The Eye

Summary

• Parts of the eye
  • Layers of the eye
  • Chambers of the eye
  • The retina
• The visual pathway

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Parts of the eye

• There are 3 layers of the wall of the eye that extend around the eye:
  1. Sclera – the tough, fibrous layer
  2. Uveal tract  – the vascular layer
  3. Retina –  the sensory layer

• • There are also numerous cavities and structures within the eye. These can be split into three chambers:
    1. Anterior chamber 
       • Aqueous humour
    2. Posterior chamber
       • Lens
    3. Vitreous chamber
       • Vitreous humour

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The Sclera

• = the whites of the eyes

• This a continuation of dura mater around CNII
  • Tough, fibrous collagen outer layer of the eye
  • Becomes transparent at the cornea which covers the iris/puil (therefore the two are continuous)

• Functions
  • Maintains eye shape
  • Maintains intraocular pressure
  • Protective vs infection and trauma

• The Cornea at the very front of the eye covering the lens/pupil/iris is essentially the window to the eye
  1. Protective function
  2. Used in refraction of light
     • Lens then focuses this onto retina

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Uvea

• • This is the pigmented and vascular layer of the eye
    • Gives the colour of the iris
  • 3 parts (from posterior to anterior):
    • Choroid
    • Ciliary body which is made up of
      1. Ciliary processes – secrete aqueous humour
      2. Ciliary muscles – control lens shape
    • Iris

• The choroid makes up the bulk of the uveal tract
  • Mostly located posteriorly
  • Connective tissue and vascular layer that provides nutrition to the eye

• Ciliary body anteriorly has 2 functions:
  1. Ciliary processes which release aqueous humour
     • Also give rise to the suspensory ligament which holds lens in position
  2. Ciliary muscles which adjust the shape of the lens
     • This is required in the accommodation reflex
     • Controlled by CNIII

• The Iris is the anteriormost part with a central opening called the pupil
  • Control of pupil size controls light focussing by the lens
  • The pupil is controlled by sphincter (parasympathetic) and dilator pupillae (sympathetic) muscles

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Retina

• This is the sensory interface of the eye
  • Transduces light into nervous impulses through the use of photoreceptors

• Structure
  • The retina is transparent
    • Made up of 10 layers within it

• • 2 parts separated by the ora serrata
    1. Optic part which has 2 layers
       • Outer Pigmented layer in contact with choroid (contains rods/cones)
       • Inner neural layer – deeper
    2. Non-visual part lining the iris and ciliary body

• Macula lutea lies at the middle of the posterior part of the retina
  • This is the central vision area of the retina that has the highest visual acuity
    •  Contains relatively ↑concentration of cones
    • Appears slightly darker
  • 100% concentration of cones at the fovea centralis  in the middle of the macula
    • Most acute and colourful vision focused at fovea

• Optic disc – Site where CNII passes through retina
  • The CNII forms the retinal neural layer
  • There is a blind spot here due to the absence of photoreceptors
  • When considering pathophysiology, consider colour, cup and contour
    • Colour
      • Pallor occurs in optic atrophy
    • Contour
      • Abnormal shapes in refractive errors.
      • Blurred margins in papilloedema
    • Cup
      • Widens and deepens in glaucoma

• The position of retina is maintained by aqueous humour

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Chambers of the eye

• There are 3 chambers to consider
  1. Anterior chamber
     • Directly posterior to cornea and anterior to iris
  2. Posterior chamber
     • Posterior to iris and anterior to lens
  3. Vitreous chamber
     • Posterior to the lens

• The anterior and posterior chambers are both are filled with aqueous humour (i.e. are aqueous compartments)
  • Secreted in posterior chamber by the ciliary body epithelium
  • Reabsorbed via 2 routes:
    • 85% drain at the iridocorneal angle in the anterior chamber via the trabecular meshwork into the scleral venous sinus
    • 15% reabsorbed by the ciliary body
  • Functions of aqueous humour
    1. Nourishment to eye
    2. Maintenance of intraocular pressure between 10-21mmHg
  • Clinical significance of aqueous humour:
    • Glaucoma occurs due to failed reabsorption
      • Test using tonometry, ophthlamascopy, gonioscopy

• The vitreous chamber is the largest of the three chambers
  • Contains vitreous humour which is irreplaceable
  • Vitreous humour is firmer and more viscious
  • Clinical significance:
    • Protects the ocular structures

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Visual pathways

Binocular and monocular vision

• Monocular vision – i.e. the image seen by eac hindividual eye
  • Lateral vision is better than nasal (as nose gets in way)

• Binocular vision
  • Visual fields combine to form binocular field of view

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The visual pathway

 

1. An image is received at the retina
2. This image is transmitted from retina to brain along optic nerve
   • Optic nerves meet at the optic chiasm (located at the base of the hypothalamus)
3. The nasal aspects of the optic nerves cross at the optic chiasm to become the optic tracts
   • Only the nasal hemiretinas cross over to the opposite side
     • NB the temporal hemiretina looks at the nasal hemifield (and vice versa)
4. After crossing at the optic chiasm, nerve fibres travel within optic tracts as they were divided
   • Information from right visual field (left hemiretinas) travels in left optic tract
   • Information from left visual field (right hemiretinas)travels in right optic tract
   • Basically remember as each paired hemiretina will go to the ipsilateral brain
     • Therefore, corresponding hemifields of view are sent to opposite parts of the brain 
     • I.e. right hemifields (from left hemiretinas in both eyes) are sent to the left brain and vice versa
       • Right hemifield contains the left hemiretinas fibres of both eyes which both go to left side of brain
5. Each optic tract terminates in the lateral geniculate nucleus of thalamus
6. Optic radiations carry information from the LGN to the primary visual cortex
   • This is primarily in the occipital lobe 
     • Rest of occipital lobe constitutes visual association area

Retinotopic organisation within the visual cortex

• The retina is represented in small discrete regions within visual cortex

• Macula is disproportionately represented due to it’s ↑↑importance
• Visual field is represented as quadrants
  • Laterally and vertically inverted
    • E.g. upper left represented in lower right