What are the 3 types of photoreceptors in the retina?
What does the outer segment of photoreceptors consist of?
What does the inner segment consist of?
Just as different types of mechanoreceptors result in submodalities within the somatosensory system, ROD and CONE photoreceptors (named for the shape of the photoreceptor outer segment) are the source of separate submodalities within the visual system with numerous functional and anatomic distinctions.
The outer segment consists of a stack of membranous discs, which contain the photopigment and the light transducing apparatus. The inner segment contains the cell nucleus and most of the biosynthetic machinery
State the differences btwn rods and cones.
State the differences btwn the rod and cone system.(acuity, convergence, where present in retina, pigment)
What is the purpose of rods?
Where is the blind spot in the eye? Why are we not conciously aware of our blind spots?
Rods alert us to stimuli (outside of the fovea) which causes us to look over and focus on it.
Attached pic: Note the concentration of cones in the fovea and absence of rods in the same area.
blind spot: where all axons converge to form optic nerve. doesn't bother us because our eyes do not have the same blind spot. Also, the visual system fills in holes.
T or F: Each cone has a pigment, which preferentially absorbs light in the red, green or blue wavelength.
Explain this graph. Describe the reaons for changes in acuity in the different parts of the retina.
- When using cones in bright light, acuity is highest in the fovea, but falls off rapidly.
-When using rods in dim light, acuity is very low, and it is zero in the fovea because no rods are present there.
- Note also the blind spot where there are no photoreceptors
What are the syptoms of retinitis pigmentosa?
What is the cause of retinitis pigmentosa?
What type of vision is affected?
Steady-state turnover of photoreceptor outer segments occurs by shedding of discs from the distal portion of the outer segment and manufacture of new discs at the proximal region. Under normal circumstances, the discarded disc packets are rapidly phagocytized by the retinal pigment epithelium. Retinitis pigmentosa is a genetic disorder wherein these packets are not properly phagocytized and accumulate in the subretinal space -- eventually producing retinal detachment and blindness.
What is macular degeneration?
What type of vision is affected?
What population is most affected by this disease?
macular degeneration-AMD, the most common cause of blindness in the elderly). - Due to retinal dysfunction preferentially affecting cone vision.
Visual fields might show bilateral central scotomata. Peripheral vision may be normal.
In Microbiology you and a friend are reviewing gram stained slides for laboratory. He sees the organisms clearly, but can't identify them as gram positive or gram negative. Why?
How does one get this disorder?
Who is more commonly affected?
What are the effects on visual acuity?
- He has a color vision defect and is unable to differentiate red and green.
- Common genetic condition (~8% of males).
- Gram stains are differentiated by color, not shape.
- Visual acuity is usually unaffected, (there is a normal number of cones and normal amount of photopigment, but the photopigments are not adequately differentiated)
Explain the cellular response of photoreceptors to light.
Two transmembrane protein molecules define the unique physiology of the vertebrate photoreceptor. Together they are responsible for the transduction of light into an electrochemical signal in the nervous system.
1. Visual pigment
a. Chromophore Vitamin A aldehyde (retinal)
b. G-protein coupled receptor
c. Controls cGMP phosphodiesterase
d. Absorption spectrum determines color vision
2. cGMP gated channel
a. Kept open by bound cGMP
b. Permeable to sodium and calcium ions
c. Dark current modulated by light
d. Adaptation related to calcium influx
When light strikes the photoreceptor (illustrated in pic by a rod cell, same process occurs in cones) the cGMP-gated channels are closed by a three-step process. (1) Light is absorbed by and activates pigment molecules (rhodopsin in rods) in the disc membrane (the green rectangle in the rhodopsin molecule represents the light absorbing portion, retinal). (2) The activated pigment stimulates a G protein (transducin in rods), which in turn activates cGMP phosphodiesterase. This enzyme catalyzes the breakdown of cGMP to 5'-GMP. (3) As the cGMP concentration is lowered, the cGMP-gated channels close, thereby reducing the inward current and causing the photoreceptor to hyperpolarize.
Explain the amount of NT release by photoreceptors in light compared to dark. What is the reason for the change in NT release?
What NT is released by photoreceptors?
In the dark, due to the depolarizing Na (and Ca2+) current, the photoreceptors constantly release glutamate at their terminals. Light induced hyperpolarization reduces the release of glutamate. Note that photoreceptors are the only sensory neurons that hyperpolarize in response to the relevant stimulus.
The brain senses firing of APs. It doesn't matter whether or not the cell is hyperpolarized or depolarized by its stimulus.
What cell types are present within the retina?
State what cell types synapse on other cell types.
Fibers of what cells form the optic nerve?
The retina has five major classes of neurons arranged in layers: photoreceptors (rods, r, and cones, c), bipolar cells (b), horizontal cells (h), amacrine cells (a), and ganglion cells (g). The ganglion cell axons leave the retina to form the optic nerve. Photoreceptors synapse on the bipolar cells and the horizontal cells. Bipolar cells in turn synapse on the ganglion cells and amacrine cells.
