Faculty of Medicine Dr Zad Mansour The Eye The Eye Light Light is the electromagnetic radiation that is visible to our eyes The electromagnetic spectrum detectable by our visual system: visible light (wavelengths of nm) Isaac Newton: the mix of wavelengths in this range emitted by the sun appears to humans as white, whereas light of a single wavelength appears as one of the colors of the rainbow. Optics: The study of light rays and their interactions Gross Anatomy of the Eye The Eye in cross-section Ophthalmoscopic Appearance of the Eye Refraction and Accommodation The cornea is the site of most of the refractive power of the eyes Lens: Accommodation which is greater refraction for near objects and provided by the lens refractive power: reciprocal of the focal distance RF of the cornea = 42 diopters Accommodation Strabismus: Paralytic (incomitant): acquired defect of the movement of an eye, diplopia is maximal in the direction of action of the weak muscle. Non-paralytic (concomitant): Imbalance in the EOM leading to misalignment or lack of coordination between the two eyes. Esotropia: the directions of gaze of the two eyes cross, and the person is said to be cross-eyed. Exotropia: the directions of gaze diverge, and the person is said to be wall-eyed. The deviating eye usually has a defective vision; this is called amblyopia ex anopsia Exotropia Hyperopia (farsightedness) Myopia: nearsightedness, shortsightedness Hyperopia: farsightedness, longsightedness or hypermetropia Vision Correction Cataract: is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently impacting vision in the affected eye(s) and progressing to complete blindness if untreated. It is associated with increased pressure of the fluid in the eye (aqueous humour).aqueous humour The Pupillary Light Reflex: -Direct -Indirect (consensual) Visual Acuity: -The ability of the eye to distinguish two nearby points -Snellen eye chart Visual Field: Microscopic Anatomy of the Retina Information about light flows from the photoreceptors to bipolar cells to ganglion cells, which project axons out of the eye in the optic nerve. Horizontal cells and amacrine cells modify the responses of bipolar cells and ganglion cells via lateral connections. Photoreceptors: Rods: Contain more disks, higher photopigment concentration, they are 1000 times more sensitive to light, they contain the same photopigment, they contribute to scotopic conditions Cones: Less disks, three types of photopigment, they contribute to photopic conditions. The Fovea Cones are found primarily in the central retina (the fovea). Rods are absent from the central fovea and are found mainly in the peripheral retina. In the central retina, relatively few photoreceptors feed information directly to a ganglion cell; in the peripheral retina, many photoreceptors provide input. This arrangement makes the peripheral retina better at detecting dim light but the central retina better for high-resolution vision. Phototransduction Conversion of light energy into a membrane potential Rods to Cones: 20 to 1 Phototransduction in Rods In complete darkness, the membrane potential of the rod is 30 mV. This depolarization is caused by the steady influx of sodium through special channels in the outer segment membrane. This movement of positive charge is called the dark current. Sodium channels are stimulated to open by cGMP cGMP is continually produced in the photoreceptor by the enzyme guanylyl cyclase, keeping the sodium channels open. Light reduces cGMP, causing the sodium channels to close, and the membrane potential becomes more negative, thus photoreceptors hyperpolarize in response to light. The hyperpolarization of photoreceptors in response to light The activation of Rhodopsin by light Rhodopsin The activation of Rhodopsin by light (a)In the dark, cGMP gates a sodium channel, causing an inward Na + current and depolarization of the cell. (b)Light causes a change in the conformation of retinal so that it activates the opsin (this process is called bleaching as the photopigment changes colour from purple to yellow), the bleaching of rhodopsin stimulates G-protein called transducin which in turn activates the effector enzyme phosphodiesterase (PDE), which breaks down the cGMP. The reduction in cGMP causes the sodium channels to close and the membrane to hyperpolarize. Photoreceptor cell Bipolar cell Ganglion cell Horizontal C. Amacrine C. Transmission: photoreceptor cells, horizontal cells & Amacrine cells to Bipolar cells by direct electric current flow Bipolar Cells: Two types of bipolar cells provide opposing excitatory and inhibitory signals in the visual pathway: (1)The depolarizing bipolar cell (2)(2) the hyperpolarizing bipolar cell. That is, some bipolar cells depolarize when the rods and cones are excited, and others hyperpolarize. There are two possible explanations for this difference. 1) The first is that the two bipolar cells are of entirely different typesone responding by depolarizing in response to the glutamate neurotransmitter released by the rods and cones, and the other responding by hyperpolarizing. 