Chapter 9 Neuro Eye

Central/Peripheral Retina (Rods & Cones)

* Retina Structure varies from the fovea to the retinal periphery *In Central Fovea*, -there are no rods in and many cones are there -This is the area of highest visual acuity - We have greater spatial sensitivity on our central retina when its light outside - Our central vision is blind at scotopic light levels -In central retina, few photoreceptors feed information to a ganglion cell. This arrangement makes the central retina better for high resolution vision *In Peripheral Retina* - there are many more rods than cones -So we are poorer at discriminating colors in peripheral vision and we are mores sensitive to low levels of light on peripheral retina - Also because we are more sensitive to low levels of light on our peripheral retina because there are more rods, we are unable to perceive color differences at night when rods are active but cones are not - higher ratio of photoreceptors to ganglion cells so that many more photoreceptors can provide input. This makes the peripheral retina better at detecting dim light

Rod Photoreceptors

* These have long cylindrical outer segment which contain light sensitive photopigments, containing many disks. * They are thus more sensitive to light than cones and there are much more of these than cones *All of these have the same photopigment

Cone Photoreceptors

* These have shorter, tapering outer segment with fewer membranous disks. *There are three different types of these, each containing a different pigment *The variations among pigments make these sensitive to different wavelengths of light *Thus these AND NOT THE RODS are responsible for seeing color (Green, Red or Blue)

Tapetum Lucidum

A reflective layer beneath the photoreceptors that bounce light back at the photoreceptors if it passes through the retina. Nocturnal animals that have this are more sensitive to low light levels at the expense of reduced activity

How Sodium Channels open in rods

Sodium channels are stimulated to open and are gated by an intracellular second messenger (cGMP). cGMP is produced in the photoreceptor by an enzyme, keeping the Na+ channels open. Light reduces cGMP, causing the Na+ channels to close and membrane potential becomes more negative. *Thus photoreceptors hyperpolarize in response to light*

Lens

This a transparent thing that is located behind the iris. It contributes a dozen or so diopters to the formation of a sharp image of a distant point (although cornea performs most of eye's refraction). *However, it is involved more importantly in forming crips images of objects located closer than about 9m*

Pupillary Light Reflex

This involves connections between the retina and the neurons in the brain stem that control the muscles that constrict the pupils. This reflex is consensual; shining a light into one eye causes this in both eyes.

Color Opponent Theory

This is another theory of color vision. This one has to do with ganglion cells rather than codes. This says that people perceive three primary colors: yellow, blue and red Also says that the response to another color in the receptive field center is canceled by sowing another color in the receptive field surround: Red/Green and Blue/Yellow are complementary Yellow is a primary color rather than a mixture of red and blue-green light *FIGURE OUT P 326*

Retina

This is in the back of the eye, and contains photoreceptors which are specialized to convert light energy into neural activity. It is able to detect differences in intensity of light *Remember, this is technically part of the brain*

Visual Angle

This is the distance across the retina that can be described in terms of degrees.

Process of Phototransduction in Cones

This is virtually the same as in rods The only major difference is in the types of opsins in the membranous disks of the cone outer segments

Parallel Processing

This means that different visual attributes are processed simultaneously using distinct pathways These two parallel streams of information are used to determine depth

How does retina connect to brain?

Axons of retinal neurons are bundled into optic nerves, which distribute visual information (in the form of action potentials) to several brain structures that perform different functions (including the LGN)

Light Adaptation

Because of tremendous increase in sensitivity when you go from light to dar, when you go from dark to light, the eye is temporarily saturated. This essentially reverses the changes in the retina that accompanies dark adaptation

Antagonistic Center Surround Receptive Fields

Bipolar cells are said to have ______ because the response of a bipolar cell's membrane potential to light in the receptive field center is opposite to that of light in the surround. _______ (w/o antagonistic part) is passed on from bipolar cells to ganglion cells via synapses

Color Perception

Color that we percieve is largely determined by the relative contributions of short, medium and long wavelength cones to retinal signal

Dark Adaptation

Depending on how high the initial light level is, it can take minutes to nearly an hour to reach the great light sensitivity in the dark (or getting used to the dark)

What is light?

