T6 (Fys) The case of the colour-blind man

Question 0

What is the reason and result of glaucoma?

Answer 0

Decreased drainage of aqueous humour from the anterior chamber causes increased intraocular pressure, limiting the blood flow in the eye which leads to retinal damage.

Question 1

Which structures are denoted by ‘the uveal tract’?

Answer 1

The sclera, ciliary body and the iris.

Question 2

Explain the following: emmetropia, myopia, hyperopia, presbyopia.

Answer 2

Emmetropia: normal accomodation
Myopia: nearsighted (eyeball too long)
Hyperopia: farsighted (eyeball too long)
Presbyopia: age-related farsightedness

Question 3

What is a cataract?

Answer 3

Opacities in the lens caused by for example UV light.

Question 4

What layers does the retina concist of?

Answer 4

1. Pigment epithelium
2. Photoreceptor outer segments: cones and rods
3. Outer nuclear layer: receptor cell somas
4. Outer plexiform layer: receptor - bipolar cell synapses, horizontal cell projections
5. Inner nuclear layer (lateral information flow): horizontal cell soma, bipolar cell soma, amacrine cell soma
6. Inner plexiform layer: amacrine cell - ganglion cell synapses, bipolar cell - ganglion cell synapses
7. Ganglion cell layer: ganglion cell somas and axons (to optic nerve)

Question 5

Why are the light-sensitive cells part of the outermost layer of the retina, instead of being in the innermost layer (closest to the light)?

Answer 5

1. The membranous disks which houses the light sensitive photopigment are constantly regenerated and shed (into the pigment layer where they are phagocytosed).
2. The pigment epithelium regenerates photopigment after light exposure, supplying the retina with fresh pigment.
   (3. Optimum blood flow exists in the pigment layer only, vital for photoreceptor function.)

Question 6

What is the basic principle for visual signaling of the retina action potentialwise, and what values can the n. opticus membrane potential take?

Answer 6

Light reception leads to hyperpolarization. In total darkness the membrane potential is at ca - 40 mV, the hyperpolarization extreme is ca -65 mV.

Question 7

What ions, receptors, and how, is the receptor membrane potential controlled in darkness vs. light?

Answer 7

In darkness: Levels of cGMP in the outer segment are high, opening membrane channels that allow influx of Na and Ca ions which depolarize the cell. Simultanious efflux of K ions acts to counter bakance the charge difference.

In light: cGMP levels decrease, closing the cation channel. The K ion efflux then causes hyperpolarization.

Question 8

What is the basic structure of the photopigment rhodopsin?

Answer 8

Retinal (aldehyde of vitamin A) coupled woth several opsin proteins (there are many different).

Question 9

How does phototransduction occur in rods?

Answer 9

1. Rhodopsin in the disk membrane encapsulates retinal
2. A photon turns retinal from 11-cis form into all trans form
3. Transducin is activated, which activates a phosphodiesterase which hydrolyzes cGMP
4. The outer membrane hyperpolarizes

Question 10

How can little light cause even a noticeable change in the membrane potential?

Answer 10

Signal amplification:

• one molecule of activated rhodopsin can activate 800 tranducin molecules (ca 8% of transducin on disk surface)
• each phosphodiesterase can catalyze breakdown of 6 cGMP molecules
• ultimately ca 200 ion channels close from the action of one photon (1 mV change)

Question 11

How is phototransduction ended?

Answer 11

Rhodopsin kinase phosphorylates activated rhodopsin, after which arrestin binds to it, hindering further phototransduction.

Question 12

How does the retinoid cycle work?

Answer 12

1. All-trans retinal dissociates from opsin into cytosol of the outer segment
2. Conversion into alk-trans retinol
3. Transportation into pigment epithelium by IRBP (interphotoreceptor binding protein)
4. Enzymatic conversion into 11-cis retinal
5. Transportation into outer segment by IRBP (interphotoreceptor binding protein) and combination with opsin in receptor disk

Question 13

Explain the ‘light adaption’ of phototransduction amplification.

Answer 13

Light induced closure of cation channels leads to a decrease in inner Ca ion concentration. This increases the activity of guanylate cyclase (synthesises cGMP), rhodopsin kinase and affinity of cGMP-gated channels for cGMP.

Question 14

Explain scotopic, photopic and mesopic vision.

