LEC 2 Flashcards
The heart is composed of three types of fibers
- Atrial Muscle Fibers
- Ventricular muscle Fibers
- Conductive fibers:
the only difference between Atrial and. Ventricular muscle Fibers
the ventricular muscle fibers are thicker than the atrial fibers producing stronger contraction
Conductive fibers
Specialized fibers because they are neither nerves nor muscles
Conductive fibers اهم ميزه
Self-excitable
Conductive fibers Not nerves because
they don’t conduct the action potential as fast as the nerves.
Conductive fibers Not muscles because
they are non-contractile fibers
Self-excitable means
They are able to generate the impulse by their own and transmit it.
SA Node (Sinoatrial node) near
the Superior vena cava.
AV Node
Internodal (sub nodal) fibers – between the SA node and the AV node.
AV Bundle (Bundle of His) divided into
A. Right Branch B. left Branch
a fibrous tissue is considered
the point of origin and insertion for the cardiac muscle.
the highest rate of excitation
SA node
SA node has the highest rate of
excitation for a couple of reasons
- The Resting membrane potential is around ( -60 mv ) which is higher than
the other fibers and nearer to the threshold
While in other tissue it is -70 mv so needs more time to reach the threshold - the SA node cells membrane is leaky to ( Na+ )
الوقت اللازم لحتى ال atruim يتحفز
30 milliseconds
duration of action potential to reach the AV node,
90 milliseconds
duration of action potential in AVbundle
40 milliseconds
emptying process of the atrium takes
130 milliseconds.
in Purkinje fibers stimulate the ventricles
60 milliseconds
The whole process from stimulating the SA node until ventricular contraction
needs about
220 milliseconds )
The Highest rate of Excitation
SA Node
The Highest velocity rate
Purkinje fibers
plateau
after slow Ca2+ channels open allowing
Ca2+ ions to leak to the inside of the cell
( leak slowly ). This leakage takes a long
period of time (about200 ms
Refractory period divided into two periods
- Absolute refractory period
2. Relative refractory period
When we can’t re‐stimulate the heart
Absolute refractory period
Absolute refractory period represent
about 95% of refractory period
Relative refractory period represent
the last 5% of refractory period.
When we can re‐stimulate the heart
Relative refractory period
but we need a stronger stimulus
Refractory period in the ventricles
0.25-0.3 seconds
Refractory period in the atria.
0.15 s
Refractory period includes
time periods during which the depolarization & the plateau
premature contraction
Extra systolic beat
systole occurs (stimulation) during the
relative refractory period
Frank-starling law
The greater the heart muscle is stretched
during filling, the greater is the force of
contraction and the greater the quantity
of blood pumped into the aorta
