visual systems

Question 1

how many fibers are there in the optic nerve?

Answer 1

over 1 million

Question 2

what is visual perception?

Answer 2

the flow of visual information from the retina to the primary visual cortex

Question 3

Which structures are responsible for the absorption of light + transduction into electrical signals?

Answer 3

rods and cones in the retina

Question 4

The retina has 3 major functional classes of neurons, what are they?

Answer 4

1. photoreceptors (rods and cones) i
2. interneurons (bipolar, horizontal, amacrine cells)
3. ganglion cells

Question 5

What are photoreceptors?

Answer 5

(rods and cones) in outer nuclear layer. Transmit visual information to interneurons.

Question 6

What are interneurons?

Answer 6

(bipolar, horizontal, amacrine cells) in inner nuclear layer

Question 7

What are ganglion cells?

Answer 7

cells in ganglion cell layer (axons form optic nerve), 1st level of visual analysis

Question 8

What are the 2 groups of ganglion cells?

Answer 8

• -> specialized for detection of contrast and rapid changes in visual image, have circular receptive fields centrally and around
• ->Able to analyze color, form and movement

Question 9

What are the 2 groups of cells specialized for detection of contrast and rapid changes in visual image, have circular receptive fields centrally and around?

Answer 9

• on-center cells exited when light falls in center of receptive field, inhibited when stimulated in surroundings
• off- center cells exited when light falls in surroundings of receptive field, inhibited when stimulated in center

Question 10

What are the 2 groups of cells specialized in colour vision?

Answer 10

1. M-cells: large receptive fields (large dendrites), respond to large objects, able to follow rapid changes –> analysis of gross features of a stimulus and its movement.
2. P-cells: smaller, more numerous, small receptive fields, specific wavelength response–> perception of form and color, analysis of fine detail (some M-cells aswell)

Question 11

What is the visual field?

Answer 11

the view seen by the two eyes without movement of the head

Question 12

What is the retinal image?

Answer 12

an inversion of the visual field: projected on the retina smaller, inverted and laterally reversed due to refraction of light through convex lens.

Question 13

What happens to a single point in the binocular portion of one visual hemifield ?

Answer 13

it projects onto different regions of the two retinas

Question 14

the retina shows a retinotopic organization of the visual field, what is it?

Answer 14

fibres of the upper retina contain information on lower visual field
fibres of the lower retina contain information on upper visual field.
Inverted + laterally reversed all the way of the pathway to the cortex.

Question 15

What is the Binocular portion of visual field?

Answer 15

seen by 2 eyes

Question 16

What is the monocular portion of visual field?

Answer 16

seen by 1 eye

Question 17

The retina is subdivided into regions, there correspondence between regions of the visual field and the retinal image, how is it divided?

Answer 17

surface of retina is divided in midline: nasal hemiretina lies medial to fovea//
the temporal hemiretina lateral to fovea.
Each half of the retina further divided into dorsal (or superior) and ventral (or inferior) quadrants.

Question 18

What does the optic nerve carry?

Answer 18

carries all visual information from one eye, from both nasal and temporal hemiretina.

Question 19

The optic nerve (CNS) through optic canal in orbit, how does the cranial nerve form and leave?

Answer 19

unmyelinated axons of ganglion cells collect the optic disc (the region where ganglion cell axons leave the retina, free of photoreceptors –> blind spot in visual field of each eye)

Question 20

in normal vision we are unaware of blind spots, we experience blind spot only by using one eye, why is that?

Answer 20

Since disc is nasal to fovea in each eye, light coming from single point in binocular zone never falls on both blind spots simultaneously

Question 21

Explain the Meningeal covering of the optic nerve

Answer 21

sclera continues as its dural sheath, lined in turn by arachnoid and pia. Subarachnoid space around optic nerve dead-ends at back of eye but communicates with subarachnoid spaces around other parts of brain.

Question 22

Why are increases in intercranial pressure full of risks?

