Smell comes from tiny molecules released by substances around us, which stimulate specialized sensory cells in the nose. These cells, called olfactory sensory neurons, are located in a small patch of tissue high inside the nose and connect directly to the brain. Each neuron has one odor receptor that can selectively capture a specific set of odor molecules. When we breathe in these molecules, they stimulate the receptors, which send messages to the brain. The brain identifies the smell by processing the combination of activity from the neurons, which forms a coded message.
The olfactory nerve, also known as cranial nerve 1 (CN I), is the nerve that enables our sense of smell. It starts in the brain and ends in the upper, inside part of the nose. The olfactory bulb, a structure in the forebrain, receives signals from the sensory neurons in the nose and processes the odor initially. It then sends the information to the piriform cortex, the main structure of the olfactory cortex, for more comprehensive processing.
The complexity of the receptors and their interactions with odor molecules allows us to detect a wide variety of smells. For example, what we think of as a single smell may actually be a combination of many odor molecules acting on different receptors, creating a neural code that we can identify as the scent of a rose or freshly-cut grass
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