11/23/11. A few words about light nm The electromagnetic spectrum. BÓDIS Emőke 22 November Schematic structure of the eye

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1 11/23/11 A few words about light nm nm BÓDIS Emőke 22 November 2011 The electromagnetic spectrum see only 1/70 of the electromagnetic spectrum The External Structure: The Immediate Structure: 1.Sclera: 1. Pupil: A dark, adjustable opening through which the light enters (aperture). It changes its size as the amount of light entering the eye. -outher wall, sclerotic coat that covers 5/6th of the surface of the eye -white substance, dense connective tissue with collagen and elastic fibers 2. Iris: ring of muscle tissue (diaphragm) that controls the size of the pupil opening, It contains the color pigments and the color which the eyes possess are due to the iris muscles 2. Cornea: - Transparent external surface at the front of the eyeball - a five-layered membrane that covers both the pupil and the iris. 3. Lens: changes its own thickness in order to focus image properly on retina, called accomodation The human eye is appr. 2.5 cm diameter and almost spherical and there is an overpressure inside ( kpa). The Immediate Structure: The Immediate Structure: 4. Fluids in the Chambers of the Eye: 5. Choroid: Anterior and posterior chambers are filled with aqueous humor, vitreous chamber is filled with a more viscous fluid, the internal pressure mmhg ( kpa) to maintain the shape, lines most (2/3) of internal region of sclera, serves two important functions: carries blood to the retina and the humors in order to provide nourishment to the eye, light that passes through the rods and cones does not reflect back these liquids are nearly transparent, so that they can nourish the cells of the eye without interfering with the light that enters in the eye 1

2 Accommodation The Internal Structure: - is part of the CNS - begins the interpretations of visual information, converts the electromagnetic energy of the light into useful information for the brain - contains about 130 million nerve cells - two layers: - Outer plexiform layer: - a web of optical nerves that carry signals of rods and cones to the optic nerves - located between the photosensitive cells and the vitreous humor - Rods and Cones Optical Imaging in the Eye Refractive index: n 21 = v 1 v 2 Knowing the difference between the refractive indexes on the tissues (n-n ) and the curvature of the separating surfaces (r), the refraction power of the surface (D) can be calculated: D = n n' r Air-cornea: 48 dptr Cornea-aqueous humour: -6 dptr Aqueous humour-lens: 8 dptr Lens-vitreous humour: 12 dptr Eye is a dioptre optical system! Optical Imaging in the Eye Healthy eye: 25 cm, Let s determine the change in the refractive power between the two extremes during accommodation! D 25 = 1 o i D = 1 o + 1 i 1 ΔD = D 25 D = 0.25m 1 = 4dptr The young eye can form a sharp image of objects at 8-10 cm, which corresponds to a change of dioptre. Schematic structure of the retina Rods and 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. 2

3 Rods and cones in retina: colourized scanning electron micrograph (SEM) of rods and cones, showing the structure of the eye retina. Structure of photoreceptors Photoreceptive disks Outer segment Plasma membrane Outer segment Inner segment mitochondria nucleus Inner segment Synaptic end Rod Cone Synaptic end Rods Cones - detect black, white and gray - twilight (scotopic) vision - peripheral vision, i.e. the objects that are outside the main center of focus, and for night vision million cells - Protein portion is Rhodopsin (scotposin) = opsin + retinal retinal is derived from vitamin A - takes 5 minutes to regenerate 50% of bleached rhodopsin in rods - vision at normal daylight (photopic) million cells - concentrated near the center of the retina that is concerned with sharp focusing - fine details and color sensation they work well in well- lit conditions i.e., bright or sufficient light - protein portion is photopsin Iodopsin = photopsin + retinal - takes 90 seconds to regenerate 50% of iodopsin in cones - 3 different types of photopsin red (erythrolobe)- 64% nm green (chlorolobe)- 32% nm blue (cyanolobe)- 2% nm Distribution of receptors on the retina Characteristics of receptor cells Rods Senses very low light intensities (even 1 photon!) Saturates at middle level light intensities Found mainly on periphery of retina More rods are coupled to one ganglion cell (higher sensitivity, lower spatial resolution) Not color sensitive Cones Less sensitive, but sensitivity range is wide No saturation Found in fovea, especially center of fovea Less converging neural connections (better spatial resolution) Color sensitive 3

4 mv 11/23/11 The light sensing function of rod is mediated by rhodopsin Questions Rhodopsin (vision purple) The molecule that lets us see: RHODOPSIN - Belongs to the large family of G- protein coupled receptor - Two parts: a protein and a covalently bound pigment - Integral membrane protein - The absorption maximum is around 500 nm - One of the most studied receptor Illustration of Rhodopsin (blue) with 11-cis retinal A (red) Rhodopsin 2.8A resolution; Science 389,739 (2000) Science 289, (2000) Schematic diagram of the cyclic GMP cascade of vision Light hyperpolarizes the plasma membrane of a retinal rod cell Membrane potential 4

5 Overview of the process Vision defects normal, hyperopic and myopic vision 1 rhodopsin absorbs 1 photon 500 transducin molecules become activated 500 phosphodiesterase molecules become activated and they hydrolyze 10 5 cgmp molecules 250 Na + -channels close influx of Na + ions is inhibites during 1 second the cell becomes hyperpolarized (with 1 mv) transmitter secretion decreases. Vision defects astigmatism Vision defects chromatic and spherical aberration Astigmatism Spherical aberration Chromatic aberration Corrected by glases with cylindical lenses Sensitivity Resolution of the eye The smallest angle under which the eye can still discern two separate points is called the limit angle of view. Appr. 1 (minute of arc) Wave character of light: the image of individual points of the object ia a set of concentric circles (Airy-discs). Wave optical limit of resolution depends on the wavelength and the diameter of the pupil. - if λ = 800 nm, d = 2 mm if λ = 400 nm, d = 2 mm 0.8 5

6 Biological limit of the Resolution of Eye Object Image on receptors Perception 2µ m Color is a sensation and not a physical parameter! The smallest angle under which the eye can still discern two separate points is called the limit of view, which is app. 1 minute of arc (1 ). The biological and the wave-optical resolution limits are approximately equal! Sensitivity of human color receptors Color mixing Additive color mixing Subtracting color mixing Optical illusions size and direction Optical illusions size and direction 6

7 The letters of the word LIFE appear to be tilting backwards and forwards. They're not, of course. Incredibly, the two arrowed squares are identical in colour. In this image we get the effect of three "drums" rotating left and right. The white edge shading on the blue discs determines which way each drum appears to turn. 7

8 Optical illusions impossibile objects There are three main types of optical illusions: literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, tilt, colour, movement), and cognitive illusions where the eye and brain make unconscious inferences. Why don t we see the blood vessels? Color and language Read aloud the words written below Read the color of the words below (not the actual words), try to do it as fast as you can. Is it difficult? Because images stabilized on the retina fade! 8