Download PID *****2515 1.Why is it difficult to understand olfactory neural

20.5/15
PID *****2515 1.Why is it difficult to understand olfactory neural encoding compared to other sensory modalities, such as vision or audition? Because unlike light and sounds, odors cannot be classified by a simple parameter such as wavelength or frequency (p45). It is much easier to decompose visual input and it is to decompose odor input. We rely on a large number of diverse odor receptors to understand the 5/5
complexity of odor receptors. The multiplicity of receptors allows detection of a vast number of odors. Discrimination depends on combinatorial coding and on circuit­level interactions at multiple steps of olfactory processing. (p45). However, it is hard to classify receptors because it is hard to know their exact function. 2.Explain the relationship among physical space, neural space, and perceptual space in olfactory processing. Physical odor space is based on structural difference. For example, in one study, 1664 molecular descriptors for more than 1500 odorants were used to construct a 1664­dimensional odor space, in which dimension represents one feature of odorant structure ( p55). The physicochemical relationship btw two odorants can be quantitated as the Euclidean distance btw them in this space. Neural odor space maps out how odors are represented by ORNs. It is 5/5
constructed from direct measurements of odor responses from neurons of the system, or in the case of ORNs, from responses of odor receptors in an expression system (p55). For example, one study was able to map out most of the Ors expressed in Drosophila antenna. Perceptual space describes perceptual experience of an odorant, it is subjective from individual to individual. These three spaces are related: PCA reveals a correlation btw odorant structure (physical space) and its perceived pleasantness (perceptual space) among humans; and odors that are close in neural space are likely to be close in physical space and be close in perceptual space as well (p55). 3. Enumerate three ways in which the olfactory system limits the temporal extent of neural response to an odorant. What are the advantages and disadvantages of this temporal limitation? Binding of an odorant to an odorant receptor is loose compared to other GPCRs (p50). The looseness of OR­odorant binding is consistent with the finding that odorant dwell times are extremely short (<1ms). Unlike phototransduction amplification, the Cl­ influx provides the major amplification step in olfactory transduction, and is enabled by a chloride transporter that maintains a high Cl­ concentration in the cilia. This enables short odorant dwell times. Expeditious signal termination is required to ensure faithful temporal representation of odor stimuli, and it is achieve through Ca2+ ­mediated feedback inhibition of the cyclic­nucleotide gated channel, activation of phosphodiesterase, and inhibition of the adenylyl cyclase, along with extrusion of Ca2+ (p50). When translating chemical signals to electrical signals, acuity relis upon adaptation, which allows extension of the dynamic range of ORNs such that they are informative over a broader range of odorant concentrations and it enbables an animal to detect new scents above a background odor. Due to the lateral spread of M/T secondary dendrites, granule cells contact multiple M/T cells associated with different glomeruli, and thus can mediate both intra­ and interglomerular inhibition. Advantages: loose binding, fast transduction amplification, short dwell time, and expeditious signal termination are in agreement with the broad tuning of many odorant receptors; they enhance combinatorial coding, and increase temporal acuity; they allow quick binding and let go, so that we can sample quickly and more frequently. However, this brings disadvantages such as lower sensitivity, and lower SNR (signal to noise ratio), because the response cannot be modulated by time. 4. (Extra) What is combinatorial encoding? Where is it found in the olfactory system? What are the computational advantages and disadvantages of combinatorial encoding? In the olfactory system, individual odorants activate subsets of receptors, so odorants are identified not by the activation of a single receptor, but rather by the pattern of receptors that are activated (p49). Thus, combinatorial encoding is the representation of information across many neurons such that the code is described by a pattern of activation. Advantages: The relative response of different neurons can encode different information; combination of responses of different neurons can be nonlinear; and combinatorial encoding is more robust. Disadvantage: this makes odor hard to decompose, (compared to the space of visual). 5. (Extra) What is “sparse coding” and how is it achieved in third­order neurons? In the pyramidal neurons of the olfactory system, an odor stimulus elicits responses from only a small fraction of spatially dispersed neurons, and these responses consist of only few action potentials (p53). Sparse coding is achieved in the third­order neurons (such as Kenyon) because pyramidal neurons (second­order neurons) act as coincidence detectors, they only fire AP when a certain subset of M/T cells is synchronously active (p53). The coincident activity of several presynaptic M/T cells is required to overcome widespread inhibition mediated by local interneurons. We need the precise combo to fire at the pyramidal level; an individual odor is unlikely to cause a pyramidal neuron to fire. Due to the requirement of synchronized inputs from multiple second order neurons (coincidence detection), many fewer third order neurons are activated (sparsening) with more distinct activation pattern (decorrelation) . 6. (Extra) What is “pseudogenization”? Give two examples in different species. Pseudogenization is the process by which mutation renders genes nonfunctional (p48). The examples of pseudogenization in mammals are: (1). Aquatic vertebrates genomes nearly exclusively contain older Class I ORs that are tuned toward water­soluble odorants; (2). Terrestrial vertebrate genomes contain both Class I and Class II ORs which are tuned toward hydrophobic odors. Thus, we see in dolphins, Class II receptors have been pseudogenized. In addition, all V2R genes have been pseudogenized in humans. 5/5
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