Conduction velocity distribution of afferent fibers innervating the rat urinary bladder

The conduction velocities of individual afferent fibers innervating the rat urinary bladder were determined by the antidromic stimulation of dorsal roots while recording from bladder postganglionic nerves. Conduction velocities ranged from 0.5 to 21.0 m/s; 70% of the velocities were less than 2.5 m/s. The distribution within the dorsal roots was ipsilateral with 84% in L6 and 16% in S1. Neuroanatomical tracing with horseradish peroxidase applied to individual bladder postganglionic nerves resulted in ipsilateral labeling of dorsal root ganglion cells with 77% in L6, 20% in S1, and 3% in L1-L2. Ultrastructural examination of bladder postganglionic nerves revealed some myelinated fibers (average diameter: 2.6 microns) and many unmyelinated fibers. Therefore, in the rat, most of the bladder afferent fibers appear to be unmyelinated, although a population of myelinated afferent fibers is also present. Bladder afferent fibers enter the spinal cord mainly in segment L6 with a minor fraction entering in S1.