Behavior of left ventricular mechanoreceptors with myelinated and nonmyelinated afferent vagal fibers in cats.

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Abstract
The purpose of this study was to determine the behavior of left ventricular mechanoreceptors with myelinated vagal afferents and to compare them with endings with nonmyelinated vagal afferents. Single unit activity was recorded from 13 endings with nonmyelinated vagal afferents (conduction velocity 2.1 +/- 0.3 m/sec) and from 16 endings with myelinated vagal afferents (conduction velocity 7.3 +/- 1.3 m/sec). Resting discharge frequencies of nonmyelinated afferents and of myelinated vagal afferents were 1.7 +/- 0.3 and 2.7 +/- 0.5 imp/sec (P less than 0.1), respectively (at left ventricular end diastolic pressure of 6 mm Hg for both groups). Ten of 16 myelinated vagal afferents had pulse synchronous discharge under basal condition, whereas only 3 of 13 nonmyelinated vagal afferents had such activity. During aortic occlusion, the discharge of myelinated vagal afferents increased 1.7 +/- 0.3 imp/sec per mm Hg, whereas nonmyelinated vagal afferents increased significantly (P less than 0.05) less (0.5 +/- 0.1 imp/sec per mm Hg). Discharge for both groups was linearly related to left ventricular end-diastolic pressure but not to left ventricular systolic pressure. Increases in left ventricular systolic pressure alone did not increase firing for either group. During aortic occlusion, the maximum discharge rates of myelinated vagal afferents (43 +/- 7 imp/sec) were significantly higher than those of nonmyelinated vagal afferents (14 +/- 3 imp/sec) at left ventricular end-diastolic pressure of 30 +/- 2 and 24 +/- 2 mm Hg, respectively. Both groups increased their discharge during volume expansion with myelinated vagal afferents showing greater sensitivity than nonmyelinated vagal afferents. All endings studied were in the inferoposterior wall of the left ventricle. All nonmyelinated vagal afferents were in or near the epicardium. In contrast, myelinated vagal afferents were equally distributed between the endocardium and the epicardium. Myelinated vagal afferents had discrete receptive fields (1-2 mm2) whereas those of nonmyelinated vagal afferents were much larger (1 cm2). In conclusion, the discharge of left ventricular endings with nonmyelinated vagal afferents and myelinated vagal afferents both appear to be determined mainly by changes in left ventricular end-diastolic pressure. They may be located at different depths in the left ventricular wall. Myelinated vagal afferents have greater sensitivity and maximum firing frequencies than nonmyelinated vagal afferents.