Neurobiology - Lecture 13: Olfaction

Part of Exam 3

Animal Pheromone Roles

Reproductive Behavior

Territorial Boundaries

Identification of Individuals

Signal Aggression or Submission

Human Pheromone Roles

Synchronization of Menstrual Cycles

Mother-Child Relationship (Mediated by Oxytocin)

VNO

Vomeral Nasal Organ

Erection Pathway in Rats

VNO → Medial Amygdala (MA) → mPOA → Midbrain → Reticular Formation → Spine → Erectile Muscles

Wedekind et al. (1995)

Theorized that Major Histocompatibility Complex (MHC) may influence mate selection as certain combinations could be more beneficial in offspring fighting off pathogens.

Women smelled T-shirts the men had worn for two nights and found the scent of the most dissimilar MHC to be the most pleasing.

Anatomy of Scent System

Nasal Cavity → Olfactory Epithelium → Cribriform Plate → Olfactory Bulb

Olfactory Epithelium

Small, thin sheet of cells high in the nasal cavity

Cells of the Olfactory Epithelium

1. Olfactory Receptor Cells: Sites of transduction, neurons, last 4-8 weeks; Have cilia on dendrite.
2. Supporting Cells: Like glial cells; produce mucus
3. Basal Cells: Divide to produce new receptor cells

Purpose of Mucus

Odorants dissolve in mucus layer before contacting olfactory receptor cells.

House antibodies to protect the brain from viruses and bacteria

Anosmia

The inability to smell

Scent Signaling Pathway

Odorant Binds to Receptor → G`olf` Activated→ Upregulate cAMP → Open cAMP-gated Ion Channel → Na+ and Ca2+ flow into the cell → Ca2+ - gated CL- channels open → Cl- flows out of the cell → Membrane is depolarized → Receptor Potential

Why does Cl- efflux from olfactory receptor cells?

Because the intracellular concentration of CL- is higher than the mucosal concentration of Cl-, so the Cl- flows out along the concentration gradient.

Reasons for Termination of Olfactory Response

Odorants Diffuse Away

Scavenger Enzymes in Mucus Break Down Odorants

cAMP activates other signaling pathways that stop transduction

Adaptation

Decreased Olfactory Response despite the continued presence of the stimulus. Occurs within one minute.

Population Coding

The combined activity of multiple types of olfactory receptors creates the experience of an individual smell.

Allows for 1000 receptors to produce 1 trillion unique smells

Glomeruli in Olfactory Bulbs

Like bronchi in the lungs but for olfactory receptor axons.

50 - 200 um diameter

25,000 axons converge on the dendrites of 100 second order olfactory neurons

Glomeruli Specificity

Each glomeruli receives input from only one type of olfactory receptor cell.

Layers of the Olfactory Bulb

Granule Cell Layer→ IPL → Mitral Cell Layer → External Plexiform Layer → Glomerular Layer → Olfactory Neuron Layer → Olfactory Cortex

GCL of Olfactory Bulb

Granule Cell Layer; GABAergic Signaling

MCL of Olfactory Bulb

Mitral Cell Layer; Contains Mitral Projection Cells

EPL of Olfactory Bulb

External Plexiform Layer; Contains Tufted Projection Cells

GL of Olfactory Bulb

Glomerular Layer; Contains Glomeruli, Periglomerular (PG) cells, & Superficial Short Axon (sSA) cells

Projection to Olfactory Cortex

Mitral and Tufted Cells receive information from the glomeruli and send their axons to the olfactory cortex.

Olfactory Tract

Made up of Second-order Olfactory Neurons; Projects to the Olfactory Cortex and the Thalamus.

Olfactory Cortex

Reached by a Pathway that Synapses in the Medial Dorsal Nucleus of the Thalamus; Represents the Reward Value of Odor.

Olfactory Ties to Emotion and Memory

Olfactory Tract connects with Hippocampus and Amygdala

Medial Forebrain Bundle (Carries Reward Circuitry)

Olfactory Association Cortex in Temporal Lobe

Temporal Coding

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The Representation of Information by the Timing of Action Potentials. May encode Odor Quality.

Note: Scent is relatively slow.