Exam 3: VISION

Question 1

adequate stimulus for vision

Answer 1

light

Question 2

what is the first neuron for vision?

Answer 2

• the retina
• contains the neurons that will turn the light stimulus into a neural signal
• The neurons that detect this light and turn it into an action potential send their axons via the optic nerve to the brain
• The spot at which the optic nerve leaves the eye ball has no retina, meaning that there is a blind spot in this region. Your vision therefore has a blind spot that you normally do not notice

Question 3

Photoreceptors

Answer 3

• Photoreceptors are located in the inner most layer of the retina, but are the neuron that directly respond to light and depolarize in response
• There are two types: rods and cones. Rods are for low light stimuli and cones are bright light stimuli
• Photoreceptors project their axons to bipolar cells

Question 4

Bipolar cells

Answer 4

depolarize or hyperpolarize in response to photoreceptors and communicate with ganglion cells

Question 5

Ganglion Cells

Answer 5

activate in response to bipolar cells and send their axons to the brain via the optic nerve.

Question 6

Horizontal Cells and Amacrine Cells

Answer 6

supports sensitivity to light contrast, over a wide range of light intensities

Question 7

Cones vs. Rods

Answer 7

• The convergence for the rod system makes it poor for visual acuity whereas the cone system is excellent for that
• There are more cones in the fovea, which is the region of the retina this best for visual acuity
• Cones contain three different types of opsins, which allow for detection of short, medium and long wavelengths of light. Rods only have one photopigment.

Question 8

key properties of photoreceptors:

Answer 8

• Light changes the membrane potential in a graded fashion, meaning you get graded potentials and not action potentials in photoreceptors. While action potentials are not generated, the graded potentials do cause neurotransmitter release from photoreceptors
• Photoreceptors are tonically active, meaning in the dark they release neurotransmitter (glutamate). When light shines on the photoreceptors, they hyperpolarize and decrease neurotransmitter release.

Question 9

Phototransduction: Cone sequence

Answer 9

• cone
• bipolar cell
• ganglion cell

Question 10

Phototransduction: cGMP

Answer 10

• The graded potentials are created by cyclic GMP (cGMP), a second messenger. In the dark there is a lot of cGMP, the function of which is to open ion channels that let in sodium and calcium. This is how the cell becomes depolarized
• When light strikes the photoreceptor, it decreases levels of cGMP, meaning the cation channels close and the cell does not depolarize

Question 11

Phototransduction: potassium leak channels

Answer 11

There are potassium “leak” channels always open (in both light and dark). In the dark, the influx of calcium and sodium cations overpowers the potassium efflux. In the light, when the cation ion channel closes, potassium efflux now causes a hyperpolarization

Question 12

Phototransduction: opsins

Answer 12

• mechanism by which light causes a reduction in cGMP levels
• There is a family of proteins on the membrane of photoreceptors called opsins.
• The opsin molecule responds to light by causing transducin to activate, which in turn cleaves cGMP. This prevents ion channels from opening and depolarizing the receptor.

Question 13

Phototransduction: Rod sequence

Answer 13

• rod
• bipolar cell
• amacrine cell
• ganglion cell

Question 14

what causes color blindness?

Answer 14

• Most so-called color-blind humans (actually color-deficient humans) have dichromatic vision and can distinguish short-wavelength stimuli (blue) from long-wavelength stimuli (not blue).
• Most color-blindness in humans is due to the absence of cones sensitive to medium-wavelength light (M cones).
• Because women have two X chromosomes, a defective gene encoding for the medium-wavelength pigment on the X chromosome is compensated for by the other normal gene X. This is why men are much more likely to be dichromats than women are

Question 15

optic nerve

Answer 15

The axons of the retinal ganglion cells make up the optic nerve for each eye

Question 16

optic chiasm

Answer 16

About 60% of the axons from one eye cross over to the other hemisphere at the optic chiasm, the remainder 40% of axons stay on the side of the eye.

Question 17

optic tract

Answer 17

Once the axons pass the optic chiasm, the optic nerve becomes the optic tract. This distinction is important because now the tract contains axons from both eyes.

Question 18

There are multiple locations that retinal ganglion axons project to:

Answer 18

1) Lateral Geniculate Nucleus in the thalamus
2) From there, thalamic neurons project to the primary visual cortex (also called striate cortex)
3) suprachiasmatic nucleus
4) superior colliculus in the midbrain

Question 19

superior colliculus responsibility

Answer 19

Finally, some retinal ganglion neurons also synapse onto midbrain neuron in the superior colliculus, where they are responsible for reflexive orientation movements of head and eyes

Question 20

suprachiasmatic nucleus importance

Answer 20

This region is important for circadian rhythms, i.e. our internal clock that is dictated by the light/dark cycle

Question 21

what is prosopagnosia?

Answer 21

The inability to recognize faces, sometimes including ones own face, is known as prosopagnosia

Question 22

what causes prosopagnosia?

Answer 22

• There is an area in the temporal lobe called the fusiform gyrus that is crucial for pattern recognition. Visual information gets processed in the occipital lobe, but more complex pattern recognition (i.e. an object versus the face) occurs in this gyrus
• Damage to this era can leave individuals without the ability to recognize a face as a face—they see the face in its components parts and not as a “whole”

Question 23

detecting a change in light

Answer 23

• In order to convey light information, there are different circuits of neurons in the retina that are “on center” or “off center”
• Based on which glutamate receptor is expressed on the bipolar cell, glutamate release (or lack of release with light) can have opposing effects
• mGluR binds glutamate and hyperpolarizes whereas AMPAR binds glutamate and depolarizes
• The purpose of this arrangement is so that changes in light information can be detected with great sensitivity—think of it like the nociceptors and their receptive fields

Question 24

Central visual pathway

Answer 24

Photoreceptors to bipolar cells to Ganglion cells to LGN to primary visual cortex

Question 25

Central visual pathway: LGN and PVS highly ordered

Answer 25

1. The thalamus and cortex are layered, meaning that within each structure, there are distinct neurons forms a banding of layers
2. In the LGN (A), retinal ganglion neurons synapse onto specific regions, with monocular ocular dominance columns being formed
3. From the LGN, all of the inputs go to a specific layer in the primary visual cortex called layer 4. In this layer, the ocular dominance columns are binocular.