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Muscle contraction

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Title: Muscle contraction


1
Muscle contraction
2
The term "excitation-contraction coupling" refers
to the mechanism by which the action potential
causes the myofibrils of muscle to contract
3
  • Each muscle fiber behaves as
  • a single unit, is multinucleate,
  • and contains myofibrils.
  • The myofibrils are surrounded by
  • sarcoplasmic reticulum and are
  • invaginated by
  • transverse tubules (T tubules).
  • Each myofibril contains interdigitating thick and
    thin filaments, which are arranged longitudinally
    and cross-sectionally in sarcomeres
  • The repeating units of sarcomeres account for the
    unique banding pattern seen in striated muscle
    (which includes both skeletal and cardiac muscle

4
  • surrounding the myofibrils of each muscle fiber
    is an extensive reticulum called the sarcoplasmic
    reticulum.
  • This reticulum has a special organization that is
    extremely important in controlling muscle
    contraction .

5
Transverse Tubules and the Sarcoplasmic Reticulum
The transverse (T) tubules are an extensive
network of muscle cell membrane (sarcolemmal
membrane) that invaginates deep into the muscle
fiber. The T tubules are responsible for carrying
depolarization from action potentials at the
muscle cell surface to the interior of the fiber.
The sarcoplasmic reticulum is an internal tubular
structure, which is the site of storage and
release of Ca2 for excitation-contraction
coupling .
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  • the myosin heads have an actin-binding site,
    which is necessary for cross-bridge formation,
    and a site that binds and hydrolyzes ATP (myosin
    ATPase(
  • Actin has myosin-binding sites. When the muscle
    is at rest, the myosin-binding sites are covered
    by tropomyosin so that actin and myosin cannot
    interact.
  • If contraction is to occur, tropomyosin must be
    moved out of the way so that actin and myosin can
    interact.

9
  • the tropomyosin molecules lie on top of the
    active sites of the actin strands so that
    attraction cannot occur between the actin and
    myosin filaments to cause contraction.
  • When calcium ions combine with troponin C, the
    troponin complex supposedly undergoes a
    conformational change that in some way "uncovers"
    the active sites of the actin, thus allowing
    these to attract the myosin cross-bridge heads
    and cause contraction to proceed.

10
It is the interaction between these cross-bridges
of myocin and the actin filaments that causes
contraction.
11
Arrangement of Thick and Thin Filaments in
Sarcomeres
The sarcomere is the basic contractile unit, and
it is delineated by the Z disks. Each sarcomere
contains a full A band in the center and one
half of two I bands on either side of the A band
12
at sarcomere length of about 2 micrometers the
muscle is capable of generating its greatest
force of contraction.
13
The initiation and execution of muscle
contraction occur in the following sequential
steps.
1- An action potential travels along a motor
nerve to its endings on muscle fibers
2- At each ending, the nerve secretes a small
amount of the neurotransmitter substance
acetylcholine.
3- The acetylcholine acts on a local area of the
muscle fiber membrane to open multiple
"acetylcholine-gated" cation channels through
protein molecules floating in the membrane
14
4- Opening of the acetylcholine-gated channels
allows large quantities of sodium ions to diffuse
to the interior of the muscle fiber membrane.
This causes a local depolarization that in turn
leads to opening of voltage-gated sodium
channels. This initiates an action potential at
the membrane.
5- The action potential travels along the muscle
fiber membrane in the same way that action
potentials travel along nerve fiber membranes
6- The action potential depolarizes the muscle
membrane, and much of the action potential
electricity flows through the center of the
muscle fiber. Here it causes the sarcoplasmic
reticulum to release large quantities of calcium
ions that have been stored within this reticulum.
15
7- The calcium ions initiate attractive forces
between the actin and myosin filaments, causing
them to slide alongside each other, which is the
contractile process
8- After a fraction of a second, the calcium ions
are pumped back into the sarcoplasmic reticulum
by a Ca membrane pump and remain stored in the
reticulum until a new muscle action potential
comes along this removal of calcium ions from
the myofibrils causes the muscle contraction to
cease.
16
Excitation-Contraction Coupling
17
EXCITATION-CONTRACTION COUPLING IN SKELETAL
MUSCLE
  • shows the temporal relationships between an
    action potential in the skeletal muscle fiber,
    the subsequent increase in intracellular free
    Ca2 concentration and contraction of the muscle
    fiber.
  • These temporal relationships are critical in
    that the action potential always precedes the
    rise in intracellular Ca2 concentration, which
    always precedes contraction.

