Ocular Anatomy and Physiology

OPTOMETRY · SEMESTER 2 The Retina and Vitreous Ocular Anatomy and Physiology START READING NOTES Contents of This Topic The Retina and Vitreous ANATOMISTS CLINICIANS (HOW DOES THE EYE SEE?) The Retina and Vitreous CHAPTER 11. THE RETINA AND THE VITREOUS BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING TOPICS: 1. DEFINITION 2. GROSS ANATOMY 3. HISTOLOGY 4. THE RETINAL BLOOD SUPPLY 5. RETINAL BIOCHEMISTRY AND PHYSIOLOGY 6. COLOUR VISION 7. CLINICAL CONSIDERATIONS 8. THE VITREOUS. 1. DEFINITION. The BEEN is the inner neural layer or the nervous coat of the eye. It contains receptors that sense the light from the outer world and transmit it to the brain for higher processing. It is a thin transparent membrane with a purplish red colour in a living subject. Its thickness varies from 0.6mm near the optic disc to 0.1mm at its peripheral termination called the ora serrata. The outer surface is in contact with the choroid and the inner surface with the vitreous. The retina is firmly attached to the optic disc margin and at its peripheral a Externally, the peripheral termination of the retina corresponds with the site of insertion of the medial and lateral recti muscles. 2. GROSS ANATOMY. There are several prominent structures in the retina that can be identified with the help of an instrument called the ophthalmoscope. The procedure is called ophthalmoscopy or funduscopy , and the part of the eye visible on funduscpoy is called fundus. 68 2.1. The optic disc. This is most prominent structure of the retina and it is located nasally. It is a yellow-pink ,oval to round structure, also called papilla or optic nerve head . It represents the beginning of the optic nerve,( the second cranial nerve- the nerve that is responsible for vision ). The optic nerve head or the optic disc measures 1.75 mm vertically and 1.5mm horizontally in diameter. The centre of the disc has a circular depression that appears whiter than the rest of the disc. This is called the cup of the disc and is a location where the retinal arteries Fine vessels are seen to pass over the surface of the disc, which give it the pinkish appearance. The retina is firmly attached to the margins of the disc. There is no retinal tissue over the disc,thus it is insensitive to light and is referred to as the blind spot. 2.2. The area centralis. ‘Temporal to the disc is an area called the area centralis. This is the most posterior part of the globe and central part of the retina. Clinically, this area is called the posterior pole. It measures about 5-6 mm in diameter and is enclosed within the At the centre of the posterior pole is an area measuring about 1.5 mm called the macula lutea by the clinicians and fovea by the anatomists. It has a yellowish appearance when seen by an ophthalmoscope due a yellow pigment called the xanthophyll. The macula lutea or the fovea is located about 3mm temporal and 1mm inferior to the disc. The photoreceptor layer of the fovea contains only cones. The centre of the fovea is depressed and is called the fovea centralis by the clinicians and foveola by the anatomists. It measures about 0.35mm in diameter. 69 It is thus apparent that anatomists and clinicians differ in their terminologies as follows: ANATOMISTS CLINICIANS Area centralis Posterior pole Fovea Macula lutea (or just the macula) Foveola Fovea centralis ( or just the fovea) As clinicians, we will stick to the clinical terminologies henceforth to avoid confusion. The macula is responsible for central and colour vision. The sharpest central vision is achieved at the fovea 2.3. The peripheral retina. The remainder of the retina outside the posterior pole is termed peripheral retina, although further subdivision exists but that is beyond the scope of this work. The peripheral termination of the retina shows teeth like projections called the ora serrata. The retinal cells at the ora serrata continue over the ciliary body to form the non- pigmented epithelial layer of the pars plana. In the peripheral retina the predominant photoreceptors are the rods. The retina is divided into temporal and nasal halves by an imaginary line that runs vertically through the fovea. _ The centre of the optic disc is used to divide the retina into 4 quadrants: e Supero- nasal; ° supero-temporal; e infero-nasal; e infero-temporal. This division helps the clinician locate and document the position of lesions on the surface of the retina. With this division in mind one can visualise the retinal vessel's distribution on the surface of the retina. 