1. Special Senses
12.1 Olfaction

2. Special senses: sensations
Sensations: conscious or subconscious awareness of changes in the internal or external environment
4 conditions must be satisfied in order for a sensation to occur
Stimulus(change in environment) has to activate sensory receptors
Sensory receptors convert the stimulus to an electrical signal which produces a nerve impulse
Nerve impulse is conducted to the brain
Region of the brain must receive and integrate the impulse into a sensation
Adaptation: decrease in strength of sensation during a prolonged stimulus

3. Special senses: Olfaction
Olfaction: sense of smell
Chemoreceptors: detect chemicals in the mouth (taste) and nose (smell)
Nose contains million receptors for the sense smell in the Olfactory Epithelium
Nerve impulses for smell and taste propagate(travel to) the limbic system
Because of this smell and taste can evoke memories or emotional responses

4. Special senses: Olfaction
Structure of the Olfactory Epithelium
occupies the upper portion of the nasal cavity
consist of a 3 different cells olfactory receptors, supporting cells and basal stem cells

5. Special senses: Olfaction
Structure of the Olfactory Epithelium Continued
Olfactory Receptors: first- order neurons of the olfactory pathway
Olfactory hairs project off the receptors
Respond to inhaled chemicals
These chemicals are known as Odorants

6. Special senses: Olfaction
Structure of the Olfactory Epithelium Continued
Basal Stem Cells: continually undergo cell division to produce new olfactory receptors
Olfactory receptors only last for a month before they are replaced
Olfactory Glands: produce mucus that acts as a solvent for inhaled odorants

7. Special senses: Olfaction
Stimulation of Olfactory Receptors
There are hundreds of primary odors
Our ability to recognize 10,000 different odors depends on the patterns of activity in the brain
Brain activity arises from the activation of different olfactory receptors
When stimulated by an odorant olfactory receptors produce an electrical signal that triggers nerve impulses

8. Special senses: Olfaction
Olfactory Pathway
On each side of the nose there are 40 unmyelinated axons of olfactory receptors
Axons form the right and left olfactory (I) nerve
Both olfactory nerves terminate in an area on the brain called the olfactory bulbs

9. Special senses: Olfaction
Olfactory Pathway Continued
Axons coming off the olfactory bulb neuron form the olfactory tract
Olfactory tract projects to the primary olfactory area
Where conscious awareness of smell begins
Other olfactory tract axons project to the limbic system
Accounts for emotional and memory evoked responses from smell

10. Check for understanding 1
Detect chemicals in the nose and mouth
System which accounts for emotional responses or memories from food or smell
Fill in the olfactory pathway starting at the axons of the olfactory receptors, _________, olfactory bulbs, ________, primary olfactory area
First- order neurons of the olfactory pathway
Chemicals olfactory hairs respond to
Decrease in strength of sensation during a prolonged stimulus

11. Special Senses
12.2 Gustation

12. Special senses: Gustation
Gustation: sense of taste
Chemoreceptors: detect chemicals in the mouth (taste) and nose (smell)
Nerve impulses for smell and taste propagate(travel to) the limbic system
Because of this smell and taste can evoke memories or emotional responses
Only 5 primary taste can be distinguished
Sour, sweet, bitter, salty, umami

13. Special senses: Gustation
Gustation: sense of taste
Olfaction is more sensitive than gustation
Food can stimulate the olfactory system thousands of times more strongly than the gustatory system
When you have a cold or stuffy nose and you can’t taste your food it’s the olfactory system that is blocked NOT the gustatory system

14. Special senses: Gustation
Structure of Taste Buds
Receptors for taste are located in the taste buds
10,000 taste buds are located on the tongue
There are also taste buds on the roof of the mouth and throat
Taste buds are located in papillae
Elevations found on the tongue
3 types
Circumvallate/Vallate papillae
Fungiform papillae
Filifrom papillae

15. Special senses: Gustation
Structure of Taste Buds Continued
Circumvallate/vallate papillae
Form an inverted V-shaped row at the back of the tongue
Fungiform papillae
Mushroom-shaped elevations scattered over the entire tongue
Filifrom papillae
Found over the entire surface of the tongue
Contain touch receptors but no taste receptors

16. Special senses: Gustation
Structure of Taste Buds Continued
Each taste bud contains 3 different epithelial cells
Supporting cells: surround gustatory cells
Gustatory Receptor Cells:
Contain a single gustatory hair that projects to the external surface through the gustatory pore
Basal Stem Cells: produce supporting cells WHICH BECOME gustatory receptor cells

17. Special senses: Gustation
Stimulation of Gustatory Receptors
Tastants: chemicals which stimulate gustatory receptor cells
Tastant is dissolved in saliva then it enters the taste pore
Dissolved tastant makes contact with the gustatory hairs
Gustatory receptor cell releases a neurotransmitter
Neurotransmitter binds to dendrites of the first-order sensory neuron

18. Special senses: Gustation
Stimulation of Gustatory Receptors Continued
Individual gustatory receptor cells respond to more than one of the five primary taste
Some individual gustatory cells respond more strongly to some tastants than other
Different taste arise from activation of different groups of taste neurons
Adaptation to a specific taste can occur in 1 to 5 minutes of continuous stimulation

19. Special senses: Gustation
The Gustatory Pathway
Facial nerve(VII), Glossopharyngeal nerve(IX), Vagus nerve(X) all contain axons of first-order gustatory neurons that innervate the taste buds
From the taste buds nerve impulses travel along the cranial nerves to the medulla oblongata
From the medulla oblongata some axons go to the limbic system (Accounts for emotional and memory evoked responses from taste) and some axons go to the thalamus
Taste signals go from the thalamus to the primary gustatory area
Primary gustatory area gives rise to the conscious perception of taste

20. Check for understanding 2
Contains the receptors for taste
Release a neurotransmitter which then triggers the sensory neuron
Area which gives rise to the conscious perception of taste
Mushroom-shaped elevations scattered over the entire tongue
Chemicals which stimulate gustatory receptor cells
Contain touch receptors but no taste receptors
Where taste buds on the tongue are located