The Visual Pathway

Note: This Page was written as an introduction to the Page on Retinitis pigmentosa. If you are familiar with the anatomy of the eye and the normal functioning of the visual pathway please go to: Retinitis pigmentosa

The Anatomy of the Eye
The Fundus of the Eye
The Cell Layers of the Retina
The Photoreceptor Cell Layer
The Proteins of the Outer Segments
Rhodopsin
The Phototransduction Cascade
The Role of cGMP in Vision
Metabolism of Vitamin A in the Retina
Acknowledgements

The Anatomy of the Eye

Figure 1. Shows a cross-section of an eyeball where the various anatomical structures of the eye are depicted.

Looking at a cross-section of the eyeball we see the following:

• The Sclera (the white part of the eyeball): a tough tunic which encases the eye and consists of rod-like unstructured collagen , molecules interspersed with a special type of sugars, called glycosaminoglycans (GAGs), connected together in long chains that intertwine to form globules.

• The Cornea: a crystal clear structure located at one end of the sclera. It consists of:
  • an outermost layer of cells called the epithelium which sits
  • on the Stroma, a highly structured lattice-like arrangement of collagen molecules interspersed with GAGs
  • a basement membrane called Descemet's membrane made up of a specialized type of collagen, and
  • a single layer of Endothelial cells.

  Damage to the stroma, Descemets membrane or the endothelium would cause permanent damage to the cornea. This damage would interfere with the transmission of light to the retina and thus, lead to blindness.

  The cornea acts like a window which allows light to pass through several tissues in the eye before it arrives at the retina.

• Aqueous Humor: In its path to the retina, light passes through the fluid known as the aqueous humor. An increase in the volume of the Aqueous Humor would cause an increase in the intraocular pressure and lead to what is known as Glaucoma. If untreated, Glaucoma would damage the retina thus reducing vision or in severe cases cause blindness.

• Pupil and Iris: Light then passes through the Pupil, the circular aperture in the middle of the Iris (the colored tissue seen through the cornea). The pupil contracts in bright light to decrease the amount of light reaching the retina, thus preventing damage to the retina by excessive light or it dilates in dim light to ensure that enough light reaches the retina for vision to occur.

• The Crystalline Lens: together with the cornea, focuses light and allows it to pass through the

• gel-like Vitreous Body to the retina.

  If cataract develops in the lens, light will not pass through to the retina and vision is blocked.

• The Retina: surrounds most of the cavity of the eyeball and abuts on the

• Retinal Pigment Epithelium (RPE): a single layer of cells which contains pigment similar to that found in the skin.

• Behind the RPE is the Choroid: a highly enriched source of blood vessels which supply the retina with some of its high energy requirements.

• The sclera surrounds the entire eyeball.

The Fundus of the Eye

Figure 2.- The fundus of a normal Eye. Note the uniform color of the fundus, and the blood vessels which traverse the fundus starting at the pale colored Optic Disc seen at the top of the picture. The Optic Disc is the area from which the Optic nerve leaves the retina to connects with the brain. (Original slide provided by Dr. Gilles Desroches).

Looking into the eye with an ophthalmoscope, one sees the retina or what is referred to as the fundus. The retina surrounds the cavity of the eyeball. A number of blood vessels come out (the arteries) and leave (the veins) through a pale area in the retina, called the Optic Disc.

The Optic Nerve passes through the Optic Disc to transmit nerve impulses to the brain.

The Macula , is located in the fundus. Inside the macula is the Fovea where most of our sharp vision is located because of the concentration of visual cells, called the Cones, in that area (see later).

Pathological changes to the macula lead to the disease called Macular Degeneration.

The Cell Layers of the Retina

Figure 3. - The Cell Layers of the retina. Abbreviations: CH = choroid, PE = pigment epithelium, OS = outer segment layer, ONL = outer nuclear layer, OPL = outer plexiform layer, INL = inner nuclear layer, IPL = inner plexiform layer, ILM = inner limiting membrane.

