Gustatory and Olfactory Systems

Taste (Gustatory System)

There are only 4 sensations that can be processed by taste (receptors on tongue, palate and epiglottis):

(1) sweetness (tip of tongue), (2) salt, (3) sourness (lateral tongue), and (4) bitterness
distribution of any one sensation is not limited to one area but receptor for a given taste do concentrate

individual Papilla on the tongue are covered by multiple taste buds within which are multiple (~50) receptor cells (cilia)

innervation: Anterior 2/3 [chorda tympani of CN VII from geniculate ganglion] Post. 1/3 [lingual branch of CN IX]

two pathways:

secondary gustatory nucleus neurons Þ VPM Þ taste center of postcentral gyrus and posterior piriform lobe (all ipsolaterally)

secondary gustatory nucleus Þ parabrachial nucleus Þ amygdala and hypothalamus

Labeled Line Coding vs Population Coding – gustatory uses population coding only; olfactory uses both

Labeled line coding: integrity of information maintained from receptor to cortex along non integrated paths (i.e., vision)

primary afferent from receptor cells responding to the same stimuli synapses on nucleus which project to Þ

Dorso-medial Thalamus Þ primary sensory cortex

this is not found in processing taste sensation; found in olfaction

Population coding: involves reading out population activity of many different neurons along the pathway

this is how taste and olfaction sensations are processed

within a taste bud are many mixed receptor cells that synapse with a single primaryafferent neuron (loss of integrity)
this pattern is called population or computation coding = topography (specificity) of system is lost along path
in taste, the specificity of info is lost at the first synapse

Olfaction

Psychophysics of Olfaction: distinction between normal odorants and pheromones

pathway for standard odorants: nasal epithelium Þ olfactory bulb Þ pyriform cortex Þ different cortical areas

pathway for pheromones: vomer nasal organ Þ accessory olfactory bulb Þ etc; debatable if exists in humans

no debate that humans can sense pheromones (McClintock effect: room mates synchronize menstrual cycle)
McClintock effect due solely to chemicals in sweat mediated through pheromones (proven experimentally)

smell is the most memorable sense

olfactory centers strongly associated with memory (limbic system, amygdala) but weakly associated with language
contrasted to taste in that it uses both Population and Labeled line coding

Transduction of olfactory information (receptor cells Þ glomeruli Þ mitral cells)

receptor cells have axons that go into olfactory bulb on one end; cilia (similar to taste) containing receptor proteins are on the other end

stimulated receptor protein causes G-protein to split Þ cAMP cascade Þ opened Ca⁺⁺ and Na⁺ channels Þ depolarization

depolarization is enhanced by Cl^- ion channel which is stimulated by Na⁺ influx

this is a generation of receptor potential as is seen in the retina and other types of sensory transduction

Mitral cells in the olfactory bulb receive stimuli onto their dendrites via synapse with receptor cells in glomerular tufts

glomeruli are glial wrappings, which synapse on the distal end of dendrites from 8-10 mitral cells

there is a clonal line of receptor cell proteins: each olfactory receptor cell makes only 1 protein; 1000’s of lines though

tremendous specificity at the receptor level

axons from receptor cells that express the same receptor protein converge on the same glomerulus (labeled line)

however, when testing a pure odor, a wide range of the olfactory bulb is activated

therefore the receptors are broadly tuned and respond to many different odors; odor discrimination then becomes a matter of

discriminating patterns of glomeruli stimulation (population coding)

inhibition at the mitral cell occurs along its long secondary dendrites (distinct from inhibition in cortex and hippocampus)

occurs through granule cell inhibitory neurons (GABA transmitter); mitral cells are excitatory (glutamate transm.)

granule cells have no axons but dendritic spines at dendodendritic synapses on mitral dendrites

these junctions suggest reciprocal synapse: mitral cells excite granule cells / granule cells inhibit mitral cells

granules cell dendrites are both pre- and postsynaptic: glutamate release by mitral cells Þ GABA release by granule cells

recurrent inhibition is granule cell inhibition of mitral cell that excited it

lateral inhibition is granule cell inhibition of mitral cell next to or other than the one that excited it

if lateral inhibition is "knocked out," chemically distinct odors can still be detected (labeled line wiring OK); but discrimination of chemically similar odorants (5 vs 6-carbon alcohol) is lost (computation integration lost)

Mitral cell axons go to Pyriform cortex: 3-layer cortex that projects to Entorhinal cortex and Limbic System (Amygdala)

this path is associated with remembering odors; bypasses thalamus entirely

Pyriform cortical axons also go to the Olfactory Tubercle Þ Medial dorsal Thalamus Þ Frontal Orbital Cortex

this pathway is associated with being conscious of an odor (being able to distinguish odors)
note difference from other sensory systems in that the relay through thalamus is after several processing synapses
olfactory bulb may mimic thalamus because both demonstrate distinguishable electrical rythmicity from distinct stimuli

Cell assemblies = population of cells firing together in response to odor stimuli

rythmicity results from subgroups of population firing at different phases of cycle (facilitated by inhibition- see above)

olfactory system distinguishes odors by comparing population of cells at specific phase; electrical rhythm insufficient

Anosmia: loss of sense of smell through damage to nerve, no treatment; loss of smell-triggered memory most significant