D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
CHEMICAL SPECIAL SENSES
Lecture 16: Chemical senses, Taste & Smell
Chemical Senses: (chapter 53)
• gustatory & olfactory senses detect information that establishes food
preference & safety; smell also aids in detection of other animals
- linked to pathways governing primitive emotions & behaviors (especially
smell)
• taste – leading role in perception of flavor of foods, augmented by smell &
touch (texture of food); receptors are taste buds (ppts. 1 & 2)
- qualities: now appears to be five primary tastes:
- sour - related to acid; specific hydrogen ion receptor has been found
- salty - mainly sodium; 2 sodium receptors, 2 potassium receptors and 1
chloride receptor have been found
- sweet - associated with numerous organic chemicals such as sugars,
alcohols, aldehydes & certain amino acids; 2 sweet receptors, also adenosine & inosine
receptors associated with sweet taste
- bitter - alkaloids (such as quinine) or organic nitrogen compounds,
which are often poisonous; 2 bitter receptors known
- ‘umami’ - poorly understood; believed to be associated with
glutamate & aspartate (beefy, aging cheese taste); specific glutamate receptor has been
recognized
- sensitivity: relatively low concentrations of substances can be detected,
(second messenger pathway that amplifies tastant stimulus), especially bitter
substances, which often are toxic
Bio 659, lec. 16
- p. 2 -
- structure & location of taste buds: located mainly along sides of lingual
papillae (circumvallate, fungiform & foliate), projections of mucosa on dorsal surface
of tongue; also found on palate, epiglottis, mucosal arches & esophagus; receptor
organ resembles cluster of bananas – sustentacular cells mingled with taste cells,
whose microvilli surround an apical taste pore (fig. 53 – 1 & ppt. 3)
- transduction (generation of taste nerve signals): tastant interacts with
protein receptor on microvillus membrane causing opening of ion channel directly (salty
or sour tastants) or via a second messenger pathway (sweet or bitter tastants)
- causes depolarization; receptors excite taste nerve fibers to deliver
strong signal initially, followed by sensory adaptation (weaker tonic signal during
continuous tastant stimulation)
- pathways (fig. 53 – 2 & ppt. 4): oral taste signals (Cr. N. VII – chorda
tympani fibers of facial n., carried by lingual n.), posterior tongue & pharyngeal signals
(Cr. N. IX – glossopharyngeal n.) & some pharyngeal signals (Cr. N. X – vagus n.) are
conveyed to solitary tract in brainstem; synapse in gustatory nucleus of solitary
tract
- collaterals connect with brainstem salivatory nuclei, instigating
salivation reflex
- second order fibers ascend to ventral basal nucleus of thalamus (in
proximity with medial lemniscal fibers carrying somatosensory signals from tongue to lateral
post central gyrus); third order fibers from thalamus terminate in cortex of anterior
insular lobe (ppts. 5 & 6)
• smell: olfactory sense informs of presence of other animals even recognition
of individuals (not well developed in humans); contributes to perception of flavor &
influences emotional responses
- olfactory area of nasal cavity: sensory field at summit of vestibule in each
nasal cavity, spreading laterally over superior and some middle concha & spreading
medially, over upper median septum (about 5 sq. cm. total for both nasal cavities)
(ppt. 7)
Bio 659, lec. 16
- p. 3 -
- olfactory sensory cells: olfactory hair cells (modified bipolar neurons),
surrounded by sustentacular cells; hair processes (modified cilia) spread into matlike mesh in mucous coating (product of Bowman’s glands)(fig. 53 – 3 & ppt. 8)
- qualities of smell: current research has discredited former popular theory
of seven primary odors; approx. 100 genes coding for specific odorant receptors &
approx. 50 specific anosmias (loss of specific odorant sensitivity) have been
identified
- transduction: chemicals dissolve in mucus and bind to protein receptor
that triggers second messenger pathway (fig. 53 - 4 & ppt. 9); second messenger
(cyclic AMP) opens sodium channels causing depolarization; resting membrane
potential (-55 mv.) causes background tonic signal (slow volley of action potentials)
– receptor excitation results in increased signal strength (more rapid volley of APs)
- pathways: receptor axons traverse cribriform plate of ethmoid bone to
synapse with mitral cells & tufted cells in olfactory bulb; synaptic fields clustered
into glomeruli – possibly sensitive to specific odorants (fig. 53 –3 & ppt. 8)
- some second order fibers (fig. 53 – 5 & ppts. 10 to 12) in olfactory tract
enter medial olfactory area (very old system), which governs primitive reflexes such as
licking the lips & salivation; some sources have thrown doubts on existence of this area
- other second order fibers in olfactory tract enter lateral olfactory area,
which is subdivided into a ‘less old’ and a ‘newer’ pathway
- less old pathway passes through portions of limbic system
(amygdaloid nuclei and hippocampus) as well as medial cortex of temporal lobe
(paleocortex), bypassing thalamus; appears responsible for development of food preferences
or aversions
- newer pathway passes through thalamus (dorsomedial nucleus) en
route to frontal cortex (inferior surface); appears to facilitate identification of odorant and
perceptual analysis (like other sensory cortex areas)
- centrifugal (corticofugal) fibers pass in retrograde fashion (from
higher brain levels back to olfactory bulb); deliver inhibitory signals that perform lateral
Bio 659, lec. 16
- p. 4 -
inhibition (sharpens input signal) and screening of sensory inputs (likely the major basis
of sensory adaptation); mediated by granular cells of olfactory bulb that secrete GABA