BS3033 Block 2: Transducing the Senses.
Paul Glynn (pg8@leicester.ac.uk)
Sensory Transducers: Chemoceptors & Mechanoceptors.
Sense
Olfaction
Transducer Cell
Olfactory neuron
Transducer Molecule(s)
GPCR
Taste
Taste-cell
GPCR or Ion-channel
Temperature/Pain
Ad- or C-neuron
TRP-channel and ?
ITCH
C-neuron
GPCR + TRP-channel
Touch
Ab-neuron +
non-neural cell
Piezo2-channel or ?
Hearing
Inner Hair Cell
Mechanosensitive channel
Vision
Photoceptor cell
GPCR (Rhodopsin)
The Chemical Senses.
Evolutionarily ancient: eg, C. elegans has chemical sense.
Mammals have chemoreceptors:
On external/internal surfaces  detect H+, capsaicin, etc.
(Somatosensory transducers).
In specialized organs:
Olfactory epithelium (in nose)  detects odorants.
Vomeronasal organ  detects pheromones (vestigial in humans).
Taste buds (on tongue)  detect ingested chemicals.
Chemical Senses: (i) Taste (Gustation).
Gustatory neural pathway.
• Three cranial nerves (cell bodies in
trigeminal ganglia) convey signal
from taste-buds on tongue and
epiglottis, past ganglia, to synapse in
medulla (gustatory nucleus).
• Secondary afferents carry signal
to synapse in thalamus (ventral
posterior medial nucleus).
• Tertiary afferents carry signal
to synapse in cortex (primary
gustatory).
Most taste (gustation) cells are on tongue taste buds.
There are 5 basic tastes:
1. Saltiness (Na+).
2. Sourness (H+).
3. Sweetness (eg, Sucrose).
4. Bitterness (eg, Quinine).
5. Umami (eg, Glutamate).
So how do humans perceive such
a wide variety of tastes and flavours?
Neural coding of taste.
* Humans have 2000-5000 taste buds each with 50-150 taste cells.
* Most taste cells have rather broad tastant specificity.
* Each gustatory afferent axon receives input from several taste buds.
* Each secondary gustatory neuron in medulla receives input
from several primary afferent axons.
* Responses of a large number of broadly-tuned neurons specify
properties of a particular taste.
Specificities of taste cell responses to different tastants.
Expose taste bud to tastants.
Record Vm of cells and afferent
gustatory axons.
Cell 1 and axon 1: responds
to NaCl only.
Cell 2 and axon 2: responds
to NaCl, Quinine and HCl.
General finding:
Taste cells respond broadly
to bitter, sour and salt, but more
selectively to sweet tastants.
Signal transduction in taste cells.
Na+ directly permeate
constitutively-open
Na-channel  membrane
depolarisation.
H+ permeate Na-channels
and block constitutivelyopen K-channels 
membrane depolarisation.
Sucrose activates GPCR
AC activation  cAMP
 PKA activation 
phosphorylation and block
of constitutively-open K-channels
 membrane depolarisation.
In all, depolarisation opens VOCC  [Ca2+]i  vesicular release of Glutamate
 activation of afferent gustatory axon.
Signal transduction in taste cells – 2.
Analogous mechanisms operate in taste cells
for bitter and umami tastants.
Taste (Gustation): Summary.
* Tongue taste cell transducers are GPCRs or ion-channels.
* Tastant binding to receptor  [Ca2+]i  Glu release
 action potential in afferent axon of cranial nerve...
 medulla  thalamus  cortex (primary gustatory).
* Specific taste represents response of large number
of broadly-tuned neurons.