Our 5 major sensory systems
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OUR 5 MAJOR SENSORY SYSTEMS. Vision - the detection of light Olfaction - (sense of smell) the detection of small molecules in the air Taste or Gustation - the detection of selected organic compounds and ions by the tongue Hearing -The detection of sound (or pressure wave in the air)

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Our 5 major sensory systems


Vision - the detection of light

Olfaction- (sense of smell) the detection of small molecules in the air

Taste or Gustation- the detection of selected organic compounds and ions by the tongue

Hearing-The detection of sound (or pressure wave in the air)

Touch- the detection of changes in pressure, temp. and other factors by the skin

S e n s o r y s y s t e m s


When fully adapted to darkness our eyes allow us to sense very low levels of light, down to a limit of less than 10 photons. With more light we are able to distinguish millions of colors.

Through our senses of smell and taste we are able to detect thousands of chemicals and sort them into distinct categories

Our 5 major sensory systems

Each of these primary sensory systems contains specialized sensory neurons that transmit nerve impulses to the CNS

In the CNS theses signals are processed and combined with other information to yield a perception that may trigger a change in behavior.

By these means, our senses allow us to detect changes in our environments and adjust our behavior appropriately

Our 5 major sensory systems

Photoreceptor molecules in the eye

detectvisible light

Vision is based on the absorption of light by photoreceptor

cells in the eye

Photoreceptor cells are sensitive to light in a relatively

narrow region of the electromagnetic spectrum between


Two kinds of photoreceptors

Rods (100 million) and Cons (3 million)

Rods function in dim light and do not perceive color

Cons function in bright light and are responsible for color vision

Our 5 major sensory systems


Pigment epithelium

Neuronal layers

The retina

The Retina

  • Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones

Vision rod cones

The neural circuits in the retina

of a primate


-The incoming light reaches the photoreceptor cells (rods and cones) only after passing through several thin, transparent layers of other neurons.

-The pigment epithelium absorbs the light that is not absorbed by the photoreceptor cells and thus minimizes reflections of stray light.

The ganglion cells communicate to the thalamus by sending action potentials down their axons.

However, the photoreceptor cells and

other neurons communicate by graded synaptic potentials that are conducted electronically.

Our 5 major sensory systems

The Rod Cell

Scanning electron micrographs of retinal rod cells

Our 5 major sensory systems

Schematic representation

of a rod cell


1000 disks, 16nm thick

100,000,000 rod cells

in human retina

Rod cell


Biochemistry. L. Stryer

Our 5 major sensory systems

The disks which are membrane enclosed sacs are

densely packed with photoreceptor molecules

The photosensitive molecule is called the

visual pigment because it is highly colored due to

light absorption

The photoreceptor molecule in the rods is rhodopsin

consists of opsin linked to 11-cis-retinal

Our 5 major sensory systems


The electromagnetic spectrum

Our 5 major sensory systems

Absorption spectrum of rhodopsin



How does the cell respond to photons?

What mechanism converts light into a cellular signal?

Our 5 major sensory systems






Our 5 major sensory systems

(polyene- with 6 alternating double and single bonds)

Our 5 major sensory systems

Illustration of

Rhodopsin (blue)


11-cis retinal (red)

Our 5 major sensory systems

(440nm absorption)

The protonated form of the 11-cis retinal absorbs at 440nm

Unlike 380nm of the non-protonated.

The positive charge of Lys296(VII) is compensated by Glu113(II)

Our 5 major sensory systems

Activation of rhodopsin by a photon-converting a light energy of

A photon into atomic motion

-The isomerization causes the Shiff-base nitrogen to move

approximately 5A, assuming that the cyclohexane ring

of the cis-retinal group remains fixed/

-Inverse agonist- 108 Rhodopsin molecules /cell

Our 5 major sensory systems






Helix VIII











Our 5 major sensory systems

The three dimensional structure of rhodopsin

Rhodopsin 2.8A resolution; Science 389,739 (2000)

Science289, 739-745 (2000)

