Different types of sensory nerve endings in the urinary bladder of mice arising from dorsal root ganglia (DRG) at the thoracolumbar region of vertebral column

The urinary bladder is innervated by two anatomically distinct populations of sensory neurons originating from dorsal root ganglia (DRG): one from the thoracolumbar (TL) and another from the lumbosacral (LS) spinal region. While we previously characterized the morphology and distribution of sensory endings arising from LS DRG, the terminals of TL afferents remain undefined. In this study, we employed anterograde neuronal tracing to map these endings in mice. Following bilateral injection of dextran biotin into T12–L3 DRG and a 7–9-day recovery, whole bladders were processed to visualize spinal afferent axons and their calcitonin gene-related peptide (CGRP) immunoreactivity. We identified four morphological types of sensory endings, similar to those described for LS DRG: simple and complex types in both the sub/urothelium and the detrusor muscle. However, their distribution was fundamentally different. Most TL afferents (79%) terminated within the sub/urothelium, with a minor proportion (21%) in the detrusor. This pattern is the inverse of LS innervation, which predominantly targets the detrusor (81%). Furthermore, nearly all TL endings (96%) were peptidergic (CGRP +). This stark anatomical and neurochemical segregation suggests that TL and LS spinal afferent populations are functionally specialized. We propose that the dense TL innervation of the sub/urothelium may constitute a protective surveillance system sensitive to chemical irritation and inflammation, whereas the predominant LS innervation of the detrusor is likely specialized for detecting mechanical stretch during bladder filling.