Answer 22

thy are transmitted to optic nerve, compress it, and choke off both venous flow and axoplasmic transport

Question 23

What are central visual pathways?

Answer 23

o Fibres from nasal hemiretina of each eye cross the midline and move to the contralateral side at the optic chiasm.
o Fibers from the temporal hemiretina do not cross.
o Each optic tract carries complete representation of one half of the binocular zone in the visual field

Question 24

What is the lateral geniculate nucleus?

Answer 24

layered, dome-shaped, main terminus for input to the Visual Cortex.

Question 25

What are the 6 layers of the lateral geniculate nucleus?

Answer 25

Layers 1 and 2 = magnocellular layers layers 3 to 6 = parvocellular layers. The parvocellular and magnocellular layers project to separate layers of the primary visual cortex. All project to area 17, the primary visual cortex.

Question 26

What do the parvocellular and magnocellular layers do?

Answer 26

they project to separate layers of the primary visual cortex.

Question 27

What does the optic tract do?

Answer 27

curves posteriorly around cerebral peduncle and most of its fibres terminate in the LGN.

Question 28

What does each lateral geniculate nucleus have?

Answer 28

has a retinotopic representation of the contralateral half of the visual field.

Question 29

- Projections from the CONTROLATERAL NASAL HEMIRETINA reach layers _______

Answer 29

1 4 6

Question 30

- Projections from the HOMOLATERAL TEMPORAL HEMIRETINA reach layers ________

Answer 30

2 3 5

Question 31

What is the course of visual information starting in the LGN?

Answer 31

- Visual information from LGN --> through Optical radiation --> passes through the retro-lenticular part of internal capsule --> primary visual cortex.

Question 32

- Lesions in the retino-Geniculate-Cortical pathway are associated with __________________

Answer 32

specific and predictable gaps in the visual field

Question 33

lesions in optic nerve of one eye leads to

Answer 33

complete blindness of ipsilateral eye

Question 34

Lesions at optic chiasm leads to ___________

Answer 34

bitemporal hemianopsia

Question 35

Lesions in the optic tract (carries information from the contralateral portion of the visual field) leads to __________

Answer 35

controlateral homonymous hemianopsia. (loss of visual field on contralateral side)

Question 36

Lesions of large tract which may be represented by genicular calcanean radiation leads to ____________

Answer 36

Contralateral homonymous quadrantopsia

Question 37

What are visual reflexes?

Answer 37

``` The Retina (besides projections to cerebral cortex) to subcortical Regions in the Brain, some nuclei of the brainstem eg.Superior colliculus: this projection contributes to visually-guided eye movements. alternating gray cellular and white (axonal) layers on roof of the midbrain. Retinal ganglion cells project directly to the superficial layers and form a map of contralateral visual field. ```

Question 38

What does the Pretectum of the Midbrain do?

Answer 38

controls Pupillary Reflexes (Light shining in one eye causes constriction of pupil in that eye (the direct response) as well as in the other eye (the consensual response))

Question 39

How does the pupil contrict or dilate? Whate are the 5 steps?

Answer 39

1. A set of fibers leave the optic tract and terminate in the pretectal nuclei, located just rostral to the superior colliculus on each side. 2. The cells in the pretectal area project bilaterally to preganglionic parasympathetic neurons in the Edinger-Westphal (or accessory oculomotor) nucleus, which lies immediately adjacent to the neurons of the oculomotor (cranial nerve III) nucleus. The contralateral nucleus is reached though the POSTERIOR COMMISSURE. 3. Preganglionic neurons in the Edinger-Westphal nucleus send axons out of the brain stem in the oculomotor nerve to innervate the ciliary ganglion. 4. This ganglion contains the postganglionic neurons (the short CILIARY NERVE) that innervate the smooth muscle of the pupillary sphincter that constricts the pupil. 5. A sympathetic pathway innervates the pupillary radial iris muscles that dilate the pupils.