Hello to all teachers and students,
I come to you once again with lessons about vision and eyecare, yesterday we learnt about how perception occurs basically and about the path that light stimulus takes to reach our brain. In Today's Lesson, we are going to look more into the biochemistry that goes on in the retina and how the impulses are generated
Depolarization and Hyperpolarization
Before we begin lets recap on some basic physiology principles on stimulus or impulse transmission. When we talk about depolarization or hyperpolarization we talk about changes that occur in a cell's potential that causes a change in the electrical potential within the cell. Depolarization causes the cells to become more positive whiles hyperpolatization causes the cell to become more negative.
What happens is that at any point in time when the cell is resting, the internal charge is about -55mV, the cell has lots of potassium inside with sodium outside which are all positively charged particles. When a stimulus or impulse is to be fired which is the depolarization process, more of the sodium particles outside move into the cell through channels known as sodium potassium gated channels and as they move into the cell, the cell becomes more and more positive until the point it reached where any more movement does not cause any other change in the potential. You can compare this to when you got so much angry that nothing else could provoke you any further than you already are, or when you got brokenhearted that you could not feel any emotion again. After a while some mechanism come into place and brings everything back to normal, so in the process of depolarization, when one cell starts the impulse travels from that cell to the next with the help of chemicals known as neurotransmitters and so each of the cells in transmitting the charge along also undergoes this depolarization process we have talked about.
Now in the case of hyperpolatization the positively charged potassium ions which we have already learnt are within the cell at all times move out through through their own channels known as potassium channels and they keep doing this such that the internal potential within the cell drop below the -55mV. This also lasts for a while before everything returns back to is normal state.
Let's recap on what phototransduction is, we said the the process or the mechanism which the eye changes photons or light energy emanating from an object into electrical impulses for interpretation by the brain. So understanding depolarization and hyperpolatization we now know that it is by one of these mechanism that the body generates electricity or electrical impulses to be transmitted through the body and so is it going to be for the eye too. The main cells responsible for this initiation are what we called the photoreceptors, which are found in the 1st of the 10 layers of the retina. These cells have two segments, outer and inner and the process we are learning about occurs involves both however the outer segment contribute more.
So when light hits your retina, there is change in permeability of these cell. When we talk about permeability we are simply looking at the ability of things to move in and out of the cell easily, mostly molecules and particles like sodium and potassium. So just like any normal cell, more potassium inside, sodium outside, and then there is the almighty sodium potassium gated channel that would determine when to let someone go inside. This channel is mostly concentrated in the inner segment thus sodium is mostly pumped out of the cell via the inner segment and diffuse into the cell via the outer segment where some other gated channels that like positively charged particles also exist.
When light comes and the cell changes permeability, the cation channels in the outer segment closes thus preventing more positive charge from entering the cell. But whiles this is still happening, the equilibrium is not been kept because the sodium potassium channels at the inner segment continues to pump sodium out as it function demands, so guess what will happen. The cell will loss potential and become more negatively charged in other words hyperpolarization, this potential activate the neurotransmitters around which would cause the subsequent cells to also transmit impulses to start working and so the next set of cell that would take up the next action is the bipolar cells and they also undergo changes in their potential as already described in the depolarization and hyperpolarization chapter but this time round depolarization occurs instead of hyperpolatization. From here the impulse goes to the ganglion cells which extends to form the optic nerve, the final nerve that transmit all the accumulated electrical energy to different aspects of the brain for various interpretation including seeing. So the eye is a little special in that whiles almost every cell starts the electrical impulse transmission process with depolarization it starts with hyperpolarization and due is attributed to the fact that glutamate is the main neurotransmitter in the retina. (Hopefully we will get to learn more about neurotransmitters in the future and understand the mechanism behind why it could excite or hyperpolarize based on the receptors present.)
I hope you did not get lost along the way somehow, anyway if you did kindly take a second read and am sure things would look more clearer a better as i did my best to make it as simple as possible for even individual with less background in science. In our other lessons we will be looking at the brain areas responsible for the various aspects of vision like seeing color, seeing shapes, moving the eye to look at objects etc. Until then i would leave you here and have a wonderful time. Remember that COVID19 is real and to take all precautionary measures into advisory. Stay home and Stay Safe.