What Makes Me Tick - Part 2

Part 2 – Our Sight's Reception and Interpretation

A Chemical Pathway in Reception

Sight, like flight, is an astounding outcome in biology. Sight requires both a method of receiving visual input and a method of interpreting that input. Today, the retina in the human eye receives the visual input of light, and the cerebral cortex in the brain interprets that input into what we experience as vision.

According to scientific naturalism, these systems of reception and interpretation must have evolved over time. Either one evolutionary path caused both our receptive system and our interpretive system to develop simultaneously, or the two systems developed independently. If they developed independently, they originally had separate functionality (or carryover), and then later changed to interact and be functional together.

To get a better grasp on what this means, the following is one of the chemical pathways which occurs in the retina of the human eye.

For humans to see with our present-day eyes, a photon of light must enter our eye and land on our retina. Photoreceptor cells in the retina interact with these photons and result in an electrical current sent to the brain. Below details one chemical pathway that is involved in accomplishing this first step. This pathway can be studied in the article by the National Library of Medicine at https://www.ncbi.nlm.nih.gov/books/NBK10806/. (To aid in simplicity, I have organized my summary following the details outlined in Darwin's Black Box, pages 18-21.)

The human retina contains photoreceptor cells. When light in the visible spectrum lands on these cells, a photon interacts with a molecule called 11-sisretinal, rearranging it to become transretinal. When this happens, the protein rhodopsin (which was bound to 11-sisretinal) also changes. Rhodopsin becomes metarhodopsin II, which now behaves differently. Metarhodopsin II combines with another protein called transducin. Prior to exposure to metarhodopsin II, transducin was bound to the molecule GDP. But when metarhodopsin II is introduced, the GDP breaks off, and is replaced with GTP.
This new molecule composed of metarhodopsin II, transducin, and GTP now binds to phosphodiesterase in the inner membrane of the retina cell. With these 4 molecules combined, the phosphodiesterase portion is able to hydrolyze cGMP molecules (breakdown the cGMP molecules through reaction with water). This reduces the number of cGMP molecules available to interact with the ion channel in the cell membrane.
Normally the ion channel controls the amount of Na+ ion in the cell through osmosis and a molecular pump. When cGMP is reduced, the ion channel closes, causing the amount of Na+ ions to be reduced. This causes an imbalance in charge across the cell membrane. This charge differential causes an electrical current to be transmitted down the optic nerve to the brain. This current is received by different nerve cells in the brain, the interpretation system, to then be processed and interpreted as vision.
If the chemical reactions above were the only ones activated in the cell, the supply of needed proteins would quickly be depleted. The proteins and ions must be restored to their original state. 
When cGMP levels drop, Ca+2 ions are also reduced through their ion channels. When Ca+2 levels drop, two things happen: the activity of phosphodiesterase is slowed down and guanylate-cyclase begins to resynthesizes cGMP. At the same time, metarhodopsin II is modified by the enzyme rhodopsin kinase which causes it to bind to a protein named arrestin. Now the modified rhodopsin stops activating more transducin.
This process limits the signal that results from the interaction of a single photon. Then transretinal is chemically modified by an enzyme to have to more H+ atoms. Another enzyme reconverts it to 11-sisretinol. Finally, a third enzyme removes the extra two H+ atoms, returning it to 11-sisretinal.


This cascade of reactions starts with a photon of light, involves a chain of chemical reactions, results in an imbalance in the charge across the cell membrane, sends a current to a neighboring cell, and restores the chemistry needed to do it again. The current passed from the photoreceptor cell to the optic nerve cell must now interact with a system that is able to respond to or interpret the current.


The Reception System

Before taking a glimpse at the interpreting system, the reception system is far more complicated than the chemical pathway delineated above. At the following sites, https://www.ncbi.nlm.nih.gov/books/NBK10885/ (National Library of Medicine) and https://www.sciencedaily.com/releases/2013/07/130718130458.htm (Science Daily), one learns that the retina is actually composed of five different types of retinal cells with additional sublevel types. These five main types of cells are in alternating layers with different processes and synaptic contacts. Altogether there are 60 different types of cells in the retina. The instructions to build the 60 cells found in our retina comes from more than 10,000 distinct genes in our DNA.

These retinal cell types control the lateral interactions in the retina, including the system's sensitivity to contrasts in brightness and color, the communication from rods to ganglion cells, and other required contributions to visual function. Each of these retinal cell interactions have their own chemical pathways and methods of protein restoration. Altogether the retina has approximately150 million light-sensitive cells which transfer signals to the retinal cells called the ganglion cells which form the optic nerve. The optic nerve is insulated, has a blood supply, has passage through the skull, and then branches to multiple sections of both sides of the brain.  


The Interpreting System

For human sight, the interpreting system is found in the brain. An introduction to the anatomy of the interpreting system can be found in the National Library of Medicine article, https://www.ncbi.nlm.nih.gov/books/NBK482504/. The optic nerve extends back into the brain in a branchlike structure to the visual cortex. The visual cortex subdivides into five different areas based on structural and functional classifications. The first area V1 is divided up into six distinct layers, each comprising different cell-types and functions, and interprets orientation and direction. The second area has more complex cells that respond to differences in color, spatial frequency, moderately complex patterns, and object orientation. Sections V3-V5 are more specialized and focus on spatial tasks and visual-motor skills.

The brain synthesizes the information from the more than 20 different cells types of the visual cortex. The brain interprets an image that the human can then respond to with cerebral processing and muscular responses. The details involved in these processes are still being studied today, and each step of processing and response is dependent upon a sequence of chemical pathways, which must be initiated and restored for each of their different outcomes.


Stepping To Our System Today

From the photoreceptor cells in the retina to the visual cortex in the brain, there are 80 different types of cells that participate in the process of reception and interpretation of light. These component cells have varying shapes, membranes, organelles, molecular pathways, and functions.  

The different types of retinal cells are dependent on an array of proteins which are used to construct the cells and run the chemical processes. The coding for the manufacturing instructions for these proteins are found in over 10,000 distinct genes. Assuming an evolutionary model of development, the dispersal and vastness of this protein coding demands a more primitive system of reception and interpretation to have previously existed.

The first primitive reception-to-interpretation system which was stimulated by photons of light must have yielded some kind of survival response. The development of vision would not be significant without some kind of beneficial result increasing organism survival. Sight was an unforeseen product of DNA replication, and the outcome of sight on its own would not be significant. The development of vision must have been coordinated with an ability to respond to that input in a beneficial way.

Once this precursor evolutionary model of sight generated, then over time, the complex sight systems we have today encoded in over 10,000 genes must have evolved stepwise. The evolutionary path of DNA changes must have generated stepwise beneficial alterations that worked with the established chemical dependence, and also increased structural and chemical complexity. Today over 270 variants in the DNA of genes are known to cause inherited retinal diseases.

The copy variants must have increased the number of genes used in encoding, caused the differentiation of the cell types used the visual system, and generated the protein diversity in such a way as to build interconnected chemical sequences of reactions, all the while, simultaneously resulting in measurable benefits to the species rate of survival.

Human vision today is dependent upon this vast array of proteins manufactured from these genes; therefore, in Part 3 we will take a closer look into what is involved in forming these proteins.

What Makes Me Tick - Part 3

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