Lecture 10

list the neurotransmitter used for EXCITATION of the primary afferent neuron by the taste cell (5)

serotonin, glutamate, acetylcholine, noradernaline, GABA

describe the sensory transduction in olfaction (8)

1) odorant (dissolved in the mucus layer) binds to receptor in the olfactory receptor cell's cilia, that receptor is g-protein coupled by the g-protein 'g-olfactory' (G-olf); 2) upon binding the alpha-G-olf activates adenyl cyclase; 3) that activation causes adenyl cyclase to synthesise cAMP; 4) cAMP binds to cAMP-gated channels which is selective for Na+ and Ca2+, activating that; 5) Na+ and Ca2+ rush in and depolarise the cell; 6) the Ca2+ binds to Ca2+-gated Cl- channels; 7) in the olfactory receptor cells there is greater concentration of Cl- in the ICF than ECF therefore Cl- rush out further depolarising the cell; 8) the AP travels to the olfactory bulb

describe sensory transduction for sweetness, umami and bitterness (6)

1) sweet, bitter and umami (glutamate) molecules bind to receptors T1R2/T1R3, T1R1/T1R3 and T2R receptively; 2) these receptors are coupled to the G-protein gustducin, upon receptor binding the alpha-gusducin is activated which leads down a cascade; 3) one of the actions is membrane depolarization, another action is the activation of phospholipase C; 4) which goes down another cascade part of which which leads to the Ca2+ being induced out of the ER; 5) that Ca2+ induces the non-selective cation channel TPRM5 to open which causes Na+, Ca2+ influx; 6) eventually leading to NTs being exocytosed to the primary afferent neuron

describe sensory transduction for saltiness and sourness (4)

1) when food is salty/sour it has a high concentration of Na+/H+, there are open ion channels on the apical membrane of the taste cell which let Na+/H+ diffuse in; 2) when enough gets in (as would happen in a salty/sour meal) the membrane depolarises and causes depolarisation of the basolateral membrane; 3) there Ca2+ voltage-gated channels open and the Ca2+ induce exocytosis of NTs; 4) which leads to excitation/ inhibition of a nearby primary afferent neuron

describe the CNS pathway from the taste cells

afferent neurons from the taste cells synapse at the 'nucleus of the solitary tract' in the medulla, here there are neurons that go between it and the hypothalamus and the amygdala respectively (conditioning against a particular taste could be done at the amygdala), after the nucleus of the solitary tract the neurons synapses at the ventral posterior medial nucleus of the thalamus and from there synapse at the primary gustatory cortex in the insula and frontal cortex (from there are some neurons go to the amygdala)

describe the interaction of tastes

each of the tastes enhance some or all of the other tastes (e.g. without a little bit of fat, the other tastes won't be activated well and taste will be bland)

what are papillae

in regions where taste is detected (e.g. back of tongue, lateral edges of tongue) there are structures called papillae, each of these contain multiple taste buds (total of 2000 to 5000 taste buds across the entire tongue)

describe the traditional map of the distribution of tastes on the tongue

sweet - tip of tongue, salty - anterior lateral edges of tongue, sour - posterior lateral edges of tongue, bitter - back of tongue, umami - all over, fatty acids - palate; the more towards the front of the tongue a taste is the more evolutionarily it is desired (i.e. sweet is nutrients, salt is necessary, sour is spoilt food, bitter is poison)

lists the tastes (6)

sweet, sour, salty, bitter, umami (similar structure to glutamate), fat (fatty acids) [all but fat are well defined tastes and considered the basic tastes]

describe the structure of taste buds

taste buds are structured with microvilli projecting from taste cells at one end which has the receptors which bind the chemicals, basal cells (support cells) below the taste cells, then afferent neurons taking information into the gustatory pathways

describe the presence of taste cells outside of mouth

taste cells are present along entire GI tract, but the afferents from these taste cells end at the nucleus of the solitary tract or spinal cord or take a different path from there - serving a completely different purpose than flavour

describe the locational positioning of the olfactory receptor cells

the cell body lies in the olfactory epithelium, but axon extend out through the Cribiform plate to the olfactory bulb; at its apical end (olfactory epithelium end) dendrite-type process extend out of the olfactory epithelium and into the mucus layer underneath olfactory epithelium, there it splits into cilia that detects molecules that are dissolved in that mucus

describe the selectivity of the odorant receptors on the olfactory receptor cells

the human genome encodes around 1000 different receptors that each bind a small range of similarly structured molecules; each olfactory receptor cells expresses 1 or 2 different types of receptors; all this results in each different neurons having different activation profiles (which gives the receptive field); the olfactory bulb will analyse which neurons were activated to distinguish the smell

describe the main synaptic structure of the olfactory bulb

the olfactory receptor cells come into the olfactory bulb and synapse at glomeruli, each individual glomerulus encodes only one odor (i.e. multiple olfactory receptor cells that have the same receptors across the plane of the olfacotry epithium all synapse at the same glomerulus); synapsing with the glomeruli are projection neurons called mitral cells and at the other end of mitral cells there are synapses with granule cells (which have no axon or AP, but have synaptic inputs and outputs); the granule cells act to tune/ focus the glomeruli onto the main odor and ignore small activations

describe flavour

the sensation of food/ drink with each of the senses, smell, taste, texture, appearance, temperature, pain (chilli) and fat, in context of each other (however smell and taste are the dominant senses)

describe olfactory activation during food tasting in mouth

there is a passage that connect the mouth to the nasal cavity, food in mouth especially moving around/ being chewed will cause chemicals from the food to shoot up into the nasal cavity and olfaction will be activated

describe the properties of taste cells (3)

these are epithelial cells; 2 weeks turn over (this is good as they could be easily killed, say by hot food); of the multiple taste cells in a taste bud, each cell will detect its unique combination of taste (e.g. one taste cell may be activated by salt and inactivated by everything else, another cell may be activated by salt and acid while being inactivated all by the other tastes), when from a region (e.g. back of tongue) these signals are summated at a higher level we get regions of the tongue which detect the unique tastes (such as back of tongue detecting bitterness)

describe the action of inosine

this molecule binds to the T1R1/T1R3 (umami) receptors and in conjunction with umami bound to those receptors will cause a particularly strong activation

describe the olfactory epithelium

this sits on the roof of the nasal cavity; it is mainly made of 'olfactory receptor cells' which are neurons; the axons of the olfactory receptor cell travel through the Cribiform plate to the olfactory bulb; even though these are not epithelial cells they have fast turn over rate

describe the central olfactory pathways

unlike other sensory pathways which initially travel through the thalamus before going to the main areas/ cortices, projection neurons directly carry info from olfactory bulb to main areas (incl. pyriform cortex, olfactory tubercle, amygdala, entohinal cortex); the connection can then go to the oritofrontal cortex, thalamus and hypothalamus from any of the 4 main areas; the pyriform cortex (the primitive olfactory cortex) goes specifically to the orbitofrontal cortex, and the from the entorhinal cortex info is taken to the hippocampal formation