A. Light comes through the vitreous and through the retina to strike the photoreceptors
B. Bipolar cells connect photoreceptors to ganglion cells
C. Lateral interaction permits detection of local contrast
What is the response of off-center bipolar cells in response to glutamate release by photoreceptors?
What is the response of on-center bipolar cells in response to glutamate release by photoreceptors?
What receptors are involved in the different repsonse of these cells to glutamate binding?
- Photoreceptors are hyperpolarized by light onset
- Bipolar cells may be excited or inhibited by the glutamate released by photoreceptors dependent on two different types of glutamate receptors.
Off-center bipolar cells have traditional depolarizing glutamate receptors (AMPA) and thus are sign conserving (e.g. when photoreceptors are inhibited by light, so also are the off center bipolar cells).
- The on-center bipolar cells have a distinct metabotropic glutamate receptor, APB, which closes a channel to cations when glutamate binds to it. This is referred to as sign reversing.
What is the general response of ganglion cells to bipolar cell input (excitatory, inhibitory)?
What is the role of horizontal and amacrine cells in input of bipolar cells to ganglion cells?
What are the only cells in the retina that have voltage gated Na+ channels? Why?
- Ganglion cells are generally excited by bipolar cells
- Horizontal and amacrine cells may modulate these responses (not shown)
- Note that only the ganglion cells have voltage gated sodium channels. These propagate action potentials through the optic nerve.
What is a receptivie field of a visually responsive neuron?
The receptive field of a visually responsive neuron is the region of the visual world in which a stimulus of the proper characteristic (i.e. position, color, shape, movement) will influence the activity of a neuron. The receptive field of each neuron reflects the convergence and divergence of its predecessor neurons and thereby takes on new characteristics. Each photoreceptor is hyperpolarized when its location on the retina is illuminated by the appropriate wavelength of light, but it does not differentiate a large from a small spot of light.
How are bipolar cell receptive fields established? Explain the center-surround receptive field organization.
What is the purpose of this type of organization of receptive fields?
Bipolar cell receptive fields are established by their photoreceptor and horizontal cell inputs. Each bipolar cell has a circular receptive field with a central portion and an opponent surround. The center may be hyperpolarized or depolarized by light dependent on the type of glutamate receptors it has. The opponent surround is provided by horizontal cells. The center-surround receptive field organization provides for contrast enhancement so that small spots or the edges of objects are best detected. Bipolar cells are further differentiated between those that receive primarily rod input and those that receive primarily cone input.
Explain how receptive fields of retinal ganglion cells are organized.
What is the difference btwn on center and off center ganglion cells?
What is the difference btwn Y/M and X/P type retinal ganglion cells?
How ganglion cell receptive fields are built in the visual system
A. Receptive fields of retinal ganglion cells maintain the center-surround organization of bipolar cells, but are further modified by amacrine cells, which provide information about change over time.
o Receptive fields of retinal ganglion cells are circular and have antagonistic portions
o Light stimulation of the center and surround have opposite effects
o On-center cells are maximally excited by light filling the center and maximally inhibited by light filling the surround (annulus). Reverse for Off-center cells
B. On center ganglion cells fire maximally to light in the center of the receptive fields
o Cells may show no response to diffuse light (large stimuli)
o Y/M type ganglion cells respond preferentially to changes in light rather than steady state light.
o Vertical lines represent action potentials in response to the visual stimulus presented
C. X/P type retinal ganglion cells show color opponent properties; inhibited by red in the center and excited by green in the surround, and another ganglion cell shows the reverse.
attached is pg 84 of course notes
What ganglion cells receive primarily rod input?
What ganglion cells receive primarily cone input?
What are the differences btwn the sizes of receptive fields for these two cell types?
What are these 2 cell types specialized for?
Due to selective bipolar cell input, ganglion cells are further differentiated between those that receive primarily rod input (Y/M type ganglion cells) and those that receive primarily cone input (X/P type ganglion cells).
The Y/M type ganglion cells receive input from many bipolar cells and thus have large receptive fields. They are specialized for change in light (onset or offset) and movement in low light conditions.
In contrast, the X/M type ganglion cells receive input from only a few bipolar cells and thus have small receptive fields. They are specialized for fine resolution, color vision and respond primarily to hi intensity light.
What layers of the LGN/LGB do parvocellular cells (X/P type ganglion cells) synapse in?
What layers of the LGN/LGB do magnocellular cells (Y/M type ganglion cells) synapse in?
What layers of the LGN are represented by the contralateral eye? The ipsilateral eye?
Lateral Geniculate Nucleus
A. Layers : 1,4, 6 contralateral; 2, 3, 5 ipsilateral
1. Parvocellular layers: 3, 4, 5, 6
2. Magnocellular layers: 1, 2
What is the function of the superior colliculus?
Pretectal Nuclei? Suprachiasmatic nucleus?
