No abstract

Interactions between Purkinje fibers and ventricular muscle were studied in canine Purkinje-papillary muscle preparations. The change in duration of action potentials across the junctions between Purkinje fibers and papillary muscle was continuously graded, not abrupt. The transmembrane action potentials were longest in the false tendons and progressively shorter in more peripheral fibers. The responses of terminal Purkinje fibers and of neighboring muscle cells differed little in duration. The shortest action potentials were found in muscle fibers located in regions devoid of specialized tissue (tip of papillary muscle). These results suggest that the intercellular connections, including those at the junctions, offer relatively low resistance to current flow. During repolarization the current flowing between neighboring elements with intrinsically different repolarization times should therefore minimize the disparity in action potential durations on the two sides of the junctional site; a continuously graded change in duration would result. Because of this continuous gradation, premature ventricular responses initiated at the tip of the papillary muscle could be blocked, depending on the degree of prematurity, at various levels in muscle fibers functionally close to terminal Purkinje fibers or within the Purkinje system.

The electrical threshold of A-V nodal cells of isolated rabbit heart preparations was estimated by techniques which permitted stimulation and recording through the same micropipette. Cells within the node were significantly less excitable than adjacent atrial and ventricular tissues; thresholds exceeded 5 X 10~~° amp in some instances. Recovery of excitability was delayed well beyond repolarization; in cells of the midnodal region, "diastolic" threshold was not attained until 0.2 to 0.5 seconds after restoration of resting membrane potential. A cumulative effect of frequency upon excitability (fatigue) was demonstrated. At high driving rates, the lag in recovery was further delayed, and the late diastolic threshold was increased. Responses to single-cell stimulation indicate that summation may be an important feature of propagation within the node. ADDITIONAL KEY WORDS fatigue of A-V transmission atrioventricular conduction summation in A-V nodeSchematic diagram of circuits used for intraceUular stimulation and recording. ME = microelectrode; P = potentiometer; C = calibrator; A 2 = amplifier for recording transmembrane action potential; Aj = current monitor.

Certain temporal patterns of A-V and V-A transmission in experimental preparations resemble phenomena attributed to "supernormal" conduction in the clinic. Detailed study of the properties of the A-V transmission system in such experiments reveals alternative explanations in which supernormality is clearly eliminated. By application of similar principles, supernormality can be eliminated as a factor in most if not all of the published examples. Three major categories can be discerned: (1) occult 2:1 A-V block, in which an idioventricular beat "retracts" an otherwise refractory barrier within the A-V node; (2) alternation between dissociated intranodal transmission pathways; and (3) "ventriculophasic" (vagal) depression of nodal conductivity.

The morphology of the sinus and atrioventricular (A-V) nodes of human hearts was studied and connecting pathways between these nodes were sought. The cells of both nodes differed from ordinary atrial and ventricular myocardium in respect to size, shape, number, and arrangement of cytoplasmic organelles. Using the criteria of fiber-to-fiber connection, shown by means of serial histological sectioning, this study confirmed previous reports of three pathways between the sinus and A-V nodes in the human heart. It was not required that these pathways be composed of classic Purkinje-type fibers, though many of the fibers did possess some of the features of Purkinje fibers. While recent evidence reveals that specialized atrial pathways do demonstrate increased conduction velocity, their prime functional significance may be in the facilitation of orderly atrial depolarization, the maintenance of sinus node control of ventricular depolarization under various physiological conditions, and the provision of an orderly "input" into the A-V node, a tissue generally acknowledged to have a low margin of safety.