Visual System Physiology (Pierce) Flashcards
What is the general organization of cells in the visual system?
- photoreceptor cells (rods/cones) receive light impulses
- bipolar cells (have special mGluR6 receptors) either propigate IPSP or no IPSP
- ganglion cells (have “nml” NMDA or non-NMDA receptors) either propigate an EPSP or no EPSP resulting in AP or no AP to the brain
- one branch of the visual system
- convergence: many rods + many bipolar cells to one ganglion cell
- allows system to operate in dim light
- sacrifices acuity to gain sensitivity
rod system
- one branch of the visual system
- less convergence: receptor may send info to one bipolar cell that contacts one ganglion cell
- maximizes acuity
cone system
Where does cone density peak in the retina?
cones peak at the fovea (focal point of the retina)
Where does rod density peak in the retina?
rods peak at about 20 degrees off center
(example Dr. Pierce gave was looking at the stars at night, if you look about 20 degrees of center from a star, the star will appear to shine brighter in your peripheral than it will looking directly at it because the density of rods (brightness/light sensitivity) is greatest at this point)
What is the general visual pathway in dark conditions?
(photoreceptor cells depolarize in the dark)
- photoreceptor cell depolarizes >
- graded potential generated >
- NT released (glutamate, highest conc in dark conditions) >
- NT acts as inhibitory signal (to bipolar cells) >
- bipolar cell does not depolarize, no impulse sent to the brain
What is the general visual pathway in light conditions?
- photoreceptor cell hyperpolarizes >
- graded potential is not generated >
- no inhibitory NT (glutamate, lowest conc in light conditions) released >
- bipolar cell spontaneously depolarizes, visual impulse sent to the brain
How is glutamate inhibitory in the visual pathway?
- glutamate is excitatory when considering its “nml” receptors (NMDA, and non-NMDA (AMPA or Kainate))
- bipolar cells express special glutamate receptor: mGluR6
- mGluR6: GPCR that closes cGMP-gated Na+ channels similar to light transduction in photoreceptors (allows glutamate to act inhibitory), glutamate from rod/cones closes Na+ channels in bipolar cell, thus the cell does not depolarize
How does glutamate act in ganglion cells?
- glutamate released by bipolar cells is excitatory in ganglion cells
- glutamate receptor in ganglion cells is nml NMDA or non-NMDA receptors
- depolarization of bipolar cell > excitation of ganglion cell > action potential generated
- ganglion cell axons become fibers of optic nerve
- in visual cortex, ganglion cells release glutamate
Photoreceptors
- retinal connectivity:
- neurotransmission:
- synpatic interactions:
- receptive field properties:
- electrical reponses to light:
Photoreceptors
- retinal connectivity: synapses w/ bipolar cells
- neurotransmission: constantly releases glutamate (only conc changes) which binds mGluR6 on bipolar cell
- synpatic interactions: rod system has lots of convergence, cones systems have less convergence
- receptive field properties: essentially the size of the photon that corresponds w/ that location on the retina
- electrical reponses to light: light hyperpolarizes
Bipolar cells
- retinal connectivity:
- neurotransmission:
- synpatic interactions:
- receptive field properties:
- electrical reponses to light:
Bipolar cells
- retinal connectivity: synapses w/ ganglion cells
- neurotransmission: EPSP results in release of glutamate which binds NMDA receptor on ganglion cells
- synpatic interactions: rod-associated paths can exhibit convergence upon ganglion cells
- receptive field properties: more complex than photoreceptors, the center of receptive field responds differently than the surrounding
- electrical reponses to light: w/o IPSP, bipolar cell depolarizes
Ganglion cells
- retinal connectivity:
- neurotransmission:
- synpatic interactions:
- receptive field properties:
- electrical reponses to light:
Ganglion cells
- retinal connectivity: synapses in brain
- neurotransmission: EPSP results in release of glutamate in cortical circuits
- synpatic interactions: interacts w/ other cortical layers in PVC
- receptive field properties: extremely complex, visual fields are rectangular
- electrical reponses to light: EPSP in ganglion cell, depolarizes, AP generated
- major direct target of the retina
- regulates the flow of info to PVC (axons of this area relay cells to ipsilateral side of visual cortex)
- retinotopic organization maintained
- has a 6 layer lamellar structure (half from nasal half from temporal fields)
- signals from two eyes are kept apart in this area
lateral geniculate nucleus (LGN)
also called lateral geniculate body (LGB)
What are the functions of the LGN?
