Neuroscience Lecture 22: Taste and Smell PDF

Taste Sensations https://www.youtube.com/ watch?v=kuyUKdJccgM There are five basic taste sensations • • • • • Sweet – sugars, saccharin, alcohol, and some amino acids Salt – metal ions Sour – hydrogen ions Bitter – alkaloids such as quinine and nicotine Umami – elicited by the amino acid glutamate...

Taste Sensations https://www.youtube.com/ watch?v=kuyUKdJccgM There are five basic taste sensations • • • • • Sweet – sugars, saccharin, alcohol, and some amino acids Salt – metal ions Sour – hydrogen ions Bitter – alkaloids such as quinine and nicotine Umami – elicited by the amino acid glutamate Taste receptor cells are constantly dying and being replaced Taste buds are found in papillae of the tongue mucosa Papillae come in three types: filiform, fungiform, and circumvallate Taste receptor cells are probably derived from skin (epithelial) cells. Each taste receptor cell lives for only a week or so and is then replaced. Because new receptor cells are constantly being formed, the nerves that contact them must constantly form new connections. Taste Buds Taste Buds Taste pore In order to be tasted, a chemical: • Must be dissolved in saliva • Must contact gustatory hairs Physiology of Taste Binding of the food chemical: • Depolarizes the taste cell membrane, releasing neurotransmitter • Initiates a generator potential that elicits an action potential apical basolateral microvilli (“hairs”) autonomic innervation N = neuroepithelia S = support cells B = basal cells Salty & Sour Taste Transduction Salty taste is mediated by an epithelial Na+ channel (ENaC) that is sensitive to amiloride. Sour is mediated by H+ entering through the same ENaC channel or by the effect of low pH inhibiting a K+ channel. The resulting depolarization opens voltage-gated Ca2+ channels, increasing [Ca2+]i and leading to transmitter release. Sweet Taste transduction Sugar binds to a 7-transmembrane receptor that activates heterotrimeric G protein, stimulating AC, increasing cAMP, and activating PKA, which then closes a K+ channel The resulting depolarization opens voltage-gated Ca2+ channels, increasing [Ca2+]i and leading to transmitter release Bitter Taste Transduction-3 Pathways A bitter compound directly inhibits K+ channels. The resulting depolarization opens voltage-gated Ca2+ channels, increasing [Ca2+]i and leading to transmitter release. A ligand binds to a 7-transmembrane receptor and activates a G protein called gustducin that stimulates phosphodiesterase. The resultant decrease in [cAMP]i somehow leads to depolarization. Bitter Taste Transduction-3 Pathways Ligand binds to a receptor that is linked to a G protein, which activates phospholipase C. The resultant increase in [IP3] releases Ca2+ from stores, raises [Ca2+]i, and leads to transmitter release Umami Transduction Glutamate binds to a glutamate-gated, nonselective cation channel and opens it. The resultant depolarization opens voltage-gated Ca2+ channels, increases [Ca2+]i, and leads to transmitter release. Adenylyl cyclase, AMP, cAMP, diacylglycerol, IP3, phosphodiesterase, phosphatidyl inositol 4,5-biphosphate Gustatory Pathway A branch of the Facial nerve transmits impulses from the anterior two thirds of the tongue A branch of the glossopharyngeal nerve transmits impulses from the posterior third. Impulses are ultimately transmitted to the gustatory cortex. Fibers also project to the hypothalamus and the limbic system Gustatory Pathway Cranial Nerves VII and IX carry impulses from taste buds to the solitary nucleus of the medulla These impulses then travel to the thalamus, and from there fibers branch to the: ◦ Gustatory cortex (taste) ◦ Hypothalamus and limbic system (appreciation of taste) Peripheral Taste Pathways Taste Pathways in the CNS Information about taste is relayed to the nucleus of the solitary tract (NTS) and from there to the thalamus and cortex. The Ascending Taste Pathway to Thalamus and Cortex SMELL The organ of smell is the olfactory epithelium, which covers the superior nasal concha Olfactory receptor cells are bipolar neurons with radiating olfactory cilia Olfactory receptors are surrounded and cushioned by supporting cells Basal cells lie at the base of the epithelium Olfactory Reception Human olfactory epithelium and the underlying lamina propria Scanning EM of the human olfactory epithelium Dendritic knob and cilia of a receptor neuron The Olfactory Pathway The Olfactory Bulb Major Efferent Projections of the Olfactory Bulb Physiology of Smell Olfactory receptors respond to several different odor-causing chemicals When bound to ligand these proteins initiate a G protein mechanism, which uses cAMP as a second messenger ◦ cAMP opens Na+ and Ca2+ channels, causing depolarization of the receptor membrane that then triggers an action potential Olfactory epithelia send axonic processes through olfactory foramina to synapse with mitral and tufted at glomeruli (as many as 103 afferent fibers may synapse with one mitral or tufted cell) Axonic processes of mitral and tufted cells constitute cranial nerve I (CNI) • Odorant dissolved in aqueous phase of mucus interacts with specific receptor • Signal transduction occurs through the classic aGs system to phosphorylate and open a Na+ channel • Often many odorants in one “smell”