The difference between olfactory system and gustation are that with olfactory system what stimulates us to produce the sensation of an odor are airborne molecules, molecules in the air that when inhaled encounter olfactory receptor cells that are high in the nasal cavity. They then communicate the message to the olfactory bulb where the sense of smell is registered. Olfactory neurons are the only neurons that directly connect the brain and the outside world.
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What stimulates our sensation of taste can be anything that we physically eat or drink or place in our mouths. When we dissolve the food or liquids with saliva the chemicals in the foods activate the taste buds.
Unlike the olfactory system where the receptors are only found at the top of the nasal cavity, with gustation the receptor cells can be found in our taste buds. Taste buds can be found all along the surface of the tongue, on the inside of our cheeks, on the roof of our mouths and in our throat. The olfactory takes the molecules to the olfactory bulb which is part of the olfactory cortex in the brain in order to produce an odor. With gustation the information provided by the receptor cells in our taste buds goes to the thalamus which processes and distributes motor and sensory information going to and from the cerebral cortex. So this information is sent to several regions of the cerebral cortex.
With olfaction we can experience sensory adaption meaning that if we are near a certain smell for a period of time after about a minute or so we can continue to smell the Oder but aren't as sensitive to it if it bothers us. Gustation uses more of our five senses to link the flavor of something by the way the food or drink smells, looks the temperature and how it feels.