Visual System

• Orbit


  • Orbital entrance averages ~35mm in height, 45mm in width and 40-45mm in depth
  • 7 bones: Frontal, Zygomatic, Maxillary, Ethmoidal, Sphenoid, Lacrimal, Palatine
• Contents of the Orbit


  • Eyeball
  • Extraocular muscles
  • Nerves: Optic, Oculomotor, Trochlear, Abducens, 3 branches of ophthalmic nerve
  • Blood vessels: Ophthalmic artery and branches, Superior and inferior ophthalmic veins
  • Lacrimal glands
  • Fat

  
• Internal carotid artery


  Supplies blood to the eye

  
• Extraocular muscles


  • Superior rectus: Elevation, also contributes to adduction and medial rotation
  • Inferior rectus: Depression, also contributes to adduction and lateral rotation
  • Medial rectus: Adduction
  • Lateral rectus: Abduction
  • Superior oblique: Depression, abduction and medial rotation
  • Inferior oblique: Elevation, abduction and lateral rotation
  • Levator palpebrae superioris: elevates upper eyelid
• Anterior segment


  Outer 1/6 of eye, includes cornea, iris, ciliary body and lens

  
• Posterior segment


  Inner 5/6 of eye, includes vitreous humor, retina, choroid and optic nerve

  
• 3 Layers (Tunics) of the Eye


  • Fibrous tunic: Sclera and cornea
  • Vascular tunic (uvea): Choroid, iris, ciliary body
  • Nervous tunic: Retina
• Components of the Eye


  • Sclera: Maintains shape, provides protection, attachment point for extraocular muscles
  • Cornea: Clear, transparent layer that helps focus light
  • Iris: Regulates amount of light entering eye
  • Pupil: Opening in iris that light enters through
  • Lens: Focuses light rays onto retina
  • Ciliary body: Accommodation, aqueous humour production and resorption

  
• Steps involved in Vision


  1. Light waves pass through cornea

  2. Passes through pupil

  3. Passes through lens

  4. Projected onto retina
• Optic disc


  Where nerve fibres exit retina and retinal blood vessels originate, no photoreceptors = blind spot

  
• Macula


  Region in middle of retina critical for central vision, yellowish hue and no large blood vessels
• Fovea


  Dark spot in centre of macula where retina is thinnest, highest density of cones
• Retina


  • Light-sensitive back portion of eye
  • Contains photoreceptors: Rods (high sensitivity, low acuity, dark vision) and Cones (low sensitivity, high acuity, colour vision)

  
• Vertical Retinal Circuitry


  Photoreceptors transduce light, bipolar cells, ganglion cells whose axons form optic nerve
• Horizontal Retinal Circuitry


  Horizontal cells sit between photoreceptor and bipolar layers, amacrine cells sit between bipolar and ganglion layers
• Focusing Images on Retina: Corneal Refraction


  Light slows down and bends as it passes from air to cornea, which is mostly water
• Focusing Images on Retina: Pupillary Light Reflex


  1. Regulates amount of light reaching retina

  2. Direct reflex: Pupil ipsilateral to light constricts

  3. Consensual reflex: Pupil contralateral to light constricts

  
• Focusing Images on Retina: Accommodation


  1. Cornea accounts for 80% of focusing, lens accounts for 20%

  2. Ciliary muscles tighten, causing lens to thicken, improving focus for close objects

  
• Image on the retina is inverted and reversed
• Transduction of Light


  Photoreceptors have an outer segment containing visual pigment molecules composed of opsin and retinal
• Accommodation


  • Similar to light reflex & bilateral, but information goes to visual cortex first. i.e. need to be conscious
  • Important for close vision
  • Under control of CN III
  • Ciliary muscle and sphincter pupillae contract (parasympathetic)
  • Lens becomes more convex (i.e. less flat)
  • Improves the focus for close objects
  • Medial recti also contracts, leading to convergence of the eyes on the object

