Taste and Olfaction


This page contains a few, relatively straightforward points about taste and olfaction.

Taste

As shown to the right, each taste bud consists of a cluster of sensory cells forming synapses with neurons with long axons. These project to the medulla in one of three cranial nerves (VII,IX,X). The sensory cells release glutamate when stimulated by their corresponding taste.

Scattered over the tongue, there are five classical types of taste buds: salt, sweet, sour, bitter, and umami. Salt and sweet are self-explanatory. The bitter respond to various plant alkaloids, sour to acid (such as might be found in spoiled foods), and the umami to glutamate and some related substances.

The receptors for sweet and bitter are in the category of 7 transmembrane domain proteins, which activate trimeric G proteins. The other types each has its own special type of receptor mechanism.

Olfaction

The olfactory system is far more sophisticated than taste. The sensory dendrites of the afferent neurons are in the nasal epithelium at the top of the nasal passages, just under the cribriform plate. Their axons extend through the olfactory foramina to the olfactory bulb. There are approximately 400 different types of olfactory neurons, each with its own receptor.

The receptors are again 7 transmembrane domain proteins, each coded by its own gene, of course. The axons from all the olfactory neurons of one type converge on one cluster in the olfactory bulb. There they form synapses with neurons with axons in the olfactory tract.

The olfactory bulb actually is a lobe of the brain. You might not guess this in a human brain, but in lower vertebrates it is obvious, with the olfactory bulbs often being larger than the cerebral hemispheres. Taste information enters the brainstem and evokes rather simple behaviors, such as secreting saliva, or perhaps spitting something bitter out. Smell information, by contrast, projects into the medial temporal lobe near the midline of the cerebrum. This places it close to areas important in emotions and motivations. And corresponding to this, smells often call up complex, emotionally rich thoughts. For example, when I was a child, people burned leaves after raking them. Even today, the smell of a burning leaf calls up childhood images of fall days. I am sure you also can think of similar smells from your own life.