Eye

• Fibrous tunic:

  cornea

  sclera
• Cornea:

  • transparent
  • helps focus light
  • need oxygen from atmosphere (due to lack of blood vessels)
  • crystalline like structure
  • constructive interference
• Sclera

  • collagen fibres and fibroblasts
  • rigid
  • protection
  • destructive interference
• Vascular tunic

  choroid

  ciliary body

  iris
• Choroid

  Highly vascular so carries nutrients and oxygen
• Ciliary body is a structure that contains ciliary muscles and suspensory ligaments

  • makes aqueous humor
• Iris regulates light entry
• Iris

  • controlled by the autonomic nervous system
  • controlled by parasynthetic nervous system
  • controlls contraction of circular and radial muscles
• Describe an eye in bright light

  • parasympathetic nervous triggered
  • circular muscles contract
  • Radial muscle relaxes
  • pupil narrows
• Describe an eye in dim light

  • Sympathetic nervous system
  • radial muscle contracts
  • circular muscle relaxes
  • pupil dilates
• Cavities of the eye:

  • anterior cavities
  • vitreous chamber
• Anterior cavity:

  • both anterior and posterior chambers are in front of lens
  • filled with aqueous humor
• Aqueous humor:

  • made in ciliary process
  • nourishment for cornea and lens
  • pressurised (interocular pressure)
  • replaced every 90 mins
• Vitreous chamber:

  • between lens and retina
  • contains vitreous body/humor
• Vitreous humour

  • jelly like
  • contains water, collagen and  hyaluronic acid
  • not replaced
• Production of aqueous humour:

  produced in the posterior chamber

  Scleral venous sinus (schlemm canal) is the exist canal for aqueous humour
• Standard eye pressure

  60mmgH

  increased pressure can lead to glaucoma
• Lens:

  • Contains crystallins (protein)
  • Cells arranged like layers of an onion
  • Transparent
  • No blood vessels
  • Shape alters via suspensory ligaments (zonular fibres), attached to ciliary process. Round for focusing on close objects and becomes flatter for focusing on distant objects
  • Light refracted – image focussed on fovea
• Accommodation for distant objects:

  • ciliary muscle relax
  • suspensory ligaments pulled taut
  • lens flatterns
  • zx ray are nearly parallel
• Accommodation in close objects:

  • ciliary muscles contract
  • suspensory ligaments tension released
  • lens more curved
  • greater convergence of light
• Accommodation of light

  • greatest at 7-8 years
  • losing ability to accommodate is called presbyopia
• Retina:

  Ganglion cells transmit signals from bipolar cells to the brain

  Horizontal and amacrine cells help integrate visual information before it is sent to the brain

  Rods 6 to 600:1 with single bipolar cell = CONVERGENCE
• fovea: a small depression in the retina of the eye where visual acuity is highest. The centre of the field of vision is focused in this region, where retinal cones are particularly concentrated.
• Rods:

  • responsible for black and white vision
  • 525nm
  • no rods in the fovea
  • more rods in periphery of retina

  A) melanin granules

  B) discs

  C) mitochondrion

  D) golgi complex

  E) nucleus

  F) synaptic terminal

  G) synaptic vesicles

  
• How do rod cells work?

  Rhodopsin, contain opsin and retinal (derivated of vitamin A)

  Retinal converts between cis and trans
• Retinal conversion in light

  • transforms between cis and trans aided by the enzyme retinal isomerase
  • When in the trans form opsin can not bind causing bleaching of rhodospin
  • trans-retinal activates rhodopsin which activates transducin to activate phosphodiesterase that hydrolyses cGMP to GMP. Causing Na+ channels to close leading to rod hyperpolarisation. So no glutamate produced
  • bipolar cell is depolarised and action potential is passes to ganglion cell to the optic nerve to the brain
• Cone cells:

  Responsible for colour vision

  three different kinds of opsin

  • blue 455nm
  • green 530nm
  • red 625nm
• The eye

  A) sclera

  B) choroid

  C) retina

  D) fovea

  E) optic nerve

  F) optic disc

  G) central artery and vein of the retina

  H) vitreous humor

  I) lens

  J) aqueous humor

  K) pupil

  L) iris

  M) cornea

  N) suspensory ligament

  
• Retina

  A) optic nerve fibers

  B) ganglion cell

  C) amacrine cell

  D) bipolar cell

  E) horizontal cell

  F) pigmented epithelium