12. Olfaction - The Chemical Senses

Olfaction - Chemical Sense

Sense of Smell (Olfaction):

• Olfactory epithelium

  • Olfactory receptor cells (neurons)

    • True neurons, each has its own axon

    • Axons project to olfactory bulb through the cribriform plate

    • Cranial nerve I = the axons of all the olfactory receptors

    • Humans have ~ 12 million olfactory receptors

Receptor Cell Turnover:

• Olfactory receptor cells are directly exposed to environmental contaminants

• Continuously replaced every 4-8 weeks via differentiation of basal cell precursors (stem cells)

• Sensory transduction takes place in the cilia, which are embedded in a mucus layer that traps odorant molecules

Olfactory Transduction:

• Odorant Receptor Proteins

  • More than 1000 types of odorant receptor proteins (ORP)

  • Each receptor cell typically expresses ONLY ONE type of ORP

  • Transduction: G-protein-coupled-receptor pathway, leading to activation of cyclic-nucleotide-gated cation (Na+, Ca2+) channels and subsequent activation of calcium-gated Cl- channels → depolarization

Olfactory Coding:

• Each olfactory receptor neuron expresses a single type of odorant receptor protein (ORP)

• Population coding: different odorants are encoded by activity on different combinations of ORPs

Olfactory Receptor Neurons:

• Generate action potentials = spikes

• Adapt to sustained stimuli

• Are diverse, broadly tuned

Olfactory Adaptation:

• Olfactory receptor neurons reduce their response to prolonged or repeated exposure to the same odor, but remain responsive to novel odors

• Adaptation occurs through a calcium-mediated feedback pathway:

  • Prolonged receptor cell activation → increased intracellular calcium levels → decreased sensitivity of cAMP-gated cation channels

Olfactory Glomeruli:

• ~ 2000 in each bulb

• Each glomerulus:

  • Receives input from a large region of the olfactory epithelium

  • Receives input only from receptor cells expressing a particular odorant receptor protein

• Functional hypotheses:

  • Convergence increases sensitivity to weak odors

  • Sort odors into broad categories

Central Pathways:

• Olfactory receptor cells → olfactory bulb → thalamus → frontal cortex (conscious perception)

• Olfactory receptor cells → olfactory bulb → olfactory cortex → hypothalamus & amygdala (motivation & emotion)

• Olfactory receptor cells → olfactory bulb → olfactory cortex → hippocampus (learning & memory)

• Olfactory cortex has a shortcut pathway to motivation, emotion, and memory (bypasses the thalamus)

Flavor Perception:

• Olfaction plays a key role in flavor perception

• Two pathways for odorants to reach olfactory epithelium:

  • Orthonasal: entering from the external environment through the external nares (nostrils), the "normal" pathway, odorants are sampled during inhalation

  • Retronasal: entering from the mouth and throat, odorants are sampled during exhalation, key for "flavor" processing

Brain Systems Involved in Smell Perception:

• Orthonasal olfaction (sniffing in)

• Retronasal olfaction (breathing out), with food in the oral cavity

• Retronasal processing is multimodal and highly integrative