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Chapter 14: Chemical Senses: Olfactory & Gustatory Systems

An individual loses his sense of smell. After receiving a medical examination, it is determined that the reason for his loss of sensation is due to degeneration of primary afferent fibers that enter the olfactory glomerulus. In a healthy individual, upon which of the following structures do these primary afferent fibers terminate?

A. Granule cell dendrites forming axodendritic synapses

B. Granule cell axon terminals forming axoaxonic synapses

C. Mitral cell dendrites forming axodendritic synapses

D. Mitral cell axon terminals forming axoaxonic synapses

E. Axon terminals of fibers arising from the olfactory tubercle, forming axoaxonic synapses

C. The olfactory receptor and its primary afferent fiber terminate upon dendrites of mitral cells. This relationship is important because it is the axon of the mitral cell that projects out of the olfactory bulb (forming the major component of the lateral olfactory stria). The granule cell processes make synaptic contact with dendrites of mitral cells, forming dendrodendritic synapses, but are not known to make synaptic contact with primary afferent terminals. Cells arising in the olfactory tubercle are not known to project to the olfactory bulb. Instead, projections of cells situated in the olfactory tubercle contribute fibers to the medial forebrain bundle and stria medullaris.

A middle-aged man is involved in an automobile accident that causes brain damage affecting a region of the cerebral cortex, resulting in loss of the conscious perception of smell. Which of the following regions of the cortex is most likely affected?

A. Temporal neocortex

B. Posterior parietal lobule

C. Cingulate gyrus

D. Prefrontal cortex

E. Precentral gyrus

D. Experimental evidence indicates the prefrontal cortex is a key region for the conscious perception of smell. This conclusion is based on 2 observations. First, the prefrontal cortex receives major inputs from the olfactory bulb by the following routes: olfactory bulb to pyriform cortex to prefrontal cortex, or olfactory bulb to pyriform cortex (and olfactory tubercle) to mediodorsal thalamic nucleus to prefrontal cortex. Second, lesions of the prefrontal cortex result in a failure to discriminate odors. Olfactory functions are not known to be associated with any of the other choices. Instead, the primary auditory receiving area is located in the auditory cortex, the posterior parietal lobule is concerned with such processes as the programming mechanisms associated with complex motor tasks, the cingulate gyrus has been associated with such functions as spatial learning and the modulation of autonomic ...

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