HISTOLOGY
LECTURE # 9
MUSCLE TISSUE
Rationale:
The field of Histotechnology is a field of understanding and developing a deep
care for patient. It is our job to analyze and prepare the tissue samples for diagnostic which
lead to treatment. We need to understand tissue at a cellular level, their structures and
functionality. Tissue component will be discussed and observed at smaller scales compare to an
anatomy class
Students are prepared to understand each tissue, their structures and functions. This will be a
beginning to learn each tissue observed under the microscope.
Objective:
Once completed this lecture, the student should be able to:
a) Distinguish the three types of muscle tissue in mammals.
b) Learn the organization of skeletal, cardiac and smooth muscle.
c) Identify the mechanism of muscle contraction.
d) Understand the relationship between muscle and energy production.
e) Learn the regeneration of muscle tissue.
MUSCLE TISSUE
Muscle tissue is composed of differentiated cells contractile proteins. The structural biology of
these proteins generate the forces necessary for cellular contraction which drives movement
within certain organs and the body as a whole. Most muscle cells are of mesodermal origin, and
they are differentiated mainly by a gradual process of lengthening, which simultaneous synthesis
of myofibrillar proteins.
Three types of muscle tissue in mammals can be distinguished on the basis of morphologic and
functional characteristics, and each type of muscle tissue has a structure adapted to its
physiologic role.
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Skeletal muscle is composed of bundles of very long, cvlindrical, multinucleated cells that show
cross~straitions. 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 that bind actin to myosin. Cardiac muscle also has
cross-striations and it composed of elongated, branched individual cells that lie parallel to each
other. At sites of end-to-end contract 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 in the light microscope. Their
contraction process is slow and not subject to voluntary control.
Some muscle cell organelles have names that differ from their counterparts in other cells. The
cytoplasm of muscle cells (excluding the myofibrils) is called sarcoplasm (Gr. sarkas, flesh, +
plasma, thing formed), and the smooth endoplasmic reticulum is called sarcoplasmic reticulum.
The sarcolemma (sarkas + Gr. lemma, husk) is the cell membrane, or plasmalemma.
SKELETAL MUSCLE (Striated Muscle)
Skeletal muscle consists of muscle fibers, bundles of very long (up to 30 cm) cylindrical
multinucleated cells with a diameter of 10-100 µm. Multinucleation results from the fusion of
embryonic mononucleated myoblasts (muscle cell precursors). The oval nuclei are usually found
at the periphery of the cell under the cell membrane. This characteristic nuclear location is helpful in distinguishing skeletal muscle from cardiac and smooth muscle, both of which have
centrally located nuclei.
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A.
Longitudinal cut of skeletal Muscle
Sections are cut longwise where the bands of the striation are visible.
B.
Cross section cut of skeletal Muscle
Sections are cross section in order to visualize the muscle fibers.
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CARDIAC MUSCLE (Striated Muscle)
Cardiac muscle (heart muscle), like skeletal muscle, is also striated but involuntary muscle
responsible for the pumping activity of the vertebrate heart. The individual muscle cells are
joined through a junctional complex known as the intercalated disc and are not fused together
into multinucleate structures as they are in skeletal muscle.
Though unlike skeletal, cardiac muscle cells are short and branched with a single, centered
nucleus. They are also involuntary or not under immediate conscious control. Rather than Zdisks, which join skeletal muscle cells, intercalated disks join cardiac muscle fibers.
Cardiac muscles are located only in the heart. Unlike skeletal, cardiac muscle can contract
without extrinsic nerve or hormonal stimulation. It contracts via its own specialized conducting
network within the heart, with nerve stimulation causing only an increase or decrease in rate of
conducting discharge. The heart also has some very beneficial features such as an increased
number and larger mitochondria, which allow it to produce more ATP. This is very important
since the heart is constantly contracting and relaxing. Cardiac muscle can also convert lactic acid
produced by skeletal muscle to ATP. This is quite ingenious since lactic acid is a by-product of
muscle when in a deoxygenated state, a state that would be detrimental to cardiac muscle. This
muscle also remains contracted 10 to 15 times longer than skeletal muscle due to a prolonged
delivery of calcium (see discussion of cardiac action potential in Circulation section). Likewise,
it also has a relatively long refractory period, lasting several tenths of a second, allowing heart to
relax between beats. This also allows heart rate to increase significantly without causing it to go
into tetanus, which would be fatal since it would cause blood flow to cease.
Intercalated Disk
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SMOOTH MUSCLE
Smooth muscle is responsible for the contractility of hollow organs, such as blood vessels, the
gastrointestinal tract, the bladder, or the uterus. Its structure differs greatly from that of skeletal
muscle, although it can develop isometric force per cross-sectional area that is equal to that of
skeletal muscle. However, the speed of smooth muscle contraction is only a small fraction of that
of skeletal muscle.
The most striking feature of smooth muscle is the lack of visible cross striations (hence the name
smooth). Smooth muscle fibers are much smaller (2-10 m in diameter) than skeletal muscle
fibers (10-100 m). It is customary to classify smooth muscle as single-unit and multi-unit smooth
muscle. The fibers are assembled in different ways. The muscle fibers making up the single-unit
muscle are gathered into dense sheets or bands. Though the fibers run roughly parallel, they are
densely and irregularly packed together, most often so that the narrower portion of one fiber lies
against the wider portion of its neighbor. These fibers have connections, the plasma membranes
of two neighboring fibers form gap junctions that act as low resistance pathway for the rapid
spread of electrical signals throughout the tissue. The multi-unit smooth muscle fibers have no
interconnecting bridges. They are mingled with connective tissue fibers.
REGENERATION OF MUSCLE TISSUE
The three types of adult muscle have different potentials for regeneration after injury.
1.
Cardiac Muscle – has virtually no regenerative capacity beyond early childhood. Defects
or damages (e.g. Infarcts) in heart muscle are generally replaced by the proliferation of
connective tissue, forming myocardial scars.
2.
Skeletal Muscle – the nuclei are incapable of undergoing mitosis, the tissue can undergo
limited regeneration. The source of regenerating cells is believed to be the satellite cells.
3.
Smooth Muscle – is capable of an active regenerative response. After injury, viable
mononucleated smooth muscle cells and pericytes from blood vessels undergo mitosis
and provide for the replacement of the damaged tissue.
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