The left inferior cardiac nerves of nine adult cats were examined with the electron microscope. Electrical recordings were also made from four of these nerves. Most of the nerves contained between 25,000 and 40,000 unmyelinated fibers and 30 to 150 myelinated fibers. The diameters of the unmyelinated fibers ranged from 0.1 μ to 1.8 μ with mean diameters ranging from 0.47 to 0.78 μ in the various nerves. The myelinated fibers ranged in diameter from 1 μ to 9 μ with a mean diameter of approximately 3 μ. Electrical recordings showed two peaks in both the C fiber and the Aδ fiber compound action potentials. Conduction velocities of the two C fiber peaks were approximately 0.7 and 0.9 m/sec. The two groups of Aδ fibers had conduction velocities of approximately 7 and 16 m/sec.
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