Vision

Question 1

Retina and photostimulation

Answer 1

1. Light photochemically stimulates rods and cones in back of retina
2. Bipolar cells relay neurochemical signals between rods/cones and ganglion cells
3. Ganglion cells activated by bipolar cells send signals along axons that become optic nerve to the brain (occipital lobe)

Question 2

Fovea centralis

Answer 2

• depressed area at the back of the retina with high density cones
• functions as primary area of focused color vision

Question 3

Macula lutea

Answer 3

includes fovea

sensitive to visual cues

Question 4

Optic disc

Answer 4

• axons from ganglion cell layer and vascular structures
• no photoreceptors here
• “blind spot”

Question 5

Photoreceptors

Answer 5

• rods and cones
• back of retina
• sensitive to wavelengths of light
• rods
  
  • detect movement and useful for vision in low light
  • ~120 million
  • photopigment: rhodopsins
• cones
  
  • more active in light
  • detect color wavelength
  • ~6 million
  • photopigment: photopsin
  • less sensitive than rods, but faster action
• rods and cones synapse on bipolar cell layer

Question 6

Optic nerve

Answer 6

• cranial nerve II (part of CNS), ~1.5mm diameter
• derived from optic stalks during development
• >1 million ganglion cell axons connect the retina to the brain
• converge at optic disc
  
  • blind spot (no photoreceptors)
• ~2 inches from eye to the optic chiasm

Question 7

Optic chiasm

Answer 7

• Gives rise to optic tracts
• Each optic tract carries visual information from the opposite visual field
• At risk from compression from tumores within the sella turcica
  
  • pituitary, meningioma…
• Injury to chiasm causes bitemporal hemianopsia
  
  • vision loss in right visual fiels in right eye and left visual field in left eye

Question 8

Optic tracts

Answer 8

• axons from ganglion cells reorganize at optic chiasm
• axons from left visual field–>right optic tract
• axons from right visual field–>left optic tract
  
  • ex: nasal field, left eye and temporal field right eye, both enter right optic tract
• optic tract axons synapse in visual area of the thalamus (lateral geniculate nucleus, LGN)

Question 9

Lateral geniculate nucleus

Answer 9

• thalamic nuclei (one on left and right sides)
• 6 layers
  
  • first 2: magnocellular pathway
  • 3-6: parvocellular pathway
  • receive input semi-alternating from ipsilateral and contralateral tracts
  • project to primary visual cortex (Brodmann Area 17)

Question 10

Optic radiations

Answer 10

project to primary visual cortex from LGN (geniculo-calcarine tract)

often affected most by stroke but can be damages by physical trauma, tumors or other lesions

Question 11

Primary visual cortex

Answer 11

• occipital lobe cortex (V1, Brodmann area 17)
• 6 layers
• highest visual acuity projected from retinal macula to the most causal portion of the occipital cortex
• interpretation of visual info. occurs in other cortical regions

Question 12

• Visual streams
  
  • Dorsal “Where?” pathway
  • Visual “What?” pathway

Answer 12

• Dorsal “Where?” pathway
  
  • motion and spacial analysis
  • to posterior parietal cortex
• Visual “What?” pathway
  
  • form and color analysis
  • to lateral temporal cortex

Question 13

Phototransduction

Answer 13

• light energy is converted to electrical signals by rods and cones
• photoreceptors have inner and outer segments
• the outer segment: membranous discs
  
  • membranes contain molecules that absorb light
• the inner segment: contains the cell nucleus, organelles and the synaptic terminal

Question 14

Phototransduction

Answer 14

• rods: photopigment=rhodopsin
  
  • associated with a G-protein called transducin and the enzyme phosphodiesterase which degrades cyclic GMP (cGMP)
    
    • cGMP levels are elevated in the dark

Question 15

Visual processing and relay

Answer 15

• light enters the pupil, strikes the retina
  
  • a visual event is triggered in photoreceptors and relayes through bipolar and ganglion cell layers
• each eye’s optic nerve carries only info. from one eye
  
  • right field visual events are detected on the left retinal side in each eye, and vice versa
• optic chiasm is where visual info. from visual field info. crosses
• optic tracts carry info. from the opposite visual field

Question 16

Visual pathway diagram

Answer 16

Question 17

Visual defects

Answer 17

• total loss of vision in one eye
  
  • optic nerve lesion
• loss of vision in temporal (lateral) fields in both eyes
  
  • bitemporal heteronymous hemianopsia
  • optic chiasm lesion
• loss of vision in nasal field in one eye and temporal field in another
  
  • homonymous hemianopsia
  • optic tract lesion on side of nasal field loss
• loss of only upper quadrant of visual field
  
  • quadrantanopsia
  • optic radiation lesion

Question 18

Glaucoma

Answer 18

• occurs due to increased aqueous humor fluid pressure in the anterior region of the eye
  
  • caused by improper drainage of fluid from drainage canal
• this often leads to increased pressure in the posterior portion of the eye, causing damage to vessels in the retina and injury to the axons in the optic nerve
• when intraocular pressure leads to glaucoma, the damage to optic nerve axons from the retinal ganglion cell layer causes these cells to eventually die

Question 19

Macular degeneration

Answer 19

• progressive eye conditions that involve deterioration of the macula, the central region of the retina and important for central focusing
• some forms of macular degeneration
  
  • age-related macular degeneration
  • juvenile macular degeneration or macular dystrophy
  • Stargardt’s disease: form of juvenile macular degeneration

Question 20

Age-related macular degeneration

Answer 20

• “Dry” (non-neovascular or atrophic)
  
  • early stage, less severe
  • 85%-95% of cases
  • most common symptom is blurry vision
  • 3 stages
    
    • early, intermediate, and advances
• “Wet” (neovascular or exudative)
  
  • more severe than “Dry”
  • early symptoms: straight lines appear wavy
  • central vision blind spots develop