Download Smell and Taste

Somatic and sensor analyzer.
How to classify sensory receptors?
A. Energy transduced
 Chemoreceptors
 Taste buds, olfactory receptors, aortic
and carotid bodies
 Photoreceptors
 Rods and cones in retina
 Thermoreceptors
 Heat and cold
 Mechanoreceptors
 Touch and pressure
B. By the type of stimulus
Sensory neurons transmit specific signals; require “adequate stimulus” to do so
Classification by Location



Exteroceptors – sensitive to stimuli arising from outside the body


Located at or near body surfaces
Include receptors for touch, pressure, pain, and temperature

Monitor a variety of stimuli

Located in musculoskeletal organs
Interoceptors – (visceroceptors) receive stimuli from internal viscera
Proprioceptors – monitor degree of stretch
C. Type of sensory information
delivered
 Proprioreceptors
 Within muscles; at joints (position and
movement)
 Cutaneous receptors
 In skin:touch and pressure; heat and cold;
pain
 Special sensory organs
 Eyes, ears, olfactory, taste buds
 Extero- and interoreceptors
 External and internal stimuli
 Lots of overlap!
Sensory adaptation
 Phasic (fastadapting) and tonic
(slow-adapting)
receptors
 Note different firing
patterns
 Stimulation pattern
resembles EPSPs
Cutaneous receptors
What is a somatesthetic
sensation?
 From cutaneous
and
proprioreceptors
 Myelinated sensory
neurons extend all
the way to medulla
oblongata; crossing
over
 Always extends to
postcentral gyrus
 Referred pains
along same
pathway
Modulating cutaneous
sensations
 Receptive field:
density of
receptors
 Some body areas
 Lateral inhibition
helps “pinpoint”
sensation or
increase acuity
 Applies to other
senses in addition
to touch
Somatic Senses
 General somatic – include
touch, pain, vibration,
pressure, temperature
 Proprioceptive – detect
stretch in tendons and
muscle provide information
on body position,
orientation and movement
of body in space
Location of pathway
terminations
video
Smell and Taste
• Smell aka Olfaction (Latin olfacere)
• Taste aka Gustation (Latin gustare)
• Minor Senses - Really?
• Chemical Senses
• Smell and Taste very closely related to each other
Where does smell figure?
In humans
• Strong link to sense of taste
• Strong link to emotional states
• Exploited by commercials/marketing
• Used for identification of gender
• Pheromones - signal sexual arousal or a readiness for
mating
Basics: What do you need for
smell?
• Odorous Substance
• Airborne
• Perceptual system that can parse or segregate
the information in airborne odorant molecules
Smell (Olfaction)
 Olfactory epithelium with olfactory receptors, supporting
cells, basal cells
 Olfactory receptors are modified neurons
 Surfaces are coated with secretions from olfactory glands
 Olfactory reception involves detecting dissolved chemicals
as they interact with odorant binding proteins
Olfactory Receptors

Bipolar sensory neurons located within olfactory epithelium



Dendrite projects into nasal cavity, terminates in cilia
Axon projects directly up into olfactory bulb of cerebrum
Olfactory bulb projects to olfactory cortex, hippocampus, and
amygdaloid nuclei
Categorizing Smell
• Aristotle: Pungent, Succulent, Acid and Astringent
• Hennings Smell Prism
• Based on verbal descriptors of smell
• Multidimensional Scaling (MDS)
• Subjects rate only the similarity of odor-pairs
• Consistent arrangement in an N-dimensional space
MDS
Eugenol (cloves)
Propionic acid (sweaty socks)
Methyl Salicylate (cloves)
Acetic acid (vinegar)
Vanillin
Aldehyde C14 (flowers)
Nitrobenzene (almonds)
Scatole (feces)
Citronelli (lemons)
Turpentine
Hydrogen Sulfide (rotten eggs)
Physiology
• Nasal Cycle
• Olfactory Epithelium
• Olfactory Receptor Cells
(Complete Neurons)
• Free Nerve Endings
• Olfactory Nerve
(Axons of the ORCs)
Physiology contd.
Olfactory Receptor Cells have cilia
Cilia are bathed in mucous
Mucous contains Odorant binding
proteins (OBP)
OBP transmits the odorant
molecules from the air to receptor
sites on the cilia
Olfactory Pathways
Neural Representations
Very little known
• No odorotopic map found.
• All cells fire for all odors but form different patterns
different odors => Population code.
• Other complications: Change in intensity leads
to increase in the firing rate of neurons but triggers
changes in the perceived quality of the odorant.
Odor Perception
Odor (as well as taste) are much more phenomenological
than vision or audition.
• Odor Detection
• Very good: Mercaptan 1 part per 50 billion parts of air.
• Varies greatly from odor to odor (molecular properties)
• Varies with a variety of other factors
• Time of day, Age and Gender
Smoking in injurious to odor detection!
• Odor Identification
• Much worse
• Shows context dependence
• Varies once again with age and gender
Odor Identification
• Determined by Gender
•Women vastly superior
• Can be improved with practise - benefits both genders
• Stimulus Saliency
• Infants and mothers can recognize each other by
smell alone
• Infants hedonic reactions to smell develops with age
• Odor Familiarity - Seen in elderly people
Smoking is injurious to odor identification!
Some more amazing facts
• Odor Constancy
• Natural sniffs lead to constant odor perception but
artificial blowing of air does not
• Common Chemical Sense
• Odors judged pleasant at moderate concentrations are
judged unpleasant at high concentrations
• Related to the action of free nerve endings - stimulated
in an indiscriminate manner
• Warning system for the organism
• Adaptation - Adapt to a particular odor
• Cross Adaptation: Adapt to a different odor
• Could have led to odor categorization but turns out that
cross-adaptation is not symmetric!
Disorders of Smell
Anosmia or “Odor Blindness”
• Often caused by a blow to the head
• Temporary since ORCs can regenerate
• Sometimes can be total but in that case is specific to
certain odors, e.g. sweat, urine
• Anosmia can have serious consequences such as loss
of apetite and weight
Taste – Gustation
 Taste receptors
 Occur in taste buds
 Most are found on the
surface of the tongue
 Located within tongue
papillae
 Two types of papillae
(with taste buds)
 Fungiform papillae
 Circumvallate papillae
Taste Buds
 Collection of 50 –100
epithelial cells
 Contain three major cell
types (similar in all
special senses)
 Supporting cells
 Gustatory cells
 Basal cells
 Contain long microvilli –
extend through a taste
pore
Taste Sensation and the
Gustatory Pathway
 Four basic qualities of taste
 Sweet, sour, salty, and bitter
 A fifth taste – umami, “deliciousness”
 No structural difference among taste
buds
Gustatory Pathway from Taste
Buds

Taste information reaches the
cerebral cortex
 Primarily through the
facial (VII) and
glossopharyngeal (IX)
nerves
 Some taste information
through the vagus nerve
(X)
 Sensory neurons synapse
in the medulla
 Located in the solitary
nucleus
Figure 16.2