NSCI Unit 9 - Visual System

What does the visual system detect?

Light

What are the visual system’s receptors?

• Photoreceptors

  • Two types are rods and cones

Explain transduction in the visual system

• Process by which light causes receptor potential (electrical resonse)

  • In darkness, Na⁺ channels are open allowing a depolarizing current to flow (dark current)

  • Light causes this channel to close and creates a hyperpolarizing effect

What molecule opens the dark current channel?

cGMP opens the Na⁺ when light is not present

Anatomy of the Retina

1. Optic nerve fibers

2. Ganglion cells

3. Bipolar cell layer (also contains horizontal and amacrine cells)

4. Photoreceptors

5. Pigmented epithelium

4 important aspects of retinal anatomy

1. Rod and cone receptors are the only light sensitive cells

2. Ganglion cells are the only source of output from the retina

3. Only ganglion cells fire APs (exception with amacrine cells), all others depolarize or hyperpolarize based on NT release

4. Duplex retina: we use both rods and cones for vision

Visual Pathway

• Photoreceptor → (horizontal cells) biopolar cell → (amacrine cells) ganglion cell → optic nerve → optic chiasm → optic tract → LGN → V1

  • Light passes through ganglion and bipolar layers before reaching photoreceptors

Horizontal cells

Receive input from photoreceptors and send information to other photoreceptors and bipolar cells

Amacrine cells

Receive input from bipolar cells and send information to ganglion cells, bipolar cells, and other amacrine cells

Photoreceptor Structure

• Detects light with photopigments in the disk membrane

• Four main regions

  • Outer segment (contains disks)

  • Inner segment

  • Cell body

  • Synaptic terminal

• Membrane potential changes determine release of glutamate NT

Rods

• Long and cylindrical, higher photopigment concentration

• Uses rhodopsin photopigment

• 92 million in retina, found on the periphery

• Very high sensitivity

• Large receptive field, low acuity

• Slow response time

• Night vision

Cones

• Shorter, tapering segment of fewer discs

• Uses 3 types of opsin photopigment

• Less pigmented than rods

• 5 million in retina, mostly in the fovea

• Low sensitivity

• High acuity vision

• Fast response time

• Day vision

Explain why visual acuity is greatest at the fovea

Only cones are present and other cellular intermediaries are pushed out of the way for direct access to light

Explain differences in photopic, mesopic, and scotopic light levels

• Photopic (daytime) - Only cones used, rods are too sensitive and become oversaturated

• Mesopic (intermediate) - Uses both cones and rods

• Scotopic (nighttime) - Only uses rods

  • Central vision becomes blind because it only contains cones which aren’t activated

Fovea

• Center region of the retina where only cones are present

• Place of greatest visual acuity since all other cellular layers are pushed out of the way

• Rods completely absent

Phototransduction in Rods

1. Light activates/bleaches rhodopsin

2. The G-protein transducin is stimulated

3. Phosphodiesterase (PDE) an effector enzymes also activates

4. PDE reduces cGMP levels

5. Na⁺ channel closes and membrane hyperpolarizes

Where does phototranduction take place?

In the disks of photoreceptors, located in the outer segment

What is different about phototranduction in cones?

• Uses 1 of 3 types of opsins instead of rhodopsin

  • Detects short, medium, or long wavelengths of light

• Opsins require more energy to be bleached (less sensitive)

What are the two main effects light causes?

• Hyperpolarization in cells

• Decreases release of glutamate from photoreceptors

What does depolarization cause in photoreceptor cells?

Release of glutamate

Ganglion Cells

• Last layer of the retina

• Axons of the ganglion cells form the optic nerve

  • THE ONLY RETINAL AFFERENT AXONS

• 3 types:

  • M-type

  • P-type

  • Non-M/non-P type

Bipolar Cells

• Receive direct synaptic input from center photoreceptor cells

• Receive indirect input from surround photoreceptor cells via horizontal cells

Receptive Field (Vision)

• The pattern of light that elicits an electrical response in a specifc area of neurons on the retina

RFs become more complex as you continue (retina to LGN etc…)

Explain how bipolar and ganglion cell receptive fields are organized to process visual information

• Both contain concentric receptive fields with a center and a surround. Two types:

  • On-center/Off-surround

  • Off-center/On-surround

• The center and surround are antagonistic to eachother

Explain the relationship between On-center/Off-surround cells and photoreceptors in the LIGHT

Light hyperpolarizes photoreceptor cells, less glutamate released

• On-center bipolar cells have glutamate inhibitory metabotropic receptors

  • Less glutamate → less inhibition → cell depolarized

• Off-bipolar cells have glutamate excitatory ionotropic receptors

  • Less glutamate → less excitatory → cell hyperpolarized

Explain the relationship between On-center/Off-surround cells and photoreceptors in the DARK

Darkness depolarizes photoreceptor cells, more glutamate released

• On-center bipolar cells have glutamate inhibitory metabotropic receptors

  • More glutamate → more inhibition → cell hyperpolarized

• Off-center bipolar cells have glutamate excitatory ionotropic receptors

  • More glutamate → more excitatory → cell depolarized

M-Type Ganglion Cells

• Burst of rapid APs

  • Adapts as a stimulus, so drops quickly even when sustained

• Not sesntivie to differences in color

• 5% of total ganglion population

• Low contrast stimuli

• Magno, bigger

• Larger receptive field

P-Type Ganglion Cells

• Sustained discharge as stimulus remains present

• Color sensitive

  • Color opponency

    • Red/Green

    • Blue/Yellow

• 90% of total ganglion population

• Color detection

• Parvo, smaller

Non-M/non-P Type Ganglion Cells

• 5% of total ganglion population

• Some aid in color detection

Parallel Processing

Information from the different ganglion cell types is kept on separate parallel streams, from the retina to the thalamus

What are the non-thalamic targets of the retinofugal pathway?

