smell

smell (olfaction)

taste (gustation)

both are chemical senses (responding to chemicals in the environment), considered “minor” senses in humans

good survival value in humans - ex. sensitive to smell of smoke, taste/smell aversion

basic transduction in smell

  • stimuli - chemicals (airborne and fat soluble)

    • chemosensory receptors not very different from inter-neuron communication

how smell/olfaction works:

  • nasal cavity has baffles so air must travel a ways to get to top

  • allows for purification, warming, and moisturizing of air before it hits olfactory epithelium

  • at the top of each nasal cavity there is the olfactory epithelium

    • patch of tissue with olfactory receptors and supporting cells - lined in mucus

  • odor molecules get absorbed in the mucus (must be fat soluble to be absorbed) and gets transported to the olfactory receptors’ cilia by binding proteins in the mucus

  • when odorant hits the cilia/dendrites it binds w a receptor protein which excites the cell

  • the excited cell sends an impulse down the axon which ends in the olfactory bulb

  • the “right chemical” determined by the match between the amino acids on the receptor and the chemical make up of the odorant

how do we identify different smells?

  • same ORN will fire for very different types of odorant

  • many different ORNs fire for the same odorant

  • we think that it is general pattern of activity across the olfactory bulb that indicated the particular smell

how this works:

  • odorants can bind w different receptors (via lock and key model)

  • pattern of receptors activated will produce a specific pattern of glomeruli activity

  • the pattern = the smell

neural connections

  • relatively direct connections to amygdala (emotions) and limbic system (hypothalamus - drives & hippocampus (memory))

  • then to medial dorsal nucleus of thalamus → orbital frontal cortex (conscious ID of smell)

how sensitive is olfaction?

  • in humans is pretty good, but varies w scent

    • smoke can smell 1 part per 50 billion

  • much more sensitive in dogs

olfaction is not that acute: not that good at identifying exactly what we are smelling

  • variability between individuals, women better than men, younger better than old

  • smoking/poor air quality diminishes ability to smell

  • learning helps, if told once people are better next time

adaptation to odor

  • quick adaptation w quick recovery

  • long term adaptation w slow recovery (maybe due to replacement of receptors)