Muscle Tissue Slides C 20152016

8/19/2019 Muscle Tissue Slides C 20152016

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Muscle TissueThe muscle tissue is an assemblage of cells which

are highly specialized in contractility by virtue of their cellular contents of contractile structural

proteins.

The contractile ability of muscle cells is responsible

for movements in some organs and the body as awhole.

Muscle cells are derived from the embryonic

mesoderm through a process of cellular elongationaccompanied by intracellular synthesis of

myofibrilar proteins.

Muscle tissue is widely distributed all over the body.


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Classification of muscles (See Fig M1)

Muscle cells are classified into four main types

based on structural and functional properties of the

cells.These include:

1. Striated skeletal muscle: Striated skeletal muscleusually exist in bundles which are made up oflong, cylindrical and multinucleated cells. Theyalso posses cross striations under the lightmicroscope and are often attached to bones(skeleton). They are multinucleated and

voluntarily controlled2. Striated non-skeletal muscle: This muscle type

share the same properties with striated skeletalmuscle except that they are not attached to

bones and are not under voluntary control.


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3. Cardiac muscle: This muscle type also exists in

bundles, posses cross striations, long,

cylindrical, multinucleated and branching cells.

The cells (fibres) run parallel to one another and

are connected end-to-end by intercalated disks.

Cardiac muscles are not under voluntary control

and are restricted to the heart and some veins.4. Smooth Muscle: Smooth muscles are made up of

bundles of small, fusiform, and mononucleated

cell which have no cross striation under light

microscopic view. This muscle type is widelydistributed in the internal organs and is not

under voluntary control.


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Other Characteristic features of the

basic types of muscles are best

appreciated by a tabular

comparison of the muscles as

follows:


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Internal Organization of the Skeletal Muscle

(See Fig M3A & M3B):

• A skeletal muscle fibre may be up to 35 cm in length

with a diameter ranging from 10 to 100 micron

• Under the light microscope, a longitudinal section of a

skeletal muscle fibre show cross-striation which

comprises alternating dark and light bands.• The dark bands are referred to as the

A band (Anisotropic band) and

•

The Light band are the I band (Isotropic band).• Under the electron microscopic view, each I band is

divided in the middle by a transverse line called the

Z line.


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• The dark bands are also formed by Myosin

contractile proteins while the light bands are

formed by Actin contractile proteins.

• The contractile apparatus of the muscle is

subdivided into subunit referred to as the

SARCOMERE

• The distance from one Z line to the next one

constitutes the Sarcomere of the skeletal

muscle (See Diagram).

• Sarcomeres are also interconnected at the Z

line by the proteins α-actinin and desmin.


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• Under the electron microscope, the A band

is also divided in the middle by a transverse

line called the M Line. This is the region at

which adjacent myosin are interconnectedlaterally. It is also formed mainly by creatine

kinase a protein involved in the synthesis of

ATP.• There are two light zones on either side of

the M line. These collectively constitute the

H band within the A band.• In this region there is no overlap between

myosin and actin (See Fig. M3).


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Contractile Proteins of the Striated muscle

(See Fig. M4):

•

Myosin and Actin are the principal proteinsof the myofibrils constituting 55% of

myofibrillar proteins. (Ratio 1-6)

•

While Myosin forms the thick filaments,Actin forms the thin filament.

• Two other contractile proteins associated

with the thin filaments are:• Tropomyosin (40 nm long) and

• Troponin (Three globular subunits)


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The Tubular systems of the Skeletal Muscle

(See Fig. M5A & M5B):

The tubular systems of the striated musclecomprise the:

• The transverse (T) tubules: These are finger-

like invaginations of the sarcolemma whichform tubular network around the myofibril at

the junction the light and dark bands. They

serve as channels through which electricalimpulse is propagated from the exterior to

the interior of the muscle cell.

S l i R ti l t b l


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Sarcoplasmic Reticulum tubules:

• The smooth sarcoplasmic reticulum of the

muscle cell form terminal cisternae on either

side of the T tubular network leading to the

establishment of a system of three tubules

referred to as the Triad complex of tubules.

• The sarcoplasmic tubules contain calcium

ions which are released from the tubules on

stimulation by the impulse passing along

the T tubular network (See Diagram).


