10/11/10

1

Gustation and Olfaction: Non-Topographic Sensory

Modalities

Importance of Taste Perception

•  Nice to have •  Evolutionary benefits

10/11/10

2

Basic Tastes:

•  Salty •  Sour

•  Bitter •  Sweet •  Umami (amino acids, savory)

• Gustatory afferents synapse on a single taste cell

10/11/10

3

• Metabotropic Pathway

• Bitter, Sweet, Umami

GPCRs Used: Bitter: T2R Sweet: T1R2 + T1R3 Umami: T1R1 + T1R3

Taste cells do not coexpress these specific sets of GPCRs

Methods of depolarization

•  Ionotropic: Sodium or protons enter the cell

•  Metabotropic: GPCRs are activated •  Both methods result in vesicle release

onto gustatory afferents

•  Gustatory afferents do not receive input from multiple taste cells

Projections of Gustatory Information

10/11/10

4

Regions of Taste Specialization

90% of taste cells respond to more than one of the five basic tastes

10/11/10

5

Population Coding

•  From an ensemble of neurons’ responses, the brain has to determine what the stimulus was

•  It cannot simply look at the response of a single cell

10/11/10

6

10/11/10

7

Population Code

•  The information from single cells is ambiguous

•  If tuning functions are known, the responses of different, broadly tuned cells, can be compared to extract information about the stimulus

10/11/10

8

Over 1000 different receptor protein genes

Adaptation occurs after about 1 minute

Signals sent to glomeruli

10/11/10

9

Olfactory Bulb

• About 100 2nd order olfactory neurons per glomerulus • Each glomerulus is innervated by about 25,000 ORNs

Receptors Don’t Project Randomly

• ORNs express a few, or one, olfactory receptor proteins • About 2,000 glomeruli per bulb

10/11/10

10

Similar distributions of glomeruli in different bulbs, and different animals of the same species

10/11/10

11

Olfactory Bulb Circuitry

Temporal Coding

•  Gilles Laurent (Caltech) •  Lateral inhibition from granule cells is

important for synchronizing responses between Mitral cells (honey bee expt.)

•  Timing of spikes in relation to other spikes is important for coding

Central Olfactory Pathways

10/11/10

12

Like taste, smell was likely a very important sense in evolution. Humans: about 5 million olfactory receptor neurons Dogs: up to 220 million (and much larger nasal epithelium)

Pheromones •  Chemicals released by an animal that

cause physiological or behavioral changes in another animal of the same species

•  Mark territory, lay trails, attract mates, promote social cohesion

•  Can be used for pest control

Light Brown Apple Moth

10/11/10

13

The Vomeronasal System (rodent)

Hypothalamus initiates release of hormones

Published in Science, 2003

Typical response of an AOB neuron

10/11/10

14

•  AOB neurons show selective responses:

•  3 test strains used: BALBc, B6, and CBA

•  Cells were selective for a combination of strain and sex

•  Mouse sex detector

Mitral cells of the AOB can also be selective for strain

• Only one cell out of 43 showed broad selectivity

10/11/10

15

T-Shirt movie

Do Humans Detect Pheromones?