If we were to look at a small thin specially treated and stained section of the wall of the eyeball, under the microscope, we see the following, starting from the outside of the eye (see Figure 3):

• the Sclera : The tough collagen and glycosaminoglycans tunic that was discussed above. (see Figure 1).

• the Choroid(CH): an area that is rich in blood vessels,

• the Retinal Pigment Epithelium (PE): a single layer of cells that is rich in black pigment, similar to that found in the skin,

• the Photoreceptor or visual Cell layer: which as its name implies, is the cell that receives light and allows vision.

  The photoreceptor cell layer can be divided into four sections:

  • the outer segment layer (OS): contains the parts of the Cone and Rod cells which contain the visual pigment, Rhodopsin, the protein that is essential for vision, and

  • an inner segment (IS) layer, the area that contains the microsomes, on which proteins are synthesized and the mitochondria which supply the energy for the fuctioning of the cell,

  • the nuclear layer in which the chromosomes containing the genes are found and

  • the Synaptic body through which electrical impulses generated after light strikes the outer segments of the photoreceptor, are transmitted to the inner retina.

• the inner nuclear layer (INL ) Contains the nuclei of the Horizontal cells, the bipolar cells, the ganglion cells and the Amacrine cells. These cells are essential for transmitting nerve impulses from the photoreceptors to the brain.

• The photoreceptor cells, the horizontla cells, the ganglion cells and the amacrine cells are connected with each other by processes called axons which come out of one cell and and dendrites, which come out from the other (OPL).

• the inner plexiform layer of nerve fibers (IPL) connects the retina with the brain.

• the inner limiting membrane (ILM)

The Photoreceptor Cell Layer

Light is reflected from the object we are viewing, goes through the cornea and the lens where it is focused onto the retina.

At the retina, light passes through the inner retina to the photoreceptor cells where it is absorbed by the visual pigment, Rhodopsin, found in the outer segments of the photoreceptor cells.

Rhodopsin then undergoes a series of chemical reactions with other proteins found in the outer segments. These interactions,activate a number of photoreceptor outer segment enzymes, whose activities lead to the "hyperpolarization" of the photoreceptor cell.

The hyperpolarization of the visual cell cause the transmission of an electrical impulse which travels through the inner retina to the brain forinterpretation.

The photoreceptor cells consist of two types of cells:

Figure 4: Diagram of a cone cell showing outer segment with comb-like discs, nuclear areas and synaptic body.

Figure 5 - Diagram of a rod cell showing the outer segment with its stack of discs separated from the cell membrane, the inner segment, the nucleus and the synaptic body.

• Cone Cells (Figure 4): which are shaped like cones, hence their name, are responsible for our vision under light conditions, our sharpest vision, and discrimination between colors.

  In spite of their importance, these cells are small in number in the mammalian retina and are mainly concentrated in the macula and particularly in the fovea.

• Rod Cells (Figure 5): get their name from their rod-like appearance. Most of our knowledge about vision comes from the study of Rod Cells which are much more numerous in the mamalian retina than the cones. Rod cells also predominate in the retinas of nocturnal animals. Rod cells allow vision in dim light and at night.

  ---

  ---

Both the cone and the rod cells, are made up of four distinct areas (see Figures 4 and 5):

• The outer segment which consists of a series of disc-like structures that contain the enzymes and proteins that capture light entering the eye and initiate the process of vision

• the inner segment (IS): contains the microsomes on which enzymes and proteinsrequired for the proper function of the cell are synthesized, the golgi body which packages the products of the microsomes for export to their proper destination inside or outside the cell, and the mitochondria which provide the energy for the various functions of the cell, in this case, the process of vision.

• the nuclear area(N): which contains the genetic codes that determine the nature of the proteins and inzymes that are to be synthesized on the microsomes.

• synapttic body: transmits electrical signals originating in the photoreceptor outer segments to the cells of the inner retina.

The cone and rod cells not only differ in their function but they also differ in the structure and shape of their outer segments. Thus:

• the cone outer segment cell membranes undulate to form a comb-like appearance so that the cone discs are connected to the plasma membrane throughout the extent of the outer segment, and thus are open to the extracellular space.