Our 5 major sensory systems

Three dimensional Model of Rhodopsin


at Helix 8


Our 5 major sensory systems

Rhodopsin photoactivation

Alcohol dehydrogenases

Our 5 major sensory systems



Transducin at 39kD; b 36kD;  8kD

In the dark transducin is in the GDP form

the binding of GTP to transducin leads to the

release of R* which enables it to catalyze the

Activation of another molecule of transducin

A single R* catalyzes the activation of 500

molecules of transducin, the first stage in

the amplification of vision

Our 5 major sensory systems

Schematic diagram of the cyclic GMP cascade of vision

Activation of phosphodiesterase by g a t

The binding of GTP switches on the phosphodiesterase (PDE) by relieving an inhibitory constraint. In the dark the two catalytic subunitsa and b are held in check by a pair of inhibitory subunits (g).By binding of Gat to the enzyme it removes the inhibitory subunits and the enzyme is activated

Activation of phosphodiesterase

by Gat














The hydrolysis of cGMP by phosphodiesterase is the second stage of

of amplification

Our 5 major sensory systems



Our 5 major sensory systems

Light hyperpolarizes the plasma membrane of a retinal rod cell


Membrane potential

The light induced hyperpolarization is transmitted by the plasma

membrane from the outer segment to the synaptic body.

A single photon closes hundreds of cation specific channels (~500)

and leads to a hyperpolarization of about 1-5mV

Our 5 major sensory systems

Cation channels (~500) in the rod cell close following the transduction of a single photon.

These represent 3% of the total number of channels that are open in the dark. The resultant hyperpolarization is about 1mV and lasts about 1 sec.

This is sufficient to depress the rate of neurotransmitter release that transmits the onward signal

Our 5 major sensory systems

The high-degree of co-operativity (3 molecules of cGMP) to open the channel increases the sensitivity of the channel for small changes in cGMP which enable it to act as a switch.

Our 5 major sensory systems

CNG- Cyclic nucleotide-gated channels

Cyclic nucleotide

binding domain

Our 5 major sensory systems

Dark Current

In the dark


In the Dark…

  • In the dark the channel is open Na+ flow in can cause rod cells to depolarise.

    • Therefore in total darkness, the membrane of a rod cell is polarised

  • Therefore rod cells release neurotransmitter in the dark

  • However the synapse with bipolar cells is an inhibitory synapse i.e. the neurotransmitter stops impulse

Our 5 major sensory systems


In the light


In the Light…

As cis retinal is converted to trans retinal, the Na+ channels begin to closei

less neurotransmitter is produced. If the threshold is reached, the bipolar cell will be depolarised


forms an impulse which is then passed to the ganglion cells and then to the brain

Our 5 major sensory systems


Rods and cones


Rods and Cones

Our 5 major sensory systems

Onerhodopsin molecule

Absorbs one photon

500Transducin molecules are activated

500Phospodiesterase molecules

are activated

105 cGMP molecules are hydrolyzed

250Na+ channels closed

106-107ions/sec are prevented from entering

the cell for a period of 1 sec

Rod cell membrane is

hyperpolarized by 1 mV

Our 5 major sensory systems

Guanylate cyclase


The enzyme Guanylate cyclase

looses its activity in high Ca2+

Color vision


Color Vision

  • 3 different cone cells. Each have a different form of opsin (they have the same retinal)

  • 3 forms of rhodopsin are sensitive to different parts of the spectrum

    • 10% red cones

    • 45% blue cones

    • 45% blue cones

Our 5 major sensory systems

The absorption spectra of the cone visual

pigment responsible for color vision

Con Cells

The cone photoreceptors are 7TM domain receptors that utilize

11-cis-retinal as chromophore. Absorption maxima (nm)

in human are 426 (blue), 530 (green) and 560 (red)

Our 5 major sensory systems

Comparison of the amino acid sequence of

the green and red photoreceptors

Our 5 major sensory systems

Color Vision

  • Colored light will stimulate these 3 cells differently - by comparing the nerve impulses from the 3 kinds of cones the brain can detect any colour

    • Red light  stimulates R cones

    • Yellow light  stimulates R and G cones equally

    • Cyan light  stimulates B and G cones equally

    • White light  stimulates all 3 cones equally

  • Called the trichromatic theory of color vision

Our 5 major sensory systems

Color Vision

  • When we look at something the image falls on the fovea and we see it in color and sharp detail.

  • Objects in the periphery of our field of view are not seen in colour, or detail.

