9 : 4/ Mechanical and Electrical Character of the Heartbeat 165 



in all directions as an electrochemical impulse over the surface of the 

 auricle, causing the muscle fibers to contract. When two pulses reach 

 the opposite side of the auricle from two directions, they annihilate each 

 other because the contracted muscle will not conduct another impulse. 



Besides causing the auricle to contract, the electrochemical pulse, 

 originating at the s-a node, also stimulates the auriculo-ventricular (a-v) 

 node (see Figure 4). This node, after a short time delay of about 0.1 

 sec or slightly less, puts out a new electrical pulse which is conducted 

 down a special group of fibers called the a-v bundle, diagrammatically 

 illustrated in Figure 4. These fibers terminate in the central muscular 

 wall between the two ventricles. From these terminals, the pulse 

 spreads over the walls of the ventricles causing them to contract. 



The s-a node resembles a free-running electronic multivibrator con- 

 trolling a second multivibrator, the a-v node, which in turn controls a 

 third multivibrator, the ventricle itself. Many factors suggest this 

 analogy. The fundamental rate of the s-a node can be varied by two 

 different sets of nerves which act to speed or slow the rate of firing of the 

 s-a node. This is analogous to tuning either the resistance or the capacity 

 of a free-running multivibrator. 



In some cases, the s-a node fails. Then the a-v node takes over control 

 of the heart. The auricular contraction is no longer properly syn- 

 chronized with the ventricular action, but this is by no means fatal. 

 The a-v node behaves as an electrical multivibrator synchronized by 

 pulses from the s-a node. When free-running, it has a slower firing 

 rate (about 50 beats per min in man). 



If the a-v node also fails, the heart neither stops, nor does the animal 

 die. Rather, the auricular and ventricular walls take over control 

 directly. Their free-running rate is still slower (about 30 beats per min 

 in man) . The ventricles and auricles are then completely independent 

 in their times of contractions. On the average, the auricular beat then 

 interferes with, rather than promotes, circulation. 



The cardiac muscle fibers, like skeletal-muscle and nerve fibers, have a 

 resting potential around 90 mv, the outside being positive relative to the 

 inside. As in skeletal-muscle and nerve fibers, the action potentials are 

 about 120 mv; that is, the outside is 30 mv negative relative to the inside 

 at the peak of the spike. All three types of fibers are also similar in that 

 the concentration of potassium ions is much higher within the cell than 

 in the surrounding medium, whereas the sodium ion concentrations are 

 just the reverse. 



The cardiac muscle fibers differ markedly from skeletal muscle and 

 nerve fibers in the kinetics of the recovery to the resting potential. In 

 the largest mammalian nerve axons, this takes a fraction of a millisecond. 

 In smaller nerve axons and skeletal muscle fibers, the recovery period is 