Electromagnetic energy/radiation that is emitted in the form of waves. It has wavelength, frequency and amplitude

Depolarization of a Rod

For a typical neuron at rest, the membrane potential is -65mV (close to equilibrium potential for K). In contrast, In compete darkness, the membrane potential of the rod outer segment is -30 mV. This depolarization is caused by the steady influx of Na+ through special channels in the outer segment membrane

Signal Amplification in Rod Phototransduction

Many G proteins (transducin) are activated by each rhodopsin, and each PDE enzyme breaks down more than one cGMP molecule. This amplification gives our visual system incredible sensitivity to small amounts of light. *Remember, rods are more sensitive to light than cones because they contain more disks in their segments and thus more photopigment and also because they amplify the response to light more than cones do*

Phototransduction

This occurs in the rods, which outnumber cones btw. (Many of what has been learned about this by rods has proven to be applicable to cones as well) This process is analogous to activity at G-protein-neurotransmitter receptor *Light energy interacts with photopigment which produces a change in membrane potential*

Young-Helmholtz Trichromacy Theory

This says the brain assigns colors based on comparative readout of three cone types. The retina contains three receptor types, each type being maximally sensitive to a different spectrum of wavelengths. When all types of cones are equally active (red blue and green), we perceive white. Novel colors arise from other mixtures.

Iris

This surrounds the pupil and has a pigmentation that provides what we call the eye's color

Direct (vertical) pathway for visual information to exit the eye

Photoreceptors --> Bipolar Cells --> Ganglion cells Photoreceptors respond to light and influence the membrane potential of the bipolar cells connected to them Ganglion cells fire action potentials in response to light, and these impulses propagate along the optic nerve to the rest of the brain

Gross Anatomy of the Eye

Pupil Sclera Iris Cornea Optic Nerve

Visual Field

amount of space viewed by the retina when the eye is fixated straight ahead

Rhodopsin

in the rods, this is a pigment in the membrane of the stacked disks in the rod outer segments. This is the pigment that absorbs electromagnetic radiation when there is light eventually leading to hyperpolarizing

Properties of Gamma Radiation/ X Rays

radiation emitted at a high frequency (Short wavelengths) has highest energy "cool colors"

Properties of radio Waves

radiation emitted at lower frequencies (longer wavelengths) has less energy "hot colors"

Optics

study of light rays and their interactions

Light rays travel in a straight line until

until they interact with the atoms and molecules of atmosphere and objects on the ground. These interactions include 1) Reflection 2) Absorption 3) Refraction

Structural Differences in Rods and Cones (Light Sensitivity)

*In nighttime lighting (scotopic), only rods contribute to vision *In daytime lighting (photopic), cones do the bulk of the work. *At intermediate light (mesopic), both rods and cones are responsible for vision

Indirect Pathway to Bipolar Cell Receptive Fields

- In addition to direct connections with photoreceptors, bipolar cells also are connected via horizontal cells to a circumscribed ring of photoreceptors that surrounds the central cluster - First, when a photoreceptor hyperpolarizes in response to light, output is sent to horizontal cells that also hyperpolarize - Second, the effect of horizontal cell hyperpolarization is to counteract the effect of light on neighboring photoreceptors *The effect of this indirect path input is to depolarize the central photoreceptor which counteracts the hyperpolarizing effect of light shined directly on it (the two surrounding photoreceptors connected to horizontal cells are all still hyperpolarized)

Features of Ganglion Cell Receptive Fields

- Most retinal ganglion cells are not particularly responsive to changes in illumination that include both the receptor field center and the receptive field surround. - Rather it appears that the ganglion cells are mainly responsive to differences in illumination that occur within their receptive fields -EG: For OFF center cells, dark in the center of the receptive field causes the cell to depolarize whereas dark in the surround causes the cell to hyperpolarize. Here are some different situations 1) In uniform illumination, center and surround cancel to yield some low level of response 2) When the edge centers the surround region of the receptive field without encroaching on the center ,the dark area has the effect of hyperpolarizing the neuron, leading to a decrease in the cell's firing rate 3) As the dark area begins to include the center, the partial inhibition by the surround is overcome and the cell response increases 4) But when the dark area fills the entire surround, the center response is canceled and firing rate is similar to the one with all light

