Photoreception

Definitions & Key takeaways

Photoreception is the process by which photoreceptor cells transduce light energy into electrical energy. Human photoreceptor cells are rods and cones found within the retina. Rods are mainly used for night vision, have a high sensitivity to light, and a low visual acuity, whereas cones are mainly used for color vision, have a low sensitivity to light, and a high visual acuity.

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Introduction0:00–0:18

Photoreception is the process that describes how photoreceptors like rods and cones absorb light waves that enter the eye and convert them into electrical signals which are then sent to the brain for visual processing.
Photoreceptors are located in the retina, which is a light sensitive neural layer of tissue at the back of the eye. The retina itself is composed of ten of its own distinct layers.

Retina0:18–3:45

Moving from the deepest layer of the retina, from posterior to anterior, the layers are as follows: the pigment epithelium, the photoreceptor layer, the outer limiting membrane, the outer nuclear layer, the outer plexiform layer, the inner nuclear layer, the inner plexiform layer, the ganglion cell layer, the nerve fiber layer, and finally the inner limiting membrane.
Since the inner limiting membrane and nerve fiber layer are the most anterior portions of the retina, you would think that as light enters the eye it would hit these layers first.
However, light actually travels right past all the retinal layers until it comes into contact with the deepest layer of the retina, the pigmented layer.
So let’s trace the pathway of a visual impulse as it travels from the pigmented layer, all the way through to the nerve fiber layer and eventually to the brain.
The first layer, the pigmented layer is only a single-cell thick and is the layer closest to the choroid, the vascular portion of the eye just posterior to the retina.
The pigmented layer contains epithelial cells which absorb light so it doesn’t scatter within the eye. The second layer, the photoreceptor layer contains photoreceptors.
Photoreceptors are specialized neurons that detect light and when they hyperpolarize, they send visual impulses in the form of electrical signals to the brain.
The third layer is the outer limiting membrane which sits at the base of the photoreceptor cells and provides mechanical support to the retina, helping it to maintain its structure.
The fourth layer of the retina is called the outer nuclear layer and it’s made up of the cell body and nuclei of the photoreceptor cells.
The axons of these photoreceptors extend into layer five, the outer plexiform layer where they synapse with the dendrites of interneurons like bipolar cells.
Next, is the sixth layer, the inner nuclear layer which is composed of the cell bodies of interneurons, which connect the outer and inner plexiform layer.
The seventh layer is the inner plexiform layer which is where the axons of the interneurons synapse with the dendrites of ganglion cells.
The eighth layer is the ganglion cell layer which is where the ganglion cell bodies reside. The nerve fiber layer is the ninth layer and is where the axons of these ganglion cells are located.
These axons travel to the posterior center of the retina and exit the eye through the optic disc to form the optic nerve or cranial nerve II.
The optic nerve then carries this visual information to the brain. The tenth, and last remaining layer of the retina is the inner, or internal limiting membrane, and it simply serves as a membrane separating the rest of the retina from the gel-like vitreous humor the fills the inside of the eyeball.
Now, let’s take a closer look at the photoreceptors which are either rods or cones. Both rods and cones are modified neurons that hyperpolarize when they absorb light, and send neural impulses that trigger a relay of neurons that deliver that impulse to the brain.

Photoreceptors3:45–4:25

Photoreceptors have three main segments. They have an outer segment, which is the portion of the receptor that detects light and an inner segment that contains the cell body.
Thirdly, photoreceptors have a synaptic terminal which connects the photoreceptor to interneurons, like bipolar cells, that help propagate the signal to the optic nerve.
Let’s take a look at rods, which are mostly located in the periphery of the retina. The outer segment of rods have folds of cell membrane that form disks, like the pleats on a curtain.

Rods4:25–5:09

These discs have a dense concentration of a transmembrane protein called Rhodopsin which gets activated by light. Rods have lots of rhodopsin so they’re highly sensitive to light and it only takes a single photon of light to activate a rod.
That’s why rods allow us to see even in dim lighting conditions, like at night. Rhodopsin doesn’t differentiate between different wavelengths of light, so it only allows for black and white vision.

