A 39-year-old man is unable to expel the last drops of urine from the urethra at the end of micturition because of paralysis of the external urethral sphincter and bulbospongiosus muscles. This condition may occur as a result of injury to which of the following nervous structures
First, I need to recall the innervation of those muscles. The external urethral sphincter is under voluntary control, so it's innervated by the somatic nervous system. The bulbospongiosus muscle is part of the perineal muscles, which are also somatically innervated.
The somatic innervation to these muscles comes from the pudendal nerve. The pudendal nerve originates from the sacral plexus, specifically S2-S4. So if there's an injury to the pudendal nerve, it could paralyze these muscles. Let me check: the external urethral sphincter is innervated by the pudendal nerve via its perineal branch. The bulbospongiosus is also innervated by the same nerve.
Now, looking at the options. The correct answer would be the pudendal nerve. The other options might include nerves like the hypogastric, which is part of the autonomic nervous system and doesn't control these muscles. The inferior rectal nerve is a branch of the pudendal nerve but mainly supplies the anal region. The perineal nerve is actually a branch of the pudendal nerve, so if the question lists the perineal nerve as an option, that's a distractor because the root is the pudendal.
Injuries to the pudendal nerve can occur from trauma, prolonged sitting, or childbirth. The key here is that the external urethral sphincter is voluntary and needs the pudendal nerve. If that's damaged, the man can't voluntarily contract the sphincter to expel the last urine. Also, bulbospongiosus is involved in ejaculation and helping with urination, so its paralysis would contribute to the problem.
Clinical pearl: Remember that the pudendal nerve is crucial for voluntary control of the external urethral sphincter and bulbospongiosus. Injury here leads to urinary dribbling and issues with ejaculation. The autonomic nerves (like hypogastric) control the internal sphincter, which is involuntary.
**Core Concept**
The external urethral sphincter and bulbospongiosus muscles are voluntarily controlled by the **pudendal nerve**, a somatic nerve arising from the **sacral plexus (S2-S4)**. Injury to this nerve disrupts voluntary contraction of these muscles, impairing the ability to fully empty the bladder.
**Why the Correct Answer is Right**
The **pudendal nerve** innervates the external urethral sphincter via its **perineal branch** and the bulbospongiosus muscle via its **dorsal nerve of the clitoris/penis**. These muscles are essential for voluntary urination and ejaculation. Damage to the pudendal nerve (e.g., from pelvic trauma, childbirth, or prolonged cycling) causes flaccid paralysis, preventing the final expulsion of urine due to loss of voluntary sphincter control.
**Why Each Wrong Option is Incorrect**
**Option