Picking Up the Scent

Evolution has a lot of resources invested in the human sense of smell, according to Jeffrey R. Martens, Ph.D., an assistant professor of pharmacology who joined the Medical School faculty three years ago. It takes the activity of at least 300 human genes — more than 1 percent of the entire human genome — to smell the difference between a banana and a steak.

Cilia are on the front lines of the body’s olfactory system. They grow from the ends of long olfactory neurons — the only neurons in the body with a direct connection between the outside environment and the brain.

Millions of intertwined olfactory cilia fill a mucus layer lining the inside of the nasal passages. When you inhale an odor, odorant molecules bind to matching receptor proteins on the surfaces of cilia from specific olfactory neurons to create a biochemical signal.

Olfactory cilia grow from the ends of long olfactory neurons like these that carry scent signals directly to the brain. Photo: Courtesy of the Martens Laboratory

The human nose needs so many olfactory cilia to sort out the seemingly infinite number of different combinations in the odorant molecules we inhale every day, according to Martens.

Genetic mutations that affect cilia or their proteins can disable this delicate sensory machinery. The result is anosmia, or the inability to smell — a condition that Martens says often goes undiagnosed by physicians and unnoticed even by people who have it.

“If your sense of smell has been deficient since birth or declines gradually over time, you may not realize that anything is wrong,” he says.

Martens adds that many diseases and medical conditions — including obesity, developmental disorders, Leber Congenital Anamosis (a form of childhood blindness), sexual dysfunction, Alzheimer’s disease and depression — may be associated with defects in olfactory cilia that affect the sense of smell.

“Olfactory function tests may be a useful, non-invasive screening tool for these and other cilia-related diseases,” Martens says.

—SP

Also:

Secrets Of The Cilia

Highway for the Light