Smell Notes

Smell (Olfaction)

• Smell and taste are special senses that can be learned from and improved upon, unlike pain, which is simply pain.

• Anatomy of Olfaction:

  • Nasal Cavity:

    • The opening is the nostril.

    • Lined with bones and tissue; not a smooth surface.

    • Ethmoid bone is visible in X-rays or CT scans.

    • Conchae (superior, middle, and inferior) create air turbulence.

    • Air doesn't flow straight; it swirls around due to the conchae, moving air toward the roof of the nasal cavity where the olfactory system is located.

    • Conchae act as "roadblocks" to create air turbulence.

  • Odorants:

    • Particles in the air that you breathe in.

  • Enlarged Smell Apparatus:

    • Ethmoid bone and cribriform plate.

    • Olfactory nerve projects from the nasal cavity into the cranial cavity through holes in the cribriform plate.

  • Nasal Cavity Components:

    • Yellow Cells:

      • Bipolar neurons with two ends.

      • One end faces inward into the nasal cavity, the other projects into the cranial cavity.

      • Distal part has dendrites with cilia (hair-like structures).

      • Cilia are dense and overlap.

    • Purple Cells:

      • Supporting cells that maintain the environment for the neurons.

      • Balance and remove toxins, provide nutritional support.

    • Basal Cells:

      • Give rise to new supporting cells through mitosis.

    • Olfactory Glands:

      • Produce mucus.

• Nose Hair and Mucus:

  • Nose hair traps odorants.

  • Irritation of hair cells triggers mucus production from olfactory glands.

  • Mucus binds trapped odorants, forming mucus balls that are expelled.

  • Mucus protects cilia and dendrites from toxins.

• Olfactory Pathway:

  • Odorant binds to cilia, activating an action potential.

  • Signal travels to the cell body and then to the olfactory nerve.

  • Synapses in the olfactory bulb (in the cranial cavity) with mitral cells.

  • Axons of mitral cells form the olfactory tract.

  • Olfactory nerve is in the peripheral nervous system; olfactory tract is in the central nervous system.

  • Olfactory tract goes to the olfactory lobe (olfactory cortex), located medially to the temporal lobe, where smell is sensed.

  • Amygdala is part of the olfactory cortex, linking smell to emotion, memory, and response.

  • Smell can trigger memories or emotional responses (e.g., pleasant scents vs. overwhelming cologne).

  • Smell acts as a protective mechanism; unpleasant smells can trigger memory recall related to negative experiences.

• Signal Amplification:

  • Odorant binds to a receptor.

  • Activates a G protein and adenylyl cyclase, leading to a second messenger cascade (metabotropic).

  • This results in the opening of multiple channels. One odorant can open approximately 10 channels, amplifying the signal.

  • Small amounts of odorant can trigger perception of the scent.