Olfactory Nerve

Cranial nerve I is composed of the set of axonal processes that arise from the olfactory bipolar cells that lie within the nasal mucosa. These sensory cells lie in a sheet-like formation across the mucosal surface rather than being condensed into a ganglion as is the case for other peripheral nerves. Unlike the situation with the visual, vestibulocochlear, and gustatory systems, there are no separate receptor cells for the olfactory system. The distal ends of the olfactory bipolar cells have the molecular machinery to bind oderant molecules and transduce the signal via various complex biochemical events. These bipolar neurons terminate on first-order multipolar neurons called mitral cells that are located within the olfactory bulbs, which lie ventral to the frontal lobes.

The bipolar cell axons form complex synaptic contacts with the mitral cell processes in a series of spherical structures called glomeruli. The glomeruli form a layer superficial to the layer of mitral cell bodies in the olfactory bulb. Different oderant molecules stimulate different sets of glomeruli, but a simple oderant "map" across the region of different oderant molecules has not been found. The mitral cells project via the olfactory tract to multiple sites in the ventral region of the telencephalon collectively referred to as the olfactory cortex. An olfactory pathway to part of neocortex is via a relay from olfactory cortex to the mediodorsal nucleus of the dorsal thalamus and then to prefrontal cortex, which occupies a large portion of the frontal lobe and is concerned with a multitude of complex, higher cognitive functions.

Damage to the olfactory system results in the loss of the sense of smell. The appreciation of the sensation of the flavors of food is also impaired since flavor is a complex sensation based on olfactory and gustatory interactions at the cortical level. In some cases of epilepsy caused by lesions involving the ventral, olfactory-related regions of the cerebral hemispheres (particularly a structure called the uncus), an aura involving an olfactory hallucination may preceed seizure activity.

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