General Ganglion Cell Receptive Fields

- Most retinal ganglion cells have essentially the same concentric center surround receptive field organization as bipolar cells - ON-center and OFF-center ganglion cells receive input from the corresponding type of bipolar cell -HOWEVER ,unlike bipolar cells, ganglion cells fire action potentials. - Actually they fire action potentials whether or not they're exposed to light, and light in the receptive field center or surround increases or decreases the firing rate (in that order if they're ON)

Bipolar Receptive Fields

- These can be categorized into two classes; ON and OFF based on the response to the glutamate released by photoreceptors - The circuitry that gives rise to ______ consists of direct input from photoreceptors and indirect photoreceptor input relayed by horizontal cells

Factors of Dark Adaptation

1) Dilation of Pupils, which allows more light to enter the eye 2) Regeneration of unbleached rhodopsin 3) Adjustment of functional circuitry of the retina so that information from more rods is available to each ganglion cell

Three Important Points to Remember about Pathway for visual information

1) With one exception (light sensitive ganglion cells - but they don't play a major role in vision) the only light sensitive cells in the retina are rod and cone receptors. All other cells are influenced by light only via direct and indirect synaptic interactions 2) Ganglion cells are the only source of output from the retina. No other retinal cell types projects an axon through optic nerve 3) Ganglion cells are the only retinal neurons that fire action potentials, and this is essential for transmitting information outside the eye. All other retinal cells de/hyperpolarize but do NOT fire action potentials

Steps of Transduction of Light by Rods

1) light activates rhodopsin 2) The bleaching of rhodopsin causes transducin, the g-protein to be stimulated 3) Phosphodiesterase (PDE), the effector enzyme, is activated 4) PDE activation reduces cGMP level 5) as a result of reduced cGMP, Na+ channels close and the cell membrane hyperpolarizes

Every Photoreceptor has four regions

1) outer segment 2) inner segment 3) Cell body 4) Synaptic Terminal

What is the transmitter released by photoreceptors when depolarized?

Glutamate (remember photoreceptors are depolarized in the dark and hyperpolarized by light) So, photoreceptors release fewer neurotransmitters in the light than in the dark. We have to remember that dark rather than light is the preferred stimulus for a photoreceptor

Besides the cells in the direct path from photoreceptors to brain, retinal processing is influenced by two additional cell types

Horizontal Cells Amacrine Cells

Refraction

Images are formed in the eye by this type of interaction. This is the bending of light rays that can occur when they travel from one transparent medium to another.

Calcium's Role in Light Adaptation

In addition to the factors discussed earlier, the ability of the eye to adapt to changes in light level relies on changes in calcium concentrations within the cones When you go from dark to light, the cones are initially hyperpolarized. If the cones stayed in this state, we would be unable to see further changes in light level It's important to note cGMP gated sodium channels also admit calcium. In the dark, Ca2+ enters the cones and has an inhibitory effect on the enzyme that synthesizes cGMP. When cGMP-gated channels close, flow of Ca2+ into photoreceptor is curtailed along with Na+ As a result, more cGMP is synthesized (b/c synthetic enzyme is less inhibited by entering calcium) allowing the cGMP gated channels to open again. *Stated simply, when the channels close, a process is initiated that gradually reopens them even if the light level does not change* Calcium also appears to affect photopigments and phosphodiesterase in ways that decrease their response to light. These mechanisms ensure that the photoreceptor are always able to register relative changes in light level, though information about the absolute level is lost

Why does Phototransduction occur in cones?

In bright light, cGMP levels in *rods* fall to the point where the response to light becomes saturated; increasing the light level causes no additional hyperpolarization Thus, vision during the day depends entirely on the cones, whose photopigments require more energy to become bleached

Where does the conversion of electromagnetic radiation into neural signals occur?

