Gustatory pathway


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 becomes detected by the taste buds. 
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 odor of the substance. 
There are lots of taste buds found on the superior surface of the tongue, these taste buds are buried within the papillae of the tongue. 
The papillae at the anterior 2/3rd of the tongue is the fungiform papillae. 

The papillae at the posterior 1/3rd of the tongue is the circumvallate papillae, while those at the sides of the tongue are the folliate papillae. 
Fungiform papillae sends neurons through the chordae tympani of the facial nerve(VII) while circumvallate and folliate papillae sends 1st order neurons through the glossopharyngeal nerve(IX). These two nerve sends impulses to the 2nd order neurons in gustatory nucleus present in the medulla. 2nd order neurons from medulla then sends 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 feeling the food on the tongue) some to the orbitofrontal region of prefrontal cortex(also involved in 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 modality of taste(salt,sour,sweet,umami and bitter). 
A taste cell contain microvilli with protein receptors called gustducins. When a gustducin binds with a ligand(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 depolarisation. 
Sweet taste cell gustducin binds sugar molecule and activates G protein which activates adenylate cyclase to produce cAMP, cAMP then causes closure of K+ channel, leading to depolarisation. 
Bitter taste cell gustducin binds quinine, which activates G protein and cause the IP3 and diacylglycerol 2ndary messengers to lead to the release of Ca2+ from endoplasmic reticulum, causing depolarisation.
After the taste cell has been activated and depolarised to a threshold it then cause an action potential on the sensory neuron that innervates it, this sensory neuron then takes this information to the particular areas of brain for that particular taste as described above.