Fundamentals of Neuroscience Exam 3

phototransduction

• physical stimuli (photons) are converted into electrical signals that can be processed by specific circuits at successive stages

• occurs in the outer segment disk membrane of photoreceptors

• photoreceptors hyperpolarize in response to light stimulus

fovea

• a small depression in the retina where visual acuity is the highest due to cone concentration

• location in the retina where light has a direct path to photoreceptors

eccentricity

the varied/various degrees of responses to light across the retina

encoding

representation of visual information is transformed and processed and becomes more abstract along the visual pathway

decoding

visual scenes are “inferred” from the encoded information to become perception

dynamic range

the ratio between the largest and smallest values of the working range of luminance values, >10^-11 fold

rods

• work better at lower light levels since they are very sensitive (less light needed to change membrane potential)

• do not contribute to color vision

• saturate at higher light levels, slower to recover

• multiple rods pooling to 1 bipolar cell loses spatial resolution/acuity

• fire graded potentials not action potentials

• rod bipolar cells On only (sign-inverting), synapse onto A2 amacrine cells, synapse onto cone bipolar cells

cones

• contribute to color vision

  • three types of cone opsins- S (blue), M (green), and L (red)

  • color is sensed by comparing signals from cones with different spectral sensitivities

    • blue-yellow, red-yellow, or red-green color opponent

• work at higher light levels since they have a higher threshold

• concentrated in the fovea for high acuity/spatial resolution

• less sensitive than rods (need more light to change membrane potential) but faster

• 1 cone to 1 bipolar cell maintains high spatial resolution/acuity

• fire graded potentials not action potentials

adaptation

• the process by which the visual system alters its operating properties in response to changes in the environment

• light adaptation- photoreceptors (visual system in general) becomes less sensitive to the same intensity of stimulation when the background illumination is higher

  • increasing background light intensity decreases light sensitivity so that more light is required to elicit the same response

  • brighter background → less Ca2+ → increased GC activity → more cGMP produced → more light is needed to have a stronger PDE activation

• adaptation expands the dynamic range of light intensity within which rods can distinguish the intensity differences between background and objects

Weber’s law

the minimum increase of stimulus which will produce a perceptible increase of sensation (just noticeable difference) proportional to the pre-existing stimulus

perception of light intensity

• brightness constancy- a white object appears to be white whether the environment is light or dark

• visual stimulus = illumination * reflectance of surface

  • visual system only cares about reflectance as a property of objects not the environment/number of photons

vertical pathway

pigment cells → photoreceptors → bipolar cells → retinal ganglion cells (RGCs)

horizontal pathway

horizontal and amacrine cells

rhodopsin (GPCR)

• composed of opsin (7 transmembrane domain protein) and 11-cis retinal (chromophore within opsin microenvironment)

• in response to light, retinal changes conformation from 11-cis to all-trans, causing opsin to open up → photoisomerization

• active rhodopsin binds with transducin (heterotrimeric G-protein) and activates it via GDP-GTP exchange

• Talpha activates phosphodiesterase (PDE), which converts cGMP into GMP

  • in dark, mostly guanylate cyclase producing cGMP; in light, mostly PDE converting cGMP into GMP

• cGMP activates a cyclic nucleotide-gated (CNG) cation channel

  • more cGMP increases opening probability of CNGs

cyclic nucleotide-gated (CNG) cation channel

• cGMP activates a cyclic nucleotide-gated (CNG) cation channel

  • more cGMP increases opening probability of CNGs

• 6 transmembrane domains per subunit, 4 subunits, each with a selectivity pore between S5 and S6 and a cyclic nucleotide binding domain (CNBD) at S6

• nonselective cation channel (Na+, K+, Ca2+)

• found in the outer segment membrane of photoreceptors

• constant outward potassium ion current so that cell hyperpolarizes when CNG channels close, maintains membrane potential in the dark

recovery

• return to dark state

• cGMP increases to reopen CNG channels via calcium-dependent guanylate cyclase (GC) activity

  • decrease in calcium causes GC to increase production of cGMP from GTP

• Talpha-GTP deactivation via intrinsic GTPase activity, converts itself to inactive Talpha-GDP state

  • Talpha then disassociates from PDE, inactivating PDE

• rhodopsin deactivation via the binding of rhodopsin kinase to the cytoplasmic tail of R*, which it phosphorylates so that arrestin can bind and prevent transducin binding

• retinal back to 11-cis via exchange with pigment cells

  • retinoid cycle, slow process

blind spot

place in retina in which all axons exit the retina to go to the brain

Macular degeneration is damage to the macular, or center, of the retina. Why is loss of this area so devastating to vision?

The macula is where the fovea is located, and damage to the fovea would affect color vision, spatial resolution, and acuity due to the concentration of cones in the fovea.

Light is absorbed by the protein ______, which changes from _____ to a _____ configuration. This causes a structural change in the ____ protein. This conformational change allows the G protein called __________ to bind, and results in the exchange of _____ for GDP. The _______subunit of the G protein then activates the protein ______, which breaks down cGMP. The reduction in cGMP causes CNG channels to ______ and the cell to ______.

retinal, 11-cis, all-trans, opsin, transducin, GTP, Talpha, PDE, close, hyperpolarize

Figure 4-15 in the textbook shows the currents from a rod and a cone in response to increasing magnitude of light flashes. How do these responses contribute to the higher sensitivity in low light and higher acuity in daylight conditions?