- control the motions of the eyes to converge on a point of interest
- control the focus of the eyes based on distance
- determine relative positions of objects to map them in space
- detect movement relative to an object
- direct target of the retina
- connects w/ tectospinal tract to send projections to cervical anterior horn cells (for eye movement)
- creates map of visual space to activate appropriate motor responses required to move eyes into their intended position within the orbits
- specifies movement intention rather than fixation of movement upon a target
superior colliculus
- direct target of the retina
- pupillary light reflex: sends projections to Edinger-Westphal then on to ciliary ganglion
pretectum
How are objects in the left visual hemifield sent to the right side of the brain and vice versa?
This has to do with the way the light from the object is refracted into the retina, left visual field is sent to left eye nasal retina (goes to right side of brain) and right visual field is sent to right nasal retina (goes to left side of brain)
What 3 areas make up the visual cortices?
- primary visual cortex (has unique 6 layers that maintains retinotopic org)
- parastriate cortex
- extrastriate cortex
What is the role of V1 visual cortex?
- identifies edges and contours of objects
- also decodes visual input to redirect info to subsequent steps of analyses to other visual areas
What is the role of V2 visual cortex?
depth perception, which is detected by analyzing disparities between two eyes
(Dr. Pierce used example of optical illusions and 3D stereograms)
What is the role of V3A visual cortex?
(anterior to V3)
- major indentification of motion
- “Is motion happening? Yes or no?”
What is the role of V4 visual cortex?
- complete processing of color inputs
- lesions here can cause achromatopsia (unilat lesions may go unnoticed)
What is the role of MT/V5 visual cortex?
(MT = middle temporal)
- tracks motion across a scene in terms of directionality and context of background/foreground
- does so by containing neurons that selectively respond to direction of moving edge
What are the unique structures present in the primary visual cortex that lead to specificity of eye laterality, angular distinction, and color vision?
ocular dominance columns (eye laterality), orientation columns (angular distinction), and blobs (color vision)
(again, these are unique to V1, as seen in the photo below these structures become much less defined in V2)
- columns that vertically span all 6 layers of the PVC
- give rise to stripes that can be seen on the cortical surface and in cross-sectioning of the PVC
- the cells in these columns preferentially respond to input from one eye or the other (ocular dominance)
ocular dominance columns
- columns that span all 6 layers of the PVC
- organized region of neurons that are excited by visual line stimuli of varying angles
- oriented perpendicular to the cortical surface (and to ocular dominance columns)
orientation columns
- span all 6 layers of the PVC
- organized region of neurons that are sensitive to color, assemble into cylindrical shapes
- all 3 color-coding cones are required for accurate color detection
- this is the ultimate destination of cone inputs
blobs
How do melanopsin ganglion (MG) cells help regulate circadian rhythm?
- 1-3% of ganglion cells directly sense light (MG cells)
- due to expression of melanopsin, these cells are sensitive to blue-wavelengths of light
- they then provide light-sensing info for regulation of circadian rhythm (considered a non-image forming light-responsive system)
- mechanism: MG cells project directly to hypothalamic suprachiasmatic nucleus (via retino-hypothalamic tract), the neuroendocrine effector in the pineal gland which then produces melatonin in a rhythmic pattern
TLDR: MG cells > hypothalamus SCN > pineal gland > melatonin
*blue light suppresses our body’s release of melatonin*
What structures are involved in the dorsal pathway?
primary visual cortex (V1) > V2, V3 > V5/MT > parietal/frontal cortex
What structures are involved in the ventral pathway?
primary visual cortex (V1) > V2 > V4 > temporal lobe