In the back of the retina at the photoreceptors

ON Bipolar Cells

Light shined onto a cone can depolarize some bipolar cells. These cells turned on by light are called _____ The cone-to-bipolar synapse inverts the signal from the cone: The cone hyperpolarizes to light, but __________ depolarize

OFF Bipolar Cells

Light shined onto a cone will hyperpolarize some bipolar cells. These bipolar cells are known as ______ because light effectively turns them off

Photoreceptor G-Protein mechanism in RODS

Light stimulation of the photopigment (*rhodopsin*) activates G-proteins (*transducin*), which in turn activate an effector enzyme (*phosphodiesterase (PDE)*) that changes the cytoplasmic concentration of a second messenger molecule (*cGMP*). This change causes a membrane ion channel to CLOSE and the membrane potential is altered (*hyperpolarizes the cell*)

ON vs OFF Ganglion Cells

ON-center ganglion cell will be depolarized and respond with a barrage of action potentials when a small spot of light is projected onto the center of its receptive field OFF center cell will fire fewer action potentials when a small spot of light s projected to the center of its receptive field. it will fire more action potentials if a small *dark spot covers the receptive field center*. **IN BOTH ON AND OFF TYPES, THE RESPONSE TO STIMULATION OF THE CELL IS CANCELED BY THE RESPONSE TO STIMULATION OF THE SURROUND**

Three Opsins in Cones

Red (long wavelengths ~560nm) Green (medium wavelength ~530nm) Blue (short wavelength ~430 nm) *Each type of cone is activated by a broad range of different wavelengths of light, and there is overlap in the wavelengths that affect the three cone types*

Types of Photoreceptors

Rod Cone

Vitreous Humor

This a more viscous jelly like area that lies between the lens and the retina. It serves to keep the eyeball spherical

Summary of Direct/Indirect Pathways of Bipolar Cells

The receptive field of a bipolar cell consists of two parts: 1) a circular area of retina providing direct photoreceptor input, called the *receptor field center* 2) a surrounding area of retina providing input via horizontal cells called the *receptive field surround* *The response of a bipolar cell's membrane potential to light in the receptive field center is opposite to that of light in the surround* - Direct Pathway depolarizes ON bipolar cell and Indirect Pathway hyperpolarizes ON bipolar cell -Direct Pathway hyperpolarizes OFF bipolar cell and Indirect Pathway depolarizes OFF bipolar cell

How can different bipolar cells give opposite responses to *direct* cone input?

There are two kinds of receptors that receive glutamate released by the photoreceptors 1) OFF bipolar cells have ionotropic glutamate receptors and these channels mediate a classical depolarizing excitatory postsynaptic potential from the influx of Na+. Hyperpolarization of the cone causes less neurotransmitter to be released, resulting in a more hyperpolarized bipolar cell 2) ON bipolar cells have G-protein coupled receptors and respond to glutamate by hyperpolarizing (glutamate binds and hyperpolarizes the cell). SO if the cone is hyperpolarized, glutamate won't be released and nothing will bind to the receptors Each bipolar cells receives *direct synaptic input* from a cluster of photoreceptors. *The number of photoreceptors in this cluster ranges from one at the center of the fovea to thousands in the peripheral retina*

Intrinsically photosensitive retinal ganglion cells (ipRGCs)

These are a small percentage of retinal ganglion cells that transduce light. They use melanopsin as a photopigment. They function as normal ganglion cells that receive input from rods and cones and send axons to the optic nerve, but they also are photoreceptors. Their photosensitivity differs from rods and cones 1) These actually depolarize to light 2) They have large dendritic fields. 3) they are important for providing input that synchronize behavior to daily changes in light level (circadian rhythms)

M-Type Ganglion Cells

These are large ganglion cells. They make up about 5% of the ganglion cell population. These have larger receptive fields and conduct action potentials more rapidly in the optic nerve. They are also more sensitive to low-contrast stimuli They respond to stimulation with a transient burst of action potentials. They lack color opponency They can detect subtle contrasts over large receptive fields and contribute to low resolution vision *These are the only thing other than cones that are sensitive to light*

P-Type Ganglion Cells

These are smaller ganglion cells. These are smaller and conduct action potentials slower than M cells. They respond with sustained discharge as long as the stimulus is on These are well suited for the discrimination of fine detail *These are the only thing other than cones that are sensitive to light*

Amacrine Cells

These generally receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells and other amacrine cells

