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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

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