• Hypothalamus (Suprachiasmatic nucleus) - entrain our circadian rhythm to the earth’s light/dark cycle

• Pretectum - Control the size of the pupil & certain eye movements

• Superior Colliculus - orients the eyes in response to new stimuli (move fovea to objects of interest)

Explain how the eyes are arranged to perceive vision

• Peripheral vision comes from the nasal parts of the retina

  • Right field peripheral vision comes from left nasal

  • Left field peripheral vision froms from right nasal

    • Peripheral vision must cross the optic chiasm because they carry opposing information

      • Nasal left has right periphery and must get onto right hemisphere

• Center vision comes from the temporal parts of the retina

  • Right center comes from left temporal

  • Left center comes from right temporal

    • Information is already organized onto correct sides, right eye catches left vision and brings to right hemisphere

What is the effect of a lesion in the optic nerve?

• Cut in left optic nerve causes loss of left peripheral vision

• Cut in right optic nerve causes loss of right peripheral vision

What is the effect of a lesion in the optic chiasm?

Loss of all peripheral vision

What is the effect of a lesion in the optic tract?

• Lesion in right optic tract - Loss of vision in left field

• Lesion in the left optic tract - Loss of vision in the right field

Lateral Geniculate Nucleus (LGN)

• Part of the thalmus in the brain, processes visual information from the retina and sends it to V1

• 6 distinct layers that keep information from different parts of the retina separate

  • Right temporal (sees left vision), and the left nasal (sees left vision) come to the right hemisphere LGN

  • Left temporal (sees right vision), and the right nasal (sees right vision) come to the left hemisphere LGN

• Each LGN takes information from both eyes

Layers of the LGN

• Layers 1, 4, 6 contain contralateral information (nasal peripheral info)

• Layers 2, 3, 5 contain ipsilateral information (temporal central info)

Organization of Retinal Inputs to the LGN

LGN is organized by ganglion cell input, and neurons are activated by one eye

• M-type ganglion cells of the optic nerve go the layers 1 and 2 (Magnocellular layers) of the LGN

• P-type ganglion cells of the optic nerve go the layers 3-6 (Parvocellular layers) of the LGN

• NonM/P ganglion cell types go to the inbetween layers of the LGN, K1-K6 (Koniocellular layers)

Magnocellular Layer

• Layers 1 and 2 of LGN

• Analysis of object motion

• Layer 4C⍺ of V1

Parvocellular Layer

• Layers 3 through 6

• Fine detail, form, and color

• Layer IVCβ of V1

What is the major target of the LGN?

The straite cortex (aka the primary visual cortex or V1)

What is retinotopy?

• The organization of information from retinal input in the LGN and striate cortex

Neighboring cells in the retina feed information to neighboring places in LGN and V1

What are the 3 key points of retinotopy?

1. Information is distorted because amount of receptive fields vary - central field of vision seen by fovea is overrepresented

2. Receptive fields overlap - One point of light in the retina can be broadly distributed in the cortex

3. Cortex doesn’t have pictures - Vision comes from brain’s interpretation of visual signals

Explain layer 4C in the visual cortex

• Main onput zone from the LGN, input from eyes remains separate, monocular

• 4C⍺ comes from magnocellular layer

• 4Cβ comes from parvocellular layer

Receptive Fields in V1

• Monocular

• Binocular

• Orientation Selective

• Direction Selective

Monocular RF

Small, center surround, only from one eye

• Layer 4C: similar to LGN cells

• Layer 4Cα: insensitive to the wavelength 

  • Magnocellular LGN input

• Layer 4Cβ: center-surround color opponency

  • Parvocellular LGN input

Binocular RF

• Respond to light in either eye, each containing a RF

Most neurons in layers superficial to IVC are binocular. (respond to light in either eye!)

Orientation Selective RF

• Respond to a moving, elongated bar of light with a specific orientation

Found in most V1 neurons outside layer IVC (and some within)

Direction Selective RF

Respond to a bar of light moving perpendicular to its orientation in a specific direction

Orientation Column

Column of neurons that all respond to the same prefered orientation

Dorsal Stream

• “Where is it?”

• Visual motion and visual control of action

• V1, V2, V3, MT, MST, other dorsal areas

  
  

Area MT (Middle Temporal aka V5)

Cells are direction and movement selective

Area MST (Medial Superior Temporal)

• Navigation

• Directing eye movements

• Motion perception

Ventral Stream

• “What is it?”

• Perception of visual world and object recognition

• V1, V2, V3, V4, IT, other ventral areas

V4

• Shape and color perception

• Achromatopsia: loss of color vision

Area IT

• Major output of V4

  • Also contains fusiform face area

• Responds to complex forms, abstract shapes, color and texture

• Damage may cause trouble identifying objects, naming them, seeing their color, visualizing faces