Answer 14

Scotopic: vision using only rods
Photopic: vision using only cones
Mesopic: vision usin both rods and cones

Question 15

How do the convergences or rods and cones compare (receptor to bipolar cell ratio)?

Answer 15

Rods: 15-30 rods to 1 bipolar cell
Cones: 1 cone to 1 bipolar cell

Question 16

How do cones and rods differ in adaption to light, e.g. brightest possible exposure?

Answer 16

Rods: 600 ms to adaption
Cones: 200 ms to adaption

Question 17

Outline the connection of rods to rod ganglion cells.

Answer 17

Unlike the cone bipolar cells, rod bipolar cells first synapse with an amacrine cell (gap junctions and chemical synapses with terminals of cone bipolars) which then synapse with ganglion cells in the inner plexiform layer.

Question 18

What specifies the foveola?

Answer 18

It is entirely rod free.

Question 19

How do the cone colours and wavelenghts correspond?

Answer 19

blue = Short
green = Medium
red = Long

Question 20

What two types of dichromatism are there? What other types of colour-blindness exists?

Answer 20

Protanopia: lack of red-sensing cones
Deuteranopia: lack of green-sensing cones

Anomalous trichromats: require higher intensity stimulation of a certain colour (protanomalous/deuteranomalous/tritanomalous trichromats)

Question 21

How do light spots and dark spots get perceived/interpreted by the cones in the retina?

Answer 21

Based on ganglion receptive fields and increments and decrements of light intensity in the center of the receptirve field (luminance contrast).

Light spot in center: cone is hyperpolarized, on-center bipolar cell depolarized, off-center bipolar cell hyperpolarized, on-center ganglion cell fires, off-center ganglion does not fire

Dark spot in center: cone is depolarized, on-center bipolar cell hyperpolarizes, off-center bipolar cell depolarizes, on-center ganglion does not fire, off-centre ganglion cell fires.

Question 22

How is the sign change achieved in the on-center, off-center circuit?

Answer 22

On centre: cone to bipolar synapse is metabotropic and closes Na ion channels, changing the sign

Off-centre: cone to bipolar synapse is ionotropic (as is bipolar-ganglion synapse for both on- and off-centered cells), opening Na ion channels, preserving the sign (sign-conserving)

Question 23

How is light adaptation achieved on ganglion/bipolar level?

Answer 23

Sensitivity of the ganglion cells for a spot of light is relative to the surrounding background lighting a.k.a. center-surround mechanism. This is based on surround antagonism mediated by horizontal cells (somas in inner nuclear layer, synapses in outer plexiform layer).

Question 24

Explain the physiological basis of the surround antagonism phenomenon.

Answer 24

Horizontal cells synapse with photoreceptor cells in the outer plexiform layer. The mechanism activates when illuminance of off-centre area matches that of on-centre area, where surrounding photoreceptors decrease the release of glutamate, causing hyperpolarization of horizontal cells. This leads to a decrease of GABA release from horizontal cells onto presynaptic membranes of the on-center photoreceptor synapses, increasing the latter’s level of depolarization and thus inhibiting light-evoked transmission by the on-center ganglion cell.

Question 25

How much of the axons of the optic nerves cross sides at the optic chiasm?

Answer 25

Ca 60%.

Question 26

What is the difference between the optic nerve and the optic tract?

Answer 26

The nerve contains ganglion axons from only one eye whilst the tract contains axons from both.

Question 27

Describe the retinogeniculostriate pathways/primary visual pathway.

Answer 27

Optic nerve - optic chiasm - optic tract - thalamus dorsal lateral geniculate nucleus - optic radiation (part of inernal capsule) - primary visual cortex (V1)/striate cortex/Brodmann’s area 17(calcarine fissure of occipital lobe)

The optic tract also targets the hypothalamus (circadian rhythms), pretectum (pupillary and lens control and reflex) and the superior colliculus (orienting movements of head and eyes).

Question 28

Explain how the pretectum plays part in pupillary constriction.

Answer 28

Projections from the pretectum synapse with the Edinger-Westphal nucleus which are close to the nucleus of CN III. These contain preganglionic parasympathetic neurons that pass via CN III to terminate on neurons in ciliary ganglion which innervate constrictor muscles of the iris.

Question 29

What is the diagnostic value of the pupillary reflex?

Answer 29

Clinical testing of left- and right-side sensory and motor neurons by observing the direct and the consensual responses of the pupils when shining light in either is possible.

