Physiology II: Vision II

Outer segment of the rids and cones: houses the discs that contain the …

light-absorbing photopigment

Inner segment of the rods and cones: houses the cells … (mitochondria and nucleus)

metabolic machinery

Synaptic terminal of the rods and cones: stores and releases …

neurotransmitter

…: a photopigment found in the rods of the retina, essential for detecting light

rhodopsin

…: a protein embeded in the membrane

opsin

…: a light-sensitive molecule derived from vitamin A

retinal

In the dark retinal is in the …, rhodopsin is inactive

11-cis-retinal configuation (bent)

In the dark the rod cell keeps …, maintaining a depolarized state → continuous neurotransmitter glutamate release

Na+ channels open

In the light retinal changes its shape to …, activating opsin

all-trans-retinal (straight)

Activated rhodopsin triggers a cascade of enzymes that: … (3)

closes Na+ channels, hyperpolarizes rod cells, reduces glutamate release

After activation enzymes convert all trans retinal back to 11 cis retinal to … rhodopsin for another round of light detection

regenerate

… signals to the brain that light was detected

reduced glutamate

Absence of light: concentration of …, Na+ channels in photoreceptors open → dark current → depolarizes the photoreceptor → passive spread from outer segment to synaptic terminal

cGMP high

Absence of light: passive spread from outer segment to synaptic terminal → … → increased release of glutamate from synaptic terminal in the dark

Ca2+ channels open

…: inward Na+ leak

dark current

In the light: … → concentration of … → hyperpolarization (receptor potential) → spread of hyperpolarization to synaptic terminal

transducin-dependent cascade, cGMP low

In the light: spread of hyperpolarizaion o synaptic terminal → … → decreased releas of glutamate in the light

closes Ca2+ channels

…: the brighter the light the greater the hyperpolarization and the greater the reduction in glutamate released

graded receptor response

Glutamate via two different receptors: …

on-center bipolar cells, off center bipolar cells

In response to the dark: On-center bipolar cells - glutamate binds to … → hyperpolarization (this is inhibition)

inhibitory (metabotropic) receptors

In response to light: On-center bipolar cells - less glutamate → … → depolarized (this is excited)

less inhibition

In response to darkness: Off-center bipolar cells - glutamate binds to … → depolarization (this is excited)

excitatory (ionotropic) receptors

In response to light: off-center bipolar cells - … → less excitation → hyperpolarization (inhibited by light)

Less glutamate

In short the perception of light or dark os to do with whether … and send them further to thalamus and visual cortex

on-center ganglion cells fire AP

Dark sequence: … → … → … → …

increased glutamate from photoreceptors, hyperpolarization of bipolar cell, no action potentials in on center ganglion cells, perception of darkness

Light sequence: … → … → … → …

decrease of glutamate from photoreceptors, depolarization (release of inhibition) of on center bipolar cells, action potentials in on center ganglion cells, perception of light

on and off center recptive fiels result in the extraction of …

luminance contrast

Retinal ganglion cells have a receptive field with two zones (center, surround), these cells do not respond to absolute brightness, but to … between the two zones

relative brightness differences

If light hits both the cener and surround equally → …

weak response

If light preferentially hits the center or surround → …

strong response

…: horizontal and amacrine cells help sharpen the contrast by suppressing neighboring signals

lateral inhibition

Lateral inhibition … (makes edges where brightness changes) stand out, it contours and shapes the visual scene

enhances contrast

…: indistinct, gray vision at night

rods

…: provide sharp, color vision during the day (low sensitivity)

cones

Cones have high acuity, there is … in the retinal pathways

little convergence

Rod signaling pathway: marked convergence onto bipolar and then ganglion cells → …

>100 rods converge on one ganglion cell

Rods have high …, pooling ight signals across space

sensitivity

…: night vision, (rods are useful in detecting even the smallest amount of light)

