Mechanosensitive Primary Bladder Afferent Activity in Rats With and Without Spinal Cord Transection

Purpose: Spinal cord injury induces functional and morphological changes in bladder afferent pathways. However, direct evidence for changes in the excitability of afferent nerve activity primarily originating from the bladder has not been clearly demonstrated. Thus, we determined the characteristics of peripheral mechanosensitive bladder afferents in the pelvic nerve and possible afferent changes in A delta and C fibers after spinal cord injury. Materials and Methods: Adult female rats were divided into 2 groups, including spinal cord injured and neurologically intact animals. In the spinal cord injury group the spinal cord was transected at Th9 at 4 weeks before functional experiments. For single unit afferent activity monitoring fine filaments were dissected from the L6 dorsal root and bladder afferent fibers were identified. Single unit afferent activity was studied during constant filling with saline. Results: Two afferent patterns were linked to small phasic increases in intravesical pressure during bladder filling, including accelerated and nonaccelerated types. The incidence of the accelerated type was significantly higher in the spinal cord injury group than in the neurologically intact group regarding A delta and C fibers. However, we found no relationship between conduction velocity and the functional properties of bladder mechanosensitive afferent fibers in neurologically intact or spinal cord injured rats. Conclusions: Results indicate that mechanosensitive bladder afferent activity has several patterns and is facilitated after spinal cord injury, especially in concert with small bladder contractions (micromotions). The functional properties of these individual afferent fibers are not related in an obvious manner to their conduction velocity and, thus, probably the afferent fiber type.