Final answer:
The pudendal nerve innervates the muscle responsible for expelling urine or semen from the urethra, controlling the external urinary sphincter's voluntary actions during micturition and ejaculation.
Step-by-step explanation:
The muscle that helps expel the final urine from the urethra or ejaculate semen is innervated by the pudendal nerve. The pudendal nerve arises from the sacral micturition center and controls the skeletal external urinary sphincter. This sphincter is under voluntary control and is responsible for maintaining continence during bladder filling as well as allowing for urination and ejaculation.
During the voluntary phase of micturition or ejaculation, somatic motor neurons associated with the pudendal nerve need to be inhibited to relax the external urethral sphincter. This process is necessary for urine to leave the body through the external urethral orifice. The role of the pudendal nerve is essential for voluntary control over these functions.