Download LGN

VS131 Visual Neuroscience
Lateral Geniculate
Nucleus (LGN)
The major targets of the retinal ganglion cells:
RETINA  LGN  CORTEX (“vision’)
 Superior Colliculus (saccades)
 Suprachiasmatic Nucleus (circadian)
 Pretectum (pupilary light reflex)
 Accessory Optic System (‘steadicam’)
Only about 10% of retinal ganglion cells do not terminate in the
Lateral Geniculate Nucleus (LGN). (Does NOT mean that 90%
do not terminate in other nuclei).
‘BLINDSIGHT’: preservation of very limited ability to
perform visually guided tasks after destruction of the
retina to LGN to cortex pathway, in the apparent
absence of conscious perception.
Bottom line: take out the LGN-cortical system and you
are for all practical purposes completely blind.
LESIONS OF THE GENICULO-CORTICAL SYSTEM
CAUSE DIRECT LOSS OF VISION IN SPECIFIC
PARTS OF THE FIELD OF VIEW. IMPORTANT FOR
CLINICAL DIAGNOSIS.
The LGN is part of the thalamus, which itself is part of
the diencephalon.
Sometimes called the “LGNd” because it is part of the
Dorsal thalamus. (No directly corresponding LGNv,
maybe a small oculomotor system in cats??).
The thalamus contains sensory relay nuclei for all the
senses: sometimes called “the gateway to the cortex”.
(Olfaction is a partial exception: some direct
connections from olfactory bulb to cortex).
Don’t panic!
(Mostly F.Y.I.)
Take home
message: axons in
these pathways are
not positioned
randomly but are
arranged in order.
So what does the LGN do?
Relay information from the retina to cortex...
… and nobody has been able to demonstrate
conclusively that the LGN does anything else!!!
LGN relay cells have properties that are very much
like those of the retinal ganglion cells that contact
them.
Probably something very important – ‘gating’ of
information to cortex? We’re working on it…
LGN layers are
in register.
Even though
inputs from the
same part of
the retinas are
brought close
together in the
LGN, they do
not talk to each
other – they are
still monocular.
(Don’t get
binocular cells
until you reach
cortex).
1,4,6: cross the
river styx.
2,3,5: stay on
the same side!
Cat! Many early
experiments,
terminology still
used.
Cat X is
homologous to
monkey parvo
Cat Y is
homologous to
monkey magno
LGN relay neurons project (I.e., send axons) to cortex.
LGN interneurons make only local connections.
There are more interneurons than relay neurons!
LGN neurons get feedback connections from cortex. (The
one-way connection from retina to rest of brain is unique in
the visual system).
LGN gets other inputs as well. For example: from brainstem
and perigeniculate.
STILL don’t know what the LGN might be doing other than
acting as a simple relay…
WARNING: There are other classes of retinal/LGN
neurons than parvo/magno (“koniocellular”,
“intralaminar”, etc), but their function has not been
determined well enough to make it into the
textbooks just yet. Be warned that 20 years from
now it will probably be a different story.
“The Vision Thing”