Download Cardiac muscle tissue

Heart walls – 3 distinct layers:
1 - endocardium - innermost layer; epithelial tissue that
lines the entire circulatory system
2 - myocardium - thickest layer; consists of cardiac
muscle
3 - epicardium - thin, external membrane around the
heart.
Cardiac muscle tissue:
striated ( consists of sarcomeres just like skeletal
muscle)
cells contain numerous mitochondria (up to 40% of cell
volume)
adjacent cells join end-to-end at structures called
intercalated discs which is cell membrane separate
indivisual cardiac muscle from one another.
Intercalated discs contain two types of
specialized junctions:
desmosomes (hold the cells tightly together) and
gap junctions (which permit action potentials to
easily spread from one cardiac muscle cell to
adjacent cells).
Cardiac muscle tissue forms 2 functional syncytia
or units:
the atria being one &
the ventricles the other.
Because of the presence of gap junctions, if any cell is
stimulated within a syncytium, then the impulse will
spread to all cells. In other words, the 2 atria always
function as a unit & the 2 ventricles always function as
a unit. However, there are no gap junctions between
atrial & ventricular contractile cells. In addition, the
atria & ventricles are separated by the electrically
nonconductive tissue that surrounds the valves. So, as
will be discussed later, a special conducting system is
needed to permit transmission of impulses from the
atria to the ventricles
In cardiac muscle, there are two types of
cells:
contractile cells &
autorhythmic (or automatic) cells.
Contractile cells, of course, contract when
stimulated.
Autorhythmic cells, on the other hand, are
self-stimulating & contract without any
external stimulation.
The action potentials that occur in these two
types of cells are a bit different:
0n the left is the action potential of an autorhythmic cell; on
the right, the action potential of a contractile cell.
Autorhythmic cells exhibit PACEMAKER POTENTIALS.
Depolarization is due to the inward diffusion of calcium .
&it begins when: the slow calcium channels open,
Repolarization is due to the outward diffusion of
potassium
• In contructile cell
• depolarization is very rapid & is due to
the inward diffusion of sodium .
• repolarization begins with a slow outward
diffusion of potassium, but that is largely
offset by the slow inward diffusion of
calcium . So, repolarization begins with a
plateau phase. Then, potassium diffuses
out much more rapidly as the calcium
channels close , and the membrane
potential quickly reaches the 'resting'
potential .
Most of the muscle cells in the heart are contractile cells.
The autorhythmic cells are located in these areas:
Sinoatrial (SA), or sinus, node
Atrioventricular (AV) node
Atrioventricular (AV) bundle (also sometimes called the bundle
of His)
Right & left bundle branches
Purkinje fibers
Various automatic cells have different 'rhythms':
SA node - 60 - 100 per minute (usually 70 - 80 per minute)
AV node & AV bundle - 40 - 60 per minute
Bundle branches & Purkinje fibers - 20 - 40 per minute
SA node = has the highest or fastest rhythm &, therefore, sets
the pace or rate of contraction for the entire heart. As a result,
the SA node is commonly referred to as the PACEMAKER.
Spread of cardiac excitation ( Conducting System of the Heart):
Begins at the SA node & quickly spreads through both atria
Also travels through the heart's 'conducting system' (AV node >
AV bundle > bundle branches > Purkinje fibers) through the
ventricles
For efficient pumping:
The atria should contract (& finish contracting) before the
ventricles contract. This occurs because of AV nodal delay
(that is, the impulse travels rather slowly through the AV node
& this permits the atria to complete contraction before the
ventricles begin contraction).
The atria should contract as a unit, & the ventricles should
contract as a unit. This occurs because the impulse spreads
so rapidly that all myocardial cells in the atria and ventricles,
respectively, contract at about the same time. The impulse
spreads rapidly through the ventricles because of the
conducting system.
