EXCITATION OF THE HEART 



295 



portion of the interventricular septum was the first 

 portion of tlie ventricle to be activated. Hypotheti- 

 cally, the electrical impulse had only to travel through 

 the connective tissue boundary in this region — a 

 distance of i mm or less — before ventricular excitation 

 began. It thus seemed that, since so long a time was 

 consumed in going so short a distance, there must be 

 a true delay, i.e., a period when the impulse was 

 moving little or not at all. The time which must be 

 accounted for by conduction within the A-V node 

 has been decreased by the realization that the Purkinje 

 fibers excite the ventricle nearer to the apex than 

 previously thought (see below). A portion of the 

 atrioventricular interval is thus consumed by conduc- 

 tion in the bundles and other peripheral Purkinje 





FIG. 1 1. Intracellular records from sinus node, atrial muscle, 

 A-V nodal region, and common bundle. For timing purposes, 

 large potential which has lowest base line is repeated and is 

 taken near common bundle. Small potential, A, which begins 

 earliest is from sinus nodal region. It shows a diastolic pre- 

 potential and slow rate of depolarization with lack of overshoot. 

 Potential B has a resting potential slightly smaller than that of 

 common bundle but depolarizes to about the same extent. 

 This is from ordinary atrial muscle and occurs somewhat later 

 than potential from sinus node. C: potential from upper A-V 

 node shows a smaller amplitude and a small diastolic prepo- 

 tential after rapid repolarization. A similar potential with a 

 diastolic prepotential, phase D, is seen occurring somewhat 

 later and closer in time to the depolarization of the common 

 bundle. This potential is from the mid A-V node. (From 

 Hoffman et al. (58).] 



tissue. The A-V conduction time has been increased 

 by the realization that the atrial cells in the A-V 

 nodal region are actually excited about two-thirds of 

 the way through the time interval occupied by the P 

 wave (92). As will be seen below, the electrical 

 impulse ma\' at times travel very slowly through the 

 tissues which link the atrium and ventricle, but 

 there appears to be no evidence that the impulse 

 stops or that any chemical transmission is involved. 

 The term delay therefore seems inappropriate, 

 implying as it does a period during which nothing 

 happens. Despite the fact that newer knowledge has 

 indicated the incorrectness of this term, it probably 

 will continue to be used for convenience. 



Recently both intracellular and extracellular 

 electrodes have been used to study A-V nodal con- 

 duction, but the information so far derived provides 

 only imperfect answers. Hoff^man and co-workers 

 (57, 58), Sano and co-workers (107, iio), and 

 Matsuda and co-workers (74), who studied mammals, 

 and Inoue (59), who studied the frog, are generally 

 agreed concerning the shape of the intracellular 

 action potentials (figs. 11, 12) recorded from the A-V 

 node itself, a) The resting and action potentials are 

 smaller than those in the rest of the atrial or ventricu- 

 lar musculature, h) The rate of change in voltage 

 during the period of initial depolarization is less 

 rapid than in other muscle, c) The action potential 

 has a shorter duration than that from the fibers of the 

 bundle of His and is usually of about the same 

 duration as the action potential from ordinary atrial 

 musculature. Sano and co-workers (107), using an 

 ingenious marking technique with the intracellular 

 electrode, were able to demonstrate histologically 

 that these potentials were indeed from the A-V nodal 

 region. They further feel that the A-V nodal potential 

 has a step on the rising phase and that potentials 

 without this step are from "transitional tissue." 



Despite the excellence of these intracellular studies, 

 there remain unanswered questions concerning the 

 shapes of the recorded action potential. In some 



FIG. 1 2. Potentials from the A-V node recorded by Sano and co-workers. Trace i from ordinary 

 atrial muscle; trace 7 from the common bundle. Traces 3 and 4 from the A-V node, and are dis- 

 tinguished by a "step." [From Sano et al. (107).] 



