The olfactory tract is the only cranial nerve that does not pass through a relay nucleus in the thalamus. The nerves carrying signals from olfactory receptors enter the skull via the cribriform plate.
The senses of proprioception, vision, hearing, and taste all enter the brain and synapse at relay nuclei in the thalamus before information is sent to higher cortical centers. By contrast, smell signals travel from olfactory receptor neurons, to the olfactory bulb, and then via mitral and tufted cells directly to the olfactory cortex without passing through the thalamus first.
Poncelet describes an approach to evaluating peripheral neuropathy in the primary care setting, with the first task being to establish whether the nerve involvement is focal, multifocal, or symmetric. Causes of neuropathies that may affect the cranial nerves include diabetes, HIV/AIDS, Lyme, Guillain-Barre syndrome, sarcoid, diptheria, and neoplasms of the skull base or meninges. However, these do not commonly affect the olfactory nerve, which is more likely to be affected by trauma or other causes such as URI, sinusitis, depression , or other medications.
Benarroch describes the organization of the olfactory system. He notes that olfactory deficits are found in diseases such as Parkinson disease, Alzheimer disease, Huntington disease, and frontotemporal dementia. The reasons for this are incompletely known, but likely to be due to degeneration of cells in the olfactory system at many levels and not just one location.
Illustration A from Benarroch is a diagram illustrating the various neurons and nuclei involved in transmission of olfactory sensory information from the olfactory receptor through to the cortex.
Answer 1: The V2 branch of the trigeminal nerve, carrying facial sensation, enters through the foramen rotundum.
Answer 2: The glossopharyngeal nerve, carrying taste from the posterior 1/3 of tongue among many things, enters via the jugular foramen.
Answer 3: The vestibulocochlear nerve, carrying hearing and vestibular information, enters through the internal auditory meatus.
Answer 4: The V1 branch of the trigeminal nerve, carrying facial sensation. enters through the superior orbital fissure (the other nerves that traverse this fissure are motor nerves, not sensory).
Poncelet AN. An algorithm for the evaluation of peripheral neuropathy. Am Fam Physician. 1998 Feb 15;57(4):755-64. Review. PubMed PMID: 9490998
PMID:9490998 (Link to Abstract)
Benarroch EE. Olfactory system: functional organization and involvement in neurodegenerative disease. Neurology. 2010 Sep 21;75(12):1104-9. doi: 10.1212/WNL.0b013e3181f3db84. Review. PubMed PMID: 20855854
PMID:20855854 (Link to Abstract)