Yuste C8 Flashcards
Odorants
Volatile, organic molecules, often lipophilic, which are mostly emitted by organic matter.
Transmitted by air currents in odour plumes.
Measured slowly and carefully with every sniff of breath.
Smell
Sensory system built to detect the presence of animals or plants
Principles of olfaction
Parallel pathways for specialised measurements of stimulus properties and top-down control to “construct” perceptions.
Smell and taste
In contact through the pharynx
Two steps of olfaction
Detecting odourants and decoding smells
Olfactory pathway overview
Starts inside the nose in the olfactory mucosa, which has an epithelium with sensory neurons that project to the olfactory bulb. The bulb then has neurons that project to the cortex and many parts of the brain.
Unique about smell
The only sense that doesn’t go through the thalamus before going to the cortex.
Mucus
Secreted by specialised cells in the olfactory epithelium, traps and concentrates door molecules, both through its particular chemical environment and via specialised binding proteins. The mucus brings odourants in contact with the cilia of the olfactory receptor neurons.
Cilia of ORNs
Have olfactory receptors, specifically binding odourants.
ORs
Part of the GPCR family. 7 TM regions. We have about 350 different ORs, which share the same structural scaffold but differ in the AA sequence that builds their binding pockets for odourants, enabling recognition of diverse chemicals.
Each odourant can bind to several ORs, with higher and lower affinity. ORs form a matrix of molecular detectors that can bind to essentially any molecule.
ORNs
Display allelic exclusion – only one allele of a gene is expressed while the other is silenced.
Each neuron expresses only one OR, so that the message it sends to the brain is specific. One ORN, one OR, ensuring that all ORN are different from one another.
Olfactory transduction
Occurs on the membranes of the cilia of the ORNs. Once an odourant binds to the receptor, it triggers 2º messenger cascade. Activation of G-olf, which activates adenyl cyclase III (ACIII), which generates cAMP. cAMP activates a cation channel that brings in Na and Ca into the cell, depolarising the neuron. Ca also starts a series of 2º events that shape the depolarisation. => Amplification of the signal.
Olfactory bulb
Axons from the ORNs project to glomeruli, where they make contact with dendrites from mitral cells (neurons that project into the cortex).
ORNs that express the same OR have axons that converge onto the same glomerulus, so each glomerulus received info from one type of neuron and one type of receptor. Transforming a chemical signal into a spatial map of active glomeruli. Encoded odourants into a map.
Each bulb has 2 glomeruli that receive axons from the neurons that express the same OR; position of these two is symmetric in each of the bulbs.
Mitral cells
Neurons that project into the olfactory cortex aka pyriform cortex. Given mitral axon contacts many cortical neurons; a given cortical neuron revives axons from many different mitral cells.
Axons also project to the enthorhinal cortex (learned responses), the amygdala (innate olfactory responses).
Combinatorial code
To detect smell, you read out the pattern of activation of all glomeruli, since each particular pattern corresponds to one door. Combination of active glomeruli encode a final => particular odour.
Scrambling of info from each OR in the olfactory cortex
Cortical neurons receive inputs form a combination of different glomeruli, and a given cortical neuron will only fire if those inputs happen to fire at the same time; thus the neuron becomes active only if a particular combination of glomeruli has turned on. That cortical neuron is detecting a particular odour. Reading the spatial code of the bulb and picking up one particular combination. Each of the cortical neurons respond to one particular odour and all of them together will decode all odours.
Olfactory cortex as a perceptron
Cortical neuron fires if a given combination of inputs are active, acting as an AND gate. This builds input selectivity.
Taste
ORNs routinely get activated by our food, so taste is also largely smell. Limited range of sensations. Our brain only recognises a very limited set of tastes, the tastands.
Taste is a hardwired sense.
The 5 tastes
Sugar – to identify food rich in C-Hs calories.
Salt – to detect sodium, essential ion.
Umami – to find out if the food is rich in proteins
Sour – the taste of acid, to detect if food is spoiled
Bitter – detecting alkaloids, to detect possibly poisonous food.
Gustatory pathway
Starts in the taste buds, located in the tongue and pharynx. There we have taste cells with taste receptors; these cells project to the gustatory nucleus in the brain stem, which then projects to the thalamus, which finally projects to the gustatory cortex.
Taste cells
Located in the groove of the tongue, in the taste buds. Their apical surface contacts the mouth cavity, which is where taste receptors are located.
Have different types of taste receptors; some are ion channels, some are GPCRs.
Ion channel taste receptor cells
Salt – amiloride-sensitive Na+ channels
Sour – H+-sensitive proton channels
Directly depolarise the neurons.
GPCR taste receptor cells
Sweet, umami, and bitter tastands.
Basal region of taste cells
Depolarisation of cells tiggers transmitter release that regenerates APs in gustatory sensory neurons, whose axons relay that signal to the nucleus of the gustatory tract in the brainstem. From there, taste info is relayed to the hypothalamus, amygdala, and ventro-posterior medial VPM nucleus of the thalamus, which projects to the gustatory area. Different tastes are mapped onto five different patches there.
Experiments with expressing different receptors in taste receptor cells
Consistent with a label-line code – individual neurons respond only to one tasting and the taste quality is then determined via an anatomical pathway.
Chemical senses optimisation and receptive fields
Optimised to single molecule detection; build with parallel processing; track the identity of the stimuli in a chemical space (chemical receptive fields).
Two basic computational principles
Olfaction – softwired; based on a neural network strategy, first encoding the stimulus in a multidimensional space, randomly scrambling it all, then decoding this combinatorial code with perceptron-type circuits and attractors that identify particular odours.
Gustatory system – simpler, hardwired. Five parallel labelled lines independently carry tastand info to the brain.