It takes less light stimulation to produce current within rods, which is their property of sensitivity. This sensitivity allows us to see even when it is dark and there is little light in the environment since rods are stimulated by small amounts of light. It takes more light stimulation to produce current within cones, meaning that they can function in bright environments with high acuity, as they will not become saturated in daylight like rods do.

receptive field

• the area of visual field (or corresponding area of the retina) from which activity of a neuron can be influenced by visual stimuli

• become larger and more complex in property along the visual pathway

RGCs

• fire action potentials

• use center-surround receptive fields to analyze local contrast

  • two parallel pathways:

    • On-center/Off-surround excited by light in center or turning to dark in surround

    • Off-center/On-surround excited by light in surround or turning to dark in center

  • antagonistic organization

• do not simply respond to light but start to analyze spatial patterns, contrasting light and dark over a small area of the retina, respond to change

• tiling of On and Off RGCs- at any given location in the retina, a signal reaches both On and Off RGCs to be processed in parallel pathways

bipolar cells

• use glutamate

• either depolarized or hyperpolarized by light depending on the glutamate receptors they express

  • On bipolar cells express metabotropic glutamate receptors → less glutamate = less Gi activity = cation channels are less inhibited/more active = depolarization (sign-inverting)

  • Off bipolar cells express ionotropic glutamate receptors → less glutamate = more receptors close = fewer cations enter cell = hyperpolarization (sign-preserving/sign-conserving)

horizontal cells

• use glycine (inhibitory)

• mediate lateral inhibition

  • sign-conserving when receiving signal from surround cones

  • sum the activity of many cones in a region

  • sign-inverting when synapsing back onto center cones

• responsible for the antagonistic surround of ganglion cells

computational

• what does the system do?

  • RGCs- detect contrast

  • DSGCs- detect motion direction

algorithm

• how does the system do what it does?

• RGCs- lateral inhibition (enhances contrast detection)

• DSGCs- asymmetric inhibition (enhances motion detection)

implementational

• how is the system physically realized?

  • RGCs- horizontal cell connections

  • DSGCs- direction-selective GABA release at synapses between SACs and DSGCs

On-Off DSGCs

• can have both On and Off center-surround receptive fields

• direction selective- encoding the direction of the stimuli, responding more to a specific preferred direction and less or not at all to a null direction

• delayed excitation causes signals to align with and add to each other when the stimulus travels in the preferred direction

• asymmetric inhibition suppresses the null direction due to the alignment of the inhibitory stimulus with the excitatory stimulus, which cancels out to produce no output.

starburst amacrine cells (SACs)

• use both GABA and acetylcholine

• can be ON or OFF, visually responsive inhibitory cells

• modulate DSGCs

• do not have axons, instead have an elaborate system of dendrites

• have more synaptic contacts on the DSGC’s null side than preferred side because of asymmetric inhibition (excitatory stimulus from bipolar cells can occur before inhibitory stimulus from SACs)

ipRGCs

• intrinsically photosensitive- have their own phototransduction pathway with the GPCR melanopsin

• depolarize in response to light

• project to suprachiasmatic nucleus (SCN)

• important for Circadian rhythm and mood regulation

which visual system cells use action potentials?

RGCs and amacrine cells

A photoreceptor cell is exposed to a flash of light. How does the membrane potential of this cell and its corresponding ON-center bipolar and ganglion cells change?

hyperpolarize, depolarize, depolarize

How would an OFF-center ganglion cell’s firing rate change when a light was turned on, turned off, and then turned on again?

decrease, increase, decrease

What do you think is the advantage of the retina using graded potentials?

More variation in magnitude, amplitude summation, and their ability to be depolarizing or hyperpolarizing allow for more complex signaling over short distances

How does the influence of horizontal cells help explain the optical illusion in the Mach bands in which the edge of a gray band appears darker on one side and lighter on the other?

Lateral inhibition mediated by horizontal cells is the cause of the antagonistic nature of center-surround in RGCs, which enhances local contrast that allows us to see illusions like the Mach bands in which the bands have contrasting shades of gray.

binocular vision

• the binocular visual field is seen by both the right and left eyes

  • the nasal retina of the left eye and the temporal retina of the right eye take in information from the left visual field

  • the nasal retina of the right eye and the temporal retina of the left eye take in information from the right visual field

  • temporal retina axons stay on the same optic tract side (ipsilateral), while nasal retina axons cross over to the optic tract on the other side (contralateral)

  • information from the left visual field is processed by the right hemisphere of the brain while information from the right visual field is processed by the left hemisphere of the brain

lateral geniculate nucleus (LGN)

• has layers organized by eye- anatomically segregated

  • layers 1 and 2 have larger cell bodies and are called the magnocellular layers

  • layers 3-6 have smaller cell bodies and are called the parvocellular layers

  • layers 1, 4 and 6 receive contralateral input

  • layers 2, 3 and 5 receive ipsilateral input

• parallel processing- 3 layers represent left eye and 3 layers represent right eye

• receptive fields are similar to those of RGCs (center-surround, ON-OFF antagonistic organization)

• “relay cells”

primary visual/striate cortex (V1)

• 6 layers

  • layer 4 monocularly segregated in alternating bands → ocular dominance columns

  • other layers have convergence, respond to both eyes (binocular)

  • topographically and systematically represents retinal information

• information flow

  • layer 4 → layers 2/3 → layer 5 → layer 6

• cells with similar preferred orientation are organized in vertical columns in large mammals

• cortical modules

simple cells (V1)

• orientation selective- respond to specific light orientations much more strongly than others

• feedforward model- V1 neurons respond to lines and edges of certain orientation, LGN neurons converging onto a V1 cell have specifically aligned receptive fields that form the preferred receptive field orientation for that V1 cell

complex cells (V1)

• still orientation selective but no clear on/off regions in receptive field- position invariant, more abstract, respond to motion

• feedforward model but with simple cells that have the same orientation selectivity