Transcript BCH 443 Muscle Tissues

BCH 450
Muscle Tissues
Dr. Samina Hyder Haq
Dept of Biochemistry
King Saud university
Functions of Muscle tissues
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Body movement (Locomotion)
Maintenance of posture
Respiration
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Communication (Verbal and Facial)
Constriction of organs and vessels
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Diaphragm and intercostal contractions
Peristalsis of intestinal tract
Vasoconstriction of b.v. and other structures (pupils)
Heart beat
Production of body heat (Thermogenesis)
Types of Muscle Tissue
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Skeletal
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Smooth
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Attached to bones
Makes up 40% of body weight
Responsible for locomotion, facial expressions, posture, respiratory
movements, other types of body movement
Voluntary in action; controlled by somatic motor neurons
In the walls of hollow organs, blood vessels, eye, glands, uterus, skin
Some functions: propel urine, mix food in digestive tract, dilating/constricting
pupils, regulating blood flow,
In some locations, autorhythmic
Controlled involuntarily by endocrine and autonomic nervous systems
Cardiac
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Heart: major source of movement of blood
Autorhythmic
Controlled involuntarily by endocrine and autonomic nervous system.
Connective Tissue of a Muscle
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Epimysium. Dense regular c.t. surrounding entire muscle
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Perimysium. Collagen and elastic fibers surrounding a
group of muscle fibers called a fascicle
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Contains b.v and nerves
Endomysium. Loose connective tissue that surrounds
individual muscle fibers
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Separates muscle from surrounding tissues and organs
Connected to the deep fascia
Also contains b.v., nerves, and satellite cells (embryonic stem
cells function in repair of muscle tissue
Collagen fibers of all 3 layers come together at each
end of muscle to form a tendon or aponeurosis
Skeletal Muscle Structure
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Composed of muscle cells
(fibers), connective tissue,
blood vessels, nerves
Fibers are long, cylindrical,
and multinucleated
Tend to be smaller diameter in
small muscles and larger in
large muscles. 1 mm- 4 cm in
length
Develop from myoblasts;
numbers remain constant
Striated appearance
Nuclei are peripherally located
Muscle Fiber Anatomy
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Sarcolemma - cell membrane
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Surrounds the sarcoplasm (cytoplasm of fiber)
Contains many of the same organelles seen in other cells
An abundance of the oxygen-binding protein myoglobin
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Punctuated by openings called the transverse tubules (T-tubules)
Narrow tubes that extend into the sarcoplasm at right angles to the surface
Filled with extracellular fluid
Myofibrils -cylindrical structures within muscle fiber
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Are bundles of protein filaments (=myofilaments)
Two types of myofilaments
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Actin filaments (thin filaments)
2.
Myosin filaments (thick filaments)
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At each end of the fiber, myofibrils are anchored to the inner surface of the
sarcolemma
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When myofibril shortens, muscle shortens (contracts)
Sarcoplasmic Reticulum (SR
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SR is an elaborate, smooth endoplasmic reticulum
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runs longitudinally and surrounds each myofibril
Form chambers called terminal cisternae on either side of
the T-tubules
A single T-tubule and the 2 terminal cisternae form a
triad
SR stores Ca++ when muscle not contracting
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When stimulated, calcium released into sarcoplasm
SR membrane has Ca++ pumps that function to pump Ca++
out of the sarcoplasm back into the SR after contraction
SARCOLEMMA
Parts of Muscle
Sarcomeres: Z Disk
to Z Disk
Sarcomere - repeating functional units of a
myofibril
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About 10,000 sarcomeres per myofibril, end
to end
Each is about 2 µm long
Differences in size, density, and distribution of
thick and thin filaments gives the muscle
fiber a banded or striated appearance.
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A bands: a dark band; full length of
thick (myosin) filament
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M line - protein to which myosins
attach
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H zone - thick but NO thin filaments
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I bands: a light band; from Z disks to
ends of thick filaments
Thin but NO thick filaments
Extends from A band of one sarcomere
to A band of the next sarcomere
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Z disk: filamentous network of
protein. Serves as attachment for actin
myofilaments
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Titin filaments: elastic chains of amino
acids; keep thick and thin filaments in
proper alignment
Structure of Actin and Myosin
Myosin (Thick)
Myofilament
Many elongated myosin molecules shaped
like golf clubs.
Single filament contains roughly 300
myosin molecules
Molecule consists of two heavy myosin
molecules wound together to form a
rod portion lying parallel to the
myosin myofilament and two heads
that extend laterally.
Myosin heads
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Can bind to active sites on the
actin molecules to form crossbridges. (Actin binding site)
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Attached to the rod portion by a
hinge region that can bend and
straighten during contraction.
3.
Have ATPase activity: activity
that breaks down adenosine
triphosphate (ATP), releasing
energy. Part of the energy is used
to bend the hinge region of the
myosin molecule during
contraction
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Thin Filament: composed of 3 major
proteins
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F (fibrous) actin
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Tropomyosin
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Troponin
Two strands of fibrous (F) actin form
a double helix extending the length of
the myofilament; attached at either
end at sarcomere.
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Composed of G actin monomers
each of which has a myosinbinding site (see yellow dot)
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Actin site can bind myosin during
muscle contraction.
Tropomyosin: an elongated protein
winds along the groove of the F actin
double helix.
Troponin is composed of three
subunits:
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Tn-A : binds to actin
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Tn-T :binds to tropomyosin,
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Tn-C :binds to calcium ions.
