What does the development of taste and smell disorders in the olfactory system involve?

Updated: Jan 08, 2021
  • Author: Eric H Holbrook, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Once an odorant binds to its receptor, a signaling cascade depolarizes the neuron, which sends the signal along its axon, which then converges together within the bundled axons of the fila olfactoria deep to the epithelium.

These axons project through the cribriform plate to the ipsilateral olfactory bulb. The olfactory bulb cells contacted by the olfactory receptor cells include the mitral and tufted cells, arranged in specialized areas termed glomeruli. The axon terminals of receptorlike neurons synapse within the same glomeruli, forming an early topographical odorant map. Therefore, an odor is thought to activate a set of odorant receptors based on its chemical composition. The corresponding glomeruli of the olfactory bulbs are in turn activated, creating a unique pattern of excitation in the olfactory bulb for each odorant.

The glomerular cells are the primary output neurons of the olfactory bulb. Axons from these cells travel to the olfactory cortex, which is divided into 5 parts, including (1) the anterior olfactory nucleus, connecting the 2 olfactory bulbs through the anterior commissure, (2) the olfactory tubercle, (3) the pyriform cortex, which is the main olfactory discrimination region, (4) the cortical nucleus of the amygdala, and (5) the entorhinal area, which projects to the hippocampus.

The olfactory pathway does not involve a thalamic relay prior to its cortical projections. Relays from the olfactory tubercle and the pyriform cortex project to other olfactory cortical regions and to the medial dorsal nucleus of the thalamus and probably involve the conscious perception of odors.

Conversely, the cortical nucleus of the amygdala and the entorhinal area are limbic system components and may be involved in the affective, or hedonic, components of odors. Regional cerebral blood flow (measured with positron emission tomography) is significantly increased in the amygdala with introduction of a highly aversive odorant, and it is associated with subjective ratings of perceived aversiveness.

The vomeronasal organ (VNO), or Jacobson organ, is a bilateral membranous structure located within pits of the anterior nasal septum, deep to the nasal respiratory mucosa and next to the septal perichondria. Its opening in the nasal vestibule is visible in 91-97% of adult humans, and it is 2 cm from the nostril at the junction of the septal cartilage with the bony septum. Unlike lower animals, axons projecting from the VNO have not been found in postnatal humans.

The VNO is believed by some to detect external chemical signals termed pheromones or vomeropherins through neuroendocrine-type cells found within the organ. These signals are not detected as perceptible smells by the olfactory system and may mediate human autonomic, psychologic, and endocrine responses.

Free trigeminal nerve endings, which are stimulated by aversive or pungent stimuli (eg, ammonia, menthol), exist in the nasal mucosa. Odors that stimulate the trigeminal system can induce a sensation of the presence of a strong chemical but without the quality of smell. These stimuli are processed via separate pathways from those in the olfactory system, described above.

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