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Lecture 11: Chemical Senses
Transcript of Lecture 11: Chemical Senses
Smell is a potent wizard that transports you across thousands of miles and all the years that you have lived.Helen Keller
Odors are detected by nasal olfactory sensory neurons. These neurons have projections that extend from the nasal cavity, through the porous cribriform plate, and synapse in the glomeruli of the olfactory bulb
Signals originating at the cilia are transmitted along the olfactory sensory neurons, across the cribiform plate to the olfactory bulb
The signal is then relayed from the olfactory bulb to several regions of the cortex
Different sensory receptors respond to different types of odors
Specific odors will activate a certain population of these receptors
Those specific receptors make up the signal “signature” for that specific odor
Olfactory receptors are coded for by one of the largest gene families in the genome
When the appropriate chemical "odorant" interacts with the G-protein-coupled receptor, the alpha-subunit disassociates from the receptor and activates the enzyme cAMP adenyl cyclase
cAMP adenyl cyclase activation results in the production of cAMP
Increased levels of cAMP in the cell opens the nucleotide-gated channel and allows cations (e.g. Na+ and Ca2+) to flow into the cell
Let’s take a minute to compare this signal transduction system with that of the visual system!
Stimulus results in G protein disassociation of alpha-subunit
Alpha-subunit of G protein activates enzyme
Enzyme changes internal concentration of some ligand
Change in ligand concentration alters membrane potential by opening or closing ion channels
Stimulus in visual system is light, which activates G-protein, and activates cGMP phosphosiesterase which decreases cGMP concentrations and closes sodium channels, resulting in hyperpolarization
Stimulus in olfactory system is chemical, which activates G-protein, and activates cAMP adenyl cyclase, which increases cAMP concentration, which opens ligand-gated sodium channels and depolarizes the cell.
Third level of olfactory information sorting:
The periglomular cell forms inhibitory synapses with the mitral cell dendrites
This offers another mechanism by which olfactory signals are “cleaned”
If a single smell activated all of the receptors, one would be unable to distinguish between smells
However, if all but the correct signals are inhibited by mechanisms like the periglomular cells, then the message can be appropriately interpreted by the brain.
First level of olfactory information sorting:
The axon of each olfactory sensory neuron synapses in a single glomerulus
Within each glomerulus, several thousand olfactory sensory neurons converge and synapse on some 20-50 mitral and tufted cells (a.k.a. relay cells).
Second level of olfactory information sorting:
Granule cells communicate with mitral cells
When granule cells receive an excitatory stimulus from one mitral cell, it can inhibit signals from other mitral cells
This two-way communication between the granule and mitral cells enables "olfactory pruning"
Only the strongest chemical signals will be relayed to the olfactory cortex, and further processed by the brain
Central Olfactory Pathways
Mitral cells send their axons to a number of brain areas, including the anterior olfactory nucleus, piriform cortex, the medial amygdala, and the entorhinal cortex
The piriform cortex identifies the odor
The medial amygdala is involved in social functions such as mating and the recognition of animals of the same species
The entorhinal cortex is associated with memory, e.g. to pair odors with proper memories.
Fluid-filled organ with an opening into the nasal cavity
Lined with a sensory epithelium similar to that of the olfactory bulb
More prominent in nonhuman mammals
Functional significance questionable in humans
Sweaty T-shirt experiment
Claus Wedekind: Swiss Immunologist
Female subjectsSniff t-shirts that had been worn by men for 2 full days
Rate the smell on pleasantness
Rate their attraction to the person wearing the shirt
Wedekind et al. 1995, Proc. R. Soc. Lond. 260, 245-249
The results of the study suggested that women were most attracted to those men that were most different from them in terms of their MHC genes. MHC stands for major histocompatibility complex, and it is integral to several properties of immune function. For example, MHC genes must match between an organ donor and an organ recipient. Otherwise, the donated organ will be attacked by the recipient’s immune system. However, when it comes to sexual selection, theory states that dissimilar MHC genotypes are most likely to result in the highest viability of offspring. So this study suggests that women were able to perceive the genetic integrity of the men wearing the t-shirts, just by smelling their sweat. More specifically, the women were able to (subconsciously) identify and assess the degree to which the males were similar or dissimilar to them in their genetic makeup for the MHC genes.Since this study, a number of other studies have suggested that humans may be capable of evaluating pheromones, however, the idea is not fully accepted.
One part of this study that was particularly interesting was that the sample population of women included women both on and off of birth control
Only those women that were cycling naturally (i.e. were NOT on birth control) showed an ability to distinguish between similar and distinct MHC complexes.
If there is a lot of similarity between two people’s MHC genes, those people will not be attracted to one another because they would be passing on a disadvantageous immune system to their offspring
However, if their respective MHC genes are not similar, they should be more attracted to each other
The ability to taste is enabled by specialized bunches of cells known as "taste buds"
Various cell types that are present in each taste bud
Taste pore at the surface, where the various molecules will translocate from the saliva into the receptive cells of the tongue
Taste cells: ~50-150 taste cells in any given bud
extend all the way from the base of the taste bud to the apical end
apical end of the taste cells are equipped with microvilli that extend into the taste pore
microvilli are initial intake location for molecules entering the taste cells
Chemicals that are received by the microvilli on the taste cells generate action potentials within the taste cells
Action potentials induce the release of neurotransmitters from the taste cells onto the gustatory afferent nerves.
Four basic tastes
To demonstrate the response to sour, enjoy...
Information about taste is initially converted into an electrical signal in the taste buds
Signal is relayed through the nucleus of the solitary tract (within medulla) to the ventral posterior medial nucleus of the thalamus
Neurons from the thalamus then synapse on the gustatory cortex
(no babies were harmed in the making of this video)