Final answer:
The external urethral sphincter does not have three parts but is a single muscle controlled by the somatic nervous system, allowing voluntary control of urination.
Step-by-step explanation:
The student has asked about the three parts of the external urethral sphincter, which is involved in the process of urination. However, the question seems to imply a slight misconception as the external urethral sphincter itself is not divided into three distinct parts. Instead, it is better classified as a single striated muscle that plays a key role in controlling the passage of urine. This muscle can be consciously controlled thanks to its regulation by the somatic nervous system, which allows for voluntary control of urination through the relaxation of this sphincter. It is innervated mainly by the pudendal nerve arising from the sacral plexus, particularly via the S2-S4 nerves.
The urethra also includes an internal urinary sphincter made of smooth muscle, which operates involuntarily under autonomic nervous system control. This sphincter relaxes as the bladder fills, allowing urine to enter the urethra. Upon urination, the smooth muscle lining of the urethra relaxes, while closing the external urethral opening involves voluntary contraction of the external urethral sphincter.