Visual System Physiology (Pierce)

Question 1

What is the general organization of cells in the visual system?

Answer 1

• photoreceptor cells (rods/cones) receive light impulses
• bipolar cells (have special mGluR6 receptors) either propigate IPSP or no IPSP
• ganglion cells (have “nml” NMDA or non-NMDA receptors) either propigate an EPSP or no EPSP resulting in AP or no AP to the brain

Question 2

• one branch of the visual system
• convergence: many rods + many bipolar cells to one ganglion cell
• allows system to operate in dim light
• sacrifices acuity to gain sensitivity

Answer 2

rod system

Question 3

• one branch of the visual system
• less convergence: receptor may send info to one bipolar cell that contacts one ganglion cell
• maximizes acuity

Answer 3

cone system

Question 4

Where does cone density peak in the retina?

Answer 4

cones peak at the fovea (focal point of the retina)

Question 5

Where does rod density peak in the retina?

Answer 5

rods peak at about 20 degrees off center

(example Dr. Pierce gave was looking at the stars at night, if you look about 20 degrees of center from a star, the star will appear to shine brighter in your peripheral than it will looking directly at it because the density of rods (brightness/light sensitivity) is greatest at this point)

Question 6

What is the general visual pathway in dark conditions?

Answer 6

(photoreceptor cells depolarize in the dark)

• photoreceptor cell depolarizes >
• graded potential generated >
• NT released (glutamate, highest conc in dark conditions) >
• NT acts as inhibitory signal (to bipolar cells) >
• bipolar cell does not depolarize, no impulse sent to the brain

Question 7

What is the general visual pathway in light conditions?

Answer 7

• photoreceptor cell hyperpolarizes >
• graded potential is not generated >
• no inhibitory NT (glutamate, lowest conc in light conditions) released >
• bipolar cell spontaneously depolarizes, visual impulse sent to the brain

Question 8

How is glutamate inhibitory in the visual pathway?

Answer 8

• glutamate is excitatory when considering its “nml” receptors (NMDA, and non-NMDA (AMPA or Kainate))
• bipolar cells express special glutamate receptor: mGluR6
• mGluR6: GPCR that closes cGMP-gated Na+ channels similar to light transduction in photoreceptors (allows glutamate to act inhibitory), glutamate from rod/cones closes Na+ channels in bipolar cell, thus the cell does not depolarize

Question 9

How does glutamate act in ganglion cells?

Answer 9

• glutamate released by bipolar cells is excitatory in ganglion cells
• glutamate receptor in ganglion cells is nml NMDA or non-NMDA receptors
• depolarization of bipolar cell > excitation of ganglion cell > action potential generated
• ganglion cell axons become fibers of optic nerve
• in visual cortex, ganglion cells release glutamate

Question 10

Photoreceptors

• retinal connectivity:
• neurotransmission:
• synpatic interactions:
• receptive field properties:
• electrical reponses to light:

Answer 10

Photoreceptors

- retinal connectivity: synapses w/ bipolar cells

- neurotransmission: constantly releases glutamate (only conc changes) which binds mGluR6 on bipolar cell

- synpatic interactions: rod system has lots of convergence, cones systems have less convergence

- receptive field properties: essentially the size of the photon that corresponds w/ that location on the retina

- electrical reponses to light: light hyperpolarizes

Question 11

Bipolar cells

• retinal connectivity:
• neurotransmission:
• synpatic interactions:
• receptive field properties:
• electrical reponses to light:

Answer 11

Bipolar cells

- retinal connectivity: synapses w/ ganglion cells

- neurotransmission: EPSP results in release of glutamate which binds NMDA receptor on ganglion cells

- synpatic interactions: rod-associated paths can exhibit convergence upon ganglion cells

- receptive field properties: more complex than photoreceptors, the center of receptive field responds differently than the surrounding

- electrical reponses to light: w/o IPSP, bipolar cell depolarizes

Question 12

Ganglion cells

• retinal connectivity:
• neurotransmission:
• synpatic interactions:
• receptive field properties:
• electrical reponses to light:

Answer 12

Ganglion cells

- retinal connectivity: synapses in brain

- neurotransmission: EPSP results in release of glutamate in cortical circuits

- synpatic interactions: interacts w/ other cortical layers in PVC

- receptive field properties: extremely complex, visual fields are rectangular

- electrical reponses to light: EPSP in ganglion cell, depolarizes, AP generated

Question 13

• major direct target of the retina
• regulates the flow of info to PVC (axons of this area relay cells to ipsilateral side of visual cortex)
• retinotopic organization maintained
• has a 6 layer lamellar structure (half from nasal half from temporal fields)
• signals from two eyes are kept apart in this area

Answer 13

lateral geniculate nucleus (LGN)

also called lateral geniculate body (LGB)

Question 14

What are the functions of the LGN?

