Micturition and urine storage are complex activities primarily driven by parasympathetic innervation from the sacral spinal cord and sympathetic innervation from the lower thoracic and upper lumbar spinal cord respectively. Additionally, we previously found that the detrusor is also innervated by a small number of direct inputs originating in the lower thoracic and upper lumbar ventral horns, the function of which is not yet understood. To characterize these direct fibers, we conducted in vivo and ex vivo pharmacological experiments.

Three female mixed-breed hounds underwent a laminectomy from spinal level T10 to S4. Roots originating from each level were stimulated and changes in pressure were recorded. Sacral roots and hypogastric nerves were then transected bilaterally. Neuromuscular nicotinic receptor antagonist atracurium was administered at 0.25 mg/kg and lower thoracic/upper lumbar roots were stimulated. Data were analyzed using an unpaired t-test. At euthanasia, the bladder was harvested and strips of smooth muscle devoid of mucosa were isolated from bladders. The contractile response to electric field stimulation was determined in the presence of 5 µM atracurium (n=18-24 strips from 5 animals). Data were analyzed using 2-way ANOVA followed by Tukey’s multiple comparisons test. Remaining bladder tissue was stored for future assays.

All three animals exhibited detrusor contractions from lower thoracic/upper lumbar stimulation. IV administration of atracurium significantly blocked detrusor contraction induced by stimulation of L2 ventral root in vivo (p=0.0029, n=3; Figure 1). In contrast, treatment with 5 µM atracurium did not block EFS-induced contractions in isolated detrusor smooth muscle strips ex vivo (Figure 2).

The presence of detrusor contractions after lower thoracic/upper lumbar root stimulation regardless of sacral root and hypogastric nerve transection suggests that the contractions are mediated by nerves other than the traditional sacral parasympathetic or hypogastric sympathetic innervation. A significant decrease in detrusor contraction after treatment with atracurium indicates that activity of this subpopulation of nerves is mediated by neuromuscular nicotinic receptors. Furthermore, the nicotinic receptors are not located in the intramural ganglia due to the absence of change in contractility of isolated bladder smooth muscle after treatment with atracurium ex vivo.

Functional motor innervation of the bladder from above the lumbosacral spinal cord may provide new options for treating urinary incontinence following lower spinal cord injury.