Refractory period of contractile cell
Lasts about 250 msec (almost as long as contraction period)
The long refractory period means that cardiac
muscle cannot be restimulated until contraction is
almost over & this makes summation (& tetanus)
of cardiac muscle impossible. This is a valuable
protective mechanism because pumping requires
alternate periods of contraction & relaxation;
prolonged tetanus would prove fatal
Coronary artery disease (CAD) is a condition in
which plaque builds up inside the coronary
arteries that supply heart muscle with oxygenrich blood. Plaque is made up of fat, cholesterol,
calcium, and other substances found in the
blood. When plaque builds up in the arteries, the
condition is called atherosclerosis. Plaque
narrows the arteries and reduces blood flow to
your heart muscle. It also makes it more likely
that blood clots will form and partially or
completely block blood flow. When coronary
arteries are narrowed or blocked, oxygen-rich
blood can't reach the heart muscle. This can
cause angina or a heart attack. Angina is chest
pain or discomfort that occurs when not enough
blood flows to an area of heart muscle. A heart
attack occurs when blood flow to an area of
heart muscle is completely blocked.
This prevents oxygen-rich blood from
reaching that area of heart muscle and
causes it to die. Without quick treatment, a
heart attack can lead to serious problems
and even death. Over time, CAD can
weaken heart muscle and lead to heart
failure and arrhythmias. Heart failure is a
condition in which your heart can't pump
enough blood throughout your body.
Arrhythmias are problems with the speed
or rhythm of your heartbeat. CAD is the
most common type of heart disease
• Heart Valves:
• Atrioventricular (AV) valves - prevent backflow of
blood from ventricles to atria during ventricular systole
(contraction)
– Tricuspid valve - located between right atrium & right
ventricle
– Mitral valve - located between left atrium & left ventricle
• Semilunar valves - prevent backflow of blood from
arteries (pulmonary artery & the aorta) to ventricles
during ventricular diastole (relaxation)
– Aortic valve - located between left ventricle & the aorta
– Pulmonary valve - located between right ventricle & the
pulmonary artery (trunk)
• All valves consist of connective tissue (not cardiac
muscle tissue) and, therefore, open & close passively.
Valves open & close in response to changes in
pressure:
• AV valves - open when pressure in the
atria is greater than pressure in the
ventricles (i.e., during ventricular diastole)
& closed when pressure in the ventricles is
greater than pressure in the atria (i.e.,
during ventricular systole)
• Semilunar valves - open when pressure in
the ventricles is greater than pressure in
the arteries (i.e., during ventricular systole)
and closed when pressure in the
pulmonary trunk & aorta is greater than
pressure in the ventricles (i.e., during
ventricular diastole
• What happens in the heart during each
'mechanical' event:
– Atrial systole :
• no heart sounds (because no heart valves are opening or
closing)
• a slight increase in ventricular volume because blood
from the atria is pumped into the ventricles
– Ventricular systole:
• the first heart sound (lub) (labeled S1 below) - this sound is
generated by the closing of the AV valves (& this occurs
because increasing pressure in the ventricles causes the AV
valves to close)
• initially there is no change in ventricular volume (called
the period of isometric contraction) because ventricular
pressure must build to a certain level before the semilunar
valves can be forced open & blood ejected. Once that
pressure is achieved, & the semilunar valves do open,
ventricular volume drops rapidly as blood is ejected
Heart valve disease is a condition in which one or
more heart valves don't work properly, making
the heart work harder and affecting its ability to
pump blood. Malfunctioning heart valves can
create two basic problems: (1) Regurgitation, or
backflow, occurs when a valve doesn’t close
tightly. Blood leaks back into the chamber rather
than flowing forward through the heart or into an
artery. Backflow is most often due to prolapse
(the flaps of the valve flop or bulge back into an
upper heart chamber during a heartbeat).
2) Stenosis occurs when the flaps of a valve
thicken, stiffen, or fuse together. This
prevents the heart valve from fully
opening, and not enough blood flows
through the valve. heart valve disease
coulde be congenital or acquired later in
life. Although a valve may be normal at
first, disease can cause problems to
develop over time. Many people have
heart valve defects or disease, but don't
have symptoms, and not cause any
problems.
For other people, the condition can worsen slowly
over time until symptoms develop. If not treated,
advanced heart valve disease can cause heart
failure, stroke, blood clots, or sudden death due
to cardiac arrest. Lifestyle changes and
medicines can relieve many of the symptoms
and problems linked to heart valve disease, and
can also lower the risk of developing a lifethreatening condition, such as stroke or sudden
cardiac arrest. Eventually, however, faulty heart
valves may have to be repaired or replaced