Chemical Senses Gustation (taste)+ Olfaction (smell) = Flavor

Smell

Olfactory Epithelium (~5 cm 2 ) Turbinates

Olfactory neurons live only a few weeks (the ONLY neurons that are directly exposed to the external environment)

basal cell olfactory neuron support cell, or sustentacular cell

Amoore’s Stereospecific Theory of Smell Minty Floral Musky Putrid Ethereal Camphoraceous Pungent Now thought to have ~1,000 receptor types

Odorant-Binding Proteins (OBPs) and Phermone- Binding Proteins (PBPs) receptor recognizes complex, but not ligand alone ligand transferred to receptor only with assistance of OBP ligand spontaneously dissociates from the complex and binds to the receptor

Receptors for odorants resemble receptors for neurotransmitters Figure 32-4

Four zones in olfactory epithelium defined functionally Neurons with the same receptors are restricted to a single zone, but Neurons with different receptors are found within each zone

The 3-D model shown on the right (from Couto & Dickson, 2005) is that of a fly antennal lobe (AL), the equivalent of the vertebrate olfactory bulb. As you can see, the AL is composed of spheroidal structures, the glomeruli. While vertebrate olfactory bulbs may contain thousands of glomeruli, the fly AL contains only ~43 glomeruli. Furthermore, each glomerulus is uniquely identifiable, based on its size, shape and position in the AL neuropil. The glomeruli are so invariant in these features that they have been given specific names. Each glomerulus receives innervation from olfactory receptor neurons (ORNs) expressing a single odorant receptor. The VA1v glomerulus for example, receives input from ORNs expressing the OR47b receptor. A glomerulus may thus be thought of as representing a single unit of smell. The constellation of unique glomeruli forms the Olfactory Map of the animal.

Two Parallel Pathways Lateral –Direct connections between olfactory bulb and entorhinal cortex, pyriform cortex –Connections to amygdala and other limbic structures Medial –Olfactory tubercle –Medial dorsal nucleus (thalamus)‏ –Orbitofrontal cortex

Taste

Primary Tastes Salt Sour Sweet Bitter Umami Low pH Na+ cation, small anion Large, non-dissociating organic molecules Alkaloids Amino acids, esp. glutamate