The olfactory nerve, or cranial nerve one (CN I) is a special visceral afferent nerve, which transmits information relating to smell. It is the first and shortest cranial nerve and embryologically, the olfactory nerve is derived from the olfactory placode (a thickening of the ectoderm layer). This article shall look at the anatomy of the olfactory nerve - its structure, anatomical course and clinical relevance.
The sense of smell is detected by olfactory receptors located within the nasal epithelium. These receptors are composed of a dendritic process, which projects to the surface of the epithelium, and a central process, or axon, which projects in the opposite direction through the basement membrane. The dendritic process projects a number of short cilia, known as olfactory hairs, into the mucous membrane and these cilia react to odors in the air and stimulate the olfactory cells.
The axons of the olfactory receptors, known as fila olfactoria, assemble into small bundles of true olfactory nerves, which penetrate the small foramina in the cribriform plate of the ethmoid bone and enter the cranial cavity. This process is known as the anatomical course of the olfactory nerve and describes the transmission of special sensory information from the nasal epithelium to the primary olfactory cortex of the brain.
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. These mitral cells form synaptic glomeruli with the olfactory nerve fibres and from these glomeruli, second order nerves pass posteriorly into the olfactory tract.
The olfactory tract travels posteriorly on the inferior surface of the frontal lobe. As the tract reaches the anterior perforated substance (an area at the level of the optic chiasm) it divides into medial and lateral stria:
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 sensory function of the olfactory nerve is achieved via the olfactory mucosa, which is located in the roof of the nasal cavity. This mucosal layer not only senses smell, but it also detects the more advanced aspects of taste. It is composed of pseudostratified columnar epithelium which contains a number of cells:
In addition to the epithelium, there are Bowman’s glands present in the mucosa, which secrete mucus.
Anosmia is defined as the absence of the sense of smell and can be temporary, permanent, progressive or congenital. Temporary anosmia can be caused by infection (e.g. meningitis) or by local disorders of the nose (e.g. common cold), while permanent anosmia can be caused by head injury, or tumours which occur in the olfactory groove (e.g. meningioma).
The olfactory nerve (CN I) is essential for the sensory function of the sense of smell. It is composed of the olfactory epithelium, Bowman's glands, olfactory bulb, and olfactory tract. The olfactory nerve fibres synapse with the mitral cells, forming synaptic glomeruli. From the glomeruli, second order nerves then pass posteriorly into the olfactory tract. The olfactory tract travels posteriorly on the inferior surface of the frontal lobe and divides into medial and lateral stria as it reaches the anterior perforated substance. The lateral stria carries the axons to the primary olfactory cortex which is located in the uncus of temporal lobe. The medial stria carries the axons through the medial plane of the anterior commissure, to the olfactory bulb of the opposite side. The primary olfactory cortex sends nerve fibres to many other areas of the brain, including the piriform cortex, the amygdala, olfactory tubercle and the secondary olfactory cortex, which are all involved in the memory and appreciation of olfactory sensations.
The sensory function of the olfactory nerve is achieved via the olfactory mucosa, which is located in the roof of the nasal cavity. This mucosal layer senses smell and more advanced aspects of taste. It is composed of pseudostratified columnar epithelium which contains three main cells: basal cells, sustentacular cells, and olfactory receptor cells. Basal cells form the new stem cells from which the new olfactory cells can develop while sustentacular cells provide structural support, analogous to the glial cells located in the CNS. Olfactory receptor cells are bipolar neurons which have two processes: a dendritic process and a central process (also known as the axon). The dendritic process projects to the surface of the epithelium, where a number of short cilia, the olfactory hairs, are present and react to odors in the air and stimulate the olfactory cells. The central process projects in the opposite direction through the basement membrane.
Clinical relevance of anosmia and olfactory nerve examination also exist. Anosmia is the absence of the sense of smell which can be temporary, permanent, progressive, or congenital. It can be caused by infection, head injury, or neurodegenerative conditions such as Parkinson's or Alzheimer's disease, as well as genetic conditions such as Kallmann syndrome and Primary Ciliary Dyskinesia. For olfactory nerve examination, the patient should be asked if they have noticed any changes in their taste or smell, then test each nostril by asking the patient to identify a certain smell. This is usually done with the help of smell strips or samples of different odors. Additionally, a patient with anosmia may be referred to an ear, nose, and throat (ENT) specialist for further tests or treatment.
The olfactory nerve is essential for the sense of smell. It is composed of the olfactory epithelium, Bowman's glands, olfactory bulb, and olfactory tract. The olfactory nerve fibres synapse with the mitral cells, forming synaptic glomeruli. From the glomeruli, second order nerves then pass posteriorly into the olfactory tract, travelling posteriorly on the inferior surface of the frontal lobe and dividing into medial and lateral stria as it reaches the anterior perforated substance. The primary olfactory cortex sends nerve fibres to many other areas of the brain, which are involved in the memory and appreciation of olfactory sensations. Clinical relevance of anosmia and olfactory nerve examination also exist, and should be tested when a patient experiences any changes in their taste or smell.