Scotopic

Rods have low ,,,: differentiating two nearby points

resolution

COne signaling pathway there is almost no convergence: …

each cone has a private line to ganglion

Cones have high acuity: each cone transmits information about small receptive field → … → recognition

spatially-resolved (highly detailed) vision

Cone have low sensitivity: ganglion cells receives signals only from one cone light needs to have …, photopic vision

high luminance

Dark adaptation: going from bright sunlight into a darkroom, initially cannot see anything clearly because intense light before … → decreased photoreceptor sensitivity

breakdown of photopigments

In the dark, … and sensitivity to light gradually increases, only the highly sensitive rods can response to dim light

photopigments gradually regenerate

Light adaptation: when moving from dark into the light → at first there is … evrythign appers bright.Rapid breakdown of rod photopigment by intense light → rod sensativity decreases → …

rods burn out

Other mechanisms involved in adaptation: … and … rather than luminance

pupil size, perception of contrast

…: strongly excite cone pathways suppress activity in surrounding weakly excited pathways

horizontal cells

4 photopigments (1 rod 3 cones) all have the same retinal but different …

opsin

The absorption maximum of each photoreceptor (4) is at a …, but absorption functions overlap

different wavelenth

3 cones respond to a given wavelength at a …

different extent

…: ratios of stimulation of the three cone types

Trichromatic theory of color vison

Pigments in various objects selectively absorb particular wavelengths from light-emitting sources → unabsorbed light is … → that is what is percieved

reflected

Color perception is due to the …

ratios of cone stimulation

color perception is a …

subjective experience

…: lack of a particular cone type, or differential sensitivity of only two types of cones

color blindness

…: perceive certain colors differently and unable to distinguish some colors

red gree color blindness

Red green color blindness is more preventent in males because it is …

x-linked recessive

…: retinal ganglion cells have receptive fields for color contrast, they dont just detect “more red” or “more green” they compare inputs

opponent color cells

If red light hits an R/G opponent cell → … excites and … inhibits

red signal, green signal

This competition enhances …, making it easier to distinguish subtle color boundaries

color contrast

opponent signals go from: … → … → … final color perception is reconstructed in the brain

retina, lateral geniculate nucleus in thalamus → primary and associative visual cortices

…: when you stare at one color for a long time, the opponent system fatigues, when you look away you perceive the opposite color

Afterimages

..: the field of view that can be seen without moving the head and eyes

visual field

In the visual cortex (occipital lobe), the image of the visual field is mapped point by point, each par of the retina sends information to a specific area in the visual cortex preserving …

spatial organization

…: ensures the brain knows where in space the visual signals are coming

retinotopic mapping

In the visual cortex, a disproportionately large number of neurons are devoted to processing signals from the … compared to the peripheral retina

fovea

…: separation of movement, form and color pathways (separation maintained also in the cortex)

lateral geniculate nucleus of thalamus

…: very specific receptive fields, built by converging connections from afferent pathway

cortical cells

…: in the occipital lobe, first cortical area to receive visual input from the LGN of the thalamus, processes basic visual features (edges, orientation, movement, special frequency), it has a precise retinotopic map of the visual field

striate cortex

…: specific receptive fields, they respond to particular types of visual input like a line of certain angle or a moving edge

cortical cells (V1)

at each … neurons have a greater capacity for abstraction

level of visual processing

…: receptive fields elongated, sensitive to specific orientation a light bar or edge, precise position within their receptive field (line detectors)

simple cells

…: larger, less sensitive to exact position, sensitive to specific orientation, but anywhere in the receptive field, direction sensitive, they detect oriented lines or edges

complex cell

…: sensitive to specific orientation (like simple and complex), specific lenth or particular edge (line endings or corners)

hypercomplex cells

…: functionaly unit of the V1 cortex, contains a complete set of orientation column, ocular dominance columns for both eyes, color blobs, fully processes all visual features for one small peice of the visual scene

hypercolumn

…: each eye sees slightly differtn images because they are spaced apart

binocular disparity

…: in the primary cortex they compare input from the left and right eyes,

ocular dominance columns

…: estimation of distance and create a sense of 3D space

stereopsis

…: light sensative pigment → control of light-dark circadian cycles (control of biological clock)

melanopsin

Light input from the eyes influences the … (bright light by suppressing melatonin production)

alertness and attention

External eye muscles move the eyes quickly and percisilry inder the control of: … (3)

oculomotor (III), trochlear (IV), abducens (VI)

The info reaching the visual cortex is not a replica of the visual field, various aspects of visual information such as form, movment, color, and depth are spperated and proejected in … to different cortical regions

parallel pathways

…: integration of these seperate its of processed information → reassembled picture of the visual scene

perception

Pateions with lesion in specific area have …

highly specific impairment