2 Human Chemical Senses
Tom Finger
I. Introduction
A. Role of Chemical Senses in Behavior
B. Chemical Senses of humans
1. Smell
2. Taste
3. Chemesthesis: Trigeminal chemoreception
C. Clinical Complaints & how to distinguish between the chemical senses
D. Comparison of Chemical Senses
1. Receptor Cell & sensory endorgans
2. Cranial Nerve
3. Primary Sensory Nucleus in the Brain
4. Sensations & Utilization
II. Olfaction
A. Peripheral Anatomy
1. Orthonasal vs retronasal stimulation
2. Olfactory Epithelium
a. mucus layer & cilia
b. olfactory receptor neurons
B. Olfactory Transduction
1. G-protein coupled receptors
2. cAMP & cAMP-gated channels
C. Olfactory Bulb & Representation of Odors
1. Glomeruli: Odor Map
2. Output Cells: Mitral Cells
D. Central Olfactory Pathways
1. Olfactory Tract as tract of CNS
2. Piriform Cortex Orbitofrontal Cortex (Conscious appreciation of odor)
3. Entorhinal Cortex Hippocampus (Odor-evoked memories)
4. Olfactory Tubercle & Amygdala (Limbic/visceral responses to odors)
III. Taste
A. Taste qualities (bitter, sweet, salt, sour, “umami” [savory])
B. Peripheral Taste Apparatus
1. Distribution of Taste Buds: lingual vs extra-lingual
2. Lingual Papillae
3. Organization of Taste Buds
C. Diversity of Taste Transduction Mechanisms
1. ion channels (sour, salty)
2. second-messenger systems (bitter, sweet, umami)
D. Central Taste Pathways
1. nuc. solitary tract – orotopic map
2. VPMpc
3. taste cortex (in insular cortex)
4. “flavor” cortex = orbitofrontal cortex
IV. Chemesthesis: Trigeminal chemoreception
A. Free Nerve endings
B. Trp Channels (chemically-sensitive ion channels)
Fig. 1: The olfactory epithelium can be stimulated
either orthonasally, i.e. by odors entering the nostril, or
retronasally, i.e. by odors from substances in the mouth
which curve backward past the soft palate the enter the
nasal cavity from the rear.
Introduction
The ability to detect chemicals probably first evolved because of the necessity for
primitive organisms to detect sources of food and to avoid noxious compounds. However,
chemical cues provide not only information on the nature of the environment, but also serve as a
means of communication between organisms.
Traditionally the chemical senses are described as playing only a minor role in human
behaviors compared to other vertebrates. While it is certainly true that some species (e.g. dogs)
have a much more acute ability to detect smells, a close study of the chemical senses in humans
reveals that they are sophisticated systems that play important biological and social roles. The
sense of smell in humans can detect quantities of odorants in air that are below the detection
threshold of modern gas chromatographs. The parallel computation necessary for recognition of
most smells rivals that of the most sophisticated computers currently available.
Olfaction and taste are important clinically because they play a role in the regulation of
food intake, and contribute greatly to our quality of life, including p