the cell.
Clinical correlation:
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. 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 (e.g, 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