• the cone outer segments are shorter, have a cone-like appearance (hence their name), a wider base and tapering shape compared with those of rods.

The rod outer segmets, on the other hand are:

• longer and more slender and

• their discs consist of double membranes that are separated from each other and from the outer segment membrane and

• their discs are stacked one on top of the other like a stack of coins (see Figure 5 and 6).

Figure 6 - An electron micrograph of a section of a rod outer segment. Note the double membranes of the discs and the fact that the discs are separated from the cell membrane.

The Proteins of the Outer Segments

The proteins of the outer photoreceptor segments are synthesized in the inner segments and exported by the Golgi Body to the outer segments. In the cones, the new proteins are layed in the outer segment area closest to the inner segments. In the rod cell the exported proteins are formed into discs which are layed at the bottom of the outer segment.

As new proteins are delivered to the outer segments the older ones are pushed towards the apex of the outer segments. Eventually, the oldest proteins are shed and are removed that is, phagocytised by the neighboring pigment epithelial cells (RPE), thus keeping the area between the tips of the outer segments and the RPE clean. In the RPE the phagocytised discs are digested and moved to the back of the RPE cell.

The process of phagocytosis occurs in a 24 hour circadian rhythm. In Rod cells it takes place daily at light onset and at sunset for the cones.

The pigment epithelium appears to have other importantfunctions. Thus, under intense light conditions, it extends long processes around the photoreceptor outer segments presumably to protect the outer segments from light damage. Under dim light conditions These processes are retracted to allow for more light to reach the outer segments.

RPE also plays a role in the process of vision whereby it is involved in the conversion of all-trans retinal to 11-cis retinal (see below)

Rhodopsin

The undulating membranes of the cones contains the photopigment, Photopsin, while the discs of the rods contain the visual pigment, Rhodopsin. Chemically, Photopsin and Rhodopsin seem to have little differences between them and often, they are referred to as Rhodopsin.

Rhodopsin forms about 80 to 90% of the proteins of the rod outer segment disks. It consists of a protein, called opsin, and a special form of vitamin A, called 11- cis retinal.

Opsin consists of a single polypeptide chain of 348 amino acids that winds itself seven times around the outer segment disc membrane forming four loops facing the outside of the ROS disc and three loops facing the inside of the disc.

The rhodopsin molecule is folded in a three dimensional manner so that the first and seventh loops are in close proximity with each other, thus forming a a pocket that contains the 11-cis retinal that is linked to the amino acid lysine in position 296 of the opsin molecule.

The Phototransduction Cascade

The Phototransduction Cascade is initiated when light strikes rhodopsin and isomerizes the chromophore of rhodopsin, 11-cis retinal, to all-trans-retinal to cause a conformational change in the rhodopsin molecule which results in the Photo- excited rhodopsin, Metarhodopsin II.

There are two routes that Metarhodopsin II can be directed:

• phosphorylation by rhodopsin kinase (see later),

• react with Transducin, a tripeptide consisting of an alpha (a), a beta (ß) and a gamma (y) polypeptide.

In its inactive state (under dark conditions of illumination)transducin binds GDP in the form (Ta.GDP and TßY

When Metarhodopsin II reacts with transducin several reactions take place:

• a conformational change in the transducin molecule to cause the exchange of the GDP bound to Ta for GTP

• the break-up of transducin into:

  • The active form of transducin, Ta.GTP,

  • and TßY,

• in the meantime, Metarhodopsin II is now released to react with more Transducin molecules or be phosphorylated by rhodopsin kinase.

cGMP PDE is an enzyme consisting of four polypeptides, alpha (a, beta ßand two gamma yy polypeptides (PDEaßyy).

The reaction of Ta with PDE aßyy causes the following changes:

• hydrolysis of the GTP which is bound to Ta forming Ta.GDP. The latter is now free to react with Tßy to form Tßy.Ta.GDP and react with another molecule of the active rhodopsin, Metarhodopsin II.

• releases the two inhibitory gamma polypeptides from cGMP PDE to form the active PDEaß that rapidly hydrolyzes cGMP.

