Spinal mechanisms contributing to urethral striated sphincter control during continence and micturition: "How good things might go bad"

The external urethral sphincter motoneurons in the sacral ventral horn control the striated external urethral sphincter muscles that circle the urethra. Activity in these motoneurons and muscle normally contribute to continence but during micturition, when urine must pass through the urethra, the motoneurons and striated muscle must be silenced. Following injury to descending pathways in the spinal cord, the ability to inhibit sphincter activity is disrupted or lost, resulting in bladder-sphincter dyssynergia and functional obstruction of the urethra during voiding. This chapter will first review the various reflex pathways and neuronal properties that contribute to continence, and which must be modulated during micturition in the spinal intact animal. A discussion about how the dyssynergia seen with spinal cord injury may be produced will then be presented.