2. Muscle Tissues
• Definition: muscle is a tissue specialized for contraction.
• Always associated with C.T
• Almost always associated with nervous tissue.
• Functions of Muscle
1. Produce movement
2. Maintain posture
3. Stabilize joints
4. Generate heat
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3. The muscular Tissue
The cells that constitute this tissue are referred to as muscle fibers .
Classification of muscle tissue:
Three types of muscles are usually distinguished according to striations into
striated (skeletal and cardiac)and unstriated (smooth).
1. The most familiar is the skeletal ,voluntary .This type is named skeletal muscle
because it is connected to the skeleton and is concerned with body movements.
2. The second type forms the contractile portion of the heart and is therefore called
cardiac muscle .It is striated but involuntary in action. They contract
spontaneously.
3. The third type is also involuntary and because it does not exhibit striations, it is
known as smooth muscle. It is found as part of the walls of the viscera.
5. The muscular Tissue
Each type of muscle has its own particular characteristics, but all types of
muscles exhibit common features which are:
Muscle cells are elongated in shape.
1. The plasma membrane of the muscle fiber is called sarcolemma and its
cytoplasm is called sarcoplasm.
2. Organoids and inclusions are present in the cytoplasm (mitochondria and
smooth ER )which is called sarcoplasmic reticulum. Glycogen is the main
muscle inclusions.
3. Sarcoplasmic reticulum (SR) associated with the initiation and cessation of
muscle contraction .
4. Muscle fibers are mesodermal in origin.
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6. The Skeletal Muscular Tissues
Attached to and covers bony skeleton and Responsible for overall body mobility.
The entire muscle is enclosed by a sheath of dense CT called epimysium .
Thin fibrous septa ,that constitute the perimysium exetnd the epimysium and divide the
muscle into bundles of muscles fibers called fascicles .
The epimysium sends delicate CT sheets to surround individual muscle fibers in each
fascicle. These connective tissue sheets constitute the endomysium.
Blood vessels and nerves penetrate the epimysium to the perimysium and endomysium
but Lymphatic vessels are only present in the epimysium and perimysium.
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7. Skeletal muscle fibers
Long cylindrical cells, enclosed by the sarcolemma.
The fibers are multinucleated ,pale oval nuclei that lie under its sarcolemma
(peripheral situation).
The sarcoplasm is filled with numerous myofibrils.
The myofibrils, The thick filament (myosin) and the thin myofilaments (actin).
Muscle contraction involves the sliding of filaments past each other, stimulated
by nerve impulses.
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10. The muscle fiber and the basis of striations
(The myofibrils)
Two principal types of myofilaments have been identified ;thick and thin ones .
1. The thick filament is composed of a protein called myosin.
2. The thin myofilaments are attached to the Z line ,and are composed of a protein called
actin together with two other associated protein called tropomyosin and troponin.
The above arrangement of thin and thick myofilaments shows that:
• The I-band is formed of thin myofilaments (actin filaments).
• The A-band is formed of thick filaments (myosin) and actin.
• The H-band has myosin filament only
• The M line is a site where fine transverse filaments appear to interconnect the thick myosin
filaments.
Transvese tubules (T-tubules) :
These are tubular invaginations of the sarcolemma that penetrate deep into the interior of
the muscle fiber 10
17. Muscle contraction mechanism
1. Muscle fiber is stimulated to contract.
2. sarcolemma depolarizes (a wave of depolarization passes to the T tubules)
3. In the sarcoplasm, sarcoplasm reticulum (SR) undergoes a sudden
permeability changes that release the stored calcium ions to sarcoplasm.
4. Calcium ions bind to one kind of troponin, causing a change in the
configuration of the troponin complex.
5. Which causes a change in position of the tropomysin which lead to expose
the myosin interacting sites on the actin molecule .
6. This activation induces the myosin heads (levers)to exert a pull on the
actin filament causing it undergo a sliding movement.
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22. Excitation-Contraction Coupling
• Skeletal muscles contraction controlled by the nervous system.
• The transmission of electrical excitation to the actomyosin system is called
Excitation-Contraction Coupling.
• Converting an electrical stimulus (action potential ) to a mechanical response
(contraction).
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24. • At the neuromuscular
junction, the
neurotransmitter released
from the motor neuron is
acetylcholine (ACh).
• The ACh is received by a
nicotinic cholinergic
receptor.
• The nicotinic receptor is a
non-selective cation
channel, that allows Na+ to
enter the cell; Na+ entry
causes depolarization.
Neuromuscular Transmission
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25. Neuromuscular Transmission
Fig. 11.7
• This depolarization, called
an end plate potential, is
an example of an
excitatory postsynaptic
potential (EPSP).
