martin chapter 7 Flashcards
optic nerves connect what?
the eyes to the visual processing centers in the brain
where is the retina positioned compared to the eyeball?
posterior inner surface of the eye ball
where are photoreceptors located within respect to the eye ball?
in the posterior inner surface of the eye ball as well
photoreceptors synapse on?
retinal interneurons
retinal interneurons synapse on?
ganglion cells
what are ganglion cells and what do they synapse on?
ganglion cells are retinal projection neurons and synapse in the thalamus and brain stem
where do the axons of ganglion cells travel?
into the optic nerves. axons of ganglion cells from each eye contribute to the formation of the optic nerve of the same side
where do some ganglion cells decussate?
into the optic chiasm. then they enroute to the thalamus and brain stem
where do the crossed and uncrossed ganglion cell axons course?
into the optic tract. each optic tract contains axons coming from both eyes
what is the principal thalamic target for ganglion cells?
the lateral geniculate nucleus
where does the lateral geniculate nucleus project?
to the primary visual cortex
what’s the name of the pathway by which the lateral geniculate nucleus project to the primary visual cortex?
optic radiations. it comprises multiple functional pathways whose axons intermingle and carry informations about multiple attributes of visual stimuli including their form, color, location and speed of motion.
where is the primary visual cortex (or v1) located?
in the occipital lobe, within the depths of the calcarine fissure
the primary visual cortex is also referred as?
the striate cortex. because myelinated axons form a prominent striation visible on the gross brain
where do the axons of the second path from the primary visual cortex decussate?
in the corpus callosum and terminate in the contralateral primary visual cortex. This helps to unify images from the two eyes into a perception of a single visual world.
efferent projections from the primary visual cortex follow?
one of three pathways
Certain retinal ganglion cells project directly to the midbrain, especially in 2 structures:
pretectal area and the superior colliculus
the retinal ganglion cells that skirt the lateral geniculate nucleus and travel to the midbrain directly course in a tract named?
brachium of superior colliculus
where can we find the superior colliculus?
in the tectum of the midbrain
which is the major role that the superior colliculus has in humans?
the superior colliculus has a major role in the control of saccades, rapid eye movements.
where are the pretectal nuclei? what’s their major role?
at the midbrain-diencephalic junction. rostral to the tectum.
they participate in pupillary reflexes which regulate the amount of light reaching the retina (they are also involved in other visual reflexes)
what is the binocular zone?
the part of the visual field that both eyes can see at the same time
visual field
the total area seen by the two eyes when they are fixed straight ahead.
the total visual field of one eye is not equal to one hemifield. why?
because the visual field includes a binocular zone and two monocular zones
what is the cornea?
the transparent avascular portion of the sclera
when the light enters the eye through the cornea, it is focused on the retinal surface by?
the lens
what is the fovea?
A specialized high resolution portion of the retina.
it is centered into a morphologically distinct region of the retina called macula lutea
the brain accurately controls the positions of the eyes to ensure that the key portions of images fall on the fovea of each eye
a vertical line passing through the fovea divides the retina into 2 halves:
- temporal hemiretina
- nasal hemiretina
what are the temporal crescents?
the portions of each nasal hemiretina closest to the front of the eyes. they are monocular zones receiving visual informations from the temporal parts of the visual fields.
The temporal crescents are the areas on the far side of your visual field (relative to each eye) that are not shared between the two eyes.
visual information from the visual hemifield on one side is processed by the same or the opposite side of the visual cortex?
the opposite side of the visual cortex
where do retinal axons leave the eyeball and vessels serving a part of the retina enter and leave the eye?
in the optic disk
it has no photoreceptors so it corresponds to the blind spot
two classes of photoreceptors
rods= for night vision
cones=for high acuity daylight vision
in which layers of the retina are the cell bodies of cones and rods located?
in the outer nuclear layer
in which layer of the retina are the cell bodies and dendritic processes of retinal interneurons located?
in the inner nuclear layer
in which layer of the retina are ganglion cells (retinal projection neurons) located?
in the innermost retinal cell layer, the ganglion cell layer
name of three types of specialized interneurons in the retina
horizontal cells
bipolar cells
amacrine cells
where are retinal interneurons and ganglion cells (the retinal projection neurons) displaced?
at the fovea
so the light falls directly into the photoreceptors
cones
-contain the photopigments for color Vision
-3 classes based on their absorption spectra
1. maximally sensitive to long wavelengths (560 nm)
2. max. sensitive to middle wavelengths (530nm)
3. max sensitive to short wavelengths (420nm)
-cone density is highest at the fovea and decreases continuously in the peripheral retina
rods
-contain the photopigment rhodopsin
-suited for detecting low levels of illumination
-a single photon can activate a rod cell
-absent in the fovea
-densest along an elliptical ring in the perifoveal region (the location of max light sensitivity)
what is a photopigment?
a light-sensitive molecule found in the photoreceptor cells (rods and cones) of the retina.
bipolar cells
-connect photoreceptors directly with the ganglion cells
2 principal classes
1. cone bipolar cells
receive input from a low number of cone cells to Gove high visual acuity and color vision
2. rod bipolar cells
receive convergent input from many rod cells to give less visual acuity but increased sensitivity to low levels of illumination
horizontal and amacrine cells
-enhance visual contrast by interacting with laterally located photoreceptors and bipolar cells
-horizontal cells are located in the outer part of the inner nuclear layer whereas amacrine cells are found in the inner portion
-amacrine cells contain dopamine which plays a role in adapting retinal synaptic activity to the dark
retinal ganglion cells, 2 major classes
- M cells (magnocellular)
large dendritic arbor
integrate visual info from a large portion of the retina
major input to the visual circuit for the analysis of stimulus motion as well as gross spatial features of a stimulus - P cells (parvocellular)
small dendritic arbor
process visual informations from a small portion of the retina
these cells are color sensitive and provide a major input to the vision circuit for discriminative aspects of vision, such as distinguishing form and color
where do ganglion cell axons leave the eye?
they collect along the inner retinal surface and they leave the eye at the optic disk where they form the optic nerve.
in which layer do we have the connections between photoreceptors and retinal interneurons?
