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Monday, September 19th 2011

Olfactory Nerve

The sense of smell is a special afferent skill facilitated by the first numbered cranial nerve, the olfactory nerve.

The first order neurons of the olfactory nerve are the olfactory sensory neurons. These are bipolar cells whose dendrites project g-coupled olfactory receptors into the olfactory mucosa. Dissolved molecules within the mucosa act as ligands at these receptors and trigger an action potential which travels ‘up’ the bipolar olfactory sensory neurons. The axons of these cells colasce into processes which head through the cribiform plate. These processes truly represent the olfactory ‘nerve’.

The processes which make up this short nerve end as they synapse on the second order neurons at intracranial olfactory bulb. The olfactory bulb is complexly organized however the primary secondary neurons are the mitral and tufted cells. The axons of these secondary neurons make up the olfactory tract. Of note, a very small number of these mitral and tuft cells will synapse onto the neurons of the anterior olfactory nucleus, just at the junction of the olfactory bulb and tract. The role of the anterior olfactory nucleus is extremely poorly understood.

The majority of the second order neurons however continue through the olfactory tract and just anterior to the anterior perforating substance the tract splits into striae, primarily lateral and medial striae, however many texts also list an intermediate striae.

The lateral striae ends in the primary olfactory area where the second order neurons synapse on third order neurons of the pyriform area; primarily in the cortex of the amygdala and entorhinal area in the temporal lobe. These third order neurons provide the primary projections from the olfactory system to the rest of the brain. Meanwhile the third order neurons found at the end of the axons of the medial striae are found in nuclei of the subcollasal and preseptal areas. We should note that some fibers from the medial striae cross at the anterior commisure to synapse in the contralateral subcollasal regions.

Projections from the primary olfactory area, the termination for the second order neurons of the lateral striae, include Brodmann Area 28.

From subcallosal nuclei projections extend through the medial forebrain bundle to the hypothalamus. Other projections carry through the retroflex fasciculus to the habenular nucleus. From the habenular nucleus and the hypothalamus projections go the reticular formation, superior salivatory nucleus, inferior salivatory nucleus, dorsal vagal nucleus and elsewhere.

Beyond fractures through the ethmoid bone which may lead to transection of the olfactory nerve and anosmia, the primary clinical consideration (at least as far as the Board Exam goes) is likely Foster-Kennedy Syndrome [PDF]. Here you get a collection of typical symptoms:

  • Anosmia
  • Optic Atrophy
  • Papilledema
  • Unilateral central scotoma
  • Sometimes frontal lobe injury signs such as emotional lability or personality changes

This related to a large anterior skull base mass causing, typically unilateral, olfatory tract compression.