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Muscle Tissue
Ch. 9
Three types of muscle tissue
1. Skeletal:
- Voluntary
- Multi-nucleated cells with nuclei on the periphery
- Found usually attached to long bones
- Short contractions
- Quick twitch
- Many mitochondria.
2. Smooth muscle:
- Involuntary
- Spindle shaped cells
- No striations
- Single ovoid nucleus per cell
- Found in blood vessels
- walls of hollow organs and GI tract and is associated
with peristalsis; long contractions; slow twitch
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3. Cardiac muscle:
- Involuntary
- striated
- Single nucleus per cell
- Intercalated discs
- Found only in the heart.
Skeletal Muscle
Smooth Muscle
Cardiac Muscle
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Muscle tissue is derived from the mesoderm
layer (middle germ layer)
Characteristics of muscle tissue:
1. Excitability (irritability): ability to respond to
certain stimuli
2. Contractibility: ability to shorten and thicken
3. Extensibility: able to stretch and extend without
damage
4. Elasticity: ability to return to original shape after
being stretched
Functions of muscle tissue
1. Movement: muscle pushes against bone for
locomotion, the heart pumps blood through
the blood vessels, urinary bladder empties
2. Thermogenesis: 85% of body heat is produced
by skeletal muscle contraction
3. Maintenance of posture: constant contractions
to keep us sitting or standing straight.
4. Stabilize joints: as muscles pull on bone they
help to strengthen joints.
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Organization of muscle tissue
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Muscles are covered in connective tissue
wrappings called epimysium
Bundle of muscles cells are termed a fascicle.
This fascicle is covered in perimysium
Muscle (cell) fiber is covered in endomysium.
Muscle fibers are composed of elongated
myofibrils
Basic contracting unit of a myofibril is sarcomere
Microscopic anatomy of a skeletal muscle fiber
Muscle cell is also called a muscle fiber
Muscle fiber contains large number of rod shaped
myofibrils.
Myofibrils contain the contractile units called
sarcomeres.
Structures:
a. Sarcolemma: plasma membrane
b. Sarcoplasm: cytoplasm containing stored sugars
(glycogen), mitochondria and myoglobin (O2
binding pigment in muscle).
c. Sarcoplasmic reticulum - smooth ER
forming interconnecting tubules
surrounding myofibrils.
d. Transverse (T) tubules - tubules running
between sarcoplasmic reticulum and
penetrating deeply into cell; conducts
"stimulus" into cell.
e. Terminal cisternae - terminal portions of
sarcoplasmic reticulum adjacent to
transverse tubules.
f. Triads: one T-tubule and two terminal
cisterna
Sarcomere anatomy:
1.
A bands: area overlapping myosin and
actin filaments
2.
I bands: contains actin filaments only
3.
Z discs (lines): separates sarcomeres
and anchors the thin filaments.
4.
H zone: part of the A band that
contains only myosin fibers.
5.
M line: center of the H zone that holds
the myosin fibers in place.
Myofibril filaments:
Thick filaments: myosin containing a tail and two
(globular proteins) heads. Heads interact with
thinner filaments called actin.
Thin filaments: composed of actin.
- tropomyosin: two strands of protein that
spiral around the actin filament.
- troponin: contains three subunits that
helps bind calcium.
Sliding filament theory of contraction
1954: Hugh Huxley
The sacromeres shorten and the distance
between Z lines is reduced
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Motor unit: motor neuron (somatic
nervous system) plus all of the muscle
fibers it innervated.
Point of innervation is termed
neuromuscular junction.
When motor neuron fires, all muscle fibers
innervated by that motor neuron will
contract
Anatomy of the neuromuscular junction:
Terminal axon: axonal ending of the motor
neuron
Synaptic cleft: space between the terminal
axon and the sarcolemma of the muscle
fiber
Synaptic vesicle: vesicles located in the
terminal axonal bud contain the
neurotransmitter acetylcholine (ACh)
Motor end plate: section of sarcolemma that
is folded upon itself. Millions of ACh
receptors are found in the folds.
Initiation of the muscle contraction
I) Nerve impulse reaches the terminal axon and
Ca++ is allowed to enter via voltage gated
calcium channels. Ca++ floods in from the
extracellular fluid.
II) Calcium triggers vesicles of ACh to fuse with
the axonal membrane and release (exocytosis)
Ach into the synaptic cleft.
III) ACh binds to ACh receptors on the
sarcolemma and creates an impulse that travels
throughout the sarcolemma and down T-tubules
IV) To prevent excessive contraction an enzyme,
acetylcholinesterase, breaks down ACh into
acetic acid and choline thus stopping the flow of
Ach into the binding sites located on
sarcolemma.
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V) Resting sarcolemma is polarized (voltage)
(-) on the inside, (+) on the outside
Na+ normally is not allowed to enter cell
VI) When ACh binds to receptors, gated ion
channels allow Na+ to flood into the cell
while K+ diffuses out. More Na+ is pumped
in than K+ leaving. This creates an
electrical charge across the membrane:
depolarization
VII) Depolarization sets off an action
potential (propagation) down the
membrane surface.
VIII) Repolarization is resetting the
membrane surface back to normal.
Na+ channels close while K+ channels
remain open (K+ continues pump outside).
The refractory period is when the muscle
fiber is insensitive to further stimulation
until repolarization is complete.
Action potentials are considered an all or
none response because once initiated,
they are unstoppable.
Excitation-Contraction coupling: Electrical impulse
does not act directly on the myofibrils. They
stimulate CA++ to be released from within
muscle cell.
 From motor neuron the action potential travels
along to axon terminal; here ACh is released
causing depolarization of the motor end plate;
action potential propagates along sarcolemma
down T tubules.
 Action potential triggers Ca++ release from
terminal cisternae of sarcoplasmic reticulum
(calcium ion gates open).
 Calcium ions bind to troponin causing troponin
to change shape and expose actin active sites.
 Contraction occurs
 Calcium levels decrease (due to change in
permeability) and tropomyosin blockage is
restored
Muscle twitch:
Myogram can record the phases of contraction
of a muscle fiber.
Single action potential acting on a motor unit is a
muscle twitch.
1. Latent period: muscle tension begins to
increase, but contraction has not occurred.
2. Contraction period: muscle shortens
demonstrating actin/mysoin activity
3. Relaxation period: Muscle fiber returns to
resting state.