changes in the types or numbers of ion channels in sensory neurons and pathways that render them more excitable (Hains et al., 2003b);
sprouting of sensory fibers entering the spinal cord;
alterations in post-receptor signal transduction mechanisms; and
switching of the identities of sensory fibers from non-pain fibers to pain fibers (Bryce and Ragnarsson, 2002; Hulsebosch, 2002).
Many of these mechanisms also apply to other pain conditions not associated with spinal cord injuries (Woolf and Mannion, 1999).
The brain plays a large role in modulating and interpreting the sensation of pain, so much so that experts describe pain as an “experience” rather than a sensation. Multiple areas of the cerebral cortex process pain information relayed there by a certain tract in the spinal cord, which receives its information from incoming peripheral nerves. The brain, in turn, modulates the incoming messages through several descending pathways from nuclei in the midbrain, including the periaqueductal gray.
Three common types of bladder dysfunction accompany spinal cord injuries, depending on the level of the injury (Kaplan et al., 1991). Understanding of the types of dysfunction first requires some understanding of the anatomy of the bladder and its control by the spinal cord and the brain. Two main muscle groups surrounding the bladder control urination: the detrusor muscle, which controls bladder contraction, and the external sphincter muscles at the base of the bladder, which control bladder outflow. The two muscles normally work reciprocal to one another: the detrusor muscle contracts while the sphincter muscles relax, allowing urine to flow from the bladder. Because each is fed by separate nerves, their coordination—i.e., detrusor muscle contraction with sphincter muscle relaxation—is integrated at a higher level, which, in this case, is performed by the pons region of the brain. That portion of the brain sends its axons to the sacral region of the spinal cord (S2 and S3), which also receives sensory input from the bladder (via the pelvic nerve) about bladder distention. When the pelvic nerve conveys the message that the bladder is full, the information is relayed up to the pons, which then coordinates the motor messages necessary to empty the bladder. This process is called the voiding reflex.
In individuals with complete spinal cord injuries above the level of the sacral cord, disruption of the pathway from the spinal cord to the brain can lead to bladder problems related to the lack of coordination between the detrusor and the sphincter muscles (see below). If the sacral cord or the cauda equina is injured directly, the bladder detrusor muscle becomes flaccid—a condition known as areflexia. The detrusor muscle loses its ability to