bio 397 exam 3
The Opsins are G-protein coupled receptors (GPCRs)
- Levels of light are proportional to hyperpolarization and changes in glutamate release. - more light absorbed, more hyperpolarization, less Glu release - the phototransduction pathway amplifies light signal. - A rod photoreceptor is capable of sensing a single photon of light
neurons in the magnocellular layers (mLGN cells)
- process M-retinal ganglion cell inputs - behave like M-retinal ganglion cells - have relatively large center-surround receptive fields - color insensitive - most sensitive to movement of visual stimuli
The range of wavelengths in the visible electromagnetic spectrum is...
-400nm to -700nm
Why do we say that ganglion cells are mainly responsive to differences in illumination that occur within their receptive fields?
Center-surround ganglion cells respond best when a light or dark edge falls at the boundary between the center and surround regions of the receptive field.
Most of the eye's refractive power is accomplished by what structure?
Cornea
The M channel or magnocellular pathway is involved in the analysis of object motion and the guidance of motor actions. Which of the following receptive field characteristics reflects this specialization
Direction selectivity
Why does the fovea appear like a pit in a cross-sectional view of the retinal?
Due to the lateral displacement of cells above the photoreceptors in the fovea that allows light to strike the photoreceptors without going through the other layers.
Which layer of retinal neurons has axons that leave the retina?
Ganglion cell layer
Which retinal neurons fire action potentials?
Ganglion cells
Phototransduction
Phototransduction - the process through which the absorption of photons is converted into electrical signals. · The photosensitive proteins are Rhodopsin in rods and opsin in cones (red, green, blue ,(UV)) · the vitamin A derivative, retinal, is linked to the Rhodopsin and Opsins and is required for light activation
• Intrinsically photosensitive Ganglion cells (ipRGCs) express...
a light sensitive protein Melanopsin • project to the suprachiasmatic nucleus, which is important for modulating circadian rhythms
The rod pathway is very light-sensitive because of these features:
a. rods have larger outer segments with more phototransduction proteins than cones b. many rod photoreceptors can converge and synapse onto a single rod bipolar neuron
In the central part of the retina, high _________ is achieved by a one-to-one synapse between a cone and cone bipolar neurons (BP) • in regions more peripheral to the fovea, _________ is high, and many rods converge into a rod bipolar neuron (high convergence)
acuity, sensitivity
Orientation Selectivity
derives from the added (integrated) input of layer IVCb neurons (that receive input from LGN neurons that relay single ON-center surround RGCs) that converge onto layer IIIB neurons.
Striate cortex (primary visual cortex: V1)
first binocular neurons are in layer III in striate cortex • striate cortex (primary visual cortex: V1) has 6 layers (I-VI) o layer IVC: spiny stellate neurons (spine covered dendrites) o layers III, IVB, V, VI: pyramidal neurons (spines and thick apical dendrite) o all layers: inhibitory interneurons that form local connections
Ocular Dominance:
first binocular neurons are in layer III, these alternate with left and right dominant columns
Neighboring RPE cells are crucial for
maintaining opsin activity by 'regenerating' 11-cis retinal for bleached opsins.
LGN
most RGC axons go to the LGN (other targets include the hypothalamus, pretectum, superior colliculus). • Input into the LGN is segregated into layers representing eye (right/left) and ganglion cell type (P, M, konio)
Photoreceptor Maintenance and Retinal Degeneration Disease
most cases of vision loss is due to degeneration of photoreceptors or retinal ganglion neurons. • glaucoma causes retinal ganglion neuron and axonal degeneration (too much intraocular pressure) • night blindness (retinitis pigmentosa) is due to rod degeneration (loss of peripheral vision) • macular degeneration is due to cone degeneration (loss of central vision)
koniocellular neurons (kLGN)
process P-retinal ganglion cell inputs - behave like P-retinal ganglion cells - have the smallest concentric receptive fields - have stronger color sensitivity than P-retinal ganglion cells - well suited for detecting colors that aid in shape/form discrimination
• neurons in the parvocellular layers (pLGN cells)
process P-retinal ganglion cell inputs - behave like P-retinal ganglion cells - relatively small center-surround receptive fields - color sensitive - well suited for detecting contrasts that form the basis for shape/form discrimination.
A ganglion cell's 'receptive field' can be measured by...
recording it's action potential firing rate, while moving a light stimulus across the retina.
dark currents
rod outer segments are depolarized in the dark because of a steady influx of Na+ (because a lot of cGMP-gated Na+-channels are open)
Central Processing
the left hemifield projects to the right side of the brain; the right hemifield to the left side of the brain • retinal ganglion cell (RGCs) axons from the nasal retina cross in the optic chiasm. • the central most visual field (the fovea) is overrepresented in the retinotopy map in the brain
The Eye
· Eye structures work together to increase visual acuity and sensitivity · Visual Field- the amount of space viewed by the retina when eye is fixated straight ahead · Develops from multiple germ layers (endoderm, mesoderm, ectoderm): - the retina and retinal pigmented epithelium (RPE) are CNS derived tissues
The Visual System
· Perceives form and movement and distinguishes color · identifies objects in complex visual field · used for guiding our movements
In the process of signal amplification, a single G-protein is activated by each photopigment molecule, and each PDE enzyme breaks down a single cGMP molecule.