• If the EPSP causes the
area next to the motor
end plate to depolarize to
threshold, an action
potential is generated.
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26. The action potentials on the sarcolemma spread to the
interior of the muscle cell along the transverse tubules 26
27. • T-tubules are tube-shaped invaginations of
the sarcolemma (skeletal muscle plasma
membrane) that penetrate throughout the
muscle fiber.
• The action potential in the muscle fiber is
conducted into the interior of the muscle cell
along the T-tubules.
• The lumen of the T-tubule is continuous with
the extracellular fluid, and the membrane
depolarization during an action potential occurs
across the T-tubule membrane.
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28. How does depolarization in the T-tubule
membrane open a Ca++ channel in the
sarcoplasmic reticulum membrane?
Dihydropyridine receptor
++++
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29. The muscle action potential activates the T-tubules (voltage sensors) the
dihydropyridine (DHP) receptors.
• DHP is a prototypical calcium channel blocker.
• In cardiac and smooth muscle, the DHP receptor is a functional voltage-
gated Ca++ channel.
• In skeletal muscle, the DHP receptor does not function as a Ca++ channel.
It is only a voltage sensor.
The activated DHP receptors cause the calcium release channels (ryanodine
receptors) of the SR to open.
• direct coupling?
• the favored model for skeletal muscle
• via a second messenger (e.g., calcium-induced calcium release)?
• the favored model for cardiac muscle
Calcium enters the cytosol.
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31. End of Excitation
• The ACh lasts only a short time because it is broken down by an
acetylcholinesterase.
• The ACh-esterase is anchored to the postsynaptic membrane.
Fig. 11.10 31
32. 2. Smooth muscle
• Individual smooth appears as spindle shaped cell has a single elongated rod
nucleus, centrally located
• Sarcoplasm is abundant, eosinophilic .
• Does not have clearly defined striations visible on the cells. This is because
smooth muscle cells are organized in a different way than other muscle cells.
• The actin and myosin fibers are arranged an angles to each other as they run
through the cell.
• The actin filaments in smooth muscle run from one side of the cell to the
other, connecting at dense bodies and at the cell membrane.
• In skeletal and cardiac muscle, the actin filaments are attached to Z plates,
which hold many actin filaments and show up as dark bands under the
microscope.
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34. Deference between smooth muscle and skeletal muscle
contraction
• In skeletal muscle, a signal from the somatic nervous system traverses to
the muscle, where it stimulates organelles in the muscle cell to release
calcium. Since there was always available ATP, the myosin uses it to quickly
contract the cell.
• In smooth muscle, the contraction is not controlled voluntarily by the
somatic nervous system, but by signals from the autonomous nervous
system, such as nerve impulses, hormones, and other chemicals released
by specialized organs.
• Smooth muscle is specialized to contract persistently, unlike skeletal muscle
which much contract and release quickly.
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35. Smooth muscle location and function
• Like all muscle tissue, the function of smooth muscle is to contract.
• In the circulatory system, smooth muscle plays a vital role in maintaining
and controlling the blood pressure and flow of oxygen throughout the
body. While the majority of the pressure is applied by the heart, every
vein and artery is lined with smooth muscle.
• lines the majority of the digestive system, such in swallowing process. The
cells on that side contract in reaction, a wave begins to propagate itself
down your digestive tract. This phenomena is known as peristalsis, which
responsible for moving food.
• Smooth muscle is also responsible for contracting the irises, raising the
small hairs on your arm, contracting the many sphincters in your body,
and even moving fluids through organs by applying pressure to them.
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36. 3. Cardiac muscle
• Each muscle fiber is enclosed with a sarcolemma.
• Each muscle fibers composed of several short columnar cellular units join
end to end by special surface specializations, intercalated discs, While
skeletal muscle cells can have multiple nuclei, cardiac muscle cells typically
only have one nucleus (large oval ,central position).
• Sarcoplasm of a cardiac muscle fiber is more abundant and its
mitochondria are much more numerous.
• They branch and faintly striated.
Purkinje muscle fibers:
modified cardiac muscle fibers present in the interventricular septum.
They specialized to conduct the cardiac impulse four and half times faster.
• Contraction of cardiac muscles is spontaneous.
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37. Cont.
• Intercalated discs: are small connections that join cardiac muscle cells
(cardiomyocytes) to each other.
• Gap junction: are part of the intercalated discs. When one cardiac
muscle cell is stimulated to contract, a gap junction transfers the
stimulation to the next cardiac cell. This allows the muscle to contract in
a coordinated way.
• Desmosomes: Like gap junctions, desmosomes are also found within
intercalated discs. They help hold the cardiac muscle fibers together
during a contraction.
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