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muscle contraction occurs by a sliding filament
mechanism
This is caused by forces generated by interaction
of the cross-bridges from the myosin filaments
with the actin filaments.
20
Interaction Between the "Activated" Actin
Filament and the Myosin Cross-Bridges- The
"Walk-Along" Theory of Contraction
The head attaches to an active site, this causes
forces between the head and arm of its
cross-bridge. this causes the head to tilt toward
the arm and to drag the actin filament along with
it. This tilt of the head is called the power
stroke. Then, immediately after tilting, the
head automatically breaks away from the active
site. Next, the head returns to its extended
direction. In this position, it combines with a
new active site farther down along the actin
filament then the head tilts again to cause a
new power stroke, and the actin filament moves
another step
21
Thus, the heads of the cross-bridges bend back
and forth and step by step walk along the actin
filament, pulling the ends of two successive
actin filaments toward the center of the myosin
filament
22
A, At the beginning of the cycle, no ATP is bound
to myosin, and myosin is tightly attached to
actin in a "rigor" position. In rapidly
contracting muscle, this state is very brief.
However, in the absence of ATP, this state is
permanent (i.e., rigor mortis
23
B, The binding of ATP to a cleft on the back of
the myosin head produces a conformational change
in myosin that decreases its affinity for actin
thus, myosin is released from the original
actin-binding site. C,
24
C, The cleft closes around the bound ATP
molecule, producing a further conformational
change that causes myosin to be displaced toward
the plus end of actin. ATP is hydrolyzed to ADP
and Pi, which remain attached to myosin
25
D, Myosin binds to a new site on actin (toward
the plus end), constituting the force-generating,
or power, stroke. Each cross-bridge cycle "walks"
the myosin head 10 nanometers along the actin
filament
26
E, ADP is released, and myosin is returned to its
original state with no nucleotides bound (A).
Cross-bridge cycling continues, with myosin
"walking" toward the plus end of the actin
filament, as long as Ca2 is bound to troponin C.
27
Relaxation occurs when Ca2 is reaccumulated in
the sarcoplasmic reticulum by the Ca2 ATPase of
the sarcoplasmic reticulum membrane ) SERCA(
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Rigor Mortis Several hours after death, all the
muscles of the body go into a state of
contracture called "rigor mortis" that is, the
muscles contract and become rigid, even without
action potentials. This rigidity results from
loss of all the ATP, which is required to cause
separation of the cross-bridges from the actin
filaments during the relaxation process. The
muscles remain in rigor until the muscle proteins
deteriorate about 15 to 25 hours later, which
presumably results from autolysis caused by
enzymes released from lysosomes. All these
events occur more rapidly at higher temperatures
30
The Amount of Actin and Myosin Filament Overlap
Determines Tension Developed by the Contracting
Muscle
31
Effect of Muscle Length on Force of Contraction
in the Whole Intact Muscle
32
Relation of Velocity of Contraction to Load
when the load has been increased to equal the
maximum force that the muscle can exert, the
velocity of contraction becomes zero and no
contraction results, despite activation of the
muscle fiber.
33
Isometric Versus Isotonic Contraction
Muscle contraction is said to be isometric when
the muscle does not shorten during contraction
and isotonic when it does shorten but the tension
on the muscle remains constant throughout the
contraction
34
Characteristics of Isometric Twitches Recorded
from Different Muscles
35
Fast Versus Slow Muscle Fibers
every muscle of the body is composed of a mixture
of so-called fast and slow muscle fibers, with
still other fibers gradated between these two
extremes
36
Slow Fibers (Type 1, Red Muscle(
  1. Smaller fibers.
  2. innervated by smaller nerve fibers.
  3. More extensive blood vessel system and
    capillaries to supply extra amounts of oxygen.
  4. Greatly increased numbers of mitochondria, also
    to support high levels of oxidative metabolism.
  5. Fibers contain large amounts of myoglobin, an
    iron-containing protein similar to hemoglobin in
    red blood cells. Myoglobin combines with oxygen
    and stores it until needed this also greatly
    speeds oxygen transport to the mitochondria. The
    myoglobin gives the slow muscle a reddish
    appearance and the name red muscle.

37
  • Fast Fibers (Type II, White Muscle )
  • Large fibers for great strength of contraction.
  • Extensive sarcoplasmic reticulum for rapid
    release of calcium ions to initiate contraction.
  • Large amounts of glycolytic enzymes for rapid
    release of energy by the glycolytic process.
  • Less extensive blood supply because oxidative
    metabolism is of secondary importance.
  • Fewer mitochondria, also because oxidative
    metabolism is secondary. A deficit of red
    myoglobin in fast muscle gives it the name white
    muscle.

