It's a great time to be here again today. Over the past few days, we have been looking at the human sense organ, starting with The Anatomy, and Physiology of the Ear where we looked at the human ear, explaining the outer, middle and inner ear. In the future, I will be explaining the Cochlea in an entire post, I hope to have you on my blog when the post is made. We move forward to Anatomy and Physiology of the Eye, where the eye was explained. The post touched specific areas of the eye such as the lacrima gland, Lacrima punti, the rectus muscle, oblique muscles and other part of the eyes that were not directly part of the eyeball. In the post The Anatomy and Physiology of the Eyeball, the eyeball was explained in details. I will encourage that if you missed or didn't read any of the post, you can click on the links to the post and just enjoy yourself as you read them. In today's post, we will be discussing Phototransduction. With any further ado, let's get to the business of the day.
Good day Ladies and gentlemen, welcome onboard fight Phototransduction, from stemsocial to the world. We are currently ready to take off and will be in the air immediately. We ask that in this flight, you tighten your seatbelt now, you should turn off anything that will distract you, and please ensure that you have your coffee, soda, or any other thing that will keep you refreshed during the duration of this flight. Thank you for choosing to fly with us on Phototransduction, It is your captain 
Wow!! I will do well as a pilot, or what do you think? Away from that. What is Phototransduction in simple terms is the conversion of light that gets into the eyes into electrical signals. To be able to explain this process properly, I will look at the Retina singly. The Retina of the eye recieves light from the lens and converts it to neural signals. Sure the Retina does this, how does it happen?
The Retina is made up of different components/layers, starting with the Retinal pigment epithelium which is the outer part of the retina, which is pigmented due to melanin, which helps to absorb light rays thereby preventing the scattering of light rays into the eye. It is the closest layer of the retina to the blood vessel of the choroid (a tissue layer of the wall of the eyes between the sclera and the retina which carries blood that gives nutrient to the eye) which helps to bring in nutrient and oxygen for the retina..
After the Retinal pigment epithelium are the photoreceptors, which are very important in Phototransduction. It is made up of the Rods and the Cones. The Rods are specifically meant for scotopic visions (dark/dim light). It is required during night vision. It is a cylindrical structure of plasma membrane, made up of pigment known as Rhodopsin which is a combination of Retinal (11-cis retinal) and opsin. Cones on the other hand are responsible in photopic vision. Cone is made up of a pigment known as the cone opsin called photopsin which is made up of photopsin 1, photopsin 2, and photopsin 3, iodine, and opsin with cone visual pigments, allowing the eye to be able to pick up color from different wavelengths. This involves spectrums such as Rlue, Red, and Green, with difference in wavelengths., providing visual accuracy, and good vision during the day or in bright light..
Just between the photoreceptors and the Bipolar cells are the Horizontal cells which is made up of GABA. The function of the Horizontal cells are to control the output of images from the photoreceptor, moderating the contrast, visual processing, and modulation of dark light and bright light..
The Bipolar cells connect the outer retina and the inner retina together. They recieve photoreceptor signals known as glutamate and transform them in form of polarity. Off-center Bipolar cell occurs when there is a reduction in light, where the glutamate released to the bipolar cell is reduced, while On-Center Bipolar cell occur when there is an increase in light causing maximum release of glutamate to the bipolar cells.. Between the Bipolar cells, and the Ganglion cells are the Amacrine cells which collects bipolar cell signals, decode them and re-code them as synaptic release. The Ganglion Cells connect with the Bipolar cells, and with connective axons at the end that forms the optic nerve (Cranial Nerve 2)..
Good day ladies and gentlemen, this is your captain
When the light ray enters into the retina's photoreceptors, either a Rod or a Cone, they get to the Rhodopsin/photopsin. For this post, I will be using the Rod, so we are looking at Rhodopsin(both photoreceptors follow the same mechanism). When the light ray reaches the Rhodopsin, the 11-cis retinal is converted to All Trans-Retinal which activates the opsin to become metarhodopsin II (rhodopsin R). Normally without light rays, the GDP in the rod is converted to GTP and then to cyclic-GMP (cGMP) which binds to CNG channels allowing the channel to open, and allowing for Sodium (Na+), and Calcium (Ca2+) influx. With the penetration of light rays, phosphodiesterase enzyme is activated which breaks down cGMP causing a decrease in it. Reduction in the cGMP causes the closure of the CNG channel leading to the cessation of the Na+ and Ca2+ influx into the photoreceptor, causing hyper-polarization where little glutamate is being released from the axon to the Bipolar Cells. Think of the opposite process for cones with dark. Cones will be depolarization..
While glutamate is being sent to the Bipolar cell, it is also sent to the horizontal cells to produce GABA which inhibits the transmission from the photoreceptors after releasing glutamate. GABA helps to modulate the sensitization of the photoreceptor.
With light, the photoreceptors are hyperpolarized and releases little glutamate, which stimulates the Bipolar cell neuron depolarizing the cell. In the dark, the photoreceptor (cone) is depolarized, releasing maximum glutamate, causing the bipolar cell neuron to be hyperpolarized. The Bipolar cell responds to glutamate by excitation, leading to the Off-center bipolar cell when there is no light. When there is light, the bipolar cell is referred to as the On-center bipolar cell. On either end, they produce glutamate to the ganglion cells..
In the Ganglion cell, the glutamate stimulates action potentials (it only codes which of the bipolar cell releases the glutamate) which moves down the axon to the optic nerve which then sends the action potentials to the primary visual cortex in the Occipital lobe to perceive the image.,.
Image Reference
Image 1 || Wikimedia Commons
