A 24-year-old man presents with two hours of palpitations. He is alert, has no chest pain, and his blood pressure is 124/76 mmHg. The radial pulse is regular at 150 beats per minute. His electrocardiogram shows regular atrial deflections at 300 per minute; every second atrial deflection is followed by a ventricular complex, and each ventricular complex is of normal width and configuration.
Prolonged refractoriness of which one of the following structures best accounts for this man's ventricular rate of 150 per minute?