Fibers from the LGN synapse in what layer of the cortex? What is the Brodmans number for this part of the cortex?
Visual perception depends on primary visual cortex as well as what other cortex?
A. Visual attention, eye movements
A. Pupillary responses
Suprachiasmatic nucleus (in anterior hypothalamus)
A. Circadian pacemaker
Primary Visual Cortex
A. Most fibers from the lateral geniculate synapse in layer 4 of the cortex (area 17, calcarine cortex)
B. Visual perception depends on primary cortex and secondary visual cortex (visual association areas)
Fibers traveling through Meyer's loop represnt what part of the visual field? Lesion of this loop leads to what type of anopsia?
Optic radiations: Fibers representing the superior potion of the visual filed travel from the lateral geniculate nucleus into the temporal lobe (through Meyer’s loop) on their way to the visual cortex. Damage to this loop causes a superior quadrantanopia in the contralateral visual fields of both eyes.
How is the receptive field for a cortical neuron determined?
What is the responsiveness of cortical neurons to diffuse light and small spots of light?
Receptive field determination for a cortical neuron
- Record action potentials from a neuron while presenting various visual stimuli to different parts of the visual field with the eye still.
- Determine which stimuli increase or decrease the firing of action potentials.
The recording from this cortical neuron shows the strongest response a bar of light at a particular orientation at a particular location in the visual field. For simplicity only a neuron with a receptive field with a vertical axis of orientation is shown in this figure. In fact, all axes orientation-vertical, horizontal and various obliques- in each region of the retina are represented in the primary visual cortex. This elongated receptive field apparently is based on convergence of selected neurons in the lateral geniculate nucleus to neurons in the cortex with partially overlapping receptive fields. The cortical cells are relatively unresponsive to diffuse light or small spots of light.
What do simple cells of the cortex respond to?
What are features of effective visual stimuli?
Simple cells of the cortex respond most strongly to elongated lines with a particular orientation. This may explain why perceptions preferentially recognize linear forms. It appears that the nervous system builds the visual world from a concatenation of straight lines.
Features of effective stimuli:
a. position in the visual field
What is ocular dominance (within the cortex)?
How is ocular dominance arranged within the cortex?
Answer the same questions for orientation.
How are neurons sensitive to color represented within the visual cortex?
Where in the cortex is intensity represented?
What cells detect motion within the cortex?
Cortical Organization: Cortical Columns: variations on a common theme used to represent different features (attributes) of a stimulus
1. Ocular dominance: cortical neurons tend to be driven more strongly by one eye or the other. For a given portion of the visual field neurons responding preferentially to the left eye are located together in a cortical column or slab which is perpendicular to the cortical surface: ocular dominance column
2. Stimulus orientation preference: neurons responding to the same orientation at a given portion of the visual field are arranged in a column which is perpendicular to the cortical surface: orientation column
a. Adjacent columns of cells respond most strongly to lines at other orientations in the same portion of the visual field.
b. The orientation of small portions of the visual world appears to be the basic substrate of form vision.
3. Color representation: neurons sensitive to color are arranged in columns, which are perpendicular to the cortical surface. When cut like a salami these columns appear as "blobs" in various parts of the visual cortex.
4. Motion detection: a property that is distributed primarily among the achromatic cells
5. Intensity: a property that is distributed among all cells. Cortical neurons are more sensitive to contrast, than absolute light intensity.
What are hypercolumns?
What do hypercolumns contain?
Hypercolumn: a collection of vertical columns in the visual cortex roughly 500-1000 um on a side in which each column represents the processing of a particular visual feature. All of the cells within the hypercolumn have receptive fields in the same part of the visual field. The size of the visual field represented by a given hypercolumn is one receptive field in width. Cells in hypercolumns representing the central part of the visual field have much smaller receptive fields than those in hypercolumns representing the periphery.
Each hypercolumn contains:
a. Two ocular dominance columns: one column for each eye of dominance.
Each ocular dominance column contains:
i. stimulus orientation columns: a column for each orientation
ii. cortical "blobs" (columns) concerned with color
After reaching the primary visual cortex, informatin is further projected to two major cortical regins, one to the parietal occipital cortex and the other to the occipital temporal cortex.
What functions do each of these cortices have?
What will damage to these areas do?
Which cortex are magnocellular layers located in? Parvocellular layers?
After reaching primary visual cortex information is further projected to two major cortical regions, one to the parietal occipital cortex to deal with eye movements and spatial localization (including integration of external and internal representations of space) (magnocellular layers), and the other to the occipital temporal cortex to deal with color and form recognition (including faces and words) (parvocellular layers).
Damage to these areas has different effects: parietal occipital: Neglect or deficits in spatial orientation; temporal-occipital: color or form agnosia. The form agnosia may be specific for faces (prosopagnosia) or inanimate objects.
Face recognition cell in the temporal lobe
Individual neurons in the temporal lobe respond most strongly to complex, biologically significant stimuli, especially faces.