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I t l O i ti f th C di


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Internal Organization of the Cardiac

Muscle (See Fig. M6A & M6B):•

Cardiac muscle has an average diameter of 15µand length range of 85-100µ.

• Most cells are mononucleated but a few are

binucleated.

• In all cases the nuclei are centrally located.

• Cardiac muscle consists of branching muscle

fibres and adjacent fibres are connected end to

end via these branches by intercalated disks.• Intercalated disks are specialized junctional

complexes which are peculiar to cardiac muscle.

Th d f th t i


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They are made up of three components viz.

• Fasciae Adherentes which serve as anchoring

site for actin fibres

• Maculae Adherentes which bind cardiac muscle

fibres together and

•

Gap Junctions which provide channels for ionicand electric flow between cardiac muscle cells

(fibres).

• The arrangement of myofibrils in the cardiac

muscle is similar to the arrangement in skeletal

muscle.


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• Other Contents of the sarcoplasm of

cardiac muscle:

• Glycogen: This is the energy source of

the muscle and is stored in granules.

• Myoglobin: This is an oxygen binding

protein similar to hemoglobin in blood. It

imparts dark red color on the muscle


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Cardiac Tubular System:

•

The T-tubular systems are larger and morenumerous compared with skeletal muscle.

• However, only one sarcoplasmic reticulartubule is associated with the T-tubule thus a

diads tubular system is established in thecardiac muscular system.


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Other features of the sracoplasm of cardiac

muscle include:

• Abundance of mitochondria which constitute

40% of the sracoplasm

• Large deposit of fatty acids as glyceride as

the major fuel of cardiac muscle

• Small amount of glycogen

• Granules of lipofuscin pigments

1. Granules of the precursor of Atrial

natriuretic factor which is stored in large amount(around the nucleus) in the right atrium and less

quantity in other cardiac muscles.


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Internal Organization of the Smooth Muscle

(Fig. M7A & 7B)

•The length of the spindle shaped smoothmuscle cell varies from one organ to the other.

In the vascular wall it is about 20µ while in the

gravid uterus it could be as long as 500µ.• Smooth muscle cell is surrounded by a basal

lamina and a network of reticular fibres which

ensures that a bundle of smooth muscle

contract in unison.

• Smooth muscle lacks T-tubular system but

widely distributed sarcoplasmic reticulum.


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• The myofibrils which comprise myosin actin andtropomyosin are arranged in a crisscrosspattern.

• The contractile filaments are attached to focaldensities in the sracoplasm and to attachmentdensities on the sarcolemma.

•

The mechanism of contraction of actin andmyosin filament is similar to the mechanism inskeletal muscle.

• Intermediate filaments called desmin are also

found in the smooth muscle.• Electrical and ionic communication between

smooth muscle cells are effected through Nexus(Gap) junctions.


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Other sarcoplasmic contents of the smooth

muscle include:

• Calmodulin: This is a calcium binding

protein which forms a complex with calcium

ions. The complex so formed activates the

interaction between myosin and actin.

• Mitochondria

• Polyribosomes

• Rough endoplasmic reticulum

• Golgi complex

•

Pinocytotic vesicles


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Regeneration of muscle tissue:

• Striated skeletal and non-skeletal muscles

increase in size and number by proliferationof inactive myoblasts called satellite cells.

• Smooth muscles also regenerate from

mononucleated cells.• Cardiac muscles are incapable of

regeneration

The Neuromuscular Junction (See Fig M8):


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The Neuromuscular Junction (See Fig. M8):

• This is the contact point between the muscle and its

nerve supply. It is also referred to as the

Myoneuronal junction or Motor end plate:The principal features of this structure are:

• Demyelination of the synaptic bulb

•

Synaptic Cleft (gap) containing neurotransmitter • Synaptic junctional folds on the sarcolemma

• Presynaptic vesicles containing neurotransmitter

•

Pre and Post synaptic abundance of Mitochondria• Several nuclei, ribosomes and glycogen granules

beneath the junctional folds

• Neurotransmitter receptors on the sarcolemma


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Muscle Tissue Slides C 20152016