70 The retinal artery emerges as a single vessel called the Central Retinal Artery , which then divides in to 2 branches the superior and inferior retinal arteries Each of these then further divides into 2 branches, one for the temporal and the other for the nasal quadrants of the retina. The same applies with the distribution of the Central Retinal Vein. Fig. 11.1. Anatomical landmarks of the left eye fundus. A- anatomical macula / clinical posterior pole; B- anatomical fovea / clinical macula; C- anatomical foveola / clinical fovea; D- optic disc; E- optic cup. 71 GD ait with wes offic 3. HISTOLOGY. The retina, the layer which develops from the inner and outer layers of the embryological optic cup, is divided into two major portions: e The Retinal Pigment Epithelial layer ( RPE); e The Neurosensory layer. 3.1. The Retinal Pigment Epithelium (RPE ). a. Anatomical features. The RPE is a GifigléllayerOnicellS deriving embryologically from the outer — layer of the optic cup. It is located between the choroid and the neurosensory retina, the apices of the cells pointing towards the vitreous while the base rests on its basement membrane towards the choroid. The RPE is a continuous monolayer of cuboidal / columnar cells which extend from the optic disc margin to the ora serrata. From ora serrata it continues over the ciliary body as the pigmented

OPTOMETRY · SEMESTER 2 Neuro-ophthalmic Anatomy and Physiology Ocular Anatomy and Physiology START READING NOTES Contents of This Topic Neuro-ophthalmic Anatomy and Physiology THE PHYSIOLOGY OF VISION Pituitary Chiasma Edinger Westphal Neuro-ophthalmic Anatomy and Physiology CHAPTER 12. NEURO-OPHTHALMIC_ ANATOMY AND PHYSIOLOGY BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE FOLLOWING ASPECTS OF THE TOPIC: THE ANATOMY OF VISUAL PATHWAYS THE PHYSIOLOGY OF VISION COMMON SYMPTOMS IN NEURO-OPHTHALMOLOGY OCULAR MOTOR NERVES ANATOMY AND PATHOLOGY ASPECTS PUPILLARY PATHWAYS AND REFLEXES ARWONS> 1. THE ANATOMY OF THE VISUAL PATHWAYS. The route taken by the light-generated impulses the eye is called the visual pathway. The visual pathway is effectively a tract within the central nervous system and it is made up of the: optic nerves; optic chiasm; optic tracts; lateral geniculate bodies; optic radiations; visual cortex. Disorders affecting different parts of the visual pathway produce characteristic changes in the field of vision. The nature of visual field disturbances , therefore, can help in determining which part of the visual pathway is affected. 84 Temporal retina Optic tract Lateral ‘eniculate ody Optic radiations Visual cortex Fig. 12.1. Diagramatic representation of the visual pathways. 1.1. The Optic Nerve ( Cranial_Nerve II ). The optic nerve is made up by the axons of the retinal ganglion cells . It may be divided anatomically, into four portions: intraocular (1mm); intraorbital (25mm); intracanalicular (9mm); intracranial (16mm). 85 a. The intraocular portion. This part includes the optic disc and the portion of the optic nerve that lies re The optic disc lies in the nasal retina, medial to the macula. Also called the optic nerve-head or papilla, it represents the confluence of approximately 1.2 million ganglion cell axons. It's pale pink in colour, slightly oval vertically . It has a central cup and a peripheral rim of neural tissue The central retinal vessels emerge at the centre of the optic disk, pass over the rim, and radiate out to supply the retina. The optic disk derives its blood supply via the short ciliary arteries from the ophthalmic artery, while the retina is fed by the central retinal artery. The nerve fibres of the intraocular portion are not myelinated. They traverse the sclera through the lamina cribrosa . Just posterior to the sclera, the fibres acquire a myelin coating. b. The intraorbital portion. The orbital portion of the nerve extends from the globe to the optic canal. It is 3-4mm thick, double the diameter of the intraocular portion as a result of the acquisition of myelin and meningeal sheath. Posteriorly , the meningeal sheath is continuous with the meningeal sheath around the brain. Thus, a rise in the intracranial pressure will be communicated to the subarachnoid space around the optic nerve, and may result in papilloedema. The length of the intraorbital portion of the optic nerve is longer than the anteroposterior dimensions of the orbit. The extra length of the intraorbital optic nerve allows unimpeded globe rotation as well as axial shifts within the orbit. c. The intracanalicular portion. 