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OUR 5 MAJOR SENSORY SYSTEMS
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  1. 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

  2. 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

  3. 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

  4. 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 300-850nm 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

  5. VISION Pigment epithelium Neuronal layers

  6. The Retina • Contains photoreceptor cells (rods and cones) and associated interneurones and sensory neurones

  7. The neural circuits in the retina of a primate Vision---rod/cones -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.

  8. The Rod Cell Scanning electron micrographs of retinal rod cells

  9. Schematic representation of a rod cell Photoperception 1000 disks, 16nm thick 100,000,000 rod cells in human retina Rod cell (1x40µm) Biochemistry. L. Stryer

  10. 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

  11. 300-850nm The electromagnetic spectrum

  12. Absorption spectrum of rhodopsin

  13. Questions How does the cell respond to photons? What mechanism converts light into a cellular signal?

  14. Ligand-activated Receptor Light-activated Receptor Rhodopsin

  15. (polyene- with 6 alternating double and single bonds)

  16. Illustration of Rhodopsin (blue) with 11-cis retinal (red)

  17. (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)

  18. 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

  19. RHODOPSIN D(E)RY Cys322 Cys323 In Helix VIII (311-321) Glu113 Asp2 Out Met1 Cys110 Cys187 Lys296 Glu181 Asp15

  20. The three dimensional structure of rhodopsin Rhodopsin 2.8A resolution; Science 389,739 (2000) Science289, 739-745 (2000)

  21. Three dimensional Model of Rhodopsin Palmitoyl at Helix 8 Retinal

  22. Rhodopsin photoactivation Alcohol dehydrogenases

  23. a b 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

  24. Schematic diagram of the cyclic GMP cascade of vision

  25. 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 g g g Gat g Gat Gat a b a b Active Inactive The hydrolysis of cGMP by phosphodiesterase is the second stage of of amplification

  26. 11-trans-retinal 11-cis-retinal

  27. Light hyperpolarizes the plasma membrane of a retinal rod cell mV 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

  28. 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

  29. 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.

  30. CNG- Cyclic nucleotide-gated channels Cyclic nucleotide binding domain

  31. Dark Current

  32. BiologyMad.com 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

  33. BiologyMad.com

  34. BiologyMad.com 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 i forms an impulse which is then passed to the ganglion cells and then to the brain

  35. BiologyMad.com

  36. BiologyMad.com Rods and Cones

  37. 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

  38. Guanylate cyclase GTP cGMP +PPi The enzyme Guanylate cyclase looses its activity in high Ca2+

  39. BiologyMad.com 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

  40. 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)

  41. Comparison of the amino acid sequence of the green and red photoreceptors

  42. 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

  43. 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