D21 - Refractive media of the eye, optical accomodation and the visual pathways

Question 1

Define refractive media of the eye

Answer 1

• The structures of the eye that deflect light:
  
  • Cornea
  • Aqueous humor
    
    • ​In camera bulbi anterior and posterior
  • Lens
  • Corpus viterum
• Light → cornea (transparent, bends light on retina) → camera bulbi anterior/posterior (filled with aqueous humor) → lens → corpus vitreum → retina

Question 2

Refractive media of the eye

Draw

Answer 2

Question 3

Refractive media of the eye

Cornea

Answer 3

• The transparent front part of the eye
• Where light enters the eye
• Covers:
  
  • Iris
  • Pupil
  • Anterior chamber

Question 4

Refractive media of the eye

Aqueous humor

Answer 4

• Present in:
  
  • Camera bulbi anterior
  • Camera bulbi posterior
• The camera bulbi anterior/posterior fills the space between cornea and lens
• Role:
  
  • Refractive proeperties
  • Maintainance of intraocular pressure
• Enters the system in the camera bulbi posterior → pupil → camera bulbi anterior

Question 5

Refractive media of the eye

Lens

Answer 5

• Transparent
• Biconvex:
  
  • Anterior pole
  • Posterior pole
  • Equator
  • Ventral axis (= optic axis)
• Role: helps to refract light to be focused on the retina
  
  • Accommodation: By changing chape, functions to change the focal distance of the eye so that it can focus on objects at various distances

Suspension of the lens

• Ciliary muscles
• Iris: regulates diameter of the pupil, separates the anterior chamber from the posterior chamber
  
  • Pupil
    
    • Mm. sphinter
    • M. dialator

Question 6

Refractive media of the eye

Corpus vitreum

Answer 6

• A transparent jellylike substance filling the interior of the eyeball behind the lens
• Largest
• Membrana vitera: dense surface of cells at the surface

Question 7

Give the division of the interior of the eyeball

General knowledge, not directly linked to the topic

Answer 7

• The interior of the eyeball is divided into 3 chambers (camera bulbi):
  
  • Camera anteror (anterior chamber)
    
    • Between cornea and iris
    • Filled with aqueous humor
  • Camera posterior (posterior chamber)
    
    • ​Between iris, ciliary body and lens
    • Filled with aqueous humo
  • Camera viteria (viterous body)
    
    • Largest of the 3 chambers
    • Behind the lens
    • Surrounded by retina

Question 8

Optical accomodation

Answer 8

• The ability of the eye to change its focus from distant to near opjects
• Achived by the lens changing its shape
• Infinite: tonus of m. ciliaris
  
  • Contraction of m. ciliaris → thicker lens → shorter focal length
• Nearest point of focus: relaxation of eye
• Central segments:
  
  • Optic radiation
  • Rostral colliculi
  • Geniculooccipital tract
  • Optic area of cerebral cortex
• Peripheral segments:
  
  • Retinal neurons
  • N. opticus
  • Optic chiasm
  • Optic tract
  • Thalamus
  • Lateral geniculate bodies
• Retinae

Question 9

Visual pathway

Answer 9

• After decussation in the optic chiasm the axon of the retinal ganglion cells project to the lateral geniculate body, from where the next fibers reach the primary visual cortex
• The primary visual area (Brodmann’s area 17) projects to further brain regions
• Retina:
  
  • The temporal visual field and nasal visual field project to the opposite sides of the retina
  • The dorsal and ventral parts of visual field are reversed in the same way
  • The same mapping occurs in the visual cortex and retina
• Optic nerve:
  
  • ​Fibers of n. opticus originating from the same quartes of retina run together
• Optic tract:
  
  • ​Formed by medial and lateral bundle
    
    • Lateral branch:
      
      • Towards lateral geniculate body
      • Perception of visual information
    • Medial branch:
      
      • Towards pretectum, superior colliculus
      • Visual reflexes (pupil reflex, accommodation, convergence)
• Pretectum:
  
  • ​Midbrain structure
  • Involved in mediating behavioral responses to acute changes in ambient light such as:
    
    • Pupillary light reflex
    • Optokinetic reflex
    • Temporal changest to the circadian rhythm
• Superiro colliculus:
  
  • ​Innervated by tracts from the visual cortex
  • Integrator of accadical eye movement
  • Main function: localization of an object
• Lateral Geniculate Nucleus (LGN)
  
  • Part of thalamus
  • Important relay station
  • Function: filter which, depending on the non-retinal afferents, does not let certain parts of the retinal stimuli pass to the higher cortical areas
  • Fibers of the LGN radiate to the primary visual cortex (Brodman’s area 17)
    
    • Location: medial and caudal regions of the occipital lobe
• Optic radiation:
  
  • ​In the medial part of the occipital lobe, the information is transmitted to the visual cortex via radiation