Taste and Smell

The taste buds of the tongue are not visible to the naked eye. The visible ridges and bumps on the surface of the tongue are called papillae. Taste buds are embedded in the folds and crevices of the papillae. The average person is born with about 10,000 taste buds. Each taste bud contains between 50 and 150 taste receptor cells. These taste receptor cells are responsible for sending information about the chemical composition of food to the brain. Different taste receptor cells respond to different chemicals. For example, the sour taste of food is a result of activation of sour taste receptor cells, which are activated by binding with the hydrogen atoms found in sour foods. As another example, the salty taste of food is a result of the activation of salty taste receptor cells, which are activated by binding with the sodium atoms found in salty foods. Some regions of the tongue have higher concentrations of certain types of taste receptor cells, leading to the "taste map" that is typically ascribed to the tongue. However, all types of taste receptor cells are found throughout the entire tongue.
Although gustation, or sense of taste, is a complex sensory process, there are five generally recognized tastes that people can perceive. These are sweet, sour, salty, bitter, and umami (savory). Umami is a taste that detects amino acids in foods such as meat broth and aged cheese. Recent research suggests there may be taste receptor cells for components such as fat and starches in food as well. Once activated, taste receptor cells send signals to the sensory relay of the brain—the thalamus—and then to the gustatory cortex, a region within the frontal lobes.

Olfaction (or the sense of smell) involves detecting the chemical composition of inhaled air. These chemicals are referred to as odorants. Odorants bind to olfactory receptor cells in the nasal cavity. These receptors have hairlike extensions, called cilia, that extend into the mucus covering the surface of the nasal cavity. There are many different types of olfactory receptor cells, and each type binds only to specific odorants. The olfactory receptor cells send neural signals to the olfactory bulb. The olfactory bulb is a part of the brain made up of many different types of neurons. The olfactory nerve carries smell information to other regions of the brain and is formed by the axons of neurons in the olfactory bulb. A specific smell is perceived due to the combination of activation from a range of different olfactory receptor cells.

The nose and the mouth are connected via the retronasal passage, meaning that the sense of smell is closely linked to taste perception. Unlike information related to vision, hearing, taste, and touch, olfactory information is not carried directly to the thalamus, which serves as the sensory relay of the brain. Instead, olfactory information is sent directly to regions of the brain involved in memory, learning, and emotions, such as the hippocampus and the amygdala. These connections have an evolutionary, or adaptive, function. Upon eating rotten or poisonous foods, which are typically bitter, olfactory information about this experience can help elicit emotions of disgust and create a vivid memory of which foods to avoid in the future. When inhaled air moves across the mucous membranes of the nasal cavity, odorants—substances that have a smell—dissolve in the mucus and bind to proteins on the chemoreceptor. Each bipolar neuron contains one of ~400 types of olfactory receptor proteins. Each receptor can bind to multiple types of odorants, and each odorant can bind to multiple receptors. This creates a combination of signals that allows humans to differentiate over 10,000 scents.