Visual System

Question 1

Photoreceptors - 2 types

Answer 1

1. Rods
   - 1000x more sensitive to light which is important for nighttime vision
   - located in peripheral retina
   - greater quantity than cones
   FUNCTIONALLY: more sensitive to low levels of light on our peripheral retina, unable to perceive color at night
2. Cones
   - important for daytime vision & visual acuity
   - most located in fovea
   - FUNCTIONALLY: see color, visual acuity,

Question 2

Visual field

- nasal vs. temporal fibers

Answer 2

Hemifield = only ONE side of visual field (not including the binocular vision where they overlap)

Nasal fibers - detect peripheral field
- cross in optic chism

Temporal fibers - detect midline
- do not cross

Question 3

Visual pathway

Answer 3

Photoreceptors in retina (travels along optic nerve, chiasm & tract –> LGN in thalamus –> Optic radiations (geniculostriate) –> V1 –> V2 –> SAA

Question 4

Optic Radiations

Answer 4

aka geniculostriate, or inferior optic radiations (Meyer’s Loop)

fan out and around lateral ventricles to reach primary visual cortex

–> INVERSION of visual field & optic radiations
so inferior optic radiations functionally transmit the superior visual field
** important to know if lesion is here

Question 5

Pupillary & Consensual Reflex

• located
• function

Answer 5

Located in the pretectal area, does NOT require cortical involvement b/c it occurs at the BS level

Pupillary reflex = constriction of eye directly stimulated by the light

Consensual reflex = constriction of the opposite eye

Question 6

Accomodation

Answer 6

allows you to see far away & close up

this DOES require cortical involvement
–> efferent limb controls the curvature of the lens to contract the pupil and moves the pupil towards the midline

Question 7

Lesion @ optic nerve

• what is damaged
• how does it present

Answer 7

the nasal AND temporal fibers on that ONE side is affected

Presents as blindness in one eye

Question 8

Lesion @ optic chiasm

• what is damaged
• how does it present

Answer 8

the nasal fibers that cross are damaged

Presents as loss of peripheral vision, b/c the nasal receptors detect the peripheral field

Question 9

Lesion @ optic tract

• what is damaged
• how does it present

Answer 9

On the side of the lesion: same side temporal fibers are affected & opposite side nasal receptors are affected (b/c they cross)

Presents as loss of vision of the same visual field on the contralateral side of the lesion

–> lesion on the right, will cause loss of left visual field in both eyes

Question 10

Lesion @ superior optic radiations

- how does it present

Answer 10

The OPPOSITE visual field is affected, and the INVERTED visual field is affected

–> so lesion on left superior optic radiations = loss of inferior visual field on the right

VICE VERSA for the inferior optic radiations & other side

Question 11

Eye movements - objectives for function (2)

Answer 11

1. maintain gaze stability
   - via VOR & optokinetic reflex
2. directing gaze at visual targets
   - sacaddes: fast eye movements to switch from one object to the other
   - smooth pursuit: allow following objects
   - vergence movements: movements of eye towards or away from midline to adjust for distances between eyes & visual target

Question 12

Thalamic Nuclei for Visual System

Answer 12

LGN

- relays info to primary visual field (area 17)