Download Takes 50ms for ap to travel from SA to AV

Ch. 20 The Heartbeat
Two types of Cardiac Muscle
1. Conducting System- Controls and coordinates heartbeat
2. Contractile System- Muscle contraction^propels blood
A. Conducting System:
1. Begins in the Rt. Atrium (poster, wall-near Sup Ven Cav)
a. SA node (Sinoatrial-pacemaker cells) action potential
generated here independent of the nervous system
regular intervals (80-100 AP/min)- although slower than
this due to chemicals of body (72 beats /min)
cells never reach resting potential & are small
b. A.P. through the Internodal Pathway to the AV node
a.p. travels through contractile cells of both atria
-> Takes 50ms for a.p. to travel from SA to AV
+20 mV
Sinoatriai
(SA) node
Internodal
-20 mv
pathways
Threshold
-40 mV
^ Prepotenttel
Atriovenlriculaf
JAV) n<Jde
AV bundle
Bundle
branct^s
Purklnje
(a) The conducting system
_
—
(spontaneous depolarization)
^
Time (sec)
_
——
(b) Depolarization at the SA node
Rate of impulse slows down due to smaller cells and
inefficient connections (40-60 AP/min)
Delay important-allows vents to fill before contraction
d. From AV node^ Bundle of HIS (the only electrical connection
between the atria and ventricles)
Bundle Branches (IV Septum)- right and left (larger)
Purkinje fibers-^ moderator band-> papillary muscles
e. Abnormalities-
Bradychardia- heart rate slow
Tachychardia- heart rate fast
B. Contractile System- Ventricular Muscle Contraction
1. Action Potentials from the Purkinje fibers stimulate the
contractile cells in the atrial and ventricular walls
2. A.P. leads to Ca binding to troponin, etc. etc.
The Cardiac Cycle- the period between the start of one heartbeat
and the start of the next - .8 seconds at rest
Systole- Contraction
Diastole- Relaxation
The two atria contract while the two ventricles relax- Atrial
Systole, Ventricular Diastole
Then, the two atria relax while the two ventricles contractAtrial Diastole, Ventricular Systole
Cardiac Cycle in 3 phases:
1. Relaxation Period (.4 seconds)- Ventricles start to relax
(pressure falls- opening AY valves again) and all 4 chambers
are in diastole
75% of ventricular filling occurs before the atria
contract
2. Atrial Systole (.1 seconds-contraction)- Action potential
from SA node causes atria to contract- forces the last 25%
of blood into ventricles
AY valves still open, semilunar closed
3. Ventricular Systole (.3 seconds-contraction)- Ventricles
receive action potential/signal & contract
closes AV valves, pressure increases forcing
semilunars open
ventricles begin to relax, pressure drops &
semilunars close
Exercise decreases the relaxation period
Heart sounds:
1. First sound (lubb)- AV valves close
2. Second sound (dub)- Semilunar valves close
3. Pause between them during relaxation period