Review of anatomy & physiology of the Retina Dr. Ayesha Abdullah 1.11.2017

Learning outcomes By the end of this lecture the students would be able to; Correlate the structural organization of the retina with its functions and development. Identify structural landmarks on retinal photographs. Name the investigations commonly employed for the assessment of various categories of retinal disorders. Ayesha Abdullah 2017

Camera and the eye The formation of focused images on the photoreceptors depends on the refraction of light by the cornea and the lens. The refractive power of the cornea is unvarying, but that of the lens is adjustable due to the ciliary muscle.Adjustments in the size of the pupil also contribute to the retinal image formation. Narrowing the pupil reduces both spherical and chromatic aberrations. It also increases the depth of field, i.e., the distance within which objects are seen without blurring. Ayesha Abdullah 2017

There are about 100 times more photoreceptors than the Ganglion cells. Unlike the image from a camera the resolution of the retinal image is not uniform. Why? What is the result? There are about 100 times more photoreceptors than the Ganglion cells. Retina compresses images as unlike the camera. Ayesha Abdullah 2017

Anatomical landmarks of the retina Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Normal dimensions Anatomical characteristics Clinically Observable characteristics Optic disc Place where the optic nerve fibers leave the retina. It is devoid of rods and cones hence the blind spot. Contains the central retinal artery and vein It’s a pale disc like structure with vessels emerging out of its center called the cup. Its about 1.5 mm in size. Macula It is the area where the ganglion cells are two layered. Contains the xanthophyl pigment giving it the pigmented look. It is about 5.5 mm in diameter (3.5 disc diameter/ 180 of visual angle). Roughly the area between the arterial arcades. Fovea Foveola A depression in the inner retinal surface. It contains cones only. The inner nuclear layer and the ganglion cell layer is absent. A concave central retinal depression about the same size as the disc (1.5mm) Parafovea The thickest part of the retina Area surrounding the fovea Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Histological structure of the retina Inner limiting membrane – Muller cell footplates Nerve fiber layer – axons of the ganglion cell nuclei Ganglion cell layer – contains nuclei of ganglion cells, the axons of which become the optic nerve fibers Inner plexiform layer – contains the synapse between the bipolar cell axons and the dendrites of the ganglion and amacrine cells. Inner nuclear layer – contains the nuclei and surrounding cell bodies (perikarya) of the bipolar cells Outer plexiform layer – projections of rods and cones ending in the rod spherule and cone pedicle, respectively. These make synapses with dendrites of bipolar cells. In the macular area this is known as the Fiber layer of Henle. Outer nuclear layer – cell bodies of rods and cones External limiting membrane – layer that separates the inner segment portions of the photoreceptors from their cell nucleus Photoreceptor layer – rods and cones Retinal pigment epithelium - single layer of cuboidal cells Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Development of the retina Ayesha Abdullah 2017

Functions of the retina Light perception Brightness appreciation Contrast sensitivity Two point discrimination and appreciation of details Colour perception Light and dark adaptation Circadian rhythms & hormonal balance The cones respond to bright light and mediate high-resolution colour vision during daylight illumination (also called photopic vision). The rods are saturated at daylight levels and don't contribute to pattern vision. However, rods do respond to dim light and mediate lower-resolution, monochromatic vision under very low levels of illumination (called scotopic vision). The illumination in most office settings falls between these two levels and is called mesopic vision. Ayesha Abdullah 2017

Some important facts 150 million receptors & 1 million optic nerve- convergence and mixing of visual signals The horizontal action of the horizontal and amacrine cells can allow one area of the retina to control another (e.g., one stimulus inhibiting another). The response of cones to various wavelengths of light is called their spectral sensitivity There are blue, green, and red cones but more accurately short, medium, and long wavelength sensitive cone subgroups- trichromatic vision The receptive fields of retinal ganglion cells comprise a central approximately circular area, where light has one effect on the firing of the cell, and an annular surround, where light has the opposite effect on the firing of the cell. In ON cells, an increment in light intensity in the centre of the receptive field causes the firing rate to increase. In OFF cells, it makes it decrease. The center-surround organization of ganglion cells’ receptive fields is due to the lateral inhibitory action of horizontal cells. Ganglion cells form two main retinal output channels : 1) the Parvo (or midget) cells, mostly in the fovea and receiving mainly cones inputs; 2) the Magno (or parasol) cells, mostly in the peripheral retina and receiving mainly rod inputs. Ayesha Abdullah 2017

