S P Ch 14 Olfaction Olfaction the sense of smell Gustation the sense of taste Olfactory Physiology Odors olfactory sensations Odorant any specific aromatic chemical The Human Olfactory Apparatus chemical compounds not every chemical is an odorant in order to be smelled molecule must be volatile able to float through air small and hydrophobic water repellent Unlike other senses smell is tacked onto an organ with another purpose the nose Primary Purpose to filter warm and humidify the air we breathe Nose contains small ridges olfactory cleft and olfactory epithelium Olfactory Cleft a narrow space at the back of the nose into which air flows where the main olfactory is located Olfactory Epithelium a secretory mucosa in the human nose whose primary function is to detect odorants in the inspired air the retina of the nose Three types of cells Supporting Cells provide metabolic and physical support for the olfactory sensory neurons Basal Cells precursor cells to olfactory sensory neurons Olfactory Sensory Neurons OSNs the main cell type in the olfactory epithelium OSNs are small neurons located beneath a watery mucous layer in the epithelium Cilia hair like protrusions on the dendrites of OSNs contain receptor sites for odorant molecules First structures involved in olfactory signal transduction Olfactory Receptor OR the region on the cilia of the OSNs where odorant molecules bind It takes seven or eight odor molecules binding to a receptor to initiate an action potential Cribriform Plate a bony structure riddled with tiny holes at the level of the eyebrows that separates the nose from the brain Anosmia the total inability to smell most often resulting from sinus illness or head trauma axons from the OSNs pass through the tiny holes to enter the brain a hard blow to the front of the head can cause the cribriform plate to be jarred back or fractured slicing off the fragile olfactory neurons causes a profound loss of taste as well as smell can lead to depression loss of libido Congenital Anosmia children with anosmia at birth often pretend to be able to smell Olfactory receptor cells are different from all other sensory receptor cells they are not mediated by any protective barrier and make direct contact with the brain Contrast with visual receptors that are protected by cornea hearing receptors protected by eardrum taste buds are buried in papillae Therefore many drugs can be inhaled OSN axons are among the thinnest and slowest in the body Takes longer to perceive odors compared to other senses around 400 ms compared to 45 ms for visual stimulus to reach the brain Olfactory Nerves the first pair of cranial nerves the axons of the OSNs bundle together after passing through the cribriform plate form the olfactory nerve Olfactory Bulb the blueberry sized extension of the brain just above the nose where olfactory information is first processed There are two olfactory bulbs one in each brain hemisphere corresponding to the left and right nostrils Connections are ipsilateral same side of body unlike vision hearing or touch which are contralateral Mitral Cells the main projective output neurons in the olfactory bulbs Glomeruli spherical conglomerates containing the incoming axons of the OSNs Each OSN converges on two glomeruli one medial one lateral 1 Limbic System involved in many aspects of emotion and memory Olfaction is unique among the senses for its direct intimate connection to the limbic system Olfaction s unique connection to the limbic system explains why scents tend to have such strong emotion associations Subtle differences between sensation and perception Sensation occurs when scent is neurally registered Perception occurs when becoming aware of sensations The feel of scent Odorants can stimulate somatosensory system through polymodal receptors touch pain temperature receptors These sensations are mediated by the trigeminal nerve cranial nerve V Often impossible to distinguish between sensations traveling up cranial nerve I from olfactory receptors and those traveling up cranial nerve V from somatosensory receptors Current dominant theory of olfactory perception e g peppermint cool ammonia burning Shape Pattern Theory different scents activate different arrays of olfactory receptors in the olfactory epithelium as a function of odorant shape to OR shape fit These various arrays produce specific firing patterns of neurons in the olfactory bulb which then determine the particular scent we perceive Combinatorial Neural Code the importance of patterns how can we detect so many different scents if our genes only code for about 1000 olfactory receptors we can detect the pattern of activity across various receptor types intensity of odorant also changes which receptors will be activated weak concentrations of an odorant may not smell the same as strong concentrations of it SPECIFIC TIME ORDER OF ACTIVATION OF OR RECEPTORS IS IMPORTANT Odor Mixtures how do we process the components in an odorant mixture Two possibilities Analyses example from auditory mixtures High note and low note can be played together but we can detect each individual note Synthesis example from color mixtures Mixing red and green lights results in yellow light but we cannot separately perceive the red and green in the yellow OLFACTION IS MOSTLY SYNTHESIS BUT ANALYTICAL ABILITIES CAN BE TRAINED Detection Olfactory detection thresholds depend on several factors Women generally lower thresholds than men especially during ovulatory period of menstrual cycles but sensitivity not heightened during pregnancy Professional perfumers and wine tasters can distinguish up to 100 000 odors Age by 85 50 of population is effectively anosmic Adaptation sense of smell is essentially a change detector examples walking into a bakery smelling fresh bread for a few minutes OR wearing perfume every day not really smelling anymore Receptor Adaptation the biochemical phenomenon that occurs after continuous exposure to an odorant whereby the receptors stop responding to the odorant and detection ceases Cognitive Habituation occurs after prolonged exposure to odorants days weeks years like living above a subway sub shop not smelling that smell anymore Odor Hedonics the liking dimension of odor perception typically measured with scales pertaining to an odorant s perceived pleasantness familiarity and intensity how much we like or dislike an odor nurtur vs nature 2 Familiarity Intensity Nature vs Nurture we tend to like odors we ve smelled