They can recognize thousands of different kinds of smells. Each survives for only about 60 days. These neurons are particularly vulnerable to damage as they are directly exposed to the outside environment. As the cells die, new olfactory neurons are generated by a layer of stem cells underneath the olfactory epithelium. Some examples of alkaloids are nicotine, quinine, morphine, strychnine, and reserpine.
Many poisons are alkaloids.
Many of the receptors for bitter taste are located at the back of the tongue. This location of the receptors may help to trigger the vomiting response, which could, in theory, prevent poisoning. Scientists who study taste are debating whether they should recognize the existence of a fifth taste, in addition to sweet, salty, bitter, and sour. Most Japanese recognize umami as a unique taste and detect it in seaweed, ripe cheeses, and some meats. Interestingly, most non-Japanese do not experience this taste, which suggests that the perception of taste is influenced by culture and language as well as by the functions of the taste and smell sensors.