Olfactory System and Sense of Smell

Overview

Olfactory System is responsible for the sense of smell (olfaction).
Detects airborne molecules (odorants) through olfactory sensory neurons at the nasal cavity's roof.
Converts chemical stimuli into electrical signals sent to the brain.

Odorant Molecules Dissolution
- Dissolved in mucus secreted by the olfactory epithelium.
- Guided to cilia of olfactory neurons where molecules bind to receptors.
Receptors and Neurons
- Each neuron expresses one type of protein receptor.
- Humans have ~400 different receptors for combinatorial usage.
- One odorant can bind multiple receptors, and one receptor can bind multiple odorants.
Signal Transduction
- Odorant receptors are G-protein coupled.
- Binding activates a signaling cascade leading to membrane depolarization.
- If the stimulus is strong enough, action potentials are generated.

Olfactory Nerve (Cranial Nerve 1)
- Axons of all sensory neurons form this nerve.
Olfactory Bulb
- Axons synapse with mitral and tufted cells in glomeruli.
- Each glomerulus receives axons from neurons expressing the same receptor.
- Second-order neurons receive inhibitory feedback from the cerebral cortex, affecting perception.
Olfactory Tracts and Cortex
- Axons of mitral and tufted cells form olfactory tracts to the primary olfactory cortex.
- The primary olfactory cortex is a group of several cortical areas in the frontal and temporal lobes.

Regeneration
- Exposed to external environment, thus replaced more often.
- Stem cells in epithelium differentiate into new neurons.
Anosmia
- Permanent loss of smell due to destruction of neurons.
- Transient anosmia due to nasal mucosa inflammation.

Loss of smell affects taste and flavor perception.
Decreases with age and indicates neurodegenerative disorders.
Seizures often originate from olfactory-related brain areas, leading to pre-seizure odor hallucinations.