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Sensory System
4-25-2016
Sensory receptors
• Somatic
• Visceral
-- Chemoreceptors (taste,
smell, smell)
-- Thermoreceptors
(temperature)
-- Photoreceptors (vision)
-- Baroreceptors (sound,
balance)
-- Proprioreceptors (muscle
stretch)
-- Chemoreceptors (chemicals
in blood, osmoreceptors)
-- Baroreceptors (blood
pressure)
Sensory transduction
• Receptors transform an
external signal into a
membrane potential
• Two types of receptor
cells:
- a nerve cell
- a specialized epithelial
cell
Sensory
pathways
• The sensory pathways
convey the type and
location of the sensory
stimulus
• The type: because of the
type of receptor activated
• The location: because the
brain has a map of the
location of each receptor
Sensory coding
• A receptor must convey the
type of information it is
sending  the kind of
receptor activated
determined the signal
recognition by the brain
• It must convey the intensity
of the stimulus  the
stronger the signals, the
more frequent will be the
APs
• It must send information
about the location and
receptive field,
characteristic of the
receptor
Receptor adaptation
• Tonic receptors
-- slow acting, -- no adaptation:
continue to for impulses as long as
the stimulus is there
(ex: proprioreceptors)
• Phasic receptors
-- quick acting, adapt: stop firing when
stimuli are constant (ex: smell)
Vision
Charles Darwin’s “Origin of the species” (1882)
under the heading of “Difficulties of the theory”:
How to explain the origins of organs of
extreme perfection, like the eyes of eagles.
Darwin proposed a very simple prototypic eye consisting of
just two cells, a photoreceptor cell and a pigment cell,
shielding the light from one side.
Protistology: How to build a microbial eye
Nature 523, 166–167 (09 July 2015)
http://dx.doi.org/10.7554/eLife.12620.004
Single-celled photosynthetic bacteria acting as lenses.
eLife, 5, e12620: 2016
Hou XG, et al. 2007. The Cambrian fossils of Chengjiang
(澄江) China, the flowering of early animal life. Oxford:
Blackwell
Pax 6 is a master control gene for
eye development
Chance and Necessity in Eye Evolution Genome Biol. Evol. 3:1053–1066.; 2011
Human eye
Retina
DRAWING FROM: Gillian Lee, Wellcome Images.
Retina
Is our eye a perfect design?
One rhodopsin molecule
Absorbs one photon
500 Transducin molecules are activated
500 Phospodiesterase molecules
are activated
105 cGMP molecules are hydrolyzed
250 Na+ channels closed
106-107 ions/sec are prevented from entering
the cell for a period of 1 sec
Rod cell membrane is
hyperpolarized by 1 mV
Termination of receptor activation
Science, 297, 529 (2002)
Classe of light receptor in human
• Cone photoreceptors:
– 440nm;
– 530nm;
– 560 nm.
• Rod photoreceptors: 500 nm.
• Retinal ganglion cells: 480 nm.
Cone cells carry three
different opsins for color
dection (with same retinal)
Rod cells carry opsin for
black/white light dection
LWS opsin
277
Phenylalanine
N
Outside cell
Membrane
Inside cell
C
180
Alanine
285
Alanine
MWS opsin
277
Tyrosine
N
Outside cell
Membrane
Inside cell
C
180
Serine
285
Threonine
1.0
SWS
MWS
LWS
0.8
Absorbance
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
Wavelength (nm)
DATA FROM: Stockman, et al, 1993.
Convergence
The rods’ greater spatial summation enables them to cause
ganglion cell firing at lower stimulus intensities than the cones’.
Rods do not
pick up on
location.
Cones pick up on
location.
