Psyc 362 Chapter 7 text Audition the body senses and the chemical senses Olfaction Function Helps us to identify food and avoid food that has spoiled and is unfit to eat Helps many species detect prey predator friend mate or foe Odors can evoke memories It is best for identifying things not for analyzing particular qualities The Stimulus odorants consist of volatile substances with molecular weights of 15 300 Mostly lipid soluble and organic originatic substances Anatomy of the Olfactory Apparatus The olfactory epithelium located at the top of the nasal cavity consists of two patches of mucous membranes containing the system s receptor cells Somas of the bipolar receptor cells lie within the olfactory mucosa lining the cribriform plate a bone at the base of the rostral part of the brain They re surrounded by supporting cells that contain enzymes to destroy odorant molecules and help prevent damage to olfactory receptor cell still new receptor cells are constantly being produced The receptor cells send a process toward the surface of the mucosa then it divides into many cilia that penetrate the layer of mucus Stimulus molecules on the olfactory cilia can be activated by odorous molecules dissolving in the mucus some free nerve endings that mediate sensations of pain for irritating chemicals the olfactory bulbs are protrusions at the end of the olfactory tract that receive input from the olfactory receptors each receptor cell sends an axon into an olfactory bulb where it synapses with dendrites of mitral cells neuron in olfactory bulb that receives info from olfactory receptors Those cells have axons that bring information to the rest of the brain The olfactory glomeruli are bundles of dendrites of mitral cells and the associated terminal buttons of the axons of olfactory receptors where these synapses take place Olfactory tract axons project directly to the amygdala and to regions of the limbic cortex the piriform cortex primary olfactory cortex and the entorhinal cortex The amygdala sends info to the hypothalamus probably important for acceptance or rejection of food Also for control of reproductive processes seen in many species of mammals The entorhinal cortex sends info to the hippocampus the piriform cortex sends info to the hypothalamus and to the orbitofrontal cortex using the dorsomedial nucleus of the thalamus orbitofrontal cortex also involved in gustatory system Vomeronasal organ is found in most mammals and plays an important role in responses to pheromones Transduction of Olfactory Information Olfactory cilia have receptors stimulated by molecules of odorants There are a family of genes that code for a family of olfactory receptor proteins Molecules of odorants bind with olfactory receptors and the G proteins coupled to these receptors open sodium channels and produce depolarizing receptor potentials Perception of Specific Odors we can recognize many different odorants while many odors go unaccounted for We have a relatively small number of receptor genes The cilia of each olfactory neuron contain only one type of receptor and each glomerulus receives info from many individual receptor cells all of which contain the same type of receptor molecules Therefore there are as many types of glomeruli as there are types of receptor molecules Location of particular types of glomeruli appear to be the same in each of the olfactory bulbs a particular odorant binds to more than one receptor allowing us to use a small number of receptors to detect many different odorants Different odorants produce different patterns of activity in different glomeruli such that recognizing a specific odor is a matter of recognizing a specific pattern of activity in the glomeruli a task of spatial recognition by knowing which pattern of receptors is activated we can predict which odorant is present So particular categories of molecules with particular types of structures activate particular regions of the olfactory bulb furthermore particular odorant categories example mint citrus etc with molecules in each categories of very different structure will produced patterns of activity on the posterior piriform cortex correlated with the perceived category of odor not its molecular structure Some odors have the ability to mask others for example spoiled food masked by spices fennel and clove Responses to bad odorants appear suppressed by the presence of spice odors of fennel and clove and takes place in the olfactory bulbs The glomeruli that respond to the spice odors most likely inhibit those that respond to the rancid ones