Micturition is initiated by activation of afferent sensory fibers located in the wall of the bladder; these fibers sense the degree of stretch of the bladder wall. In addition, these sensory fibers travel back to the spinal cord via sacral dorsal roots. The sensory information that reaches the spinal cord also travels to the micturition center in the rostral pons. When sensory activity to the micturition center is sufficient, a command is sent to the sacral spinal cord, leading to activation of parasympathetic fibers. The parasympathetic fibers travel to the bladder via the pelvic nerve. Activation of these fibers leads to bladder contraction. Destruction of the sacral dorsal roots (as occurs with tabes dorsalis) abolishes the reflex because sensory afferent fibers no longer send signals back to the spinal cord. A spinal cord injury at L1 is well above the sacral region where the micturition reflex originates. How ever, the central nervous system (CNS) plays an important role in facilitating or inhibiting the micturition reflex, and this function is lost with spinal cord injury. Although some patients with spinal cord injury can still elicit a micturition response (e.g., stroking of the skin in the genital region), the bladder in these patients has increased muscle tone and fails to empty completely. As the bladder becomes more and more distended, an involuntary micturition reflex can occur. However, the CNS can keep urine from being voided under these circumstances by maintaining a constant tonic contraction of the external sphincter. This contraction is accomplished through continued activation of so matic nerves that travel in the pudendal nerve from the sacral spinal cord to the external sphincter. The point at which an urge to void first occurs corresponds to a bladder volume of approximately 150 ml. However, bladder volume can increase more than twofold before involuntary micturition occurs. At 400 ml a marked sense of fullness is present. Parasympathetic fibers originating in the sacral spinal cord innervate the body of the bladder, and it is activation of these fibers that leads to bladder contraction.