1. Membrane Potential (mV)
Rhythmical Discharge of Sinus Nodal Fiber
Slow Ca++
Channels Open
Sinus Nodal
Fiber
Ventricular
Muscle fiber
K+ Channels
Open more
+20
Threshold
-20
-40
Membrane Potential (mV) }
-60
“Resting Potential”
-80
Na+ Leak
-100 1 2 3 4
Seconds

2. Time of Arrival of Cardiac Impulse
(0.22)
SA Node
AV Bundle H
(0.19)
(0.0) T
(0.03)
(0.12)
Left Bundle
Branch
(0.16)
(0.19)
(0.18)
AV Node
Right Bundle
Branch
(0.21)
(0.17)
Main Arrival Times
S-A Node sec
A-V Node sec A-V Bundle sec Ventricular
Septum sec
(0.18)
Copyright © 2006 by Elsevier, Inc.

3. Sinus Node is Cardiac Pacemaker
Normal rate of discharge in sinus node is 70-80/min.; A-V node /min.; Purkinje fibers /min.
Sinus node is pacemaker because of its faster discharge rate

4. Ectopic Pacemaker
This is a portion of the heart with a more rapid discharge than the sinus node.
Also occurs when transmission from sinus node to A-V node is blocked (A-V block).

5. Parasympathetic Effects on Heart Rate
Parasympathetic (vagal) nerves, which release acetylcholine at their endings, innervate S-A node and A-V junctional fibers proximal to A-V node.
Causes hyperpolarization because of increased K+ permeability in response to acetylcholine.
This causes decreased transmission of impulses maybe temporarily stopping heart rate.
Ventricular escape occurs.

6. Sympathetic Effects on Heart Rate
Releases norepinephrine at sympathetic ending
Causes increased sinus node discharge
Increases rate of conduction of impulse
Increases force of contraction in atria and ventricles