Taste receptor cells, with which incoming chemicals from food and other sources interact, occur on the tongue in groups of 50-150. Each of these groups forms a taste bud, which is grouped together with other taste buds into taste papillae. The taste buds are embedded in the epithelium of the tongue and make contact with the outside environment through a taste pore. Slender processes (microvilli) extend from the outer ends of the receptor cells through the taste pore, where the processes are covered by the mucus that lines the oral cavity. At their inner ends the taste receptor cells synapse, or connect, with afferent sensory neurons, nerve cells that conduct information to the brain. Each receptor cell synapses with several afferent sensory neurons, and each afferent neuron branches to several taste papillae, where each branch makes contact with many receptor cells. The afferent sensory neurons occur in three different nerves running to the brain—the facial nerve, the glossopharyngeal nerve, and the vagus nerve. Taste receptor cells of vertebrates are continually renewed throughout the life of the organism.
On average, the human tongue has 2,000–8,000 taste buds, implying that there are hundreds of thousands of receptor cells. However, the number of taste buds varies widely. For example, per square centimeter on the tip of the tongue, some people may have only a few individual taste buds, whereas others may have more than one thousand; this variability contributes to differences in the taste sensations experienced by different people. Taste sensations produced within an individual taste bud also vary, since each taste bud typically contains receptor cells that respond to distinct chemical stimuli—as opposed to the same chemical stimulus. As a result, the sensation of different tastes (i.e., salty, sweet, sour, bitter, or umami) is diverse not only within a single taste bud but also throughout the surface of the tongue.
The taste receptor cells of other animals can often be characterized in ways similar to those of humans, because all animals have the same basic needs in selecting food. Carnivores and not humans have taste buds that are tuned for water. This taste sense is found at the tip of the tongue, which the part of the tongue for instance dogs curl to lap water. This area responds to water at all times but when the dog has eaten salty or sugary foods the sensitivity to the taste of water increases. The guess is that this ability to taste water evolved as a way for the body to keep internal fluids in balance after the animal has eaten things that will either result in more urine being passed, or will require more water to adequately process.
The images below are of a rabbit's taste buds and were captured by Microscope World using a clinical lab microscope and a high definition microscopy camera.
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| Rabbit taste buds captured under a lab microscope at 40x. |
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| Rabbit taste buds captured under a lab microscope at 100x. |
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| Rabbit taste buds captured under a lab microscope at 400x. |
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