Relaxation of the external urethral sphincter is caused byMultiple Choicesympathetic innervation of the sphincter.parasympathetic innervation of the sphincter.signals from the brain.increased action potentials to the sphincter.

The relaxation of the external urethral sphincter is caused by signals from the brain. This process is part of the micturition reflex, which is controlled by the central nervous system. Here's a step-by-step explanation:

1. The bladder fills with urine, which triggers stretch receptors in the bladder wall.
2. These receptors send signals to the brain via sensory neurons.
3. In response, the brain sends signals back to the bladder via motor neurons.
4. These signals cause the detrusor muscle (which forms the bulk of the bladder wall) to contract, and the internal urethral sphincter (which is involuntary) to relax.
5. At the same time, the brain sends signals to the external urethral sphincter (which is voluntary) causing it to relax and allow urine to flow out of the bladder and through the urethra.

So, the correct answer is "signals from the brain".