9. Chemical Senses: Olfaction and Gustation
Part 3 of 4

Max O. Hutchins, Ph.D.
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Olfactory System

The olfactory system in humans is an extremely discriminative and sensitive chemosensory system. Humans can distinguish between 1,000 to a predicted high of 4,000 odors. All of these odors can be classified into six major groups; floral, fruit, spicy, resin, burnt, and putrid (Refer back to Figure 9.1). The perception of odors begins with the inhalation and transport of volatile aromas to the olfactory mucosa that are located bilaterally in the dorsal posterior region of the nasal cavity.

Morphology of Olfactory Mucosa and Cell Types

The olfactory mucosa consists of a layer of columnar epithelium, surrounding millions of olfactory neurons, which are the only neurons to communicate with the external environment and undergo constant replacement. Basal cells near the lamina propria undergo differentiation and develop into these neurons about every 5-8 weeks. The glial-like columnar cells surround and support the bipolar neurons. These columnar cells have microvilli at their apex and secrete mucus which is layered on the surface of the olfactory mucosa (Figure 9.8).

The bipolar olfactory neurons have a single dendrite which projects towards the apical mucosa. The terminal ending of the dendrites are flattened and have 5-25 cilia that are embedded in the mucosa on the surface. Each cilia may have as many as 40 specific receptor membrane proteins for interaction with different odorant molecules. The density of these receptors is enormous for humans, but significantly greater in many lower animals.

Dissolution of Odorant Molecules and Interaction with Sensory Receptors

Unbound hydrophilic odor molecules diffuse across the layer of mucus, whereas hydrophobic odors must become bound to a specific odorant binding protein to be transported to each cilium for interaction with specific receptors. All of these receptors have the same general structure, seven hydrophobic transmembrane regions, but the amino acid sequence within the cylinders spanning the membrane are extremely diverse which permits the discrimination of a large number of odors.

Transduction of Olfactory Stimuli

Odorant molecules bind reversibly to the diverse receptor membrane proteins which are coupled to a G-s group of proteins called G_olf. Activation of adenylyl cyclase leads to the formation of cAMP with the activation of Ca²⁺/ Na⁺ cation channels. The primary effect of influx of these ions is depolarization and the generation of a generator potential (Figure 9.9). Generated ionic currents are graded in response to the flow rate of the odorant molecules and to their concentration. Sites of summated generator potentials occur across the olfactory mucosa to produce specific spatial pattern of activity for each stimulating odorant molecules, which may contribute to neural coding of odors. These spatial responses across the olfactory mucosa can be recorded (electro-olfactograms) with surface electrodes.

Propagation of Action Potentials and Convergence upon the Olfactory Bulb

The resulting influx of Na⁺ and Ca²⁺ produces a depolarizing generator potential that spreads to the axon hillock. There, action potentials are generated, which are propagated to the synaptic endings in the olfactory bulb (Figure 9.9).

The action potential frequency is proportional to the concentration of specific odorant molecules. However, action potential frequency will be attenuated by adaptation or desensitization of the receptor and reduction in the production of cAMP.

Rapid adaptation and removal of the odorants permit continued recognition and discrimination of new aromas that are inhaled in the next respiratory cycle. Action potentials generated in the axon terminals of activated neurons are propagated into the glomeruli within the olfactory bulb. The olfactory bulbs have many different types of neurons and these have a laminar distribution. On the ventral side of the olfactory bulbs is a layer of glomeruli. This is a site at which axon terminals of several thousand olfactory neurons synapse with numerous dendrites from large mitral cells and tufted cells. Interneurons such as the inhibitory periglomerular cells synapse with the nerve endings within adjacent glomeruli.

Millions of axon fibers converge upon only a few thousand glomeruli within each bulb to synapse with about 75,000 mitral cells (see Figure 9.10) and about twice this number of tufted/periglomerular cells. Mitral cells are 2^nd order sensory neurons whose axons enter the olfactory tract and ascend to the olfactory cortex. This convergence/divergence between the axons of olfactory neurons and the specialized cells of the olfactory bulb generate excitatory postsynaptic potentials (EPSPs) in the dendrites of mitral cells and subsequent action potentials. Lateral inhibition by the periglomerular cells modulates activity in adjacent glomeruli innervated by other mitral and tuft cells. A complex pattern of neuronal integration for discrimination of various odorant molecules is indicated by the mechanisms of convergence/divergence with excitation/inhibition of these 2^nd order sensory neurons. This complexity is related to the recognition that no single odor stimulates a specific group of olfactory neurons. Rather a neural code is created from the activation of multiple receptors and neurons.

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