The Olfactory Nerve (CN I)
The olfactory nerve, CN I, is the first and shortest cranial nerve. It is the nerve which transmits special sensory information, allowing us to have a sense of smell. It is one of two nerves that do not join with the brainstem, the other being the optic nerve. It is similar to the optic nerve also in its structure, as it has a meningeal covering unlike CN III to XII.
Embryologically it is derived from the olfactory placode (a thickening of the ectoderm layer), which also give rise to the glial cells which support the nerve.
The olfactory nerve is also capable of regeneration. Olfactory nerves are unmyelinated and covered by Schwann cells.
The nerve consists of two processes:
- Peripheral olfactory processes (receptors) in the olfactory mucosa.
- Central processes that return the information to the brain.
Once the axon penetrates through the basement membrane, it joins other non-myelinated processes to form the fila olfactoria (bundles of olfactory axons). They then enter the cranial cavity through the cribriform plate of the ethmoid bone – the roof of the nasal cavity.
Once in the cranial cavity, the fibres enter the olfactory bulb, which lies in the olfactory groove, within the anterior cranial fossa.
The olfactory bulb is an ovoid structure which contains specialised neurones, called mitral cells. The olfactory nerve fibres synapse with the mitral cells, forming collections known as synaptic glomeruli. From the glomeruli, second order nerves then pass posteriorly into the olfactory tract.
The olfactory tract runs inferiorly to the frontal lobe. As the tract reaches the anterior perforated substance, it divides into medial and lateral stria:
- The lateral stria sends carries the axons to the olfactory area of the cerebral cortex (also known as the primary olfactory cortex).
- The medial stria carry the axons across the medial plane of the anterior commissure where they meet the olfactory bulb of the opposite side.
The primary olfactory cortex sends nerve fibres to many other areas of the brain, notably the piriform cortex, the amygdala, olfactory tubercle and the secondary olfactory cortex. These areas are involved in the memory and appreciation of olfactory sensations.
The olfactory mucosa is a very important structure as it not only senses smell, but also the more advanced aspects of taste. It is located in the roof of the nasal cavity and is composed of pseudostratified columnar epithelium which contains a number of cells.
- Basal cells: Form the new stem cells from which the new olfactory cells can develop.
- Sustentacular cells: Tall cells for structural support. These are analogous to the glial cells located in the CNS.
- Olfactory receptor cells: bipolar neurons which have two processes, a dendritic process and a central process. The dendritic process projects to the surface of the epithelium, where they project a number of short cilia, the olfactory hairs, into the mucous membrane. These cilia react to odors in the air and stimulate the olfactory cells. The central process (also known as the axon) projects in the opposite direction through the basement membrane.
In addition to the epithelium, there are Bowman’s glands present in the mucosa, which secrete mucus.
Clinical Relevance: Anosmia
Anosmia is defined as the absence of the sense of smell. It can be temporary, permanent, progressive or congenital.
- Temporary anosmia can be caused by infection or by local disorders of the nose.
- Permanent anosmia can be caused by head injury, or tumours which occur in the olfactory groove (e.g. meningioma).
- Anosmia can also occur as a result of neurodegenerative conditions, such as Parkinson’s or Alzheimer’s disease. In these conditions, the anosmia is progressive and precedes motor symptoms but it is not often noticed by the patient.
- Anosmia is also a feature of a number of genetic conditions such as Kallmann syndrome (failure to start or finish puberty) and Primary Ciliary Dyskinesia (defect in cilia causing it to be immobile)
Clinical Relevance: Testing the Olfactory Nerve
Ask the patient if they have noticed any changes in their food taste or sense of smell. Examining the nerve involves testing each nostril in turn, asking the patient to identify a certain smell (i.e. peppermint, coffee).