  • The fovea has high density of cones.

  • Each cone has a synapse with one bipolar cell and one ganglion  each cone sends impulses to the brain about its own small area of the retina  high visual acuity

Our 5 major sensory systems

Evolutionary relationships among visual pigments

Visual pigments have evolved by gene duplication

Color blindness

Color blindness

The genes for the green and red pigments lie adjacent on the human X chromosome. Are 98% identical in nucleotide sequence including introns and UTR

-Therefore, are susceptible for to unequal homologous recombination

-5% of males have this form of blindness

Our 5 major sensory systems

Recombination pathways leading to color blindness

Rearrangements in the course of DNA replication

A) Loss of visual pigment B) The formation of hybrid pigemnt genes that encode photoreceptors with anomalous abs. spectra

A homologous recombination: the exchange of DNA segment at equivalent positions between chromosomes with substantial similarity

Our 5 major sensory systems

Termination of the signal

One of the most important part of the signaling machinery

is termination of the signal even in the presence of the stimulus

This phenomenon is referred to as “desensitization”

Such mechanisms operate at both the level of the receptor

as well as down stream at the level of G-protein

Rapid termination of the receptor signal is controlled

by receptor phosphorylation which is mediated by second

messenger-kinases PKA and PKC or by a distinct

Receptor-kinsases (GRKs) together with arrestins

Our 5 major sensory systems

Heterologous desensitization

Second-messenger kinase regulation

PKA and PKC uncouple receptors from their respective G-proteins and serve as negative-feed-back regulatory loops.

Feed back regulation by the 2nd messenger-stimulated kinases PKA and PKC.

The phosphorylated receptor changes its conformation and no longer can activate the G-proteins.

It is an agonist non-specific desensitization

Our 5 major sensory systems

Homologous desensitizationGRK(G-ptrotein-receptor kinase)-mediated desensitizationA complex mechanism for regulating 7TM-receptor activity called GRK-barrestin systemIt is also called an agonist-specific desensitization because only the activated agonist-occupied conformation of the receptor is phosphorylated by by GRK.A two step process in which agonist-occupied receptor is phosphorylated by GRK and then binds an arrestin proteins. This leads to a rapid-agonist specific desensitization

Our 5 major sensory systems

Heterogous and homologous desensitization

Our 5 major sensory systems

The major GPCR regulatory pathway involves phosphorylation of activated receptors by G protein–coupled receptor kinases (GRKs),

followed by binding of arrestin proteins, which

prevent receptors from activating downstream heterotrimeric G protein pathways while

allowing activation of arrestin-dependent signaling pathways.

Grk g protein coupled receptor kinase

GRK - G-protein–coupled receptor kinase

As long as the agonist remains bound to the receptor, the activated receptor can continue to activate G proteins.

GRK which is catalytically activated by this interaction, also recognizes the activated conformation of the receptor.

Activated GRKs phosphorylate (P) intracellular domains of the receptor and are then released. The agonist-activated, GRK-phosphorylated receptor binds tightly to an arrestin protein, which desensitizes further G protein activation and couples the receptor to the clathrin-coated-pit internalization pathway and to arrestin-scaffolded (and G protein–independent) signaling pathways.

Grk gpcr kinase


The role of GRK-phosphorylation of the receptors in the sequestration process is to facilitate arrestin binding

Experiments to prove this idea

1)A mutated b-adrenergic receptor Y326A is a poor

substrate for b-Adrenergic receptor-kinase, and is not sequestered. Over-expression of b-arrestin restores sequestration

2)Removal of C-terminal tail (sites for GRK sites) prevents sequestration



The arrestin family includes > 6 members several of which undergo alternative splicing

The affinity of b-arrestin (selective for the b-receptors) increases 10-30 fold by GRK-catalyzed phosphorylation, whereas agonist occupancy has a much less significant effect.

The b-arrestins promote internalization by binding to clatherin

Our 5 major sensory systems

Science, 297, 529 (2002)

Our 5 major sensory systems

adrenergic receptor




Rhodopsin kinase

Homologous desensitization


Our 5 major sensory systems



Our 5 major sensory systems


RGS and GAP Activities




Neuron 20, 11-14 (1999)

Our 5 major sensory systems

11-cis vs. all-trans retinal

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