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THE VISUAL SYSTEM This is the highly specialized system responsible for the reception and processing of visual stimuli and initiation of appropriate responses to visual sensation. It is made up of three major components viz. 1. The eye and associated structures, which accommodate the peripheral visual receptor organs. These are located in the bony compartment called Orbit (Eye socket). 2. The visual neural pathway for transmission of visual information. 3. The visual cortex for processing of visual information. THE ORBIT (EYE SOCKET) • The orbits are two pyramidal-shaped cavities in the anterior aspect of the skull. Each orbit is composed of: • An apex, which is directed posteriorly and opens into the cranial cavity. • A base (the orbital margin) which is directed anteriorly and exposes the eyeball. • Four walls - superior, lateral, inferior and medial. • The orbit serves as a bony protective cage for the eyeball and its associated structures. • The orbital margin is formed by the frontal, zygomatic and maxillary bones while the four walls of the orbit are formed by the frontal, ethmoidal, maxillary, zygomatic, palatine, lacrimal and the sphenoidal bones. CONTENTS OF THE ORBIT: • A. B. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. The contents of the orbit could be organized into the following groups: The eyeball, its coverings, tunics and contents The Extraocular Muscles, which include; Superior Rectus muscle Lateral rectus muscle Inferior rectus muscle Medial rectus muscle Superior oblique muscle Inferior oblique muscle Levator palpebrae superioris Superior tarsal muscle Inferior tarsal muscle Orbitalis muscle (The orbital muscle of Muller) C. 1. 2. 3. 4. 5. 6. Nerves: These include: Ophthalmic nerve Optic nerve Oculomotor nerve Trochlear nerve Abducent nerve Ciliary ganglion D. Vessels: • Ophthalmic artery and its branches • Ophthalmic vein and its tributaries • Infraorbital vessels E. Other structures: • Lacrimal gland • Lacrimal sac • Nasolacrimal duct • Extraocular fat • • • • • • • • THE EYE (EYEBALL) The eye is the highly developed peripheral photoreceptive organ of the visual system It is enclosed in a protective bony cavity of the skull referred to as the orbit. It occupies the anterior half of this cavity. It is invested by a thin fascial sheath called Tenon Capsule. Deep to the capsule, the eye is further enclosed in three concentric coats, which consist of: The outer schlerocorneal layer, The musculovascular layer and The retinal layer. This layer accommodates the photoreceptor cells (Rods & Cones) of the eyeball Histological layers of the Retina: (See Diagram) • Pigmented epithelium • Outer and inner segments of photoreceptor cells • External limiting membrane • Outer nuclear layer of the nuclei of Photoreceptor cells (Rods and Cones) • Outer plexiform layer (A synaptic layer) • Inner nuclear layer of bipolar neurons and other retinal interneurons • Inner plexiform layer (A synaptic layer) • Ganglionic layer which is made up of the cell bodies of the ganglionic cells • Nerve fiber layer, consisting of the axons of ganglion cells • Internal limiting membrane derived from the vetreous membrane (Basal lamina) and end feet of Muller cells and astrocytes. • • • • Photoreceptor cells of the Neural Retina These are the highly differentiated, specialized neurons of the visual system, referred to as Rods and Cone Rods These are used for peripheral vision and for vision in the dark (Deem light). A retinal rod is cylindrical in shape, measuring about 50 microns with a diameter of 2 microns. Each retina contains about 130 million (20 times the number of Cones). Cones • Cones are used for color vision and visual acuity. • A retinal cone is cone-shaped, measuring about 40 microns with a diameter of 3-5 microns. • Each retina contains about 7million cones. • Cones share the same morphological subdivisions with rods • Cones are of three types, based on the color of light absorbed by their pigments. • The 3 colors are Red, Blue and Green. • • • Other Retina Cells include: Bipolar Retinal Neurons Ganglion Neurons Retinal Interneurons which include: 1. Amacrine cells (inner part) 2. Horizontal cells (Outer part) 3. Interplexiform cells (Inner part) • The Glial cells which include: 1. Muller cells 2. Astrocytes and 3. microglial cells Retinal Blood supply and Venous Drainage: • The retina is nourished from two sources: • The central artery of the retina, supplies the neural layers up to the outer border of the inner nuclear layer of the retina. • A companion vein of the same name drains blood from this region. • Capillary plexus of the choroid part of the middle tunic supplies the posterior part of the retina up to the outer border of the inner nuclear layer. • This region is devoid of capillary network. • THE VISUAL PATHWAY • Nerve impulse generated from phototransduction of light energy in the retina is conducted to the visual centres of the brain along the visual pathway. • The first order neurons of the visual pathway are the ganglionic cells of the retina. • About a million axons of ganglionic cells converge on the optic disc of each retina to form the Optic nerve. • • Each optic nerve, which consists of myelinated fibers traverses the optic canal at the apex of the orbit and passes posteromedially to meet its fellow from the opposite side just beneath the hypothalamus. • At this point, fibers from the nasal halves of the retina cross the midline at the Optic chiasma. • Thereafter, nasal fibers join uncrossed temporal fibers of the opposite side to form the Optic tract • • • Each optic tract passes posteriorly curving around the rostral part of the midbrain and terminates at the Lateral geniculate body Second order neurons of the visual pathway are located in the lateral geniculate body Lateral geniculate neurons give rise to axons, which form the Geniculocalcarine tract. This tract projects to the Primary visual area (Brodmann’s area 17) on either side of the calcarine sulcus on the medial surface of the occipital lobe. • • • • • Consequences of Lesions along the Visual pathway (See Diagram): Blindness in one eye. Bitemporal Hemianopsia. Nasal hemianopsia of one eye. Left homonymous hemianopsia. Left homonymous hemianopsia with macular sparing.