86 In addition to the optic nerve , the canal contains the ophthalmic artery and sympathetic fibres of the sympathetic carotid plexus. In the canal the nerve is firmly anchored (the dura and the periosteum are fused). As aresult, a small lesion in the optic canal can produce a compressive optic neuropathy even before it becomes easily visible upon neuroimagining. d. The intracranial portion. The optic nerve exits the posterior opening of the optic canal , enters the middle cranial fossa and continues posteriorly ascending to join the optic chiasma. Above the optic nerve lie the inferior surface of the frontal lobe, the olfactory tract , and the anterior cerebral and anterior communicating arteries. The lateral aspect of the optic nerve if often immediately adjacent to the internal carotid artery . Inferiorly and medially , the posterior ethmoid and sphenoid sinuses are adjacent to the nerve. The blood supply of the optic nerve is through the pial network of vessels. Arterial branches feeding the pial network originate from the ophthalmic artery . The venous drainage of the optic nerve is chiefly by the central retinal vein and to a lesser extent via the pial venous system. Both systems drain into the ophthalmic venous system ( superior and/or inferior ophthalmic veins ) in the orbit and less commonly directly into the cavernous sinus. 1.2. The Optic Chiasm. Fibres from the two optic nerves merge to form the optic chiasm. The chiasm is located anterior to the hypothalamus and above the sella turcica. 87 Fig. 12.2. Relation between the optic nerve and chiasm, the sellar structures and the third ventricle (III ). The chiasm overlies the sella turcica with the pituitary gland below, being The relatively large space between the chiasm and the pituitary explains the fact that only large tumours of the pituitary gland will compress on the chiasm and cause visual field defects. The chiasm forms the floor the third ventricle and carotid arteries bound it at either side. The circle of Willis surrounds the chiasm. 88 Anterior cerebral artery | Internal j carotid / artery I Posterior communicating Basilar artery artery Fig. 12.3. Relationship between the Optic Chiasm and the Circle of Willis. Fibres from each nasal retina cross in the optic chiasm , whereas fibres from each temporal retina do not cross. Slightly more than half of the fibres (65% ) decussate , because the nasal retina contains more ganglion cells than the temporal retina. Lesions of the chiasm characteristically involve crossing nasal fibres. As such , they cause bilateral temporal visual field defects, that respect the vertical line. 1.3. The Optic Tracts. The optic tracts begin at the posterior aspect of the optic chiasm , diverge laterally , and continue posteriorly to terminate primarily in the lateral geniculate bodies. A group of fibres leave the optic tract just before the lateral geniculate body for the pretectal area; the pretectal pathway controls light-mediated pupillary constriction. 89 Each optic tract contains crossed nasal fibres from the contralateral

OPTOMETRY COURSE Ocular Anatomy and Physiology — Optometry Notes Semester 2 · 12 topics. Source notes and Additional Study Notes are identified by their titles. Ocular Embryology The Orbital Cavity The Eyelids The Lacrimal Apparatus The Extraocular Muscles The Eye Globe The Cornea and Sclera The Anterior Chamber The Uveal Tract The Crystalline Lens The Retina and Vitreous Neuro-ophthalmic Anatomy and Physiology SEMESTER NOTESALL OPTOMETRY NOTES Need These Notes as PDF? Request a formatted copy for offline study, printing or revision. GET PDF NOTES ON WHATSAPP