Olfactory nerve

Number: I

Name: Olfactory

Sensory, motor, or both: Purely sensory

Origin: Nasal olfactory mucosa

Nuclei: Anterior olfactory nucleus

Function: Transmits the sense of smell from the nasal cavity to the olfactory bulb.

Description:

Olfactory nerves: The olfactory nerves originate from the olfactory receptor cells embedded in the olfactory mucosa in the upper nasal cavity above the superior nasal concha. These bipolar receptor cells, interspersed among supporting cells, have a thick peripheral process extending to the mucosal surface and a thin central process. Short cilia, known as olfactory hairs, emerge from the peripheral process, penetrating the mucous layer to react with airborne odors, thereby stimulating the olfactory cells.

Olfactory nerve fibers and fiber bundles: The central processes constitute the olfactory nerve fibers. These unmyelinated fibers, ensheathed by Schwann cells, form bundles that traverse the cribriform plate of the ethmoid bone to reach the olfactory bulb.

Olfactory Bulb: An ovoid structure housing various neuron types. The olfactory bulb's primary cells are mitral cells. Here, olfactory nerve fibers synapse with mitral cell dendrites within synaptic glomeruli. Additionally, smaller tufted and granular cells interact with mitral cells. The olfactory bulb also receives axons from the opposite bulb via the olfactory tract.

Olfactory Tract: This slender white matter band extends from the back end of the olfactory bulb beneath the frontal lobe, comprising mitral and tufted cell axons, along with fibers from the opposite bulb. Upon reaching the anterior perforated substance, the tract splits into medial and lateral olfactory striae. The lateral stria directs axons to the primary olfactory cortex in the periamygdaloid and prepiriform areas. Conversely, the medial stria converges fibers across the anterior commissure to the opposite olfactory bulb.

Olfactory Cortex: The periamygdaloid and prepiriform areas form the primary olfactory cortex, while the entorhinal area (area 28) of the parahippocampal gyrus, receiving input from the primary cortex, constitutes the secondary olfactory cortex. These cortical areas discern olfactory stimuli. Unlike other sensory pathways, the olfactory pathway comprises merely two neurons and reaches the cortex without thalamic synapsing. The primary olfactory cortex also sends fibers to various brain centers to mediate emotional and autonomic olfactory responses.

Clinical significance: Anosmia or a loss of sense of smell can occur bilaterally in diseases of olfactory mucous membrane, such as common cold or allergic rhinitis. Other causes of anosmia could include a fracture of anterior cranial fosaa involving the cribriform plate of ethmoid bone tearing the olfactory nerves, frontal cortex tumors or meningiomas of anterior cranial fossa.