The gustatory pathway is the complete sensory pathway for the sensation/perception of taste from the taste bud to the medulla to the thalamus to the brain.
Whenever we put a substance into our mouth, our saliva bathes it and certain substances from the food become detected by the taste buds.
The taste of a substance in the mouth is determined by nature of the substance, the temperature and texture of the substance placed in the mouth, and also influenced by the odour of the substance.
There are lots of taste buds found on the superior surface of the tongue, these tastebuds are buried within the papillae of the tongue.
The papillae at the anterior 2/3rd of the tongue are the fungiform papillae.
The papillae at the posterior 1/3rd of the tongue are the circumvallate papillae, while those at the sides of the tongue are the folliate papillae.
Fungiform papillae send neurons through the chordae tympani of the facial nerve(VII) while circumvallate and folliate papillae send 1st order neurons through the glossopharyngeal nerve (IX). These two nerves send impulses to the 2nd order neurons in the gustatory nucleus present in the medulla. 2nd order neurons from the medulla, then send impulses to the thalamus, the thalamus then acts like a main switch by sending impulses to the primary gustatory center in the insula (for sensation of taste) some to the gustatory region of the somatosensory cortex of postcentral gyrus (for filling the food on the tongue) some to the orbitofrontal region of the prefrontal cortex (also involved in the perception of taste).
A taste bud contains 50-100 taste (gustatory) cells, embedded by adjacent supporting cells. Each taste cell is sensitive to only one taste modality (form), so every taste bud has all the taste cells for all the modalities of taste (salt, sour, sweet, umami and bitter).
A taste cell contains microvilli with protein receptors called gustducin. When a gustducin bind with a legend (sugar, quinine, L-glutamate, H+, Na+) it activates the taste cell which either uses G protein-coupled or ion channel gustducin based on the modality of taste it detects.
Salty taste cell gustducin transmits Na+, sour taste cell gustducin transmits H+ both causes Ca2+ channels to open leading to depolarization.
Sweet taste cell gustducin binds the sugar molecule and activates G protein which activates adenylate cyclase to produce cAMP, cAMP then causes closure of K+ channel, leading to depolarization.
Bitter taste cell gustducin binds quinine, which activates G protein and causes the IP3 and diacylglycerol 2ndary messengers to lead to the release of Ca2+ from the endoplasmic reticulum, causing depolarization.
After the taste cell has been activated and depolarized to a threshold it then causes an action potential on the sensory neuron that innervates it, this sensory neuron then takes this information to the particular areas of the brain for that particular taste as described above.