Marah karablieh Osama khader -...
Transcript of Marah karablieh Osama khader -...
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Osama khader
Marah karablieh
Muhammad khatatbeh
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Cardiac Muscle Physiology
Introduction
The heart has two ventricles and two atriums. The heart wall is
composed primarily of
spirally arranged
cardiac muscle fibers
and has three distinct
layers:
o Endocardium: the
innermost thin
layer of epithelial
tissue that lines
the chambers of
the heart.
(Endothelium,
which also lines
the entire
circulatory
system).
o Myocardium: A
middle layer,
which is composed of cardiac muscle and constitutes the
bulk of the heart wall (myo means “muscle”; cardia
means “heart”)
o Pericardium: A thin external layer, and it consists of two
layers: Visceral (which is closer to the heart) and Parietal
with a space separating them. This space is called
Pericardial Cavity and contains serous pericardial fluid
which gives protection and absorbs shocks.
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Comparison between Skeletal and Cardiac
Muscles
Skeletal Muscle are cylindrical in shape, they are long since
they have origin and insertion, and under microscope they
appear striated. They are voluntary and needs to be
innervated in order to contract, they are not connected to
each other; motor nerves supply a number of fibers to cause
their contraction (a motor unit).
✓ Remember: Happens first: Electrical response: Action
potential
Then: Mechanical response: Contraction
✓ Excitation–contraction coupling is the physiological
process of converting an electrical stimulus to a
mechanical response. It is the link between the action
potential generated in the sarcolemma and the start of
a muscle contraction.
✓ Note: In addition to gap junctions, cardiac cells have
desmosomes in between.
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Cardiac muscle cells are
rectangular in shape,
connected to each other
by intercalated discs that
enable the rapid
transmission of electrical
impulses through the
network. At each
intercalated disk the cell
membranes fuse with one
another to form gap
junctions that allow rapid
diffusion of ions, that’s why when one cell becomes excited,
the action potential rapidly spreads to all of them.
Gap junctions are hexagonal proteins
with “open” and “closed”
conformations. They’re voltage-gated
channels, when voltage changes due
to depolarization the channels are
open and ions diffuse so fast that all
cells are depolarized at the same time,
so they contract at the same time.
Thus, cardiac cells are electrically
coupled.
If each cell contract by itself, this will cause ventricular
fibrillation (may also happen in atrium, atrial fibrillation). Here,
the heart pumps little or no blood.
Involuntary, although it is supplied by the autonomic nervous
system (sympathetic and parasympathetic). This innervation of
the cardiac muscle is not important for the initiation of cardiac
muscle contraction (during cardiac transplants the autonomic
Keep in mind, In cardiac
muscles:
-Plasma membrane:
Sarcolemma SL
-Endoplasmic Reticulum:
Sarcoplasmic Reticulum
SR
-Cytoplasm: Sarcoplasm
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supply is cut but the heart is still running). The contraction of
cardiac muscle follows (law All or None).
It’s striated, because of the contractile proteins inside the
cells
The Endoplasmic Reticulum in skeletal cells is well-developed
and stores Ca++ in high amounts, but Sarcoplasmic Reticulum is
less developed and doesn’t store enough Ca++. So how do
cardiac muscles contract if they have low amounts of Ca++
inside? Simply, they take Ca++ from the extracellular matrix.
That’s why during heart transplants, the heart is put into a
Calcium solution.
The cardiac muscle contracts all the time. This means it needs
a lot of energy, that’s why it has lots of mitochondria to supply
ATP compared to skeletal muscles that, on the other hand,
have much more nuclei.
The sarcolemma of cardiac and skeletal muscles has deep
invaginations called the T-tubules or transverse tubules. The T-
tubules of skeletal muscles are found in the I band, they are
slender and long; so, each sarcomere has two T-tubules.
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While in the cardiac muscle, they are wider and shorter and
occur at the Z line (disk); so, each sarcomere has one T-tubule.
The sarcomere is the distance between two Z lines.
