Cranial Nerve II - The Eye and Visual Fields

Cranial Nerve II - The Eye and Visual Fields

1. CN !!

   • fibres of optic nerve are axons of the cells in ganglionic later of retina
     • converge on optic disc
       • exit eye around 3-4mm to nasal side of its centre as OPTIC NERVE
         • OPTIC NERVE leaves orbit via OPTIC CANAL
           • Meets optic nerve on opposite side at OPTIC CHIASMA
             • CHIASM is at junction of ant. floor of 3rd ventricle
             • in chiasma fibres from
               • nasal 1/2 of retina decussate
                 • enter opposite optic tract
               • temporal fibres remain ipsilat
             • in chiasma optic tracts emerge
               • pass post. laterally around CEREBRAL PEDUNCLES
                 • Many terminate in LGB of THALAMUS
                   • optic tracts become optic RADIATIONS
                     • Pass through internal capsule
                       • terminate in visual cortex
                 • few go to PRETECTAL NUCLEI
                   • involved in pupillary light reflex

2. Eye

   • develops from interaction between forebrain and overlying ectoderm
     • optic placode is induced on each side of head
       • optic vesicles
         • placodal cells sink in towards optic vesicles
           • form an early lens
             • fits into
             • Hyaloid artery
               • supplies lens
             • space between lens and cornea
               • Aqueous chamber
         • indents to form
           • optic cup
             • two layers of optic cup forms
               • Retina
                 • outer layer forms pigmented layer of retina
                 • inner layer forms RODS AND CONES
                 • 1st site of processing visual info
                 • divided into temporal and visual portions
             • Neural crest mesenchyme around cup condesnses forms
               • layers of EYEBALL
                 • inner Choroid
                   • choroid and extensions of optic cup form
                     • Iris and Ciliary bodies
                       • Ciliary body is behind the iris
                         • contains ciliary muscle
                           • contraction alters shape and focusing power of lens
                             • ACCOMMODATION
                         • occupied by the lens
                       • Iris has a central aperture
                         • Pupil
                           • focuses light upon retina
                 • outer Sclera
         • stalk connecting vesicle to brain indents =
           • choroidal fissure
       • Optic vesicle grows outward and touches the SA of ectoderm which then differentiates =
       • optic vesicle + forebrain narrows = OPTIC STALK
   • central points of ant and post parts of eyeball are referred to as
     • Anterior and Posterior Poles
       • line joining poles is
         • optic axis
   • At birth the eye is 17.5 mm across
     • puberty - 20-21 mm
       • adult - 24 mm
         • stops growing too soon, remains at 20-21
           • Long site - HYPERMETROPIA
             • Focus of cornea and lens is behind retina
         • Continues to grow to 29
           • Short site - MYOPIA
             • Focus of cornea and lens is in front of retina
         • PRESBYOPIA
           • change in ability of the lens to focus after age of 45
             • as lens becomes less elastic with time and is not able to rebound to its rounded shape for close vision
     • COLOBMA
       • Congenital malformation of eye causing defects in lens, iris and retina
     • HETEROCHROMIA
       • difference in colouration, usually iris, result of excess/ lack of melanin
   • Light entering eye is REFRACTED
     • partially absorbed and passes through 1st and 2nd ON before reaching photoreceptors
       • series of photochemical reactions occur in RODS and CONES converting light energy into electrical impulses
         • Info is transferred from rods and cones to bipolar cells
           • to ganglion cells
         • Rods and Cones are photoreceptive cells of RETINA
           • 20x more rods than cones
           • RODS
             • highly sensitive to light
               • vision for low light levels
             • No. decrease towards macula
           • CONES
             • colour vision
               • confer high visual acuity
             • more abundant towards macula
             • at fovae - only cones
           • spread across MACULA
       • 1st ON (BIPOLAR CELLS) are within the RETINA
       • 2nd ON (GANGLION CELLS) form the OPTIC NERVE
   • Internal aspect of post part of eyeball =
     • FUNDUS
       • retina of fundus includes OPTIC DISC
         • Where sensory fibres and vessels conveyed by optic nerve to enter eyeball
         • OPTIC DISC
           • contains no photoreceptors
             • insensitive to light = BLIND SPOT
           • Lateral to disc is
             • MACULA
               • In the centre = small depression
                 • FOVEA
                   • Area of highest visual acuity
   • Eyeball has 3 layers
     • 1) FIBROUS LAYER (outer layer)
       • SCLERA and CORNEA
         • cornea - where light enters the eye
     • 2) VASCULAR LAYER (middle)
       • CHOROID, CILLIARY BODY and IRIS
     • 3) INNER LAYER
       • RETINA

3. Visual Fields

   • light passes into eye
     • retracted by cornea
       • compressed through pupil
         • refracted by lens
           • passes through vitreous humour
             • retina
           • by cornea and lens = imagine to be projected onto retina being reversed and inverted
   • Lateral visual fields project medially and vice versa
     • Lateral field = temporal
     • medial field = visual

4. Visual Pathway

   • 1st ON - bipolar cells of retina, synapse with
     • 2nd ON - retinal ganglion cells
       • pass to thalamus and synapse with
         • 3rd ON - in lateral geniculate nucleus of thalamus
           • project to primary visual cortex
       • axons converge at optic disc and form OPTIC NERVE
         • Enters cranial cavity through optic canal
         • 2 optic nerves converge at optic chiasm + axons from the NASAL HALVES
           • Decussate + pass into contralat OPTIC TRACT
             • continue as optic tract which pass around cerebral peduncle
               • terminate mainly in lat geniculate nucleus
                 • small no. leave LGN and terminate in pretectal area + sup. colliculus
                   • involved in pupillary light reflex
                   • go through retrolenticular part of internal capsule
                     • form optic radiations which terminate in primary visual cortex
           • TEMPORAL fibres remain ipsilat
   • Lesions
     • Total injury of 1 OPTIC NERVE
       • MONOCULAR BLINDNESS
     • MIDSAGITTAL INJURY e.g. pituitary tumour damages NASAL FIBRES from BOTH EYES
       • BITEMPORAL HEMIANOPIA
         • compression of optic chiasm e..g. tumour
     • TOTAL INJURY of any part of path BEHIND CHIASMA
       • Blindness in OPPOSITE HALF OF COMBINED FIELDS
         • E.g. disruption of optic radiation
           • HOMONYMOUS HEMIANOPIA
             • vascular and neoplastic lesions of optic tract, optic radiation or occipital cortex