THE SENSES - A PRIMER

Smell is the most primitive of the senses. Its role in finding food and warning of danger is vital for survival.

The receptors of the chemical sensory systems send electrical signals to the brain when they bind to airborne (for smell) and dissolved (taste) molecules.

The organization of the olfactory system is somewhat primitive too: receptors are dendrites of neurons that go directly to the brain. Within two or three synapses, the brain can recognize an odor.

Millions of olfactory receptors line the nasal passages, each designed to attach to specific molecules, activating a molecular cascade that generates action potentials within its neuron. The neurons terminate in the olfactory bulb, which computes signal patterns from 350 receptor types representing thousands of odors, and transmits this information to the piriform cortex for further processing.

The olfactory bulb connects directly to the limbic system, too. This link, which evolved to drive survival functions, is responsible for the intense emotions certain odors can instantaneously evoke. The sense of smell can also signal neurological degeneration: it is impaired early in Alzheimer’s and Parkinson’s disease.

Its simplicity notwithstanding, some mystery surrounds olfaction. Most scientists believe receptors respond to molecular shape; a minority that vibrational frequency is key.

Pheromones—chemicals processed through the olfactory system to trigger hormonal responses—are vital to animal reproduction, but their human importance is highly controversial. It is uncertain, that is, whether such chemicals influence attraction, mate selection, or other aspects of sexual behavior. No true human pheromone has been identified. But airborne molecules of a male hormone have been shown to alter cortisol levels in women, and an estrogen derivative to stimulate hypothalamic activity (which could influence hormone secretion and mood) in men.

Molecules in solution bind to receptors on taste cells gathered in taste buds—about 10,000 such projections, most on the tongue but also elsewhere in the mouth and throat. Individual receptors respond to sweet, sour, bitter, and salty taste—and “umami” (described as “meaty” or “savory”), now widely recognized as a fifth taste.

While it was once thought that zones of the tongue were taste-specific, all areas actually respond to all tastes (although some are particularly sensitive to one or another). Indeed, some conjecture that even a single taste bud may hold receptors for several tastes.

Molecules binding to taste receptors start a series of chemical reactions (or, in the case of salty and sour, simply open ion channels) to generate action potentials in cranial nerves. These impulses converge on the medulla oblongata in the brainstem, and move on to cortical and limbic areas. Via these pathways, taste signals influence digestive processes, emotions, and food decisions. Some taste responses are all but universal (preference for sweetness, for example), but others are idiosyncratic, a combination of genetics (why some people find broccoli bitter, for example) and experience. What’s usually called the “taste” of food properly refers to flavor, a complex phenomenon that also includes smell and qualities like texture.