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Spinal Cord Injury: Progress, Promise, and Priorities
contract and can be readily stretched. Large volumes of urine overfill the bladder and back up to the kidneys (Kaplan et al., 1991).
Two common types of bladder conditions occur in individuals with spinal cord injuries at levels above the sacral cord. The first is detrusor hyperreflexia, in which the bladder is overreactive. As the bladder fills with small volumes of urine, the detrusor muscle contracts prematurely, causing frequent urination. Research with animals suggests that part of the pathological process occurs in the sensory nerves coming from the bladder. Sensory fibers normally carrying other types of information actually switch their functioning: they become sensitive to bladder distention and trigger bladder detrusor contraction (de Groat, 1995). This form of sensory plasticity is mediated by changes in electrical properties of C fibers, a particular type of sensory neuron (Yoshimura, 1999).
Less is known about the biological basis of the second type of bladder dysfunction, detrusor-sphincter dyssynergia. This condition is marked by involuntary contractions of the sphincter muscles, which prevent urine from leaving the bladder. It can occur with the loss of the reciprocal relationship between detrusor muscle contraction and sphincter muscle relaxation. One hypothesis is that the condition is related to the reduced activity of the neurotransmitter nitric oxide in sphincter muscles. Nitric oxide is involved in relaxation of the sphincter. Reduced levels would therefore increase sphincter contraction (Mamas et al., 2003). Detrusor-sphincter dyssynergia can also arise from lesions to the pontine reticular nucleus and the reticular formation (Sakakibara et al., 1996).
Bowel Function Disruption
Bowel dysfunction frequently occurs after a spinal cord injury because the brain and spinal cord have major roles in stool elimination. Although the movement of feces down the length of the bowel is partly controlled by independent neurocircuits that reside within the bowel,4 the brain and spinal cord are essential for voluntary control over defecation. Loss of bowel function is so deeply distressing and embarrassing to individuals with spinal cord injuries that it affects their social interactions and their willingness to engage in sexual activities.
The impact of a spinal cord injury on voluntary control of the bowel is known as neurogenic bowel. Neurogenic bowel comes in two types—reflexic and areflexic—depending on the location of the injury. Reflexic
When the bowel wall is stretched, the local neurocircuits cause the muscles above the stretched area to constrict, whereas those below the stretched area are induced to relax, thus propelling feces down the bowel toward the anus.