1. CONDUCTION

2. The Cardiac Cycle Autonomic Control of Heart Rate
The pacemaker sets the heart rate but can be altered
Impulses from the autonomic nervous system modify the pacemaker activity
Nerves associated with the ANS innervate the:
SA node
AV node
Cardiac cells
Smooth muscles in the cardiac blood vessels

3. The Cardiac Cycle Autonomic Control of Heart Rate (continued)
Norepinephrine from the ANS causes:
An increase in the heart rate
An increase in the force of contractions
Acetylcholine from the ANS causes:
A decrease in the heart rate
A decrease in the force of contractions

4. Figure 21.13 The Autonomic Innervation of the Heart
Vagal nucleus
Cardioinhibitory center
Cardioacceleratory center
Medulla oblongata
Vagus (N X)
Spinal cord
Sympathetic
Parasympathetic
Parasympathetic preganglionic fiber
Sympathetic preganglionic fiber
Synapses in cardiac plexus
Sympathetic ganglia (cervical ganglia and superior thoracic ganglia [T1–T4])
Parasympathetic postganglionic fibers
Sympathetic postganglionic fiber
Cardiac nerve 4

5. The Cardiac Cycle
The cardiac cycle consists of alternate periods of contraction and relaxation
Contraction is systole
Blood is ejected into the ventricles
Blood is ejected into the pulmonary trunk and the ascending aorta
Relaxation is diastole
Chambers are filling with blood

6. The Cardiac Cycle
Cardiac contractions are coordinated by conducting cells
There are two kinds of conducting cells
Nodal cells
Establish the rate of contractions
Cell membranes automatically depolarize
Conducting fibers
Distribute the contractile stimulus to the myocardium

7. The Cardiac Cycle The Sinoatrial and Atrioventricular Nodes
Sinoatrial node (SA node)
Cardiac pacemaker
Embedded in the posterior wall of the right atrium
Determines the rate of heart contractions
Bradycardia: slow heart rate
Tachycardia: rapid heart rate
Impulse travels to the AV node via the internodal pathways

8. The Cardiac Cycle
The Sinoatrial and Atrioventricular Nodes (continued)
Atrioventricular node (AV node)
Sits within the floor of the right atrium
Impulse travels from the AV node to the AV bundle
The AV bundle travels along the interventricular septum and then divides to form the right and left bundle branches
The bundle branches send impulses to the Purkinje fibers
Impulses are then conveyed to the ventricular myocardium

9. Figure 21.12a The Conducting System of the Heart
Sinoatrial (SA) node
Internodal pathways
Atrioventricular (AV) node
AV bundle
Left bundle branch
Right bundle branch
Moderator band
Purkinje fibers
The stimulus for contraction is generated by pacemaker cells at the SA node. From there, impulses follow three different paths through the atrial walls to reach the AV node. After a brief delay, the impulses are conducted to the bundle of His (AV bundle), and then on to the bundle branches, the Purkinje fibers, and the ventricular myocardial cells. 9

10. Figure 21.12b The Conducting System of the Heart
SA node activity and atrial activation begin.
SA node
Time  0
Stimulus spreads across the atrial surfaces and reaches the AV node.
AV node
Elapsed time  50 msec
There is a 100 msec delay at the AV node. Atrial contraction begins.
AV bundle
Bundle branches
Elapsed time  150 msec
The impulse travels along the interventricular septum within the AV bundle and the bundle branches to the Purkinje fibers and, via the moderator band, to the papillary muscles of the right ventricle.
Moderator band
Elapsed time  175 msec
The impulse is distributed by Purkinje fibers and relayed throughout the ventricular myocardium. Atrial contraction is completed, and ventricular contraction begins.
Elapsed time  225 msec
Purkinje fibers
The movement of the contractile stimulus through the heart is shown in STEPS 1–5. 10

11. Figure 21.12b The Conducting System of the Heart (Part 1 of 2)
SA node activity and atrial activation begin.
SA node
Time  0
Stimulus spreads across the atrial surfaces and reaches the AV node.
AV node
Elapsed time  50 msec
There is a 100 msec delay at the AV node. Atrial contraction begins.
AV bundle
Bundle branches
Elapsed time  150 msec
The movement of the contractile stimulus through the heart is shown in STEPS 1–5. 11

12. Figure 21.12b The Conducting System of the Heart (Part 2 of 2)
The impulse travels along the interventricular septum within the AV bundle and the bundle branches to the Purkinje fibers and, via the moderator band, to the papillary muscles of the right ventricle.
Moderator band
Elapsed time  175 msec
The impulse is distributed by Purkinje fibers and relayed throughout the ventricular myocardium. Atrial contraction is completed, and ventricular contraction begins.
Elapsed time  225 msec
Purkinje fibers
The movement of the contractile stimulus through the heart is shown in STEPS 1–5. 12

13. Conducting System Process
SA Sinoatrial Node- pacemaker of heart (generates action potentials greater frequency)
.04 faster than to surrounding muscle
AV Atrioventricular Node- Slow to transmit action potential and allows completion of atrial contraction
AV Bundle of His
Fast
Bundle branches- right and left
IV Septum
Rt Ventrical apex
Lt Ventrical apex
Perkinje Fibers- large diameter cardiac muscle
Transmits signal to apex of ventricles
A lot of intercalated disks
Myocardium in apex contracts in a wringing action

14. Figure 21.11 The Cardiac Cycle
Start
Atrial systole begins: Atrial contraction forces a small amount of additional blood into relaxed ventricles.
Atrial systole ends, atrial diastole begins
0 msec
800 msec
100 msec
Atr
ial sy
sto le
Ven
tri
cul ar
dia
sto le
Ven
tri
cul ar sy
sto le
Cardiac cycle
Ventricular systole— first phase: Ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves.
Ventricular diastole—late: All chambers are relaxed. Ventricles fill passively.
Atr
ial
dia
sto le
370 msec
Ventricular systole— second phase: As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected.
Ventricular diastole—early: As ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria. 14

15. Autorhymicity of Cardiac Muscle
Action potentials in heart without external stimuli
After each action potential the membrane potential returns to its resting membrane potential
Unstable slow ion channels open and cause depolarization
This causes fast channels to open and increase depolarization
When depolarization reaches threshold--action potential happens more often in SA Node because more slow channels
Plateau Phase- Prolonged period of depolarization, separates contractions in the heart
Heart has long action potential so the heart will rest between contractions and not tetanic contractions
Absolute refractory period – CM insensitive to further stimuli

16. EKG
Electrokardiogram- device to record the action potential of cardiac muscle summation
Cannot detect force of contraction
Cannot detect Blood Pressure
Can detect abnormal heart rates/ rhythems
Can detect abnormal conduction pathways
Can detect hypertrophy and atrophy and relative position of damage
P Wave- Action potential depolarization of atrial myocardium
Causes Atrial contraction
QRS Complex- Ventricular depolarization
Causes onset of ventricular contraction
Also atrial repolarization masked by QRS signal
T Wave- repolarization before ventricular relaxation
U wave – repolarization of the purkinje fibers
PQ/PR Interval- .16 sec- atria contract and relax
QT Interval- .3 sec- ventricles contract and relax
1 Cardiac Cycle- from onset muscle contraction to next

18. The Cardiac Cycle ANIMATION The Heart: Cardiac Cycle
ANIMATION The Cardiac Cycle: Part 1
ANIMATION The Cardiac Cycle: Part 2
ANIMATION The Cardiac Cycle: Part 3
ANIMATION The Cardiac Cycle: Part 4
ANIMATION The Cardiac Cycle: Part 5 18