The Role of cGMP in Vision

cGMP plays a very important role in vision. Under normal dark conditions of illumination, cGMP keeps certain channels called "cGMP-gated cationic channels", located in the outer segment cell membranes open.

In the dark, these channels allow sodium and calcium ions to enter the photoreceptor cell. Calcium ions are extruded through a calcium ion/sodium ion exchanger located in the outer segment cell membrane.

Under dark conditions of illumination, sodium enters the photoreceptor outer segment through the cGMP-gated cationic channels, travels to the inner segment where it is extruded to the outside by a sodium-potassium activated adenosine triphosphatase located in the inner segment plasma membrane.

The movement of sodium ions from the outer segment to the inner segment causes the formation a small electric current called the "dark current", between the two segments of the photoreceptor cell thus keeping the photoreceptor cell depolarized.

Exposure to light, causes the activation of the enzyme cGMP PDE which hydrlyzes cGMP thus causing a decrease in the concentration of the mononucleotide in the outer segments. This:

• Closes the cGMP-gated cationic channels,

• blocks the entrance of sodium and calcium ions into the photoreceptor cell outer segments.

• stops the "dark current" from travelling between the inner and outer segments of the photoreceptor cell

• hyperpolarizeres the photoreceptor cell causing a nerve impulse to travel through the synaptic body and the inner retinal cell layer to the brain for interpretation and hence vision.

In order for vision to continue, the concentration of cGMP must be replenished. This is accomplished by

• inactivation of PDE. This takes place when the GTPase inherent in Ta.GTP hydrolyzes its own GTP to be converted to T a .GDP. The inhibition of PDE is believed to occur in about 100 mseconds under physiological conditions.

• activation of the calcium ion -sensitive enzyme, guanylate cyclase which synthesizes cGMP from GTP

As mentioned above, calcium enters the outer segments under dark conditions of illumination, through the cGMP-gated cationic channels. When light strikes rhodopsin the phototransduction cascade hydrolyzes cGMP and the cGMP-gated channels are closed and the entry of calcium ions into the outer segments is blocked.

Although calcium ceases to enter the outer segments under light conditions of illumination,it continues to be extruded via a calcium ion/sodium ion exchanger located in the outer segment membrane thus reducing its concentration within the outer segments.

The decrease of calcium concentration in the outer segments, activates the enzyme, guanylate cyclase whose activity leads to the increase in the concentration of cGMP, thus opening the cGMP-gated channels and the re-entry of sodium and calcium ions. Continued esposure to light leads to the recycling of the phototransduction cascade.

Return of Rhodopsin to the Dark State

• the concentration of cGMP must be replenished

• The active form of rhodopsin, Meta II , must be deactivated in order to regenerated rhodopsin

• all-trans retinal must be re-isomerized to 11cis retinal

As mentioned above the concentration of cGMP is increase by the activation of guanylate cyclase and the inhibition of cGMP PDE by Ta.GDP which would recombine with Tßy to react again and again with metarhodopsin II.

To return to the dark state, the ability of metarhodopsin II to combine with transducin would eventually be significantly reduced by phosphorylation with the enzyme, rhodopsin kinase followed by combination with the protein arrestin. These reaction cause the release of alltrans retinal from the protein of rhodopsin, opsin.

Metabolism of Vitamin A in the Retina

Absorption of light by rhodopsin in the disc membrane:

• converts 11-cis retinal to all-trans retinal.

• generates the active photoproduct, metarbodopsin II.

Metarhodopsin II reacts catalytically with transducin until its activity is quenched by phosphorylation by rhodopsin kinase and the binding of the phosphorylated product with the protein,arrestin.

The reactions cause the breakup of the metarhodopsin-phosphate-arrestin into:

• the protein, arrestin

• Opsin-phosphate, which is dephosphorlated by rhodopsin phosphatase to form Opsin. The latter is then free to react with 11-cis retinal to form a new molecule of rhodopsin

• all-trans retinal.

All-trans retinal now undergoes several chemical reactions in order to be converted to 11-cis retinal and then bind to opsin and become part of the rhodopsin molecule: