Cranial Nerve Review Series - CN I
The Olfactory Nerve - Anatomy and Histology
(This information is compiled from Wikipedia, Moore’s Clinically Oriented Anatomy, The NCBI Bookshelf and other sources. Please ask if you have any specific questions.)
Originating in the olfactory mucosa in the roof of the nasal cavity and along the nasal septum and medial wall of the superior concha, this special sensory (special visceral afferent) nerve terminates in the olfactory bulb, at the rostral end of the olfactory tract that attaches directly to the base of the forebrain (prosencephalon).
The above rendition (by Vesalius) shows olfactory bulbs (colored red) resting along the orbital surfaces of the frontal lobes of the cerebral hemispheres.
The olfactory epithelia house the cell bodies of neurosensory cells (also known as primary olfactory neurons or olfactory cells). The olfactory epithelium occupies about 2.5 cm2 of area at the apex of each nostril. This patch of yellowish brown mucosa is located in a small cavity off the main nasal passage. For this reason, “sniffing” provides more rapid stimulation than normal breathing. Within the lamina propria, below the olfactory epithelium, lie Bowman’s glands. These structures secrete mucus and IgA.
The central processes of the bipolar olfactory neurosensory cells form about 20 bundles of olfactory nerve fibers that together form the olfactory nerve. The arrowhead below points to the cell body of a bipolar cell.
Olfactory epithelium is a primitive type of sensory epithelium, lending support to the concept that olfaction is phylogenetically the oldest of the senses. The cell is both a receptor and a bipolar first-order neuron. A single dendrite projects from the apical pole of the cell to the surface of the epithelium. This dendrite ends in an apical dendritic knob (olfactory knob). Each knob gives rise to 5 to 20 long delicate nonmotile cilia, which extend into the mucus covering the sensory epithelium.
The olfactory neuron, unlike most other neurons, has a life span of only 30 to 40 days. New neurons differentiate from stem cells in the deepest or basal region of the olfactory epithelium. The basal pole of the neuron gives rise to a single unmyelinated axon. The axons form bundles, sheathed in Schwann cells, that traverse rostrally through the cribriform plate of the ethmoid bone, pierce the dura/arachnoid, and synapse in the olfactory bulb in the anterior cranial fossa.
The olfactory nerve fibers synapse with mitral cells (“mitral” comes from mitre meaning “bishop’s tall hat,” late 14c., from O.Fr.) in the olfactory bulb. The axons of these mitral cells form the olfactory tract. Each olfactory tract splits in to lateral and medial olfactory striae (distinct fiber bands).
The detection threshold for odorants is quite low: 10−13 to 10−4 molecules in air. Studies suggest that the volume concentration of receptor molecules in the mucus is in the range of 10−5 M. Each olfactory neuron has about 106 receptor molecules on its cilia. Odors penetrate the mucus overlying the sensory epithelium and gain access to the receptors by virtue of their partition and diffusion coefficients in the olfactory mucus. An odorant traverses the mucus in the range of a few dozen milliseconds, and forms a complex with the receptor in about the same time span. The odorant molecule combines with integral membrane proteins that form the receptor. The proteins and odorant-gated channels that mediate sensory transduction are located in the membranes of the olfactory cilia and the apical dendritic olfactory knob. Voltage-gated channels, located in the initial axonal segment and the axolemma, are associated with impulse initiation and propagation. The second messenger system is probably a G protein-adenylate cyclase cascade.
The olfactory nerve/tract has the shortest course of any cranial nerve. (Which has the longest?) Also peculiar to the olfactory nerve is that it is one of only two cranial nerves (the other being fibers from the optic nerve) which does not synapse in the brainstem but rather connects directly to the cortex. The lateral stria terminates in the piriform cortex—a highly evolutionarily conserved region from mammals to amphibians—while the medial olfactory stria projects through the anterior commissure to contralateral olfactory structures. There are also direct projections to the amygdaloid nucleus and to the anterior perforated substance. In addition, there are secondary and tertiary connections to the limbic system.
Clinical Significance -
from Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition.
Walker HK, Hall WD, Hurst JW, editors.
Boston: Butterworths; 1990.
(with some editorial comments by me)
Hyperosmia, or lowered threshold for odors, occurs with Addison’s disease and mucoviscidosis. Clinical perception of hyperosmia is ordinarily just about impossible either by history taking or by bedside testing.
Hypoosmia is usually caused by local processes that involve both the nasal and olfactory mucosa. Examples include rhinitis due to the common cold or allergy, smoking, certain industrial fumes, and intranasal polyps or carcinoma. Pernicious anemia, diabetes, and vitamin A deficiency cause diminished olfactory acuity. Pernicious anemia can also cause anosmia. Hypoosmia can occur after total laryngectomy. The reasons are not known.
Anosmia may be bilateral or unilateral. The patient can recognize bilateral anosmia, but unilateral anosmia is usually not perceived.
- Traumatic - head trauma is probably the most frequent cause, with an incidence of 7.5% in one large series. Blows to the occiput are five times more likely to produce anosmia than blows to the forehead because of the contrecoup effect. The injury can be so trivial as to go almost unnoticed.
- Neoplastic - tumors of the floor of the anterior fossa, such as meningiomas of the sphenoid ridge or olfactory groove, can produce anosmia, which is usually unilateral.
- Infectious - meningitis or abscess associated with osteomyelitis of the frontal or ethmoid bones can produce anosmia.
- Congenital absence of smell is present in albinos. (Interesting!)
- Vascular - Subarachnoid hemorrhage can cause anosmia.
- Psychiatric - hysteria is another cause for anosmia. Hysteria can be identified by comparing perception for coffee or vanilla with ammonia perception. Coffee and vanilla principally stimulate the olfactory cell receptors. Ammonia is a trigeminal nerve stimulator. In anosmia of organic cause the ammonia can be detected but the coffee or vanilla odor cannot.
Smell and Mate Preference - nanoreview by me
In 1976 Yamazaki et al published a seminal study showing that mice prefer to mate with partners that have dissimilar major histocompatibility (MHC) haplotypes. The evolutionary implications are fascinating. As Santos et al puts it,
It has been suggested, therefore, that animals use body odor as a guide to identify possible mates as MHC-similar or MHC-dissimilar from their own genotype. Preference for a MHC-dissimilar partner enhances MHC heterozygosity of an individual’s offspring. The possible adaptive advantages are clear: it is a mechanism of avoiding inbreeding and MHC-heterozygous offspring may have enhanced immunocompetence.
Numerous studies since 1976 have aimed to see if the same trend exists in Homo sapiens as it does in Mus musculus. Indeed, there is mounting evidence that mate choice in humans is influenced by “pheromones.” There is also genetic evidence showing that MHC and olfactory neuron receptor genes are linked, but the significance of this evidence is still not clear.
In the above Santos study, men and women were instructed to smell the sweat and urine of members of the opposite sex and rate whether or not they found the odors pleasing. Only one trend emerged: women find the sweat of a man who is MHC-dissimilar to have “pleasant” smelling sweat. Besides invoking the downright revolting image of smelling another person’s urine, this study suggests that there are perhaps evolutionary mechanisms in place that determine who we mate with—mechanisms which are out of our control. Fascinating.