Question 30

Where does the suprachiasmatic nucleus lie and which neural pathway is it involved in?

Answer 30

Ut lies in the hypothalamus and is involved in the retinohypothalamic pathway (“influences the broad spectrum of visceral functions that are entrained to the day-night cycle”).

Question 31

How can light-triggered circadian rhythm be maintained in animals deficient in rod- and cone-mediated reception?

Answer 31

The ganglion cells responsible for mediating circadian rhythm via projections to hypothalamus and pretectum themselves contain light-sensative photopigment (melanopsin).

Question 32

What makes up the binocular field of vision?

Answer 32

Left: the nasal visual field of the right eye and the temporal visual field of the left

Right: the nadal visual field of the left eye and the temporal visual field of the right

Question 33

How is peripheral vision perceived?

Answer 33

Monocularly, each eye’s most medial part of the nasal retina receiving it.

Question 34

The tractus opticus mediates a contralateral field of view. What does this imply?

Answer 34

It implies that the tract contains visual feed from both eyes.

Question 35

Distinguish between anopsia and scotoma.

Answer 35

Anopsia: “relatively large visual field deficit(s)”
Scotoma: a smaller visual field deficit

Question 36

What symptoms present int homonymous hemianopsia?

Answer 36

Complete loss of binocular vision of one side.

Question 37

What symptoms present in a lesion of the chiasma opticum?

Answer 37

Bitemproal hemianopsia/heteronomous hemianopsia (loss of peripheral field of vision on each side).

Question 38

What symptoms present in homonymous quadrantanopsia and why?

Answer 38

Loss of upper quarter field of vision due to contralateral lesion of Meyer’s loop.

Question 39

What kind of damage commonly presents with macular sparing.

Answer 39

Damage to the cortex.

Question 40

What types of discrimantion do cortical visual neurons make of visual input? Why?

Answer 40

Mostly based on orientation of edges (preferred orientation) but also movement direction and spatial and temporal frequency. This has been shown to be an efficient way of encoding natural scenes from nature.

Question 41

How are cells ordered in the neocortex as pertaining to a certain orientation preference?

Answer 41

Similar orientation preferring cells are found in one radial column.

Question 42

How eye-specific are the cells in a given column in the striate cortex?

Answer 42

Most cells are binocular. Only cells in layer 4 are strictly monocular (contains alternating eye-specific ocular dominance columns, geniculate neurons terminate here). Most neurons (neuron columns) are however either right or left polar.

Question 43

What are the layers of the striate cortex?

Answer 43

1, 2/3, 4A, 4B, 4C, 5, 6, white matter

Question 44

What is diplopia?

Answer 44

The phenomenon of double vision.

Question 45

How does stereopsis occur?

Answer 45

Diplopia makes for focus of an image on two non-corresponding parts of each retina. These are coordinated by neurons in the striate cortex specific to retinal disparities behind or in front of the plain of disparity (far neurons and near neurons). Tuned zero neurons “respond selectively to points that lie on the plane of fixation”.

Question 46

The retinogeniculate pathway is concisted of two lateral pathways. Which are these and where do they project?

Answer 46

These are the magnocellular (M ganglion cells) layer and parvocellular (P ganglion cells) layer. These project into layer 4Cα and 4Cβ respectively

Question 47

What differences are there between M and P cells?

Answer 47

M cells have larger receptor fields, faster conduction velocities and respond transiently to visual stimuli. P cells on the other hand respond in a sustained manner and can also distinguish colour (on-center cones sensing one wavelength, off-center cones sensing another).

Question 48

What is the role of the koniocellular pathway and where is it found?

Answer 48

The K cell pathways exists between the P and M cell layer. It transmits some short wavelength light.

Question 49

What other visual processing areas (extrastriate) of the cortex are there and what are their functions?

Answer 49

MT (middle temporal area, target of dorsal stream): responds selectively to the direction of a moving edge without noting colour, failure of results in cerebral akinetopsia

V4 (target of ventral stream): selective response to colour without noting movement, failure of results in cerebral achromatopsia (?)

Question 50

What problems arises from misfunction of the ventral and dorsal stream respectively?

Answer 50

Ventral (temporal lobe): recognition of objects and faces (?)
Dorsal (parietal lobe): recognition of objects based on position(?)