Answer:
Smell
The molecules that activate the sense of smell (the technical name is olfaction) are airborne; they enter the body via the nose and mouth and attach to receptor cells that line the mucus membranes far back in the nose. In humans, there are millions of such cells altogether but only several hundred (400 is a good estimate) different types of olfactory receptors.
One thing that makes olfaction unique among the senses is that its receptor cells are themselves neurons. Each olfactory receptor cell has filaments called cilia, with receptors designed to bind to specific molecules. Like all neurons, the cell also projects a thicker fiber called an axon. The axons come together in the olfactory nerve and go directly to the brain.
In other words, the olfactory nerve consists of neurons with one end in direct contact with the external world and the other in direct contact with the brain.
Whenever a detectable molecule, or odorant, attaches to an olfactory receptor, it generates a tiny electrical impulse. As these currents enter the complex network of the brain, it can quickly (sometimes within just two or three synapses, in a tenth of a second) recognize the odor.
How many odors can the human brain discriminate? Until recently, most scientists would have said something like 10,000; however, new research suggests a far greater number—perhaps a trillion. How this is possible with messages from just 400 receptor cell types remains something of a mystery—and a testament to the brain’s computing power.
The computation begins as signals are received and sorted out in the olfactory bulb, a structure on the underside of the front of the brain. From there, patterns are transmitted to the olfactory or piriform (meaning “pear-shaped”) cortex in the higher brain for further processing.
Explanation:
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