One-to-one wiring leads to ability to discriminate details
Trade-off is that cones need more light to respond than rods
Evolution of trichromatic vision
The Evolution of Primate Color Vision by Gerald H. Jacobs and Jeremy Nathans
Scientific American April 2009 page 60
The Evolution of Primate Color Vision by Gerald H. Jacobs and Jeremy Nathans
Scientific American April 2009 page 61
The Evolution of Primate Color Vision
by Gerald H. Jacobs and Jeremy Nathans
Scientific American April 2009 page 62
The Evolution of Primate Color Vision by Gerald H. Jacobs and Jeremy Nathans
Scientific American April 2009 page 62
Color Vision Deficiency “Color Blindness”
• Color-blind humans have dichromatic vision and can
distinguish short-wavelength stimuli (blue) from longwavelength stimuli (not blue).
• Introduction of photopigment genes into animals with
dichromatic vision to make them become trichromatic.
• We may be able to correct dichromatic vision in humans.
• What other message you get from that
observation?
“Real taste [in] the mouth, in my
theory must be acquired by certain
foods being habitual—[and] hence
become hereditary”
Charles Darwin’s Notebooks, 1836–1844
Distribution of
taste receptors on the tongue
Bitter
Salt
Sweet
Umami
Sour
AFTER: Chandrashekar, J. et al. (2006) Nature 444: 288.
Bitter
Salt
Sweet
Umami
Sour
Each taste bud is
thought to
contain all types
of receptor
AFTER: Chandrashekar, J. et al. (2006) Nature 444: 288.
Taste receptors
Bitter T2R protein
Sweet T1R2 + T1R3 protein
T1R2 T1R3
T1R1 T1R3
Umami T1R1 + T1R3 protein
IMAGES FROM: Dean Madden.
New virus
leaves cell
HIV attaches to CD4
receptors on T-Cell
1
2
3
45
6
Human
1
2
3
45
6
Panda
Mutations in exons 3 and 6
1
Dog
2
3
45
6
Umami receptor gene
PHOTO BY: J. Patrick Fischer, Wikimedia Commons.
Other examples
• For whales, mammals that migrated back to
the sea from land 25 to 55 million years ago,
extensive losses of sweet, umami, bitter, and
sour tastes were found.
– Genome Biol Evol. 2014 Jun; 6(6): 1254–1265.
"the umami and bitter tastes were
lost in the common ancestor of all
penguins, whereas the sweet taste
was lost earlier"
Make an explanation!
Molecular evidence for the loss of three
basic tastes in penguins
Current Biology 25, pR141–R142, 2015
Dynamic changes of sweet
• loss of the sweet taste receptor gene Tas1r2
in many obligate carnivores.
• all birds also have lost the Tas1r2 taste
receptor gene!
• How can one explain the appetitive behavior
of bird species such as hummingbirds that
typically consume foods that consist primarily
of sweet sugars?
Science 8/22/2014
Page 878-9
Signal transduction from the olfactory GPCRs.
Structures of olfactory receptor neurons.
The anatomy of olfaction in the mouse.
Why mosquito like to bite
humans?
Evolution of mosquito preference for
humans linked to an odorant receptor
Or4 recognizes the human odorant sulcatone
(6-methyl-5-hepten-2-one)
NATURE 515: 222 – 227; 2014
What transcriptional mechanism
can assure the random choice of
only one of the multiple OR genes
in a given neuron?
1, DNA recombination to bring one OR to the
promoter just like V/C joining of Ig.
2, one single enhancer acts trans to regulate
expression of one OR gene!
Controversial issue
• Interchromosomal Interactions and Olfactory
Receptor Choice. Cell 126, 403–413, 2006
• Results from H element knock out mice are
not consistent with the hypothesis of H as an
essential trans-acting enhancer for genomewide regulation of OR gene expression. Cell
130, 373–384, 2007.
Enhancer interaction networks as a means for singular olfactory receptor expression.
Cell. 2014 Oct 23;159(3):543-57.
Bacteria mechanosensory ion channel MscL
A molecular model of the MEC-4 touch-receptor complex in C. elegans.