38
Motor Unit- All the Muscle Fibers Innervated by
a Single Nerve Fiber
39
Force Summation
Summation means the adding together of individual
twitch contractions to increase the intensity of
overall muscle contraction .
  • Summation occurs in two ways
  • by increasing the number of motor units
    contracting simultaneously, which is called
    multiple fiber summation, and
  • by increasing the frequency of contraction, which
    is called frequency summation and can lead to
    tetanization.

40
1.Multiple Fiber Summation
When the central nervous system sends a weak
signal to contract a muscle, the smaller motor
units of the muscle may be stimulated in
preference to the larger motor units. Then, as
the strength of the signal increases, larger and
larger motor units begin to be excited as well,
This is called the size principle
41
Frequency Summation and Tetanization
as the frequency increases, there comes a point
where each new contraction occurs before the
preceding one is over. As a result, the second
contraction is added partially to the first, so
the total strength of contraction rises
progressively with increasing frequency. When the
frequency reaches a critical level, the
successive contractions eventually become so
rapid that they fuse together and the whole
muscle contraction appears to be completely
smooth and continuous. This is called
tetanization.
42
MECHANISM OF TETANUS
if the muscle is stimulated repeatedly, there is
insufficient time for the sarcoplasmic reticulum
to reaccumulate Ca2, and the intracellular Ca2
concentration never returns to the low levels
that exist during relaxation. Instead, the level
of intracellular Ca2 concentration remains high,
resulting in continued binding of Ca2 to
troponin C and continued cross-bridge cycling. In
this state, there is a sustained contraction
called tetanus, rather than just a single twitch.

43
Skeletal Muscle Tone Even when muscles are at
rest, a certain amount of tautness usually
remains. This is called muscle tone. Because
normal skeletal muscle fibers do not contract
without an action potential to stimulate the
fibers, skeletal muscle tone results entirely
from a low rate of nerve impulses coming from the
spinal cord
44
Contraction of Smooth Muscle
45
the same attractive forces between myosin and
actin filaments cause contraction in smooth
muscle as in skeletal muscle, but the internal
physical arrangement of smooth muscle fibers is
different.
46
Although most skeletal muscles contract and relax
rapidly, most smooth muscle contraction is
prolonged tonic contraction, sometimes lasting
hours or even days.
Comparison of Smooth Muscle Contraction and
Skeletal Muscle Contraction
  • Slow Cycling of the Myosin Cross-Bridges
  • Low Energy Requirement to Sustain Smooth Muscle
    Contraction
  • Slowness of Onset of Contraction and Relaxation
    of the Total Smooth Muscle Tissue
  • Maximum Force of Contraction Is Often Greater in
    Smooth Muscle Than in Skeletal Muscle
  • "Latch" Mechanism Facilitates Prolonged Holding
    of Contractions of Smooth Muscle
  • Stress-Relaxation of Smooth Muscle

47
  • Regulation of Contraction by Calcium Ions

smooth muscle does not contain troponin
MYOSIN LIGHT CHAIN KINASE
48
  • smooth muscle can be stimulated to contract by
    multiple types of signals by nervous signals, by
    hormonal stimulation, by stretch of the muscle,
    and in several other ways.
  • The principal reason for the difference is that
    the smooth muscle membrane contains many types of
    receptor proteins that can initiate the
    contractile process.
  • Still other receptor proteins inhibit smooth
    muscle contraction, which is another difference
    from skeletal muscle.

49
  • Probably half of all smooth muscle contraction is
    initiated by stimulatory factors acting directly
    on the smooth muscle contractile machinery and
    without action potentials.
  • Two types of non-nervous and nonaction potential
    stimulating factors often involved are
  • (1) local tissue chemical factors and
  • (2) various hormones

50
  • Source of Calcium Ions That Cause Contraction
    Through the Cell Membrane and from the
    Sarcoplasmic Reticulum

Role of the Smooth Muscle Sarcoplasmic Reticulum
Smooth Muscle Contraction Is Dependent on
Extracellular Calcium Ion Concentration
51
Contraction of cardiac muscle
52
The strength of contraction of cardiac muscle
depends to a great extent on the concentration of
calcium ions in the extracellular fluids. In
fact, a heart placed in a calcium-free solution
will quickly stop beating.
In contrast, the strength of skeletal muscle
contraction is hardly affected by moderate
changes in extracellular fluid calcium
concentration because skeletal muscle contraction
is caused almost entirely by calcium ions
released from the sarcoplasmic reticulum inside
the skeletal muscle fiber
53
Without the calcium from the T tubules, the
strength of cardiac muscle contraction would be
reduced considerably because the sarcoplasmic
reticulum of cardiac muscle is less well
developed than that of skeletal muscle and does
not store enough calcium to provide full
contraction .
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