Central projections of the ethmoidal nerve of the cat as determined by the horseradish peroxidase tracer technique.

The ethmoidal nerve innervates the nasal mucosa and constitutes the afferent limb of several upper airway protective reflexes. Protective reflexes, such as sneezing, coughing, and apnea, are those reflexes that either expel foreign substances from the respiratory tract or stop them from gaining access to the lungs. The afferents for nasal receptors are thought to be a part of the trigeminal system rather than olfactory in nature. The objective of this study was to localize the cell bodies of these ethmoidal afferents and to trace the central projections of these neurons. Horseradish peroxidase was applied to the ethmoidal nerve in 11 adult cats. Following a survival period of 48-72 hours, the animals were killed and the tissue was processed according to the tetramethylbenzidine method. Reaction product was localized in cell bodies within the trigeminal ganglion, concentrated caudal to the entrance of the ophthalmic trunk of the trigeminal nerve. Transganglionic projections to the spinal trigeminal nucleus were localized primarily in the subnucleus interpolaris and in layers I and II of the subnucleus caudalis. There was also reaction product in cell bodies within the mesencephalic trigeminal nucleus. These results are in keeping with projections of other ophthalmic division receptor afferents, such as the cornea and the supraorbital nerve.