Which of the following is not a mixed nerve?
**Core Concept**
A mixed nerve is a type of nerve that contains both sensory and motor fibers, allowing for bidirectional communication between the central nervous system and the peripheral nervous system. These nerves are responsible for transmitting sensory information from the periphery to the central nervous system and motor signals from the central nervous system to the periphery.
**Why the Correct Answer is Right**
Mixed nerves typically have both sensory and motor fibers, which are necessary for functions such as sensation, movement, and reflexes. The presence of both sensory and motor fibers is a defining characteristic of mixed nerves. For example, the sciatic nerve, which is a mixed nerve, contains both sensory fibers that transmit pain and touch from the lower extremities and motor fibers that control the muscles of the lower extremities.
**Why Each Wrong Option is Incorrect**
**Option A:** The vagus nerve is a mixed nerve that contains both sensory and motor fibers, making it a correct example of a mixed nerve.
**Option B:** The phrenic nerve is a branch of the cervical plexus and contains both sensory and motor fibers, making it a correct example of a mixed nerve.
**Option C:** The hypoglossal nerve is a purely motor nerve that controls the muscles of the tongue, making it an incorrect example of a mixed nerve.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that mixed nerves have both sensory and motor fibers, which are responsible for various functions such as sensation, movement, and reflexes.
**Correct Answer: C. The hypoglossal nerve is a purely motor nerve that controls the muscles of the tongue, making it an incorrect example of a mixed nerve.**