Olfactory axons

Yoshihara Y. and Mori K. (1997). Basic principles and molecular mechanisms of olfactory axon pathfinding. Cell Tissue Res 290, 457-463. 

What happens at the instant when an odor-active molecule comes in touch with our nasal cavity 205 The first interaction of odorant molecules takes place in the olfactory receptor neurons, which are embedded in the pseudostratified columnar epithelium (or simply, olfactory epithelium), which is located in the posterior nasal cavity in the case of mammals. Olfactory sensory neurons express receptor proteins on the surface membrane of the cilia, which gain access to the extracellular region covered with mucus. The airborne odorants are dissolved into the mucus, bind with the receptors, and then the receptor protein triggers a signal transduction cascade. This results in the opening of the cation channel that would depolarize the sensory neuron and eventually elicit a train of action potentials in the axon. The olfactory axon leads to the olfactory bulb through basal lamina and lamina propria. 

Several hundreds of primary olfactory axons converge on a single mitral cell, located on an olfactory synapse of the olfactory bulb. 

Studies in animals that could provide a basis for assessing the comparative distribution of PCBs when administered by the inhalation, oral, and dermal routes of exposure were not located. A recent study in ferrets by Apfelbach et al. (1998) reported for the first time that the olfactory system may be a potentially significant portal for the entry of airborne PCBs. The olfactory bulbs of the exposed ferrets had the highest total PCB concentration (642 ng/g lipids), while the liver, adipose tissue, and brain had levels of 202, 303, and 170 ng/g lipids, respectively. The data suggest that inhaled PCBs pass into the dentrites of olfactory sensory neurons and are transported via olfactory axons directly to the bulbs where they accumulate. While the olfactory system appears to be a significant site for the disposition of airborne PCBs, further studies are needed to confirm this observation and assess whether greater disposition in the brain is associated with inhalation exposure. 

Deep to the external plexiform layer is the mitral cell layer (Fig. 2). This is a thin layer that contains the somata of mitral cells (25- 35 pm diameter) arranged in almost a monolayer. These cells are the principal output cells of the bulb and, with some minor species differences (cf. Scott, 1986), have one apical dendrite that enters a single glomerulus, where it branches extensively and is synaptically contacted by olfactory axons (Shepherd, 1972a) (Figs. 4C,E, 5 and 12). 

Analysis of the functions of the olfactory bulb would be greatly simplified if the coding of information in primary olfactory axons were known. Unfortunately, widely differing concepts of olfactory coding can be supported by available information. 

Tozaki, H., Tanaka, S., Hirata, T. Theoretical consideration of olfactory axon projection with an activity-dependent neural network model. Mol. Cell Neurosci. 26, 503-517 (2004)... 

The 10 20 million olfactory sensory neurons in the human nose are confined to a relatively small patch of tissue located high in the nasal cavity. When odorants, e.g., carvone in Fig. 2, are deposited within the mucus covering the distal ends of the olfactory receptor cells, they interact with some of the membrane-bound, G-protein-coupled receptors [9]. This interaction initiates a transduction process that converts physicochemical information in the odorant, e.g., its structure or other attributes, into electrical energy that is conveyed in the form of pulses (action potentials) along olfactory axons to the brain. 

Figure 2 Depiction of some components of the vertebrate olfactory epithelium in the nose. Odorants, e.g., carvone, deposit themselves in the mucous layer and interact with molecular receptors in the membrane of cilia of the olfactory receptor cells. Subsequent to intracellular signal transduction events, action potentials are sent via the olfactory axons to the olfactory bulbs in the brain. Supporting cells provide physical and physiological support for the olfactory neurons. Undifferentiated basal (stem) cells are the source of new supporting and olfactory receptor cells.

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