basic visual pathway

Question 1

thalamus

Answer 1

• Large structure
• Divided into multiple nuclei with distinct functions
  -some visual
  -some non-visual

Question 2

lateral geniculate nucleus

Answer 2

• Primary visual part of the thalamus

Question 3

what projects to teh LGN in a primate?

Answer 3

~90% of retinal ganglion cells

Question 4

role of teh thalamus

Answer 4

• Plays a major gating/ modulatory role in the relay of sensory information
• Integrates information from cerebellum and basal ganglia, sending this information to the motor regions of cortex
• Determines whether information should reach consciousness awareness and is involved with sleep/ wake and attention
  -determines whether we ‘notice’ specific pieces of sensory information

Question 5

thalamic organisation

Answer 5

• The main visual component of the thalamus is the LGN
• Other region of processing are:
  -Pulvinar (communicates to higher visual cortex)
  -thalamic reticular nucleus

Question 6

general thalamic properties

Answer 6

• Send information to cerebral cortex for all sensory systems except olfaction
  -individual sub-nuclei that process specific senses
  -overall communicates with the entire cortex
• Every relay nucleus receives information back from the cortex
• Amount of feedback from cortex may equal or exceed that from sense organs
• Feedback may be specific or diffuse, may separately target relay cells and interneurons

Question 7

what type of structure is the LGN?

Answer 7

laminated

Question 8

how many layers of laminae in the LGN?

Answer 8

6 layers

Question 9

which layers are magnocellular?

Answer 9

1 and 2

Question 10

which layers are parvocellular

Answer 10

3,5,6

Question 11

what stain shows up cell bodies in the LGN?

Answer 11

nissl stain

Question 12

large cells

Answer 12

magnocells

Question 13

small cells

Answer 13

parvocells

Question 14

what does each cell in the LGN receive most of its input from?

Answer 14

a single retinal ganglion cell

Question 15

what does each LGN layer contain?

Answer 15

excitatory (relay) cells and local inhibitory interneurons

Question 16

what is underneath each laminae?

Answer 16

very small excitatory relay cells: koniocellular layers

Question 17

how are retinal projections ordered?

Answer 17

retinotopically

Question 18

where does each LGN get its signals from?

Answer 18

RGCs viewing the opposite visual hemifield

Question 19

what do adjacent cells within each LGN layer view?

Answer 19

adjacent portions of visual space

Question 20

what representation does the foveal region have in the LGN

Answer 20

a disproportionately large representation: half the mass of the LGN

Question 21

how many eyes does each layer of the LGN receive input from?

Answer 21

one

Question 22

what is the effect of each laminae only receiving input from one eye?

Answer 22

there are multiple representations of the same ‘retinotopic’ space in each LGN

Question 23

cells along the projection line all view…

Answer 23

overlapping region of visual space

Question 24

what is the receptive field of a sensory neuron

Answer 24

a region of space in which the presence of a stimulus will alter the firing of that neuron

Question 25

what organisation of receptive field to LGN cells have?

Answer 25

concentric centre-surround antagonistic

Question 26

what do LGN centre surround receptive fields act as?

Answer 26

local edge detectors

Question 27

spatial acuity small receptive field

Answer 27

high spatial acuity: more detail

Question 28

spatial acuity large receptive field

Answer 28

low spatial acuity: less detail

Question 29

temporal frequency of RFs that respond best to slow stimuli

Answer 29

low preferred temporal frequency

Question 30

temporal frequency of RFs that respond best to fast stimuli

Answer 30

high preferred temporal frequency

Question 31

parvocellular LGN neurons (8 points)

Answer 31

* Dorsal 4 layers have smaller cell bodies * Small receptive fields * High spatial acuity (can resolve fine detail) * Prefer low temporal frequencies (slow changes) * Receptive fields have concentric centre/surround like the retina * Both On and Off centre subtypes * Chromatic (red/green) but can respond to brightness * Input from retinal P ganglion cells

Question 32

magnocellular LGN neurons (8 points)

Answer 32

* Ventral 2 layers * Have larger cell bodies * Lower spatial acuity (can’t resolve fine detail) * Prefer high temporal frequencies (fast changes) * Receptive fields have concentric centre surround like the retina * Both On and Off-centre subtypes * Achromatic * Input from retinal M ganglion cells

Question 33

koniocellular LGN neurons (5 point)

Answer 33

* Very small cells found between laminae * Direct input from blue/yellow RGC’s * Indirect input from superior colliculus * Heterogenous types? * Functional properties/ function largely unknown

Question 34

organisation within laminae

Answer 34

eccentricity

Question 35

what is magnified with LGN retinotopic map?

Answer 35

fovea

Question 36

what changes with eccentricity

Answer 36

receptive field size/ proportion of M and P cells

Question 37

relay cells (4 points)

Answer 37

* ~90% of LGN cells are relay, single axon projections to V1 * Use glutamate- therefore are excitatory * Each has an axon contralateral just above the LGN

Question 38

where does the contralateral axon to each relay cell (just above the LGN) terminate?

Answer 38

in the visual sector TRN, known as the perigeniculate nucleus (PGN)

Question 39

what neurotransmitter do all inhibitory cells use?

Answer 39

gamma-aminobutyric acid (GABA) as a neurotransmitter

Question 40

what are the two populations of inhibitory neurons

Answer 40

feed forward inhibition and feedback inhibition

Question 41

feed forward inhibition neurons

Answer 41

modulates centre:surround inhibition intrinsic to the LGN receives retinal input project locally

Question 42

feedback inhibition neurons

Answer 42

global changes in the LGN cell responses with the perigeniculate nucleus project widely within the PGN feedback to LGN

Question 43

where does the LGN send excitatory input to?

Answer 43

V1

Question 44

modulatory systems

Answer 44

long projection systems from the brainstem and basal forebrain that adjust thalamic function

Question 45

noradrenaline

Answer 45

* Produced in locus coeruleus (LC): -small periaqueductal gray matter cell group ~10,000 cells/side * Maintains vigilance, fight or flight responses * Extensive projections incl. thalamus and cortex * Modulates visual sensitivity

Question 46

acetylcholine

Answer 46

* Basal forebrain groups: innervates entire cerebral cortex including amygdala and hippocampus * Pontine groups: innervate brainstem reticular formation and thalamus important for arousal and REM sleep

Question 47

serotonin

Answer 47

* Serotonergic cells found in the Raphe nuclei * Pons and midbrain groups project the whole of the forebrain * Role in mood, cardiovascular control, thermoregulation + modulate thalamic and cortical function * Specific function largely unknown

Question 48

dopamine

Answer 48

* Multiple cell groups, some of which provide input to thalamus not those with Parkinson’s * Roles in reward, neuroendocrine, motor control * Thalamic projections mainly avoid primary sensory areas * Dopaminergic modulation of the thalamus