Answer 14

1. control the motions of the eyes to converge on a point of interest
2. control the focus of the eyes based on distance
3. determine relative positions of objects to map them in space
4. detect movement relative to an object

Question 15

• direct target of the retina
• connects w/ tectospinal tract to send projections to cervical anterior horn cells (for eye movement)
• creates map of visual space to activate appropriate motor responses required to move eyes into their intended position within the orbits
• specifies movement intention rather than fixation of movement upon a target

Answer 15

superior colliculus

Question 16

• direct target of the retina
• pupillary light reflex: sends projections to Edinger-Westphal then on to ciliary ganglion

Answer 16

pretectum

Question 17

How are objects in the left visual hemifield sent to the right side of the brain and vice versa?

Answer 17

This has to do with the way the light from the object is refracted into the retina, left visual field is sent to left eye nasal retina (goes to right side of brain) and right visual field is sent to right nasal retina (goes to left side of brain)

Question 18

What 3 areas make up the visual cortices?

Answer 18

• primary visual cortex (has unique 6 layers that maintains retinotopic org)
• parastriate cortex
• extrastriate cortex

Question 19

What is the role of V1 visual cortex?

Answer 19

• identifies edges and contours of objects
• also decodes visual input to redirect info to subsequent steps of analyses to other visual areas

Question 20

What is the role of V2 visual cortex?

Answer 20

depth perception, which is detected by analyzing disparities between two eyes

(Dr. Pierce used example of optical illusions and 3D stereograms)

Question 21

What is the role of V3A visual cortex?

Answer 21

(anterior to V3)

• major indentification of motion
• “Is motion happening? Yes or no?”

Question 22

What is the role of V4 visual cortex?

Answer 22

• complete processing of color inputs
• lesions here can cause achromatopsia (unilat lesions may go unnoticed)

Question 23

What is the role of MT/V5 visual cortex?

Answer 23

(MT = middle temporal)

• tracks motion across a scene in terms of directionality and context of background/foreground
• does so by containing neurons that selectively respond to direction of moving edge

Question 24

What are the unique structures present in the primary visual cortex that lead to specificity of eye laterality, angular distinction, and color vision?

Answer 24

ocular dominance columns (eye laterality), orientation columns (angular distinction), and blobs (color vision)

(again, these are unique to V1, as seen in the photo below these structures become much less defined in V2)

Question 25

- columns that vertically span all 6 layers of the PVC - give rise to stripes that can be seen on the cortical surface and in cross-sectioning of the PVC - the cells in these columns preferentially respond to input from **one eye or the other** (ocular dominance)

Answer 25

ocular dominance columns

Question 26

- columns that span all 6 layers of the PVC - organized region of neurons that are excited by visual line stimuli of varying angles - oriented perpendicular to the cortical surface (and to ocular dominance columns)

Answer 26

orientation columns

Question 27

- span all 6 layers of the PVC - organized region of neurons that are sensitive to color, assemble into cylindrical shapes - all 3 color-coding cones are required for accurate color detection - this is the ultimate destination of cone inputs

Answer 27

blobs

Question 28

How do melanopsin ganglion (MG) cells help regulate circadian rhythm?

Answer 28

- 1-3% of ganglion cells directly sense light (MG cells) - due to expression of melanopsin, these cells are sensitive to blue-wavelengths of light - they then provide light-sensing info for regulation of circadian rhythm (considered a **non-image forming** light-responsive system) - **mechanism**: MG cells project directly to hypothalamic suprachiasmatic nucleus (via retino-hypothalamic tract), the neuroendocrine effector in the pineal gland which then produces melatonin in a rhythmic pattern **TLDR: MG cells \> hypothalamus SCN \> pineal gland \> melatonin** **\*blue light suppresses our body's release of melatonin\***

Question 29

What are the affects of blue light and overall health?

Answer 29

- **blindness and circadian rhythms:** some patients w/ blindess from **photoreceptor loss** still exhibit pupil light rxn to **bright blue light** and maintain circadian rhythm; patients blind from **optic nerve lesion**s (includes loss of MG cell input) often lack nml circadian rhythm - **aging**: lens becomes more yellowish thus spectrum of blue light transmission decreases; suspected to be one reason why older individuals experience sleep problems b/c of lack of blue light during daytime (decreased stimulation) - **surgery**: 24-hour exposure to bright blue light before surg reduces inflammation and organ damage at cellular level (mouse model) - **seasonal affective disorders**: blue light can treat SAD's by increasing alertness and stimulating cognition - **sleep behaviors**: blue light exposure prior to sleep can disrupt sleeping habits (increased duration to sleep onset and delayed beginning of 1st REM cycle), overall disrupting sleep quality

Question 30

What structures are involved in the dorsal pathway?

Answer 30

primary visual cortex (V1) \> V2, V3 \> V5/MT \> parietal/frontal cortex

Question 31

What is the general function of the dorsal pathway?

Answer 31

- "the where pathway" - travels from PVC to parietal/frontal cortex - **primary path associating vision w/ movement** - **completes motor acts based on visual input** - arises from V3

Question 32

What structures are involved in the ventral pathway?

Answer 32

primary visual cortex (V1) \> V2 \> V4 \> temporal lobe

Question 33

What is the general function of the ventral pathway?

Answer 33

- "the what pathway" - **primarily involved in interpreting images** (recognizing/copying shapes, forms, faces) and **complex patterns** - **copying/naming objects** are separate functions in the temporal lobe (damage to one area is possible w/o damaging other) - **facial recognition** is a specialized area