Visual Pathway Flashcards
M ganglion cells are also called
magnocellular cells, parasol ganglion cells
What are the features of M ganglion cells?
large cell bodies with large dendritic trees; synapse in the magnocellular layer in the LGN of the thalamus
M ganglion cells synapse in
magnocellular layer of the LGN of the thalamus
M ganglion cells provide information about _______ and less so about _________
motion; visual acuity
M ganglion cells make up _____% of ganglion cells
10%
P ganglion cells are also called
parvocellular, midget ganglion cells
What are the features of P ganglion cells?
small cell bodies and small dendritic trees; synapse in parvocellular layers of LGN in thalamus
P ganglion cells encode information about
colour vision and visual acuity
P ganglion cells synapse in
Parvocellular layers of the LGN of the thalamus
P ganglion cells make up _____% of ganglion cells
80%
Ganglion cell axons form the
optic nerve
What are the 5 different targets of ganglion cell axons?
LGN (thalamus); pretectum (midbrain); suprachiasmatic nucleus (hypothalamus); superior colliculus (eye movements); various thalamic nuclei
The most important target of ganglion cells is the
LGN in the thalamus
Projections of ganglion cells to the LGN form the
visual pathway
Projections to the pretectum in the midbrain from ganglion cells play a role in
pupil responses
Projections of ganglion cells to the suprachiasmatic nucleus of the hypothalamus are involved in
circadian rhythm
Projections of ganglion cells to the superior colliculus are responsible for
eye movements
Nasal retinal ganglion cells see the ______ visual field
temporal
Temporal retinal ganglion cells see the ______ visual field
nasal
T/F Nasal fibres are the only fibres that cross at the optic chiasm
True; temporal fibres run ipsilateral and DO NOT cross at the ciasm
The right visual cortex receives input from
the left visual field (L nasal retina, R temporal retina)
The left visual cortex receives input from
the right visual field (L temporal retina, R nasal retina)
Which fibres cross at the optic chiasm?
nasal
Bitemporal visual field defect is a classic sign of
lesion of the chiasm eg pituitary tumour
T/F All sensory information is relayed through the thalamus
False; all sensory information EXCEPT olfaction is relayed through the thalamus
Layers 1 and 2 of the LGN are
Magnocellular layers, targeted by M ganglion cells
Layers 3-6 of the LGN are
Parvocellular layers, targeted by P ganglion cells
How is information received at the LGN?
one of the M cell layers is from one eye, the other from the other eye; two of the P cell layers is from one eye, the other 2 are from the other eye - almost alternating
T/F M and P inputs from each eye mix at the LGN
False; there is no mixing of information from each eye at the LGN
V1 is located
on the medial surface of the occipital lobe, around the calcacrine fissure
What is the retinotopic organization of V1?
most central vision projects to the most posterior part of V1; as you move anteriorly along the calcarine fissure inputs are more peripheral
Lesions that affect only one eye must be occurring
before the chiasm
If visual field defects are present on opposite sides (bitemporal), the lesion is
at the chiasm
If visual defects are on the same side of each eye, the lesion is
behind the chiasm (optic tract, optic radiations, or in V1)
Macular sparing occurs when
a vascular accident damages the MCA (supplies more peripheral visual inputs to V1) but spares the PCA and therefore flow to the macular cortex of V1 (most posterior)
Bitemoporal heminanopia indicates
chiasm lesion
Homonymous hemianopia indicates
optic tract lesion
Homonymous quadranopia indicates
lesion of the optic radiation
T/F Ganglion cells require photoreceptors to respond to light
False; some ganglion cells contain a protein melanopsin which like rhodopsin (a photopigment) and are intrinsically light-sensitive (ipGCs)
Ganglion cells that are intrinsically light sensitive synapse in the
suprachiasmatic nucleus (hypothalamus), pretectum (midbrain), and posterior nucleus of the thalamus
ipGCs
intrinsically photosensitive ganglion cells/melanopsin ganglion cells
ipGCs contain
Melanopsin, similar to photopigments of photoreceptors
Light activation of melanopsin results in
depolarization of ipGCs
ipGCs provide information about
how much light is in the environment
Function of ipGCs is important in
circadian rhythm, sleep regulation, pupil responses, light level information, and light allodynia (photophobia)
What allows people who are totally blind to become jetlagged?
ipGCs - because they tell us how much light is in our environment and regulate circadian rhythms
What is the direct pupil response?
Constriction of the pupil in response to direct light
What is the consensual pupil response?
Constriction of the pupil in response to light shone on the other pupil
Pupil responses are dependent on
the ability to detect light (melanopsin ganglion cells), and functioning of the iris (sphincter and dilator pupillae muscles)
Which ganglion cells are responsible for pupil responses?
ipGCs/melanopsin ganglion cells
Melanopsin GCs project to which nucleus?
Optical Pretectal Nucleus (OPN)
What causes an afferent pupil defect?
A lesion on the pathway from the melanopsin ganglion cell to the midbrain
What causes an efferent pupil defect?
A lesion on the pathway between the Edinger-Westfal nucleus and the eye (via ciliary ganglion)
T/F Blind individuals do not exhibit pupil responses
False; the constriction is just less but they still have ipGCs/melanopsin GCs
From the pretectal nucleus, projections in the pupil response go to the
Edinger-Westfal nucleus
From the Edinger-Westfall nucleus, information travels via
CN III to the ciliary ganglion
From the ciliary ganglion, constriction in the pupil reflex is produced by
CN III innervating constrictor pupillae
How do ipGCs/melanopsin GCs regulate circadian rhythm?
Project to the surpachiasmatic nucleus of the hypothalamus about light levels
How do ipGCs/melanopsin GCs relate to photophobia in migraines?
ipGCs target the posterior nucleus of the thalamus; this is the nucleus important in the pain pathway of migraines
Migraine pain is attributed to
pain originating in the dura/meninges; projects to the posterior nucleus of the thalamus along with ipGCs (hence photophobia)