Actin (Thin)
Myofilament
Sliding Filament Model of Contraction
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Each myosin head binds and detaches several
times during contraction, acting like a ratchet
to generate tension and propel the thin
filaments to the center of the sarcomere
As this event occurs throughout the
sarcomeres, the muscle shortens
Three states of Muscles
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–Relaxed (low calcium, all Ca+ ions stored
in SR
–Contracting (Release of high calcium from
SR + ATP)
–Rigor (high calcium present in
sarcoplasm, no ATP
The role of Ca+ ions
When a muscle is at rest , Ca+ ions are not present in the sarcoplasm
because they are stored in sarcoplasmic reticulum.
In the absence of Ca + ions in the sarcoplasm, tropomycin prevents
the myosin head from attaching onto actin by blocking the binding
sites.
When a muscle is stimulared sufficiently by nerve impulse, calcium
ions are released from the sarcoplasmic reticulum and combine with
troponin, causing the tropomycin to change shape and unblock the
binding sites.
Ca+ ions are released from the sarcoplasmic reticulum at the end of a
sequence of events which begins when an action potential reaches the
neuromuscular junction.
Neuromuscular Junction
Muscle Contraction
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When the action potential arrives, Ca+ ion channels are opened in the
membrane of the nerve fibre and ca ion diffuses into the synaptic cleft. This
causes synaptic vesicles to move into the junction membrane and fuses with it,
releasing acetylcholine into the synaptic cleft.
This acetylcholine diffuses across the cleft and attaches onto receptor
molecules on the muscle fibre membrane. This leads to graded potential. This
action potential sweeps across the muscle fibre and passes to T tubules which
causes the sarcoplasmic reticulum to release Ca ions into the sarcoplasm. Ca
ions spread through the sarcoplasm. Enabling myosin head to bind onto actin.
Energy from ATP enables the head to take new position.
Ca ions are pumbed back again into the sarcoplasmic reticulum .tropomycin
blocks the myosin head binding site on the actin and the muscle relax.
Neuromuscular Junction
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Region where the motor neuron stimulates the muscle fiber
The neuromuscular junction is formed by :
1. End of motor neuron axon (axon terminal)
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Terminals have small membranous sacs (synaptic vesicles) that
contain the neurotransmitter acetylcholine (ACh)
2. The motor end plate of a muscle
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A specific part of the sarcolemma that contains ACh receptors
Though exceedingly close, axonal ends and muscle fibers are
always separated by a space called the synaptic cleft
Neuromuscular Junction
Figure 9.7 (a-c)
The Nerve-Muscle Functional Unit
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A motor unit is a motor neuron and all the
muscle fibers it supplies
The number of muscle fibers per motor unit
can vary from a few (4-6) to hundreds (12001500)
Muscles that control fine movements (fingers,
eyes) have small motor units
Large weight-bearing muscles (thighs, hips)
have large motor units
Motor Unit
What is rigor mortis
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Rigor mortis is the stiffening of muscles after death.
In both living and dead muscle calcium leaks
through the walls of the muscle fibers, and once
inside, causes the muscle to contract.. Dead muscle
can pump calcium out until it runs out of energy.
Once the dead muscle runs out of energy reserves,
it cannot pump calcium back out. Therefore it stays
contracted. This is called rigor mortis. Over time
the dead muscle contracts more and more until it
becomes quite tight.
Types of muscles
Staining of muscle section with special dyes shows these main type of
muscle fibre.
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Slow twitch fiber(Type 1) slowly contracting red fibre where
aerobic metabolism dominatesmore mitochondria, myoglobin &
capillaries, are resistant to fatigues.
 Fast twitch fiber(Type 11A) fiber of intermediate contractibility
where both aerobic and anaerobic processes.
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Fast twitch fiberType 11B rapidly contracting white fibres
where anaerobic metabolism is the major energy supply. rich in
enzymes for phosphagen & glycogen-lactic acid systems
Energy used in exercise
Aerobic(blue) anaerobic(Yellow)
Slow twitch fibres
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Slow twitch fibres are adapted to function
over long periods. They respire aerobically to
avoid build up of lactic acid. They use muscle
glycogen , as they are aerobic they can also
use the limitless supply of fat stores in the
body.their high content of myoglobib and
good blood supply means they obtain
sufficient oxygen. Large amounts of
mitochondria generate large amount of ATP.
Fast-Twitch fibres
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They are adapted for short burst of explosive
action. They generate ATP quickly and
anaerobically from stores of high energy
compound creatine phosphate and by lactate
fermentation.
Sources of energy Metabolism
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ATP IS the "currency" of energy metabolism.
Muscle contraction, that is coupling between actin
and myosin is powered by ATP (and ONLY ATP).
There is only a small amount of this material in
muscle cells but this is backed up by several buffer
systems. The most rapid of these is the creatine
phosphate/creatine phosphokinase system. This is
also the smallest reserve and at maximum utilization
it is exhausted in about 4 seconds. This is a major
source of high-energy phosphate for sprinters..
Sources of Energy
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The next largest energy source is anaerobic glycolysis.
Only glycogen stored in muscles and blood glucose
can serve as substrates for anaerobic glycolysis. In
quantity, aerobic glycolysis follows, being able to
supply enough energy for muscle activity over several
hours (dependent upon intensity).
Fatty acid oxidation has the largest ATP-producing
capacity. This is relatively slow but can produce
energy over many hours if work intensity corresponds
to the rate of ATP production. It is fascinating to note
that the most rapid sources of energy are also the most
limited
Energy source during exercise
Cori cycle
diagram
Cori cycle or the lactic acid cycle
Links for animation
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http://entochem.tamu.edu/MuscleStrucContra
ctswf/index.html
http://www.blackwellpublishing.com/matthew
s/myosin.html
http://msjensen.cehd.umn.edu/1135/Links/Ani
mations/Flash/0011-swf_breakdown_of_a.swf