Accomodation

With near objects, light rays originating at a point are no longer parallel so a greater refractive power is required to bring them into focus on the retina. So, this occurs to make an additional focusing power by changing the shape of the lens

Optic Disk

a pale circular region where the optic nerve fibers leave the retina. The sensation of light cannot occur here because there are no photoreceptors eg: blind spot

Energy content of electromagnetic radiation is proportional to

frequency

Cornea

glassy transparent external surface of the eye that covers the pupil and iris. It is the site of most of the refractive power of the eyes

Laminar Organization

cells in the retina are organized in layers known as this. They are seemingly inside out and light must pass through ganglion cells and bipolar cells before it reaches the photoreceptors *Don't get confused by thinking about the head instead of the eye: the photoreceptors are the outermost part of the retina even though they are the farthest from the front of the eye and the deepest inside the head* The innermost retinal layer contains the cell bodies of the ganglion cells

Aqueous Humor

fluid that lies behind the cornea (which lacks blood vessels btw) and nourishes the cornea

Conjunctiva

membrane that folds back from the inside of the eyelids and attaches to the sclera

Visual Acuity

the ability of the eye to distinguish two points near each other. This depends on several factors, but especially on the spacing of photoreceptors in the retina and the precision of the eye's refraction

Receptive Field

the area of the retina where light changes neuron's firing rate (in a ganglion cell). These change in shape and correspondingly in the sort of stimulus that makes the neurons most active

Reflection

the bouncing of light rays off a surface. Most of what we see is light that has been _______ off objects in our environment

Amplitude

the difference between wave trough and peak

Wavelength

the distance between successive peaks or troughs

How image formation by the eye occurs

the eye collects light rays emitted by or reflected off objects in the environment and focuses them on the retina to form images

Lateral Geniculate Nucleus (LGN)

the first synaptic relay in the primary visual pathway that serves visual perception occurs in a cell group of the dorsal thalamus called ________ From _______, visual information ascends to the cerebral cortex, where it is interpreted and remembered.

Duplex Retina

the large differences in the structure and sensitivity of rods and cones led investigators to say humans have this, which is two complementary systems in one eye

Ciliary Muscle

the lens is suspended by ligaments attached to this, which forms a ring inside the eye. These are contracted to sharpen an image, especially during accommodation when objects are near (EG: toothpicks to center a strawberry in the hole of a bagel - strawberry= lens toothpicks - ligaments bagel = this vocab word that attaches to the sclera

Macula

the middle of the retina that is darker colored in region with a yellowish hue. It is the part of the retina that is important for central vision (rather than peripheral)

Dark Current

the movement of positive charge across the membrane, which occurs in the dark is called this. The movement of positive charge depolarize the rod outer segments

Frequency

the number of waves per second

Pupil

the opening that allows light to enter the eye and reach the retina. It appears dark because of the light absorbing pigments in the retina

Absorption

the transfer of light energy to a particle or surface. Eg: sun warming up skin

Photoreceptors

these are in the retina and are specialized to convert light energy into neural energy (light energy is converted into membrane potential changes) . These lack axons and form synapses with dendrites of other sensory neurons. They detect energy levels of 400-700nM

Extraocular muscles

these are in the scelera and move the eyeball in the orbit

Horizontal Cells

these receive input from the photoreceptors and project neurons laterally to influence surrounding bipolar cells and photoreceptors.

Optic Nerve

this carries axons from the retina, exits the back of the eye, passes through the orbit and reaches the base of the brain near the pituitary gland

What does refractor power depends on?

this depends on the slowing of light at the air-cornea interface. If we replace air with a medium that passes light at about the same speed as the internal structures of the eye, the refractive power of the cornea would be eliminated (which is why things are blurry underwater but are fixed when you wear goggles)

Fovea

this is a dark spot where the retina is thinner than anywhere else. It is so thin because of lateral displacement of cells above photoreceptors, allowing light to strike the photoreceptors without passing through the other retinal cell layers. Has mostly cones

Scelera

this is continuous with cornea and is known as the "white of the eye" which forms the tough wall of the eyeball

Transducin

this is the G protein effector enzyme that is activated when rhodopsin is bleached. This in turn activates the effector enzyme PDE which breaks down cGMP that is normally present in the rod when it is dark