The Sense of Smell

By: Looby and Rachel

Introduction to Smell● Structure deep in the nose produces this important sense

● Using the olfactory system, humans can detect 5,000-10,000 different smells

● Vital to our everyday existence

○ Allows for humans to sense danger

○ Allows for an animal to sense the presence of food or other animals

● Smell is also known as olfaction

Anatomy of the Nose...● The nose is a structure of bone, cartilage, skin and muscle that extends from

the face and forms the anterior wall of the nasal cavity

● The nose contains nasal passages that allow outside air to flow into the nasal

cavity

● Deep to the nose is the hollow nasal cavity

○ Contains many folds of epithelium covered bone

○ Left and right passages

Anatomy of the Nose Continued...● On the roof of the nasal cavity is a special type of tissue known as olfactory epithelium

○ Similar to epithelium

○ Specialized for producing the sense of smell

○ It contains many sensory cells that extend from its surface and are embedded in

mucus secreted by goblet cells in the epithelium

● Deep to the olfactory epithelium are neurons

Anatomy of the Nose Continued...● Surrounding the nasal

cavity are paranasal sinuses○ Group of hollow

structures within

the bones of the

face

Anatomy of the Nose Continued...● The Maxillary Sinuses…

○ Pair of cavities within the maxillae of the face

○ Largest of the paranasal sinuses

○ Tiny passages connect the maxillary sinuses to

the nasal canal

■ Allows air to pass between them

Anatomy of the Nose Continued...● The Frontal Sinuses…

○ Pair of cavities found in the frontal bone superior to the orbits and superior to the nasal cavity

○ Separated from the nasal cavity by a thin layer of bone

○ Do not connect to the nasal canal

Anatomy of the Nose Continued...Frontal

Stinus

Anatomy of the Nose Continued...● The Ethmoidal Sinuses…

○ Group of small cavities within the ethmoid bone

○ Arranged symmetrically along the body’s midline

○ Provide lubrication to the inner nose

Anatomy of the Nose Continued...● The Sphenoidal Sinuses…

○ Pair of cavities within

the sphenoid bone

○ Tiny, bony wall

separating them

○ Filter and clean the

air breathed through

the nose

Anatomy of the Nose Continued...Basal Cells

● Stem cells located between the bases of supporting cells

● Continually undergo cell division to produce new olfactory receptors

When I have a cold, why can’t I smell???● Chewing releases aromas that

access the olfactory neurons

● If the channel is blocked…

○ Odors can NOT reach the sensory

cells

Physiology of Smell… ● Nose plays vital role in the conduction of air into the lungs

● Smell is a chemical sense along with taste (others are physical senses)

Physiology of Smell Continued… ● Stimulus: Evaporated substances suspended into the air

● Receptor Cell Type: Olfactory Receptor Cell

● Your ability to smell comes from specialized sensory cells, called olfactory sensory neurons, which are found in a small patch of tissue high inside the nose

● Cells connect directly to the brain

● Each olfactory neuron has one odor receptor

● Smells reach the olfactory sensory through 2 different pathways

The Two Pathways… ● Smells reach the olfactory sensory neurons through two pathways

1. Through your nostrils

2. Through a channel that connects the roof of the throat to the

nose

Physiology of Smell… ● Group of genes encodes proteins that help detect each specific odor

1. An odor reaches nasal cavity and diffuses through the air

2. These odor molecules attach to olfactory receptor cells

3. Each cell is covered in cilia (hair-like projections to increase surface area and movement)

4. Cilia are covered in a mucous membrane where the odor dissolves

5. Odor binds to these olfactory receptor cells and stimulates a protein called the G-protein

6. The G-protein’s job is to release calcium, which generates electrical potential

7. This potential creates a signal that is now sent to the brain

8. Signals are routed to the brain and are processed in the primary olfactory cortex

9. After stimulation of receptors, the odor molecule is destroyed and stimulation ends

10. The receptor is then freed for stimulation of another odor molecule

Physiology of Smell… ● The odorant attaches to the binding protein

● As this occurs, calcium enters the cilia

● Lots of calcium continues to enter the cilia,

creating depolarization

● By depolarization, action potential and ATP

is created

● After depolarization, repolarization occurs

which releases chlorine

Physiology of Smell… ● The Olfactory receptor neurons send info

to olfactory bulbs● Olfactory bulbs have sensory receptors

that are part of the brain and send messages to:

○ Primitive brain centers where they influence

emotions and memories (limbic system

structures)

○ “Higher” centers modify conscious thought

(neo-cortex)

● These brain centers perceive odors and access memories

○ Help remind us about people, places, or events

associated with these olfactory sensations

Homeostatic Diseases and Disorders...● Hyposmia: reduced ability to detect odors

● Anosmia: complete inability to detect odors

● Congenital anosmia: complete absence of the sense of smell

● Chronic Sinusitis: nasal cavities and sinuses are inflamed for 12+ weeks

● Parosmia: change in normal perception of odors (change in familiarity)

● Phantosmia: sensation of an odor that is not there

Homeostatic Disorder Causes...● Smell disorders often precede taste disorders

● Often caused by

○ Upper respiratory infections and injuries

○ Polyps in nasal cavities

○ Sinus infections

○ Hormonal disturbances

○ Dental problems

○ Exposure to chemicals (insecticides, medicines, radiation)

Fun Facts... ● 200,000+ people go to the doctor annually for problems with their smell

● Over 40 million receptors (detects odors) in upper root of nasal cavity

● Smell disorders often precede taste disorders

● The sense of smell is the most ancient

● Women have a better sense of smell than men

● Dogs have a great sense of smell, their receptors have 20x more surface areas

as humans and 100x more receptors per square cm than humans

Activity...Your nose and brain are suppose to help you find foods that are good to eat.

But how well does this system work? 6 different items have been collected

and put into brown paper bags. For this activity, we will ask for 3 different

volunteers to help us out. 3 of the items are edible, while 3 are not. The

volunteers will then have to write down the number corresponding to the bag

under either edible or not edible. While the activity is going on, Rachel and

Looby will be describing the physiology step by step.

Cites:

https://www.medicinenet.com/smell_disorders/article.htm

https://www.nidcd.nih.gov/health/smell-disorders

https://faculty.washington.edu/chudler/chsmell.html

https://www.healthline.com/human-body-maps/frontal-sinus#1

https://www.thoughtco.com/olfactory-system-4066176

http://www.bookrags.com/research/smell-physiology-of-olfactory-sense-wap/#gsc.tab=0

http://www.tsbvi.edu/seehear/summer05/smell.htm