False, each photopigment molecule activates many G-proteins and each PDE enzyme breaks down more than one cGMP molecule.
The sclera is the glossy transparent external surface of the eye that covers the pupil and the iris.
False, this is the cornea. sclera is the white part of our eye.
Which layer in the retina contains the cell bodies of the bipolar cells, horizontal cells, and amacrine cells?
Inner Nuclear Layer
Which of the following statements about LGNs is true?
LGN receptive fields are monocular.
Primate Retina has specialized ganglion neurons: M-type and P-type
M-type- responds maximally to MOVEMENT (color insensitive) • P-type- COLOR sensitive
Photoreceptors Feed Parallel Bipolar (BP) Cell Pathways (ON and OFF)
ON BP express the mGluR6 glutamate receptor that is a GPCR. In dark conditions, elevated glutamate release by photoreceptors activates mGluR6 that keeps Na+ (TRPM1) channels closed. In response to light, less glutamate released by photoreceptors, allows Na+ (TRPM1) channels to open and the ON BP DEPOLARIZES • OFF bipolar neurons express Glutamate-gated cation channels (AMPA/Kainate receptors). • Rod BP are ON (express mGluR6)
What is a difference between ON-center ganglion cells and OFF-center ganglion cells?
ON-center ganglion cells receive input from ON bipolar cells, whereas OFF-center ganglion cells receive input from OFF bipolar cells
LGN projections to V1
Parvocellular LGN neurons -> terminate in layer 4Cb • Magnocellular LGN neurons -> -> terminate in layer 4Ca • Konicellular LGN neurons -> -> terminate in patches in layer 2/3
What is dark current?
Photoreceptors are depolarized in the dark because of a steady influx of sodium ions
What is a receptive field?
The area of the retina that, when stimulated with light, changes a ganglion cell's firing rate
Which of the following does NOT occur when the center brightens while the surround remains dim:
The receptive cone depolarizes
Retinal Processing/Circuits
The retina is an important model system for studying neural circuitry: Its role in perception is clear, its cell types are relatively well defined, and its responses to light understood
What is meant by the term duplex retina?
The retina is capable of both scotopic and photopic vision
What is absorption?
The transfer of light energy to a particle or surface
A center-surround receptive field consists of two areas with antagonistic responses to the presence or absence of light
True
What is a center-surround receptive field?
Two concentric areas with antagonistic responses to light on and light off
A cone ON Bipolar Neuron...
a. Depolarizes when the cone it synapses with absorbs photons d. Expresses the mGluR6 receptor
What statement(s) are TRUE about Retinitis Pigmentosa?
a. Rod photoreceptors primarily degenerate b. Peripheral vision is lost
Which of the following ganglion cell axons cross to the opposite side of the brain?
b. All axons originating in the nasal retina
The Retina
· three layered structure (retinal ganglion cell layer, inner nuclear layer, outer nuclear layer) · neuronal layers are interspersed by synaptic layers (inner plexiform layer and outer plexiform layer) · the optic nerve is the projection of retinal ganglion cell axons to the brain · Photoreceptors transduce light, provide information (graded) about light intensity to bipolar neurons that synapse with retinal ganglion cells · Ganglion Cells are the only retinal neurons that fire action potentials. · the Fovea is a specialized cone photoreceptor dense region organized for optimal acuity
Photoreceptor Neurons
· two types of photoreceptors: Rods (used for dim light, Scotopic) and Cones (used for bright light, Photopic and color detection) · Individual cones have color (wavelength) sensitivity · The photoreceptor outer segment has stacked membranous discs that massively increase surface area to concentrate the membrane associated phototransduction machinery for maximal light sensitivity
ON/OFF Center Ganglion cells
• ON-Center Ganglion cells increase firing rates when a brighter center compared to surround is detected • OFF-Center Ganglion cells decrease firing rates when a darker center compared to surround is detected • ON-Center and OFF-Center Ganglion activity is due to specific synapses with BP neurons • ON-Center Ganglion cells synapse with (receive input from) ON BP • OFF-Center Ganglion cells synapse with (receive input from) OFF BP
photoreceptors adapt to changes in light (light and dark adaption) using several mechanisms
• dark adaption • dilation of pupils (lets in more light) • circuitry switch to rod-based vision (instead of cones) • regeneration of bleached rhodopsin • light adaption • calcium concentration changes alter the activity of the phototransduction pathway • changes in production of cGMP -> regulates cGMP-gated Na+-channel activity