The Retinofugal Projection & LGN
What is the scheme of the retinofugal projection (the neural pathway that leaves the eye, beginning with the optic nerve)? Where does the LGN lie in this projection?
ganglion axons go away from the retina
the brain stem
optic nerve
optic chiasm
optic tract
exit the left and right eyes at the optic disks 📀
travel through the fatty tissue behind the eyes
pass through holes in the floor ⬇️ of the skull
optic nerves from both eyes combine to form
lies at the base of the brain
axons originating in the nasal retinas cross from one side to the other 👃⚔
since only the axons originating in the nasal retinas (HALF the axons) cross → it's called a partial decussation of the retinofugal projection
run just under the pia along the lateral surfaces of the diencephalon
implications for the visual field
full visual field
entire region of space (measured in degrees of visual angle) that can be seen with both eyes looking straight ahead
midline: a vertical line, dividing the visual field into left and right halves
each half is known as a visual hemifield
binocular visual field
central portion of both visual hemifields viewed by both retinas
objects in the binocular region of the left visual hemifield will be imaged on the nasal retina of the left eye and on the temporal retina of the right eye
fibers from the nasal portion of the left retina cross to the right side at the optic chiasm,
the left visual hemifield is “viewed” by the right hemisphere of the brain and the right visual hemifield is “viewed” by the left hemisphere
small number of axons: connect with hypothalamus
some continue past the thalamus to innervate the midbrain
most !!! of them innervate the lateral geniculate nucleus (LGN) of the dorsal thalamus 1⃣ 👁
neurons in the LGN give rise to axons that project to the primary visual cortex
this projection from the LGN to the cortex is called the optic radiation ☢️
synchronizing biological rhythms
pretectum 👀💨 (part of the midbrain)
control the size of the pupil and certain types of eye movement
about 10% of the ganglion cells in the retina project to
the superior colliculus
part of the midbrain tectum 🗻
involved in orienting the eyes in response to new stimuli in the visual periphery
2 LGNs: right and left
organization
viewed in cross section, each LGN appears to be arranged in six distinct layers of cells
right LGN receives information about the left visual field ↔
input from the two eyes is kept separate
some CL layers - nasal retina of eye that decussates & some IL - from temporal lobe of opposite eye
numbered 1 through 6, starting with the most ventral layer ⬆
the two ventral layers, 1 and 2, contain larger neurons
contralateral, ipsilateral, IL, CL, IL, CL
magnocellular LGN layers
4 dorsal layers are called parvocellular LGN layers
P-type ganglion cells in the retina project exclusively to the parvocellular LGN, and M-type ganglion cells in the retina project entirely to the magnocellular LGN.
there are neurons that lie just ventral to each layer
koniocellular LGN layers
receive input from the nonM–nonP types of retinal ganglion cells and also project to the visual cortex
each koniocellular layer gets input from the same eye as the overlying M or P layer
e.g. layer K1 receives input from the contralateral eye just as layer 1 neurons do.
the retina gives rise to streams of information that are processed in parallel.
visual receptive fields of LGN neurons are almost identical to those of the ganglion cells that feed them
e.g., magnocellular LGN neurons' fields are just like those of M-type ganglion cells