 In this video, I will describe the sensory transduction mechanisms and sensory pathways for the gustatory modality. Gustation is commonly known as the sense of taste. To enable the gustatory modality, molecules dissolved in the saliva activate gustatory receptor cells that are specialized receptor cells located in the taste buds. There are five distinct tastes that have been identified by physiologists. These are salty, sour, sweet, savory, and bitter. The transduction mechanism for the salty taste involves sodium ions entering the gustatory receptor cells through a cation channel embedded in the plasma membrane. This influx of cations leads to depolarization of the gustatory receptor cell, stimulating the release of neurotransmitters onto the dendrites of the afferent neurons. The transduction mechanism for the taste of sour involves hydrogen ions entering the gustatory receptor cell through a cation channel, while simultaneously hydrogen ions inhibit the efflux of potassium through a potassium channel. The influx of cations with the simultaneous decrease of cation efflux leads to a net influx of cations causing depolarization. This depolarization then stimulates the release of neurotransmitters onto the dendrites of the afferent neurons. The transduction mechanism for the sweet taste involves a sugar molecule like sucrose binding to a G-protein-coupled receptor on the surface of gustatory receptor cells in the tongue. The G-protein-coupled receptor activates the enzyme adenovil cyclase, leading to the production of the second messenger molecule cyclic AMP. As cyclic AMP accumulates in the cytosol of the gustatory receptor cell, it inhibits a potassium channel. As potassium channels close, this creates depolarization that stimulates the release of neurotransmitters from the gustatory receptor cell onto the dendrites of afferent neurons. The transduction mechanism for the savory or umami taste involves the amino acid glutamate binding to a G-protein-coupled receptor on the surface of the gustatory receptor cell. The G-protein will activate the enzyme phospholipase C, leading to the production of the second messenger molecule anasatol triphosphate abbreviated here IP3. As the IP3 concentration of the cytosol in the gustatory receptor cell increases, IP3 will bind to a calcium ion channel in the membrane of the endoplasmic reticulum known as the IP3 receptor. IP3 will activate opening of this calcium channel and calcium will exit the endoplasmic reticulum moving into the cytosol, where calcium will then stimulate the release of neurotransmitters from the gustatory receptor cell onto the dendrites of the afferent neurons. The transduction mechanism for bitter is very similar to the savory taste. Bitter molecules such as the theobromine molecule found in chocolate that gives chocolate the bitter taste will bind to G-protein-coupled receptors on the surface of gustatory receptor cells. And these G-protein-coupled receptors will activate the enzyme phospholipase C to produce the second messenger anasatol triphosphate. IP3 then activates the IP3 receptor calcium channel on the endoplasmic reticulum leading to the movement of calcium from the endoplasmic reticulum into the cytosol which will then stimulate the release of neurotransmitters from the gustatory receptor cell onto the dendrites of the afferent neurons. The sensory pathway for the gustatory modality begins with gustatory receptor cells releasing neurotransmitter to stimulate the dendrites of afferent neurons. These afferent neurons relay information through axons that travel into the cranium in the facial nerve, cranial nerve number seven. The axons of the afferent neurons will form synapses with neurons in the medulla oblongata in a region known as the solitary nucleus of the medulla oblongata. Then neurons in the medulla oblongata will relay information to the thalamus and neurons in the thalamus will relay information to the primary gustatory cortex in the insular lobe of the cerebral cortex where the perception of taste will be processed.