  
• Left visual field


  Imaged on right side of retina

  
• Right visual field


  Imaged on left side of retina

  
• Upper visual field


  Imaged on the lower retina

  
• Lower visual field


  Imaged on the upper retina

  
• Photoreceptors


  • Have an outer segment which contains visual pigment molecules composed of opsin (a large protein) and retinal (an organic molecule derived from vitamin A)
• Transduction of Light


  1. Retinal absorbs one photon of light, changing shape (isomerization)

  2. This shape change increases binding of a regulatory protein called transducin

  3. Binding of transducin sets off a cascade that leads to activation of the enzyme phosphodiesterase (PDE)

  4. PDE activation causes Na+ channels to close, hyperpolarizing (inhibiting) the photoreceptor

  5. Normally, neurotransmitter release from the photoreceptor hyperpolarizes (inhibits) bipolar cells, so inhibition of photoreceptors removes this inhibition of bipolar cells, allowing them to stimulate ganglion cells to fire action potentials
• Optic nerve


  • Formed from axons of ganglion cells
  • Exits retina at optic disc (head of nerve)
  • Travels through the orbit
  • Exits via optic foramen in sphenoid bone
  • Joins with opposite optic nerve at the optic chiasm
• Damage to optic nerve

  • Loss of myelin / optic neuritis leads to blurred vision
  • Damage to nerve itself results in ipsilateral blindness
• Optic chiasm


  • Partial crossing of optic nerve fibres
  • Medial (nasal) retinal fibres, responsible for the lateral (temporal) portion of the visual field, cross
  • Lateral (temporal) retinal fibres, responsible for the medial (nasal) portion of the visual field, do not cross
• Damage to optic chiasm

  Bitemporal hemianopia
• Optic tract


  • Extends from the optic chiasm and winds round the cerebral peduncle
  • 3 sites of termination: Lateral geniculate nucleus (LGN) of thalamus, Superior colliculus, Pretectal area
  • Carries information from the contralateral visual field

  
• Damage to optic tract

  Homonymous hemianopia
• Extrageniculate Pathways


  • Terminations of optic tract into pretectal area and superior colliculus
  • Important for directing visual attention and eye movements toward visual stimuli
  • Pretectal area projections: important for pupillary light reflex and accommodation reflex
  • Superior colliculus projections: important for orienting the eyes in response to new stimuli

  
• LGN


  • 6 layers (1-6)
  • Layers 1 and 2 (innermost layers) are magnocellular layers, while layers 3-6 (outermost layers) are parvocellular layers
  • Information from each eye is segregated in the LGN, with axons from ipsilateral and contralateral eye synapsing on different layers

  
• Optic radiations


  • As axons leave the LGN, they form optic radiations
  • Fibres of the inferior optic radiations detour around inferior horn of the lateral ventricle and arc forward into temporal lobe, forming Meyer's loop
  • Carry info from inferior retina (superior visual field), terminating on lower bank of the calcarine fissure (the lingula)
  • Fibres of the superior optic radiations pass under parietal lobe, carrying info from superior retina (inferior visual field), terminating on upper bank of the calcarine fissure (the cuneus)

  
• Damage to optic radiations

  • Homonymous hemianopia
  • Damage to inferior optic radiations (Meyer's loop): Superior quadrantanopia
  • Damage to superior optic radiations: Inferior quadrantanopia
• Visual cortex

  • Located at poles and medial aspects of the occipital lobes
  • Primary visual cortex is above and below the calcarine sulcus
  • Lower bank (lingula): info from inferior retina (superior visual field)
  • Upper bank (cuneus) of the calcarine fissure: info from superior retina (inferior visual field)

  
• Damage to visual cortex

  • Damage to entire V1: Homonymous hemianopia (with macular sparing)
  • Damage to lingula: Superior quadrantanopia
  • Damage to cuneus: Inferior quadrantanopia
• Retinotopic Organisation


  • Different regions of the retina are represented in different portions of the primary visual cortex
  • Fovea is represented near the occipital pole, while more peripheral regions of ipsilateral retina and contralateral visual fields are represented more anteriorly
  • 50% of the primary visual cortex responds to information from the fovea, due to its high acuity