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Section II: Sensory Systems
9. Chemical Senses: Olfaction and Gustation
Part 4 of 4

Max O. Hutchins, Ph.D.
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Neural Pathway into the Olfactory Cortex

Axons from mitral and tuft cells project caudally into the olfactory tract. Fibers diverge and synapse with neurons of the anterior olfactory nucleus (AON). Axons from the AON cross to the opposite side of the hemisphere through the anterior commissure. The majority of the axons from the olfactory bulb diverge laterally and form the lateral olfactory tract which synapse with nuclei of the olfactory cortex. These are the piriform cortex (pc), the periamygdaloid cortex, part of the amygdala, and hippocampus. There are no direct relays from the olfactory bulb into the thalamus, but a few fibers synapse with 3rd order sensory neurons in the thalamic dorsomedial nucleus which are projected to the ipsilateral cerebral hemisphere (Figure 9.11).


Figure 9.11

Projection of olfactory bulb into the olfactory cortex.

Conclusion

In conclusion, many olfactory receptors respond to more than one odorant quality just like the taste receptor cells. Coding of the primary odor depends on the intensity of the odor and on a population response within the olfactory neurons. During neural processing in the olfactory bulb, a particular discharge occurs to one odorant and a different pattern for another odorant. This sensory input must be processed before being relayed to the olfactory cortex for perception and recognition of the individual odor.

Test Your Knowledge
1. Second-order sensory neurons for taste are located in the

A. Insula
B. Amygdala
C. Nucleus Solitarius
D. Uncus
E. Trigeminal ganglion
2. All of the following statements are correct about the olfactory receptor neurons EXCEPT:

A. These specialized neurons are replaced about every 5- 8 weeks.
B. Each neuron contains receptors which are specific for a single odorant molecule.
C. The axon of each olfactory neuron synapses in only one glomerulus in the olfactory bulb.
D. Odorant molecules interact with receptors coupled to a G protein called Golf.

3 Which of the following cells are 2nd order neurons with axons projecting into the anterior olfactory cortex?

A. Mitral cells
B. Glomerular cells
C. Periglomerular cells
D. Granule cells

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