The Olfactory Nerve
Inferior view of the human brain, with the cranial nerves labelled.
|Anatomical terms of neuroanatomy|
The olfactory nerve (Latin: Nervus olfactorius) is typically considered the first cranial nerve, or simply CN I. It transmits impulses that convey sense of smell. Derived from the embryonic nasal placode, the olfactory nerve is somewhat unique among cranial nerves because it is capable of some regeneration if damaged. The olfactory nerve is sensory in nature and originates on the olfactory mucosa in the upper part of the nasal cavity. From the olfactory mucosa, the nerve (actually many small nerve fascicles) travels up through the cribriform plate of the ethmoid bone to reach the surface of the brain. Here the fascicles enter the olfactory bulb and synapse there; from the bulbs (one on each side) the olfactory information is transmitted into the brain via the olfactory tract. The fascicles of the olfactory nerve are not visible on a cadaver brain because they are severed upon removal. :548
The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity. The olfactory nerves consist of a collection of many sensory nerve fibers that extend from the olfactory epithelium to the olfactory bulb, passing through the many openings of the cribriform plate of the ethmoid bone, a sieve-like structure.
The sense of smell (olfaction) arises from the stimulation of receptors by small molecules in inspired air of varying spatial, chemical, and electrical properties that reach the nasal epithelium in the nasal cavity during inhalation. These stimulants are transduced into electrical activity in the olfactory neurons, which then transmits these impulses to the olfactory bulb and from there to the rest of the central nervous system via the olfactory tract.
The functional components of the olfactory nerve include SVA, special visceral afferent, which carries the modality of smell.
Damage to this nerve impairs the sense of smell. To simply test the function of the olfactory nerve, one of the patient's nostrils is closed and a substance with a pungent odor (such as damp coffee essence) is placed under the open nostril. The test is then repeated on the other nostril. If the patient can smell the substance the patient’s olfactory nerve is likely functioning. There are olfactory testing packets in which strong odors are embedded into cards and the responses of the patient to each odor can be determined. 
Lesions to the olfactory nerve can occur because of blunt trauma, such as coup-contra-coup damage, meningitis, and tumors of the frontal lobe. They often lead to a reduced ability to taste and smell. However, lesions of the olfactory nerve do not lead to a reduced ability to sense pain from the nasal epithelium. This is because pain from the nasal epithelium is not carried to the central nervous system by the olfactory nerve; rather, it is carried to the central nervous system by the trigeminal nerve (cranial nerve V).
Aging and Olfaction
Decreases in the ability to smell is a normal consequence of human aging, and usually is more pronounced in men than in women. It is often unrecognized in patients except that they may note a decreased ability to taste (much of taste is actually based on reception of food odor). Some of this decrease results from repeated damage to the olfactory nerve receptors due likely to repeated upper respiratory infections. Patients with Alzheimer's disease almost always have an abnormal sense of smell when tested. 
Olfactory ensheathing cells surround the olfactory nerves and aid axonal growth and repair in the Central nervous system. They have recently been used in an Autotransplantation operation to restore the ability to walk in a man who's spinal chord was completely sectioned. The olfactory nerve is unique in the CNS as the nerve fibres are constantly regenerating. The ensheathing glia are critical in this process, suggesting their capacity to be used to aid regeneration elsewhere in the CNS.
|Wikimedia Commons has media related to Olfactory nerve.|
- cranialnerves at The Anatomy Lesson by Wesley Norman (Georgetown University) (I)
- Notes on Olfactory Nerve
- Mcgraw Hill's Anatomy and Physiology Revealed
- Vilensky, Joel; Robertson, Wendy; Suarez-Quian, Carlos (2015). The Clinical Anatomy of the Cranial Nerves: The Nerves of "On Olympus Towering Top". Ames, Iowa: Wiley-Blackwell. ISBN 978-1118492017.
- Saladin, Kenneth. "The Cranial Nerves." Anatomy and Physiology: The Unity of Form and Function. 6th ed. New York City: Mcgraw-Hill, 2012. 548.