Chapter 13. Functional Anatomy of the Olfactory System

Functional Anatomy of the Olfactory System

Interact with depictions of the structures and pathways involved in olfactory perception.

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Drag and drop each label into the correct blank box on this illustration of the olfactory system.

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What Is the Functional Anatomy of the Olfactory System?

Odorant molecules in the outside air enter the nasal cavities via the nostrils. Odorant molecules released from food or other substances in the oral cavity are carried into the nasal cavities via the pharynx. Within each nasal cavity, turbinates, bony convolutions of tissue, disperse the air evenly throughout the cavity. This brings odorant molecules into contact with olfactory receptor neurons (ORNs) embedded in the olfactory epithelium, a patch of tissue in the upper reaches of each nasal cavity. ORNs transduce odorant molecules into neural signals.

The olfactory epithelium contains supporting cells, basal cells, and Bowman's glands, in addition to ORNs. The supporting cells provide a structural matrix for the ORNs. Each ORN dies after a few weeks and is replaced by a new ORN; basal cells are the precursors of new ORNs. Bowman's glands continually secrete olfactory mucus, which covers the olfactory epithelium.

Each ORN has numerous hairlike cilia, which project into the mucus layer. The surface of each cilium is studded with olfactory receptors, which are members of the large family of GPCRs (G-protein coupled receptors). Odorant molecules dissolve into and flow through the mucus, where they contact and bind to these receptors. If enough odorant molecules bind to receptors on an ORN’s cilia at the same time, an action potential is produced in the ORN's axon. This is the process by which odorant molecules at sufficient concentration are transduced into neural signals.

The axons of the ORNs form the olfactory nerve. These axons travel to the olfactory bulb (part of the brain) through a grid of tiny holes in the cribriform plate, the part of the skull immediately above the nasal cavity. Within the olfactory bulb, the ORN axons enter small structures called glomeruli (singular glomerulus), where they make synapses with the dendrites of two types of relay neurons—mitral cells and tufted cells—the axons of which form the olfactory tract, carrying signals with olfactory information to higher areas of the brain.

The brain areas receiving signals via the olfactory tract include the piriform cortex, amygdala, and entorhinal cortex. The piriform cortex—the only region that both receives signals directly from the olfactory bulb and is known to be dedicated solely to olfaction—is considered the primary olfactory cortex. The amygdala is crucially involved in emotional responses and can be activated by olfactory stimuli (unpleasant and pleasant smells). The amygdala sends signals to the hypothalamus, which is involved in functions that include the regulation of thirst, hunger, and sexual behavior. The entorhinal cortex is the gateway to the hippocampus, where long-term memories are stored and retrieved.

The piriform cortex, amygdala, and entorhinal cortex all send signals to the orbitofrontal cortex, which, among other things, plays a role in evaluating incoming stimuli as positive or negative (i.e., rewarding or unpleasant).

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Drag each label into the correct blank box, in the order in which these events occur in the process of olfactory transduction and transmission. Then click SUBMIT.

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Drag each brain area at the left to attach to the matching description at the right. Then click SUBMIT.