Section 3, Chapter 15
An electrocardiogram, or ECG (or EKG) is a recording of the
electrical changes in the myocardium during the cardiac cycle.
• Represents atrial depolarization
• Leads to atrial contraction
• Conduction of electrical impulse
from right to left and downward
• Represents depolarization of
• Leads to ventricular contraction
• This massive wave hides the atria
• Represents repolarization of
Figure 15.22d. An ECG pattern with the corresponding
systole and diastole shown above.
ECG of a regular heart rhythm at 75 beats per minute
Atrial Flutter. Atria fire 250-350 times per minute. For every QRS
complex there may be 4 or more P waves.
Examples of Heart Arrhythmias. Arrows indicate p Wave.
Bradycardia – cardiac rhythm less than 60 beats per minute.
Tachycardia– cardiac rhythm greater than 100 beats per minute.
Examples of Heart Arrhythmias, fibrillation
Atrial fibrillation. Instead of contracting, the atria become quivering
chambers. The ventricles respond only to impulses that make it to the
Ventricular fibrillation = Life threatening arrhythmia. Ventricles quiver, and are
unable to pump blood properly. Requires immediate defibrillation.
Regulation of cardiac cycle
The heart rate is controlled intrinsically by the SA
node, but sympathetic and parasympathetic fibers
alter the rate at which the pacemaker fires.
Cardiac Control Center
•Within Medulla Oblongata
•Receives sensory impulses from throughout the
cardiovascular system and relays motor impulses to heart
•Cardioinhibitor & cardioaccelerator reflex centers
• Cardioinhibitor reflex center
– Parasympathetic fibers from vagus nerves innervate SA & AV nodes.
– Acetylcholine (ACh) released from fibers decreases the firing rates of SA
& AV nodes.
– Heart rate decreases
• Cardioaccelerator reflex center
– Sympathetic fibers from accelerator nerves innervate SA & AV nodes.
– Norepinephrine released from fibers increases the firing rates of SA & AV
– Heart rate and force of contraction increases
Cardioinhibitor & cardioaccelerator reflex centers alter the heart
rate in response to sensory impulses from receptors
Baroreceptors – monitor blood pressure
• Within aortic arch and carotid sinuses
• Rising blood pressure stimulates cardioinhibitor center
Figure 15.24b Illustration of the baroreflex arc
End of Section 3, Chapter 15