The Eye and the Visual System PDF

The Eye and the Visual System Where opportunity creates success Learning objectives Describe the vision’s organs; anatomy and histology of the eye. Correlate between the histological and functional organisation of the retina. Compare the structure, distribution and function...

The Eye and the Visual System Where opportunity creates success Learning objectives Describe the vision’s organs; anatomy and histology of the eye. Correlate between the histological and functional organisation of the retina. Compare the structure, distribution and functions of photoreceptors. evaluate the anatomical organization of visual pathways and their functional significance. Analyse visual defects based on the anatomy of visual pathways. Contents Anatomy of the Eye Refractive Errors The Retina Optic Nerves, Chiasm, Tract & Radiation Visual Pathways Visual Cortex Visual Field and its Defects Anatomy of the Eye The eye is composed of 3 main layers that enclose 3 transparent structures. The outermost layer: the fibrous tunic– (cornea and sclera). The middle layer: vascular tunic (uvea) - the choroid, ciliary body, and iris. The inner layer: the retina. Within these layers are 3 transparent structures: 1) the aqueous humor, 2) the gelatinous vitreous humor (vitreous body), 3) the lens. Anatomy of the Eye Pupil→ opening where light enters the eye to reach the retina Iris→ gives color to eyes / contain muscle can vary the pupil size Cornea→ glassy transparent external surface of the eye Lens→ adjust the focuses to different distances Aqueous humor→ fluid which nourish the cornea Vitreous humor→ give to the eye the spherical shape Three layers in the eye – Sclera→ continuation of the cornea / white part – Choroid→ pigment epithelium – Retina→ contains neuron sensitive to the light Optic nerve→ bundle of axons from the retina Anatomy of the Eye The area of highest acuity in the eye is the macula lutea. The optic disk: no photoreceptors “blind spot”. The cornea does approximately 70% of the focusing. Accommodation: The lens fine-tunes the focusing of light entering the eye. Light from a distance → flat lens. Light from a source nearby → relaxed, more convex lens. This is controlled by the third cranial nerve (CN), the oculomotor nerve. Refractive Errors Emmetropia (perfect vision) → if parallel light rays from a distant point source are focused sharply on the back of the retina Hyperopia: – when the eyeball is too short – near objects are focused at a point behind the retina. – convex glass or plastic lens. Myopia: – when the eyeball is too long. – far objects are focused before the retina. – concave lenses.  The Retina Part of the CNS Conversion the light into neural activity. Retinal Surface Macula→ central vision / highest acuity. Fovea→ dark spot where the retina is thinner / light contacts directly with photoreceptors. Optic disk→ retinal vessels and exit for optic nerves. Laminar Organization of the Retina Layers organise the cells inside-out – Cells above photoreceptor are relatively transparent – Pigmented epithelium→ critical role maintenance the photoreceptors 1. Ganglion cells layer. 2. Inner plexiform layer→ ganglion cells synapse with bipolar and amacrine cells. 3. Inner nuclear layer→ cell bodies of bipolar, horizontal and amacrine cells. 4. Outer plexiform layer→ photoreceptors make synapsis with horizontal and bipolar cells. 5. Outer nuclear layer→ cell bodies photoreceptors 6. Photoreceptor outer segments layer→ light-sensitive elements. Laminar Organization of the Retina Functional Organization of the Retina 1. The only light-sensitive cells in the retina are the rod and cone photoreceptors. All other cells are influenced by light only via direct and indirect synaptic interactions with the photoreceptors. 2. The ganglion cells are the only source of output from the retina. No other retinal cell type projects an axon through the optic nerve. 3. With the exception of certain amacrine cells, ganglion cells are the only retinal neurons that fire action potentials. All other retinal cells depolarize or hyperpolarize, with a rate of neurotransmitter release that is proportional to the membrane potential, but they do not fire action potentials. Anatomy of Rods and Cones o Rods function in dim light, responding to