2) The other possibility is that one of the bipolar cells receives direct excitation from the rods and cones, whereas the other receives its signal indirectly through a horizontal cell or an amacrine cell Ganglion Cells and Optic Nerve Fibres - Axons of ganglion cells form the optic nerve. - Each retina contains about 100 million rods and 3 million cones; yet the number of ganglion cells is only about 1.6 million. - Thus, an average of 60 rods and 2 cones converge on each ganglion cell and the optic nerve fibre. There are three distinct types of ganglion cells, designated W, X, and Y cells. The types of Galnglion Cells: W cells: - 40% of all ganglion cells - slow velocity of 8 m/sec - they receive most of their excitation from rods, - especially sensitive for detecting directional movement in the field of vision, and they are probably important for much of our crude rod vision under dark conditions. X cells: 55% of the total, the most numerous of the ganglion cells Velocity of transmission at 14 m/sec. The fine detail of visual image, Colour vision Y cells: The least numerous, 5% of all ganglion cells Velocity of transmission: 50 m/sec These ganglion cells presumably apprise the central nervous system almost instantaneously when a new visual event occurs anywhere in the visual field, but without specifying the details. Phototransduction in Cones In bright sunlight, cGMP levels in rods fall to the point where the response to light becomes saturated; additional light causes no more hyperpolarization. Thus, vision during the day depends entirely on the cones. The process of phototransduction in cones is virtually the same as in rods. 3 types of cones: "blue" cones that are maximally activated by light with a wavelength of about 430 nm, "green" cones that are maximally activated by light with a wavelength of about 530 nm, and "red" cones that are maximally activated by light with a wavelength of about 560 nm Colour Detection The Young-Helmholtz trichromacy theory: - According to the theory, the brain assigns colors based on a comparison of the readout of the three cone types. - When all types of cones are equally active, as in broad-spectrum light, we perceive white - The color that we perceive is largely determined by the relative contributions of blue, green, and red cones to the retinal signal. - It is difficult to detect colors at night because only the rods, with a single type of photopigment, are activated under dim lighting conditions. - The peak sensitivity of the rods is to a wavelength of about 500 nm, perceived as blue-green (under photopic conditions). automobile dashboard indicator lights Colour Blindness -Red, Green, Blue -Three colour system: Trichromats -Two-colour system: Dichromats -One-colour system: Monochromats -Red & Green: X chromosome -Blue: chromosome 7 -About 2% of men actually lack either the red or the green pigment (Dichromats) -The person with loss of Red Cones: Protanope -The person with loss of Green Cones: Deuteranope Ishihara colour test The central visual system The Central Visual System The Retinofugal projection, from the optic nerve: (1)to the suprachiasmatic nucleus of the hypothalamus, presumably to control circadian rhythms that synchronize various physiologic changes of the body with night and day (2) into the pretectal nuclei in the midbrain, to elicit reflex movements of the eyes to focus on objects of importance and to activate the pupillary light reflex (3) into the superior colliculus, to control rapid directional movements of the two eyes (4) into the ventral lateral geniculate nucleus of the thalamus The Retinofugal projection : The neural pathway that leaves the eye in the optic nerve Suprachiasmatic Nucleus of the Hypothalamus Optic Nerve to Pretectal nucleus of the midbrain Visual Pathway to the Superior Colliculus Lateral Geniculate Nucleus (LGN) LGN: - 6 layers - Layers (I, II): Magnocellular layers, they receive input from the large type Y retinal ganglion cells and provide a rapidly conducting pathway to the visual cortex (transmit black & white information) - Layers (III, VI): Parvocellular layers, they receive their input almost entirely from the type X retinal ganglion cells that transmit colour and convey accurate point- to-point spatial information at a moderate velocity of conduction The Visual (Striate) Cortex -Located on the medial aspect of the occipital lobes -Primary visual cortex (V1), area 17, calcarine fissure -Secondary visual cortex (V2), visual association areas, area 18, lie anterior, superior, inferior, and lateral the primary visual area Layers of the Visual Cortex slow Colour Vision Eye Movements and Their Control Fixation Movements of the Eyes (Horizontal Gaze) Fixation movements are controlled by two neuronal mechanisms: 1) the voluntary fixation mechanism: It allows a person to move the eyes voluntarily to find the object on which he or she wants to fix the vision. Prefrontal Cortex 2) involuntary fixation mechanism: It holds the eyes firmly on the object once it has been found. Superior Colliculus PPRF VI MLF Frontal Cortex VI III The eyes look toward a hemispheric lesion and away from a brainstem lesion ?? RL The Occipito-temporal Network Visual object agnosia: the inability to recognize familiar object. Prosopagnosia: the ability to recognize faces