Lecture 4: Vision

Retina

The light-sensitive layer of tissue located at the back of the eye that transduces light into neural impulses that are sent to the brain through the optic nerve.

Cornea

The protective outer layer of the eye that has the most refractive power to protect the inner structures.

Lens

The transparent structure that refracts light through movement of the ciliary muscles.

Presbyopia

The inability to focus on nearby objects due to the lens becoming less flexible over time. 

Farsightedness (hyperopia)

The refractive error in when the image focuses behind the retina, not allowing individuals to see close objects clearly. This error is corrected using a convex lens. 

Nearsightedness (myopia)

The refractive error in when the image focuses before the retina, not allowing individuals to see distant objects clearly. This error is corrected using a concave lens. 

Photoreceptor cells

The specialized cells located in the deepest layer of the retina that convert light into electrical signals. Comprised of two different types (rods or cones).

Ganglion cells

The specialized cells located in the lightest layer of the retina that transmit visual information from the eye to the brain. They are the only output cells. 

Octopus Eye

The layout of the _____ differs in that they have an “outside” to “out” orientation of the retina, meaning photoreceptors are located on the lightest layer of the retina, while the ganglion cells are located in the deepest layer. 

Rods

The photoreceptor cells that are more sensitive to light and utilize scotopic vision. ____ outnumber cones 20:1. 

Cones

The photoreceptor cells that are used in bright light and are responsible for color detection. Have cells for red, green, and blue.

Signal Transduction of Light in Dark Environment

In ___ environments, rhodopsin is inactive and does not activate the G-protein transducin. Phosphodiesterase is also in turn inactive. There are high levels of cGMP inside the cell membrane. Due to this, a cGMP-gated Na⁺ channel is open, allowing Na⁺ to enter the cell and depolarize it, eventually firing off action potentials.

Signal Transduction of Light in Light Environment

In ____ environments, rhodopsin absorbs photons and activates, which in turn activates transducin. Transducin converts GDP into its active form GTP, which phosphodiesterase uses to cleave cGMP to GMP. Due to the decreasing amount of cGMP within the cell membrane, the cGMP-gated Na⁺ channel closes, hyperpolarizing the cell and decreasing the release of neurotransmitters. 

Guanylyl cyclase and Ca²⁺ can allow for the gradual reopening of the cGMP-gated Na⁺ channel even in dark environments.

Receptive Field

The area of the retina that causes changes in membrane potential. 

Bipolar Cells

The cells that exist between photoreceptors and ganglion cells transmit signals between the two. They have center-surround receptive fields that are generated by interactions with horizontal cells.

Light ON Bipolar Cell

When light is ___ the bipolar cell, it depolarizes in response to light and hyperpolarizes in response to glutamate. ____ bipolar cells have G protein glutamate receptors that are inhibitory, and when there is less glutamate being released, there is overall less inhibition. 

Light OFF Bipolar Cell

When light is ___ the bipolar cell, it hyperpolarizes in response to light and depolarizes in response to glutamate. ____ bipolar cells have glutamate ligand gated Na⁺ channels, so when light is off, more glutamate is secreted and released and in turn opens up these ligand gated Na⁺ channels, depolarizing the bipolar cell. 

Retinal Processing

The bipolar center-surround receptive field is transferred to retinal ganglion cells

• ON Retinal Ganglion cells follow the same pattern as ON Bipolar Cells

• OFF Retinal Ganglion cells follow the same pattern as OFF Bipolar Cells

Magnocellular Retinal Ganglion Cells

These retinal ganglion cells (5%) contain large, fast-adapting receptive fields and are responsible for processing information about large, fast things related to movement. 

Parvocellular Retinal Ganglion Cells

These retinal ganglion cells (90%) contain small, slow-adapting receptive fields and are responsible for processing information about small, slow, colorful things. 

QOD: A spot of light falls onto the center of a receptive field. For the following cells, indicate how the membrane potential will change (depolarize/hyperpolarize) and whether that cell will release more or less neurotransmitter (±):

• Photoreceptor

• ON Bipolar Cell

• ON Retinal Ganglion Cell

• OFF Bipolar Cell

• OFF Retinal Ganglion Cell

Photoreceptors always hyperpolarize in response to light, as light closes the cGMP Na⁺ channels that serve to depolarize the cell. This, in turn, decreases the number of action potentials firing and neurotransmitters being released.

If a spot of light fell directly onto the center of a receptive field, the ON Bipolar cell would depolarize. ON Bipolar cells have G protein glutamate receptors that are inhibited by glutamate. If there is less glutamate release, then there is less inhibition. Less inhibition allows the cell to depolarize, leading to more action potential firing and neurotransmitter release. ON Retinal Ganglion Cells share the same pattern as ON Bipolar Cells, so they would follow something similar.

The OFF Bipolar Cell would hyperpolarize, as the glutamate ligand-gated Na⁺ channels would close as there is less glutamate to bind and open the channels. This hyperpolarization would lead to fewer action potentials firing off and fewer neurotransmitters being released. OFF Retinal Ganglion Cells share the same pattern as OFF Bipolar Cells, so they would follow something similar.