Skeletal Muscles Cardiac muscles
-Voluntary
-Attached to bones or skin
-Very long, cylindrical,
multinucleate, cells
-Striated: packed with orderly
arrangement of contractive
proteins
-Not self-stimulating: each fiber
innervated by branch of motor
neuron as part of motor unit
-Under control of nervous system
-High number of nuclei
mitochondria
-Fast Contracting
-No rhythmic contractions
-Fatigues Easily
-Involuntary
-Found only in the Heart
-Branching chains of cells
connected by intercalated discs,
with single nucleus and striations
-Striated: many contractive
proteins in orderly arrangement
-Self-stimulating: impulse spreads
from cell to cell
-Under control of nervous and
endocrine systems and various
chemicals
-Intermediate energy
requirement: higher number of
mitochondria than the skeletal
muscle cells.
-Intermediate speed of
contraction lets contraction
spreads quickly through tissue
due to intercalated discs (gap
junction)
-Rhythmic contractions
-Doesn’t fatigue
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Action Potential
Skeletal Muscles
-Rest Membrane Potential is -70 mV
-When stimulated, Na+ channels are opened and slow
depolarization begins until reaching the threshold
-After that, fast depolarization takes place due to opening of
Na+ voltage-gated channels (firing stage), trying to reach a
potential of +61 (Eq. potential of Na+) but that’s impossible
because cells contain ions other that Sodium
-Then, Na+ channels close and K+ channels open causing
repolarization (falling stage)
-This whole process takes 1millisecond – 10 milliseconds
maximum
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Cardiac Muscle
-Resting membrane potential is -90 mV (Phase 4)
o Phase 0 (Depolarization): When the cardiac cell is
stimulated, fast voltage-gated Na+ channels open and
permit Na+ to flow into the cell and depolarize it. The
membrane potential reaches about +20 – +30 mV. In this
phase, we have an increase in the permeability of Na+
and a decrease in the permeability of K+.
✓ Important note: The decrease of the K+ level to less
than what it was during the resting stage is
important, but why? Because if that didn’t happen,
K+ will diffuse to the outside, and Ca++ will take its
place. Therefore, that way, a positive went out & a
positive went in, there’s no benefit in that! As a result,
there will be no Plateau.
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o Phase 1 (Partial or Initial Repolarization): Na+ channels
close, the cell begins to repolarize, and transient K+ and
Cl- specialized channels open.
Again, the decrease in K+ permeability during Phase 0
and 1 is very important in order to maintain the Plateau.
o Phase 2 (Plateau): this phase plays the main role in
contraction as it maintains depolarization. Slow voltage-
gated Ca++ channels open, which induces the release of
Ca++ from SR, and K+ channels close.
o Phase 3 (Rapid Repolarization): Ca ++ channels close and
slow voltage-gated K+ channels open, this permits K+ to
rapidly exit the cell, ends the plateau and returns the cell
membrane potential to its resting level.
o Phase 4 (Resting membrane potential): rearrangement of
ions by Na - K pump.
-The whole process takes 250-300 milliseconds, thanks to the
long refractory period (an absolute refractory period from the
beginning to half of the repolarization stage where the muscle
cannot contract, and a relative refractory after it where the
muscle may contract to a stronger stimulus).
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Contraction and
Tetanisation
First, let us know what exactly
“Tetanisation” means,
To tetanize: to stimulate a
muscle at progressively higher
frequencies until successive
contractions fuse and cannot be distinguished from one
another, and tetanus: a state of muscular contractions without
periods of relaxations.
In the skeletal action potential: Because of the short
refractory period, if there's another stimulus, a new action
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potential and another contraction will start in the relative
refractory period of first action potential. This is called Tetanus
or Spasm. (Relative refractory period: the last half of the
repolarization phase until the action potential has ended).
In the cardiac action potential: When the heart is given an
action potential, it will NOT give the muscle Tetanus no matter
how strong the action potential is, but the question is why? That
is because it has a VERY long Absolute Refractory Period.
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مس الت ال ت غرب واألمل األبدي ة الت ال ت فت والش كن اهلم
أمل منه ...فمن ال أمل فيه ال