Cones5:09–6:59

a small depression in the posterior retina. Cones make the fovea the area of highest visual acuity.
Like rods, cones also have folds of cell membrane that form disks, but unlike rods which are shaped like a cylinder, the outer segment of these photoreceptors is shaped like an ice cream cone, which explains the name.
In cones, these disks are covered by a low concentration of a protein called photopsin, which gets activated by light. As a result, cones have a relatively low sensitivity to light and require hundreds of light photons to get activated.
That’s why cones are mainly useful in bright light conditions like during the day. In addition, there are three different types of cone cells, and each one is covered by a different type of photopsin that’s only activated by a certain wavelength of light.
S cones have photopsin that gets activated by short wavelengths of light in the 400-500 nm range which corresponds to blue.
M cones have photopsin that gets activated by medium wavelengths of light in the 450-630 nm range which corresponds to green.
And L cones have photopsin that gets activated by long wavelengths of light in the 500-700 range which corresponds to red.
So when you see a red sweater, only the red cones are activated, whereas when you see a purple hat, both the red and blue cones are activated.
Interestingly, we see yellow when red and green cones are activated, and cyan when blue and green cones are activated. with ganglion cells in the inner plexiform layer.

Vision Acuity6:59–7:41

Rods are considered low acuity because multiple rods converge onto a single ganglion cell. It’s a bit like having multiple witnesses telling a single police officer about a suspect simultaneously, creating a fuzzy, low resolution image of the suspect.
In comparison, cones provide high acuity vision because each cone connects directly to its own ganglion cell. So that’s like a witness giving a detailed report to an officer, resulting in a high-resolution image of the suspect.
When light enters the eye, it travels past the ganglionic cell layer and the interneuron layers until it reaches the photoreceptor layer and the pigmented layer deep in the retina.

Phototransduction cascade7:41–10:32

In the photoreceptors, light is absorbed by rhodopsin proteins in rods and photopsin proteins in cones. Rhodopsin and photopsin are both G protein coupled receptors and are in the opsin family.
G protein coupled receptors are cell surface receptors that receive a signal from outside of the cell and trigger changes within the cell.
When the opsin proteins, rhodopsin and photopsin, are struck by photons, a protein called retinal changes shape or isomerizes from 11-cis-retinal to 11-trans retinal, creating a conformational change in the opsin proteins.
That causes the opsin proteins to activate G proteins called transducins (Gt), which in turn activate the enzyme phosphodiesterase.
Phosphodiesterase converts cyclic guanosine monophosphate or cGMP to guanosine-5-monophosphate. Now, normally rods and cones have sodium channels which are bound to cGMP that allow sodium to continuously flow into the cell.
When phosphodiesterase converts cyclic GMP to guanosine-5- monophosphate, there’s less cyclic GMP within the cell available to bind to sodium channels, and the sodium channels close.
This decreases the inward current of sodium entering the cell and the photoreceptor becomes more negative, or hyperpolarized.
When photoreceptors are hyperpolarized, they stop sending action potentials down to the synaptic terminal and that closes the voltage-gated calcium ion channels near the synaptic terminal.
As a result, there’s less of the excitatory neurotransmitter glutamate released into the synapse between the photoreceptor and bipolar cell.
So in essence when light triggers a photoreceptor, it turns off and stops releasing neurotransmitters. Interestingly, it’s the lack of glutamate that causes the bipolar cell to open up its voltage-gated calcium channels and depolarize.
And the bipolar cell then triggers depolarization of the ganglion cell. And from there, the action potential is propagated along the optic nerve and ultimately to the primary processing centers in the brain in the occipital lobe.
##Summary Alright, as a quick recap...photoreceptor cells such as rods and cones are found within the retinal layer of the eye.
Rods are mainly used for night vision, have a high sensitivity to light, and a low visual acuity, whereas cones are mainly used for color vision, have a low sensitivity to light, and a high visual acuity.

Review10:32–11:10

Light is absorbed by rhodopsin in rods and photopsin in cones which sets off a G-protein coupled signal transduction cascade that passes from the photoreceptor to the bipolar cells and eventually to the ganglion cells which carry the signal to the brain for visual processing.
which sets up a g protein-coupled signal transduction Cascade that passes from the photoreceptor to the bipolar cells and eventually to the ganglion cells, which carry the signal to the