Week 2 - Special Senses - Smell

Special Senses breakdown

All special senses have corresponding ORGANS:

• Organ → special sense → stimuli → neural sensation

• eye → vision → light → colour

• Ear & cochlea → hearing → sound waves → pitch

• Vestibular apparatus → balance → head movement → motion

• Noes & olfactory epithelium → smell → airborne chemicals → smell

• Tongue & taste buds → gustation → tastants → flavour

Taste & Smell: Definitions

Both taste and smell are primitive senses that influence if nearby substances should be savoured (eaten) or avoided (possible toxicity).

• Both senses influence digestive secretions; apetite

• People have varying degrees of taste & smell acuity (sharpness)

Taste vs Smell

Smell is less important and sensitive than taste in humans

Taste & Smell: Function

Smell comtributes to 80% of taste

• Both stimulated by chemoreceptors, often working in tandem

• Taste receptors → excited by food chemicals dissolved in saliva

• Smell receptors → airborne chemicals coating nasal membrane

• Both receptor cells can be continuously renewed

  • Smell receptors can be damaged by toxic fumes

  • Taste receptors can be damaged by hot foods, rigorous mechanical action

• Old people have a decreased ability to replace these neurons, hence they prefer to salt their foods more, and bitter foods they disliked when young

  • THIS IS WHY DAD IS SO OBSESSED WITH COROLLA / BITTER MELON

Smell/Olfaction: Nose anatomy

Olfactory receptor neurons (chemoreceptors) are located in the superior nasal cavity, termed the “olfactory epithelium”.

Smell/Olfaction: Olfactory epithelium anatomy

Contains sensory neurons (chemoreceptors) & supporting cells.

• Dendrites of olfactory receptor neurons extend into the mucosal lining of the nasal cavity.

• Basal cells act as stem cells and constantly differentiate to replace olfactory receptor cells.

• Gaseous chemicals are inhaled (odorants) that dissolved into the mucus lining the olfactory epithelium

  • If noxious airborne chemicals are inhaled, it can damage the olfactory epithelium, including the olfactory receptor cells, hence the need for basal cells.

Smell/Olfaction: Smell

Any scent can contain hundreds of different odorants/chemicals.

• Humans can distinguish ~10,000 odours

• There are ~400 olfactory receptor types

Smell is perceived by the pattern of activation in these receptors.

Smell/Olfaction: Olfactory receptor cells

Each olfactory receptor cell can only respond to only one molecular feature of a chemical/odorant compound.

• Hence, allows one odorant to stimulate multiple olfactory receptor cells

• Thus, allows a unique pattern of stimulation to form, which can be perceived by the brain

The intensity of the smell, will increase/decrease the presence of odorants, thus sending more/less action potentials to the brain.

Smell/Olfaction: Sensation

1. Chemicals are inhaled (odorants) in a gaseous state

2. Odorants dissolve into the mucus lining the olfactory epithelium

3. Odorant binds to olfactory receptor cell (chemoreceptor) dendrite’s cilia

4. Second messenger activated

5. Second messenger opens Na+ & Ca2+ channels

6. Na+ & Ca2+ flood the olfactory receptor cell

7. Na+ & Ca2+ entry leads to opening of calcium-activated chloride channels

8. Cl- leaves the cell

9. Cell depolarises from high membrane potential (Na+ & Ca+ enter, Cl- exits), leading to an action potential

Smell/Olfaction: Neural pathways

1. Axons of olfactory receptor cells travel through the olfactory foramen (ethmoid bone)

2. Olfactory receptor cells synapse with neurons within the olfactory bulb on the ventral surface of the frontal lobe

3. Here, olfactory nerve (Cranial Nerve 1) begins

4. Axons of olfactory nerve (Cranial Nerve 1) travel to:

   1. Primary olfactory cortex in the frontal lobe

   2. Limbic system & hypothalamus to be integrated into long-term memory and sentimental emotional responses

Smell/Olfaction: Changes to sense of smell

• Sense of smell is ever changing, with a constant turnover of new olfactory receptor cells

Factors that influence sense of smell:

• Attentiveness of smell due to hunger

• Females are more sensitive

• Smoking decreases sensitivity (damage to olfactory epithelium)

• State of olfactory epithelium (can be impacted due to illness such as colds)

• Pregnant women have a heightened sense of smell

Smell/Olfaction Disorders: Anosmia

Loss of sense of smell

Cause: Damage to olfactory nerves/tracts

• Blunt force trauma

• Some pharmaceuticals can kill olfactory neurons

• Temporary loss of smell due to inflammation from respiratory illness/allergies

Effect:

• Food tastes bland (taste is 80% smell)

  • person may need more spice in food

• May lead to some depression, as loss of enjoying food can effect emotional state