Eyes: Visual Field Defects Flashcards
What makes up the photoreceptors in the retina? (2)
Rods & cones
Overview of visual pathway (to the optic chiasm)
1) Photoreceptors (rods & cones) in the retina are stimulated by photons of light entering the eye.
2) Light-sensitive surface membrane proteins (e.g. rhodopsin) of these cells are stimulated to propagate 2nd messenger responses which convert light energy into electrical signals.
3) The photoreceptors synapse with retinal bipolar cells, which in turn transmit these signals to retinal ganglion cells.
4) The retinal ganglion cells converge at the optic disc, forming the optic nerve.
5) The optic nerve exits the eye, travelling through a defect in the lamina cribrosa of the sclera.
6) The optic nerve travels through the bony orbit and enters the middle cranial fossa through the optic canal.
7) Once inside the skull, the optic nerve travels along the floor of the middle cranial fossa, through the medial aspect of the cavernous sinus.
8) Left and right optic nerves then converge at the optic chiasm.
9) At the chiasm, fibres from the nasal aspect of each retina cross over (decussate) to the contralateral optic tract, while fibres from the temporal retina remain on their respective sides (look up image).
10) In this way, left-sided post-chiasmal fibres pertain to the right side of the visual field, and vice versa.
11) Optic tracts (L and R) then extend from chiasm to thalamus.
12) At thalamus, afferent sensory fibres from the eye synapse with 2nd order sensory neurones at the L and R lateral geniculate nucleus.
13) From here, the sensory nerves radiate dorsally to the calcarine sulcus of the occipital lobe (these are the optic radiations).
14) Optic radiations loop either through the parietal lobe or through the temporal lobe (Meyer’s loop).
What forms the optic nerve?
The retinal ganglion cells converge at the optic disc, forming the optic nerve.
How does the optic nerve exit the eye?
The optic nerve exits the eye, travelling through a defect in the lamina cribrosa of the sclera.
Why may the optic nerve be considered an extension of the forebrain?
As it is covered by the meninges of the CNS (rather than by epi/peri/endoneurium like other peripheral nerves).
How does the optic nerve enter the skull?
Enters the middle cranial fossa through the optic canal (a defect in the lesser wing of the sphenoid).
What bone is the optic canal in?
Sphenoid bone
Where do the L and R optic nerves converge?
At the optic chasm
Where is the optic chasm located?
Directly above the sella turcica of the sphenoid bone.
What is located immediately behind the optic chiasm?
The pituitary gland
Where does the optic tract extend from?
From the chiasm to the thalamus
What happens at the thalamus?
Sensory afferent nerves from the eye synapse with the 2nd order sensory neurones at the L or R lateral geniculate nucleus in the thalamus.
What happens in the visual pathway from the LGN?
L and R optic radiations.
From the thalamus, the sensory nerves radiate dorsally to the calcarine sulcus of the occipital lobe.
Optic radiations can reach the visual cortex in the occipital lobe by looping through one of which 2 lobes?
1) Parietal lobe –> upper optic radiation
2) Temporal lobe (Meyer’s loop) –> lower optic radiation
Optic radiations travelling through the parietal lobe correspond to which part of the visual field?
Upper half of the retina / lower visual field.
