Visual Senses

Question 1

what does vision require detection of

Answer 1

various light wavelengths

Question 2

what is the role of lacrimal gland

Answer 2

1. secretes saline tear
2. fluid cleans and lubricates the front of the eye during a blink
3. prevents cornea from drying out and frost injury during cold
4. contains lysozomes and immunoglobulin A to protect against infections

Question 3

what is the difference between optical and visual axis

Answer 3

1. optical axis gives most optically clear image (crosses cornea, lens dead centre)
2. visual axis to fovea gives best colour vision

Question 4

what is the refraction of an image

Answer 4

light bends when passing from across the cornea, lens and vitreous humor

light information through a convergent lens are refracted and focused at a focal point

Question 5

what is distance from the focal point behind the lens dependent on

Answer 5

the distance to the image

Question 6

how do horses have full accomodation

Answer 6

they don’t have full accommodation ability

shape of retina means it can see near and far by moving head/eyes

Question 7

what are subcortical reflexes

Answer 7

do not involve visual cortex

pupillary light reflex

dazzle reflex

Question 8

what is the pupillary light reflex

Answer 8

pupillary reflexes indicate functional state of afferent and efferent that control the pupil

ex. if light stimulus directed to left eye elicits a consensual constriction in the right eye but not direct one in left eye –> afferent limb of the reflex (optic nerve is intact; but efferent limb to the left eye is damaged)

damaged oculomotor nerve

Question 9

what is the dazzle reflex

Answer 9

blink or head movement

subcortical mediated brainstem response

palpebral fissure closes in response to sudden intense ilumination of eye –> ipsilateral response is greater than contralateral response

absence of a response indicates blindness

Question 10

what are the cells in the retina

Answer 10

at back of eye there is either dark pigment or tapetum

Question 11

what are the steps in intracellular phototransduction of rhodopsin

Answer 11

1. photon hits rhodopsin (R)
2. isomerization of retinal
3. activation of alpha subunit of transducin
4. activation of phosphodiesterase (PDE)
5. decreases cGMP levels and closes ion channels (inhibitory)

–> hyperpolarizes the receptor cell

Question 12

what are the distribution of photoreceptors in the retina

Answer 12

Question 13

what are rods more sensitive to

Answer 13

light (irradiance)

one rod responds to one photon

there are multipile rods per bipolar cell

rods work in the scotopic region

rods are poor for spatial detail

Question 14

what are the functions of cones

Answer 14

provide more detailed information –> including colour

but they need more light (photopic region)

ratio of cones to ganglion cells is 1:1

Question 15

what are the function of ganglion cells

Answer 15

integrate multiple photoreceptor signals

Question 16

what is the function of melanopsin

Answer 16

signals light control for daily rhythms

ganglion cells show intrinsic photoresponsiveness

melanopsin is a photoreceptors that response to irradiance information

mutations in melanopsin have detrimental effects on daily rhythms

Question 17

what is the neural circuit from retina

Answer 17

from retina to lateral geniculate nucleus located in the thalamus

Question 18

what are the layers of the LGN

Answer 18

6 layers of retinal map

consists of parvocellular (1-4) and magnocellular (5,6) cells

parvocellular cells are small, integrate signals from cones and necessary for colour and form

magnocellular cells are large, integrate signals from rods, involved in movement, depth and irradiance

Question 19

what is the primary visual cortex

Answer 19

integrate multiple LGN cells

simple, complex and hyper-complex cells with different receptive fields

Question 20

what is the visual control of motor movement

Answer 20

dorsal stream flows from V1 to V5/V3A

guides movements such as the hand postures for grasping a mug or pen

damage to ventral stream prevents identification of objects; damage to dorsal stream results in optic ataxia

reaching movements directed toward individaul body (tactile or proprioceptive feedback) remains intact

Question 21

what are the 4 pathways of central visual and non-visual pathways

Answer 21

eye –> optic tract

1. lateral geniculate nucleus –> visual cortex
2. rostral colliculi –> dazzle reflex
3. pretectum, pons –> pupillary light reflex
4. suprachiasmatic nucleus –> hypothalamic, pineal gland

Question 22

what are the additional visual pathways for proprioception and posture

Answer 22

visual proprioceptive system

linked to the vestibular apparatus

provides information by which head movement is sense

Question 23

what is the trichromatic theory

Answer 23

colour vision that is based on the coding of the 3 basic colours (red, green and blue)

Question 24

what is the opponent process theory

Answer 24

colour vision that emphasizes the importance of the opposition of pairs of colours: red versus green and blue versus yellow

Question 25

where does colour vision start

Answer 25

start in ganglion cells

multiple colour opsins information is integrated by gangiolonic cells; then transmit info to LGN parvocellular cells

Question 26

what is colour vision represented by in the cortex

Answer 26

by blobs

complex neurons in the visual temporal cortex (V4) respond to objects in the visual field that have colour that is dependent on orientation texture and shape