06. Olfaction

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As taste, olfaction [ 5 , 133 , 141 ] which is a sense of chemical stimuli. It is through this sense that we can detect and analyze the volatile chemicals in the air and are called: Smells.

While this direction is of vital importance in other species, humans interest is very high. Indeed, humans smell is poorly developed compared to other animals [ 5 ] and runs a highly subjective which makes it difficult to study.

1. Reception:

The receiving member olfaction seat at the upper portion of the nasal cavities. This is the olfactory mucosa [ 94 , 130 ] which contains olfactory sensory cells, 10 million humans against 200 million in the dog [ 141 ]! These cells are bipolar neurons provided olfactory cilia, they have the ability to detect odors at the end of their dendrites.

Olfactory neurons are unique: They continue to proliferate in adults (neurogenesis) [ 4 , 116 ]. The lifetime of a primary olfactory neuron is from about 30 to 60 days [ 99 ].

We are capable of feeling between 4,000 and 10,000 different odors [ 5 ], each active odor preferably a particular group of neural receptors [ 5 ]. The mucus of the olfactory mucosa captures molecules that will bind to the receptor proteins at the membrane of the ciliary olfactory receptors. This connection will cause a cascade of biochemical reactions that ultimately will depolarize the membrane and cause the creation of an action potential [ 38 , 41 ].

2. Transmission:

Receptor neuron axons will cross the cribriform plate of the ethmoid bone forming the olfactory nerve [ 41 , 50 , 116 ]. The latter is a particular nerve. First, it is the shortest nerve because its length is just over the thickness of the cribriform plate, and then he was not anatomically solid structure and firm as the rest of the body's nerves, but it ' is rather a cluster beam through the nerve fibers at different locations cribriform.

Some writers confuse the olfactory nerve to the olfactory tract [ 83 ] between the olfactory bulb and strips while most authors agree on the fact that the olfactory nerve is none other than the all nerve fibers between the mucosa olfactory and olfactory bulb [ 116 ].

The olfactory bulb [ 4 , 38 , 41 ] is characterized by the presence of clusters [ 38 , 96 ], the spherical structures containing junctions synaptic neuronal receptors, mitral cells (main relay neurons of the olfactory bulb) [ 5 ] and local interneurons. There are about 1000 glomeruli in the olfactory bulb [ 5 ], each an average of 25,000 synaptic junctions.

Olfactory neurons that have the same affinity for a particular odor are grouped in the same glomerulus where they synapse with mitral cells relay. They pass through the olfactory tract [ 116 ] and the lateral olfactory tract [ 45 ] to terminate directly at the pyriform and prépyriforme cortex (primary olfactory cortex) [ 50 ] without first relay with the thalamus.

3. Perception:

Primary olfactory cortex more fibers will be projected on the hypothalamus, thalamus, amygdala, hippocampus and the orbitofrontal cortex [ 38 ].

Fibers that project to the limbic system (hippocampus, amygdala in particular) cause emotional reactions and induce the formation of memories.

Some odors such as smoke, gas or skunk stimulate the sympathetic nervous system. Appetizing odors stimulate salivation while unpleasant odors cause defensive reflexes such as sneezing, choking or vomiting.