Olfaction PDF

Odour: A general smell sensation of a particular quality Odourant: Molecules that olfactory receptors “recognize” and respond to by producing neural signals that the brain represents as perceptions of different odours Chemical compounds, but not every chemical is an odourant In orde...

Odour: A general smell sensation of a particular quality Odourant: Molecules that olfactory receptors “recognize” and respond to by producing neural signals that the brain represents as perceptions of different odours Chemical compounds, but not every chemical is an odourant In order to be smelled, molecule must be volatile, small, and hydrophobic Detection threshold: The concentration of an odourant necessary for a person to detect it. Identifying and Discriminating Odours Smell smell is tacked onto an organ with another purpose—the nose ◦ Primary purpose: To filter, warm, and humidify air we breathe Nose contains small ridges, olfactory cleft, and olfactory epithelium Olfactory cleft: back of the nose, where the main olfactory epithelium is located Olfactory epithelium: A secretory mucosa, detect odourants in the inspired air Cribriform plate: A bony structure riddled with tiny holes, at the level of the eyebrows, that separates the nose from the brain Axons from Olfactory Receptor Neurons (ORNs) pass through the tiny holes to enter the brain Anosmia: The total inability to smell, most often resulting from sinus illness or head trauma. ◦ 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. ◦ Anosmia causes a profound loss of taste as well as smell. posterior piriform cortex (PPC): The posterior (rear) portion of the piriform cortex; it produces representations of the quality of an odour as a whole, regardless of whether the odour is simple or complex Retina of the nose Three types of cells: 1. Supporting cells: metabolic and physical support for the olfactory receptor neurons. 2. Basal cells: Precursor cells to olfactory receptor neurons. 3. Olfactory receptor (sensory) neurons (ORNs/OSNs): The main cell type in the olfactory epithelium. OSNs are small neurons located beneath a watery mucous layer in the epithelium ◦ Mitral cells: The main projective output neurons in the olfactory bulbs. ◦ Tufted cells: A secondary class of output neurons in the olfactory bulbs. ◦ Glomeruli: Spherical conglomerates containing the incoming axons of the ORNs Each ORN converges on two glomeruli (one medial, one lateral) ◦ Turbinates: Bony convolutions of tissue protruding into the nasal cavities, functioning to disperse air evenly throughout the nasal cavities.  ◦ Cilia: Hairlike protrusions on the dendrites of ORNs. Contain receptor sites for odourant molecules. ◦ Olfactory receptor (OR): The region on the cilia of ORNs where odourant molecules bind. ~8 odour molecules binding to a receptor required to initiate an AP They 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. Adapt to odours Adaptation to Odours cross-adaptation: In olfaction, reduced sensitivity to odourants that are chemically or perceptually similar to odourants to which the person has been continuously or repeatedly exposed. Neural Code for Odour genome contains about 1000 different olfactory receptor genes; each codes for a single type of ORN All mammals have pretty much the same set of 1000 genes. However, some genes are non-functional pseudogenes. Dogs and mice: Only 20% are pseudogenes. Humans: Between 60%-70% are pseudogenes: meaning we have about 300-400 types of ORN Each person has a different number of pseudogenes, resulting in individual differences in sensitivity to smells. Representing Odours in the Brain piriform cortex: considered to be the primary olfactory cortex Amygdala–hippocampal complex: critical for the unique emotional and associative properties of olfactory cognition Entorhinal cortex: provides the major sensory association input into the hippocampus. Also receives direct projections from olfactory regions Pathway from Nose to Brain Odours, Emotion, Memory People not good at identifying odours without context Detection, discrimination, and recognition How much stimulation is required before we perceive something to be there? Olfactory detection thresholds: Depend on several factors 1. Sex: Women havegGenerally lower thresholds than men, especially during ovulatory period of menstrual cycles, but their sensitivity is not heightened during pregnancy 2. Professional perfumers and wine tasters can distinguish up to 100,000 odours. 3. Age: By 85, 50% of population is effectively anosmic 4. Durability: Our recognition of smells is durable even after several days, months, or years Limbic system: The encompassing group of neural structures that includes the olfactory cortex, the amygdala, the hippocampus, the piriform cortex, and the entorhinal cortex Involved in many aspects of emotion and memory. Olfaction is unique among the senses for its direct and intimate connection to the limbic system. Olfaction’s unique connection to the limbic system explains why scents tend to have such strong emotion associations. Tip-of-the-nose phenomenon: The inability name an odourant, even though it is very familiar Contrary to tip-of-the-tongue phenomenon, one has no lexical access to the name of the odourant, such as first letter, rhyme, number of syllables, etc. ◦ One example of how language and olfactory perception are deeply disconnected Anthropologists have found that there are fewer words for experience of smells compared to other sensations ◦ Olfactory information is not integrated in thalamus prior to processing in cortex ◦ Majority of olfactory processing occurs in right side of brain while language processing occurs in left side of brain Sense of smell is essentially a change detector Receptor adaptation: The biochemical phenomenon that occurs after continuous exposure to an odourant, whereby the receptors stop responding to the odourant and detection ceases Cross-adaptation: The reduction in detection of an odourant following exposure to another odourant Presumed to occur because the two odours share one or more olfactory receptors for their transduction, but the order of odourants also plays a role Odours and Environment Cognitive habituation: The psychological process by which, after long-term exposure to an odourant, one is no longer able to detect that odourant or has very diminished detection ability E.g.: Going out of town, coming back and noticing how your house smells Three mechanisms involved: 1. Olfactory receptors internalized into cell bodies during odour adaptation maybe hindered after continuous exposure, take longer to recycle 2. Odourant molecules may be absorbed into bloodstream, causing adaptation to continue 3. Cognitive-emotional factors The importance of attention We cannot smell while we are asleep Attention increases our ability to detect odours Attention is cut off during sleep, so is our ability to respond to odours Effects of Odours on Social and Reproductive Behaviour Pheromone Pheromone: A chemical substance emitted by individual organisms that evokes behavioral or hormonal responses in other individuals of the same species. Vomeronasal olfactory system: In many species, an olfactory system that senses pheromones; it is distinct from the main olfactory system used to smell most substances. Human Leukocyte Antigen Detection Human leukocyte antigens (HLAs): The genes responsible for regulating the immune system; women can detect HLA differences in odours from men, possibly influencing their selection of a mate.