In the outer synaptic layer
where do bipolar cells synapse on ganglion cells?
in the inner synaptic layer
what increases the transparency of the retina?
the fact that ganglion cells axons are unmyelinated when they are in the retina. they become myelinated once they enter the optic nerve
which type of cell forms the myelin sheet around the ganglion cells axons?
oligodendrocytes
Muller cells
-non neuronal cells in the retina
-a kind of astrocytes
-serve important structural and metabolic functions
-their nuclei are located in the inner nuclear layer and their processes stretch vertically across most of the retina
the pigment epithelium
-non neuronal element associated with the retina
-external to the photoreceptor layer
-serves metabolic and phagocytic roles
-the retina doesn’t tightly adhere to the pigment epithelium
which artery provides the supply for the inner retina? the outer retina instead?
branches of the ophthalmic artery (branch of internal carotid artery) for the inner retina
the outer retina is devoid of blood vessels, nourishment comes from arteries in the choroid
the choroid, what is it?
a layer of ocular tissue between the sclera and the retina
where are visual stimuli from one half of the visual field processed?
in the contralateral thalamus, cerebral cortex and midbrain
where do the axons of ganglion cells of each nasal hemiretina decussate?
in the optic chiasm (where the optic nerves converge)
and enter the contralateral optic tract.
instead the axons of ganglion cells of each temporal hemiretina don’t decussate and enter the ipsilateral optic tract.
each optic tract contains axons from the contralateral nasal hemiretina and the ipsilateral temporal hemiretina
where does the optic tract splits?
in the ventral diencephalic surface
where are visual informations processed in the superior colliculus?
in the dorsal layers
where are somatic, sensory and other types of info processed in the superior colliculus?
in the ventral layers. the ventral layers also contain part of the neural apparatus for neck and eye muscle control
certain superior colliculus neurons have an axons that converge to two thalamic nuclei that serve more integrative functions than sensory relay alone. name of the 2 nuclei
- lateral posterior nucleus
- pulvinar nucleus
they project to higher order visual areas and to parietal temporal occipital association areas
which is the major retinal projection?
the lateral geniculate nucleus
the lateral geniculate nucleus sends its axons to the primary visual cortex via?
optic radiations
how many principal layers does the lateral geniculate nucleus contain?
6 principal layers
where do P cells synapse in the LGN? and M cells?
P cells in the dorsal laminae of LGN
M cells in the ventral laminae of LGN
where do neurons in the magnocellular or ventral layers and in the parvocellular or dorsal layers project?
in 2 different sublaminae of layer IV of the primary visual cortex
koniocellular system
ganglion cells (other than M and P classes) project to koniocellular layers (thin gaps in the LGN in between the magnocellular and parvocellular layers)
this system process short wavelength ligth, contributing to color vision (along with the parvocellular system)
may play a role in non color vision as well
columnar organization of the cerebral cortex. what does it mean?
neurons located above and below one another have similar properties.
the primary visual cortex also has a columnar organization because local connections primarily distribute the thalamic input vertically (from layer IV to superficial and deeper layers rather than in the same one)
connections in the same layer of the primary visual cortex also happens and they mainly run in the stria of Gennari in layer IVB
the primary visual cortex has at least 3 types of columns
- ocular dominance columns= contain neurons that receive visual input from the contralateral eye or from the ipsilateral eye (in layer IV)
- orientation columns= contain neurons that are maximally sensitive to visual stimuli with similar spatial orientation (from layer II to VI, sparing a portion of layer 4 that is insensitive to stimulus orientation)
- color columns= vertically oriented neurons primarily in layer 2 and 3 that are sensitive to the color of visual stimuli
where do neurons receive informations from one eye only?
in layer 4
where do neurons receive informations from both eyes (binocular vision)?
in layers above and below the 4th one
ocular dominance columns
columns of neurons that receive inputs preferentially from one eye rather than the other one
NB: these columns also comprehends the neurons in layers above and below the 4th one that receive stimuli from both eye
they are organized based on the pattern seen in layer 4
CHAT:
Ocular dominance columns start with neurons in layer 4 that prefer input from one eye.
These columns extend through multiple layers (2, 3, 5, 6) where neurons start to integrate input from both eyes.
The organization of these columns is based on the pattern seen in layer 4, but the columns encompass neurons that handle both monocular and binocular processing.
how are the higher order visual areas collectively termed?
the extra striate cortex (because they are outside the striate cortex/the primary area that contains the stripe of gennari)
where are the higher order visual areas principally located?
in the occipital lobe (In Broddman’s area 18 and 19)
some of them are also located into the adjoining caudal, parietal and temporal lobe
what are scotomas?
blind spots
the ventral stream to the temporal lobe carries info about?
it is the what pathway
about specific features of objects and scenes
the dorsal stream to the parietal lobe carries what type of info?
carries spatial informations
it is the where pathway
