M1(2) The Sensory System (Smell)

Smell

• What: olfactory receptor neurons

• Stimulus: molecules of the air we breathe

• Where: olfactory epithelium of the nasal cavity

Olfactory Receptor Neuron

Each cell contains a tuft of ~5 olfactory cilia

• Each cell expresses only one type of receptor, but there are over 1000 different receptor types

Olfactory Bulb

Receive input from the axons of olfactory sensory neuron

• Contains glomeruli

Glomeruli

Spherical structures within the olfactory bulbs.

• Serve as processing hubs where axons of sensory neurons synapse with mitral cells

Glomeruli Function

Axons from neurons with the same receptor type converge on the same glomerulus

• Each glomerulus represents one aspect of an odor (e.g. one note in a chord)

• Unique sets of glomeruli produce a distinct odor perception (the chord)

Mitral Cell

Receive input from the glomeruli.

• Refine, amplify, and relay the signal via their axons (which form the olfactory tract) to the brain

Olfactory Pathway

1. Detection

• Olfactory sensory neurons in the olfactory epithelium detect odor molecules

2. Synapse

• Olfactory sensory nerve axons synapse with olfactory bulbs

• Within the bulbs, glomeruli synapse with mitral cells

3. Signal Refinement

• Mitral cells refine and amplify the signals

• Mitral cells form the olfactory tract and relay impulses to the brain

4. Cortical and Limbic Processing

Cortical and Limbic Processing

• Piriform lobe (olfactory cortex): first stop for olfactory information.

• Two major pathways:

  1. Frontal lobe (orbitofrontal cortex): conscious interpretation and identification of smells; some signals pass through the thalamus.

  2. Hypothalamus, amygdala, and limbic system: generate emotional responses to odors.