Olfactory

Signal transduction and flow

• receptors cells in olfactory epithelium

• transduction

• action potential generations in receptor cells themselves

• direct pathway to pyriform cortex  (amygdala/hippocampus(memory center of the brain))

• Indirect pathway to thalamus (MD)

• Conscious scent sensation in the orbitofrontal cortex

transduction - chemical transported into action potentials 

ONLY sensory system reaches two 

temporal lobe 

ALL send to the thalamus

orbital frontal cortex - sense what we are smelling - AP in the rector cells

Olfactory receptor neurons

• sick; mucus is thick - protects the brain from toxic chemicals

• Olfactory cells can be replaced

• cribriform plate - keeps axons going in the right direction (towards the olfactory bulb)

Olfactory transduction with GPCRs

• about 400 receptor cells

• ALL GPCRs receptors (G-proteins)

• NO ions pass through

• active second messengers - leads to depolarization

• going towards olfactory bulb

• ACTIVATE - Na, Ca channels flow in and DEPOLARIZE

• Cl flows out (taking its negative charge)

Each receptor express only one gene

Olfactory GPCRs are BROADLY TUNED because

1. Each GPCR type responds to multiple odorants

2. each odorant can activate multiple GPCR types

One receptor type per receptor cell

(can’t tell by only one cell by what you are smelling)

USING POPULATION CODING - LOOKING AT ALL RECEPTORS TOGETHER TO UNDERSTAND WHAT WE ARE SMELLING

All receptors are mixed in the nose, but they go to the SAME REGION in the olfactory bulb - called the GLOMERULUS

One glomerulus, one receptor type!

ORGANIZATION of the olfactory epithelium

• different sets of odorant receptors genes are expressed in 4 zones of the olfactory epithelium

• same rectors are all located in one zone,  but are scattered throughout the zone along with neurons expressing other receptors

• Sensory information is broadly organization into 4 zones prior to transmission to the brain

• different epithelial zones project to different of the olfactory bulb

Slide 12….. woahhhhhhh

Organizes information by how many carbons in the molecule

• arranges odors according to chemical structure

Lateral inhibition:

Enhance the signal to the nearby synapses that smell like wht’s in front of you 

• Olfactory receptors (OSN): Excitatory synapses - Glutamate

• Excitatory synapses:

  • tufted cells (T): glutamate

• Mitral cells (M): glutamate

• Inhibitory synapses:

synapsed horizontally 

• Periglomerular cells (PG) - 

• granule (Gr)

Central Olfactory Pathways…

Vomeronasal organ transmits information about pheromones:

Tubular structure that opens into nasal cavity

• many vertebrates have a vomeronasal system to detect pheromones - odor signals secreted by many animals

• receptor cells are found in a vomeronasal organ (VNO) near the olfactory epithelium( can detect EXTREMELY low concentrations of pheromone molecules)

• Rodents have two types of vomeronasal receptors: V1R and V2R (differ in rodents between males and females)

Do humans sense pheromones?

contribute to attraction - no absolute proof -

CODING

• population coding

receptors broadly - uses all axons active together

• sensory maps

(Olfactory bulb maps) - different maps for banana, citrus or pineapple - to get information about what we are snelling

• temporal coding

Using temporal order in order to identify what we are smelling