All of the senses are part of the nervous system as they rely on nerves to send messages to the brain. Sight is the most closely related as it travels directly by the optic nerve, while smell and taste use many of the same receptors (ever notice food tastes weird when you have a cold?) hearing works more closely with the bones in the inner ear however still making use of the vibrations picked up to let the brain interpret, and touch has multiple types of receptors all over the body. The receptors that allow taste (taste buds on the tongue) are the nerves in this case. There are different types of taste buds located on different parts of the tongue allowing us to taste different flavors (salty sweet etc.). Keep in mind that the tongue/teeth/and mouth in general are part of the digestive system. Nothing in the human body serves only one purpose so it really depends on how much detail you wish to include.