2. Smell The sense of smell is known as olfaction. It is the ability to detect and recognize chemicals that contact the membranes inside the nose.
3. Smell The neurons that are responsible for smell are known as olfactory cells. They line the olfactory epithelium in the rear of the nasal air passages. Each olfactory cell has a cilia (a threatlike dendrite) that extends from the cell body to surface of the nasal passage. Olfactory receptors are located on the cilia.
4. Smell Odor molecules trigger changes in a G-protein inside the olfactory cell, and this causes chemical activities that lead to an action potential. Humans probably have several hundred olfactory receptor proteins, but rats and mice have about a thousand types!
5. Taste Coding in the Brain When an olfactory receptor is stimulated, its axon carries an impulse to the olfactory bulb. Each chemical odor excites only part of the olfactory bulb. Smell is then coded in terms of which part of the olfactory bulb is excited. The olfactory bulb sends axons to the olfactory area of the cerebral cortex, where neurons respond to smells that cluster together. From the cerebral cortex, information also goes to areas that are responsible for eating behavior, because this behavior is very sensitive to odors.
6. Taste Coding in the Brain While receptors for vision and hearing are with us for a lifetime, olfactory receptors only survive for just over a month. That is because they are easily damaged because they are exposed to everything in the air. When a smell receptor needs to be replaced, a stem cell matures into a new olfactory cell in the same location as the old one and its axon then has to find its way to the correct place in the olfactory bulb. Each olfactory neuron axon has copies of its olfactory receptor protein, and it uses this copy to find its correct connection in the olfactory bulb.
7. Taste Coding in the Brain Some people have trouble smelling one type of chemical, and this is known as a specific anosmia. If a person cannot smell at all, this is known as anosmia.
