Pre-trematic Nerve of the Second Pharyngeal Arch
The chorda tympani is the pre-trematic nerve of the 2nd pharyngeal arch (answer c). 1
Anatomical Basis
The facial nerve (cranial nerve VII) is the nerve of the second pharyngeal (hyoid) arch and follows the classic branchiomeric pattern with both pre-trematic and post-trematic branches 1:
Pre-trematic Branch
- The chorda tympani represents the pre-trematic branch, arising just before the facial nerve exits the stylomastoid foramen 1
- This nerve passes through the middle ear cavity and distributes to structures derived from the mandibular (first) arch, though it originates from the second arch nerve 2
- The chorda tympani carries taste fibers from the anterior two-thirds of the tongue and preganglionic parasympathetic fibers to the submandibular ganglion 1
Post-trematic Branches
- The main trunk of the facial nerve after exiting the stylomastoid foramen represents post-trematic distribution 1
- These branches (temporal, zygomatic, buccal, mandibular, and cervical) supply muscles of facial expression derived from second arch mesoderm 1
- The nerve to the stylohyoid and posterior digastric muscles are also post-trematic branches 1
Embryological Context
The branchiomeric pattern is highly conserved throughout vertebrate evolution, with each pharyngeal arch having characteristic pre-trematic and post-trematic nerve distributions 3. Studies in chondrichthyes (cartilaginous fish) demonstrate that the homologue of the chorda tympani serves as the pre-trematic branch of the facial nerve, supplying the floor of the mouth 4.
Incorrect Options Explained
- Option a (nerve to submandibular gland): This is carried by the chorda tympani but is not itself a named nerve branch 1
- Option b (nerve to nasal mucosa): This is supplied by branches of the maxillary division of the trigeminal nerve, not the facial nerve
- Option d (nerve to parotid): The parotid receives parasympathetic innervation from the glossopharyngeal nerve (CN IX) via the lesser petrosal nerve 5
- Option e (nerve to lacrimal duct): Lacrimal gland innervation comes from the greater petrosal nerve (another facial nerve branch) via the pterygopalatine ganglion 1