Electrophysiological and electron microscopic analysis of the vagus nerve of the pigeon, with particular reference to the cardiac innervation

Abstract

The compound action potential components and their associated fiber contingents were investigated in the pigeon vagus nerve with a view toward identifying the vagal cardioinhibitory fibers. In the cervical vagus, the compound action potential evoked by electrical stimulation included four major components that conducted at 17.0–30.0 (A-wave), 8.0–14.5 (B1-wave), 4.4–7.0 (B2-wave) and 0.8–2.0 (C-wave) m/sec. Cardiac slowing was not elicited until activation of the B1-wave, and the bradycardiac response was maximal when this component was maximized. Electron microscopic analysis of the cervical vagus revealed myelinated fibers 1.1–6.8 μm in diameter and unmyelinated fibers 0.3–1.4 μm in diameter. A contingent of myelinated fibers approximately 2–4 μm in diameter apparently generated the B1-wave, while the prominent unmyelinated fiber contingent (37%) accounted for the C-wave. Analysis of various vagal branches indicated that approximately 20% of the cervical vagal fibers exit the main trunk between cervical and mid-thoracic levels, but few of these are the larger myelinated fibers 2 μm in diameter. The upper abdominal vagus consists largely of unmyelinated and small myelinated fibers, and consequently the vast majority of larger myelinated fibers found in the cervical vagus exit between mid-thoracic and upper abdominal levels, presumably in the cardiac branches. Direct examination of the cardiac branches confirmed this. Thus, it is concluded that the B1-wave of the compound action potential is uniquely associated with cardiac slowing, that this component is generated by myelinated fibers ranging from 2 to 4 μm in diameter, and that almost all such fibers are destined for the cardiac branches of the vagus.