The facial nerve (cranial nerve VII) has two distinct roots.
The Two Roots of the Facial Nerve
The facial nerve isn't just one bundle of fibers; it's actually comprised of two roots that join together before exiting the skull. Here’s a breakdown of these two roots:
- Motor Root: This is the larger of the two roots and is primarily responsible for controlling the muscles of facial expression. This is the nerve that allows us to smile, frown, and make other facial movements.
- Nervus Intermedius: This is the smaller root, and it has a mixed function. As mentioned in the reference, it carries:
- Sensory fibers: Responsible for carrying general sensation from parts of the external ear.
- Taste fibers: It conducts taste sensations from the anterior two-thirds of the tongue.
- Parasympathetic fibers: These fibers control the lacrimal glands (tear production), salivary glands, and mucous glands of the nasal cavity and palate.
How the Roots Join
The two roots, the motor root and the nervus intermedius, come together within the temporal bone to form the facial nerve trunk (also known as cranial nerve VII). After merging, the facial nerve travels through the facial canal, a tunnel within the temporal bone, before exiting the skull.
Summary
| Root | Function | Size |
|---|---|---|
| Motor Root | Controls facial muscles (expression) | Larger |
| Nervus Intermedius | Sensory, taste, and parasympathetic functions (tear and saliva production) | Smaller |
These two roots ultimately combine to create the single facial nerve, which then travels through the skull to perform a variety of critical functions. The reference confirms that the facial nerve is comprised of these two roots: "a motor root and a smaller mixed sensory, taste and parasympathetic root, known as nervus intermedius...".