Some important facts The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron The receptive fields of retinal ganglion cells comprise a central approximately circular area, where light has one effect on the firing of the cell, and an annular surround, where light has the opposite effect on the firing of the cell. In ON cells, an increment in light intensity in the centre of the receptive field causes the firing rate to increase. In OFF cells, it makes it decrease. The center-surround organization of ganglion cells’ receptive fields is due to the lateral inhibitory action of horizontal cells. Ganglion cells form two main retinal output channels : 1) the Parvo (or midget) cells, mostly in the fovea and receiving mainly cones inputs; 2) the Magno (or parasol) cells, mostly in the peripheral retina and receiving mainly rod inputs. Ayesha Abdullah 2017

Convergence Ayesha Abdullah 2017

vitreous Biochemistry of vision: the first step in vision is the detection of photons.  In order to detect a photon, specialized cells use a molecule called 11-cis-retinal.  When a photon of light interacts with this molecule, it changes its shape almost instantly.  It is now called trans-retinal.  This change in shape causes a change in shape of another molecule called rhodopsin.  The new shape of rhodopsin is called metarhodopsin II.  Metarhodopsin II now sticks to another protein called transducin forcing it to drop an attached molecule called GDP and pick up another molecule called GTP.  The GTP-transducin-metarhodopsin II molecule now attaches to another protein called phosphodiesterase.  When this happens, phosphodiesterase cleaves molecules called cGMPs.  This cleavage of cGMPs reduces their relative numbers in the cell.  This reduction in cGMP is sensed by an ion channel.  This ion channel shuts off the ability of the sodium ion to enter the cell.  This blockage of sodium entrance into the cell causes an imbalance of charge across the cell's membrane.  This imbalance of charge sends an electrical current to the brain.  The brain then interprets this signal and the result is called vision.  Many other proteins are now needed to convert the proteins and other molecules just mentioned back to their original forms so that they can detect another photon of light and signal the brain RPE Ayesha Abdullah 2017

Ayesha Abdullah 2017

Rods and cones differ in their degrees of convergence onto ganglion cells. Convergence makes the rod system a better light detector, but reduces its spatial resolution. The near one-to-one mapping within the cone system maximizes the discrimination of fine detail, visual acuity. The high density of cones with their one-to-one relationship with bipolar and ganglion cells allow the fovea to mediate high visual acuity. The superior foveal acuity further benefits from reduced optical distortion provided by the displacement of the inner nuclear and ganglion cell layers Ayesha Abdullah 2017

Rods & Cones Rod System Cone System Achromatic High convergence High light sensitivity Low visual acuity Cone System Chromatic Low convergence Low light sensitivity High visual acuity Ayesha Abdullah 2017

Direction of visual impulse Direction of light Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Ayesha Abdullah 2017

Investigations for retinal structural and functional assessment Clinical assessment- Ophthalmoscopy Ayesha Abdullah 2017

Ophthalmic investigations Ultrasound –B & A scans Ocular coherence tomography (OCT) Angiography Electroretinogram Electro-oculogram Ayesha Abdullah 2017

Ayesha Abdullah 2017

OCT Ayesha Abdullah 2017

Angiography Ayesha Abdullah 2017

Electroretinogram Ayesha Abdullah 2017

Home work for next week What is the blood supply of the inner and outer retinal layers? What makes the inner and outer retinal blood barriers and what is their significance? Light has to travel through all the layers before generating the sensation of vision, why is that so? Ayesha Abdullah 2017