Olfactory Nerve: What is it? Processes, Anatomical Course, Sensory Function and Clinical Relevance

It is the first and shortest cranial nerve, which transmits special sensory information, which allows us to have a sense of smell.

It is one of the two nerves that do not join with the brainstem, the other is the optic nerve. It is similar to the optic nerve also in its structure, since it has a meningeal cover unlike CN III to XII.

Embryologically it is derived from the olfactory placode (a thickening of the ectoderm layer), which also gives rise to the glial cells that support the nerve. The olfactory nerve is also able to regenerate. The olfactory nerves are not myelinated and are covered by Schwann cells.

The olfactory nerve is the first of the 12 cranial nerves located inside the head. It transmits sensory data to the brain, and is responsible for the sense of smell. The olfactory receptors of the nerve are found within the mucosa of the nasal cavity. Unlike many other nerves, CN1 does not have two trunks.

On the contrary, its sensory fibers extend through the cribrous plate of the ethmoidal bone, a part of the skull located behind the nose. Once the chemicals and particles in the air enter the nasal cavity, they interact with these neuronal receptors.

CN1 does not attach to the brainstem. She and the optic nerve are the only cranial nerves for which this is true. CN1 is the shortest cranial nerve within the human head.

It can be susceptible to injuries created by blunt traumatic damage, which can be the result of complications of frontal lobe brain tumors, meningitis and some other factors.

This will lead to a reduced or absent sense of smell. However, even if the CN1 is damaged, the nasal pain will still be transmitted through the trigeminal nerve.

The nerve consists of two processes:

  • Peripheral olfactory processes (receptors) in the olfactory mucosa.
  • Central processes that return information to the brain.

The olfactory nerve is responsible for the sense of smell. The nasociliary and nasopalatine nerves provide a general feeling.

Anatomical course

Once the axon penetrates through the basement membrane, it joins other non-myelinated processes to form the olfactory row (bundles of olfactory axons). Then they enter the cranial cavity through the cribrous plate of the ethmoidal bone, the roof of the nasal cavity.

Olfactory bulb

Once in the cranial cavity, the fibers enter the olfactory bulb, which is in the olfactory sulcus, inside the anterior cranial fossa.

The olfactory bulb is an ovoid structure that contains specialized neurons, called mitral cells. Olfactory nerve fibers synapse with mitral cells, forming collections known as synaptic glomeruli. From the glomeruli, second-order nerves then pass into the olfactory tract.

Olfactory tract

The olfactory tract runs inferiorly to the frontal lobe. As the tract reaches the previous perforated substance, it is divided into:

  • Medial estria.
  • Lateral stria

Sending the lateral stria carries the axons to the olfactory area of ​​the cerebral cortex (also known as the primary olfactory cortex).

The medial stria carries the axons through the medial plane of the anterior commissure where they meet the olfactory bulb on the opposite side.

The primary olfactory cortex sends nerve fibers to many other areas of the brain, notably the piriform cortex, the amygdala, the olfactory tubercle, and the secondary olfactory cortex. These areas are involved in memory and the appreciation of olfactory sensations.

Sensory function

Olfactory mucosa

The olfactory mucosa is a very important structure, since it not only detects smell, but also the most advanced aspects of taste. It is found on the roof of the nasal cavity and is composed of pseudostratified columnar epithelium that contains several cells.

Basal cells: form the new stem cells from which the new olfactory cells can develop.

Stem cells : high cells for structural support. These are analogous to the glial cells located in the CNS.

Olfactory receptor cells : bipolar neurons that have two processes, a dendritic process and a central process. The dendritic process is projected towards the surface of the epithelium, where a series of short cilia, the olfactory hairs, project towards the mucous membrane.

These cilia react to odors in the air and stimulate the olfactory cells. The central process (also known as the axon) is projected in the opposite direction through the basement membrane.

In addition to the epithelium, there are Bowman 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 an infection or by local disorders of the nose. Permanent anosmia can be caused by a head injury, or tumors that occur in the olfactory sulcus (for example, meningioma).

Anosmia can also occur as a result of neurodegenerative conditions, such as Parkinson’s or Alzheimer’s disease. Under these conditions, anosmia is progressive and precedes motor symptoms, but is often not noticed by the patient.

Anosmia is also a characteristic of several genetic diseases such as Kallmann’s syndrome (lack of onset or end of puberty) and primary ciliary dyskinesia (defect in the cilia that makes it immobile)

Olfactory nerve test

Ask the patient if they have noticed any change in their taste or sense of smell. Examining the nerve involves examining each nostril in turn, asking the patient to identify a certain smell (ie, mint, coffee).