Why our eye is so exquisitely sensitive to light but brain is not? - Seeker's Thoughts

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Why our eye is so exquisitely sensitive to light but brain is not?


Why our eye is so exquisitely sensitive to light, but our subconscious behaviors are comparatively insensitive to light?

Do you ever wonder, how does our brain gets the signal to maintain the body such as synchronization of circadian rhythms to light/ dark cycles?
The common answer is that the eyes only send signals to the brain. However, when researchers dug deeper- they found that the optic nerve plays an important part not eyes.

What is the Optic Nerve? 

The optic nerve is in the back of the eye. It is also called the second cranial nerve or cranial nerve II.




Also Read, 
It is the second of several pairs of cranial nerves. The job of the optic nerve is to transfer visual information from the retina to the vision centers of the brain via electrical impulses.
      A bundle of more than a million nerve fibers carrying visual messages from the retina to the brain. (In order to see, we must have light and our eyes must be connected to the brain.)

      Your brain actually controls what you see, since it combines images. The retina sees images upside down but the brain turns images right side up. This reversal of the images that we see is much like a mirror in a camera.

     Glaucoma is one of the most common eye conditions related to optic nerve damage.

What is the work of the Optic Nerve?

In the human eye, the optic nerve receives light signals from about 125 million photoreceptor cells (known as rods and cones) via two intermediate neuron types, bipolar and amacrine cells. 

In the brain, the optic nerve transmits vision signals to the lateral geniculate nucleus (LGN), where visual information is relayed to the visual cortex of the brain that converts the image impulses into objects that we see.

New research led by north-western University, has found that a subset of retinal neuron sends inhibitory signals to the brain.

Before, researchers believed the eye only sends to the brain. Before researchers believed the eye only sends excitatory signals. (Simply put: Excitatory signaling makes neurons to fire more; inhibitory signaling makes neurons to fire less.)



Does it impact on Subconscious Behaviour?

Researchers also found that this subset of a retinal neuron is involved in subconscious behaviors, such as synchronization of circadian rhythms to light/ dark cycles and pupil constriction to intense bright lights. 

By better understanding how these neurons function. Researchers can explore new pathways by which light influences our behavior.


These inhibitory signals prevent our circadian clock form resetting to dim light and prevent pupil constriction in low light, both of which are adaptive for proper vision and daily function.


According to Tiffany Schmidt, who led research – “we think that our results provide a mechanism for understanding why our eye is so exquisitely sensitive to light, but our subconscious behaviors are comparatively insensitive to light.”

Schmidt and her team blocked the retinal neurons responsible for inhibitory signaling in a mouse model. When this signal was blocked, dim light was more effective at shifting the mice’s circadian rhythms.

This suggests that there is a signal from the eye that actively inhibits circadian rhythms realignment when environmental light changes, which was unexpected.

This makes some sense, however, because you do not want to adjust your body’s entire clock for minor perturbations in the environmental light/dark cycles, you only want this massive adjustment to take place if the change in lighting is robust.

The team also found that when the inhibitory signals from the eye were blocked, mice’s pupils were much more sensitive to light.

An ongoing hypothesis is that this mechanism keeps pupils from constricting in very low light, this increases the amount of light hitting your retina, and makes it easier to see in low conditions. This mechanism explains in the least part why your pupil avoids constricting until bright light intensifies.


How the eye works?

The five senses include sight, sound, taste, hearing, and touch. Sight, like the other senses, is closely related to other parts of our anatomy. The eye is connected to the brain and dependent upon the brain to interpret what we see.

How we see depends upon the transfer of light. Light passes through the front of the eye (cornea) to the lens. The cornea and the lens help to focus the light rays onto the back of the eye (retina).

The cells in the retina absorb and convert the light to electrochemical impulses which are transferred along the optic nerve and then to the brain.

The eye works much the same as a camera. The shutter of a camera can close or open depending upon the amount of light needed to expose the film in the back of the camera.

The eye, like the camera shutter, operates in the same way. The iris and the pupil control how much light to let into the back of the eye. When it is very dark, our pupils are very large, letting in more light.

The lens of a camera is able to focus on objects far away and up close with the help of mirrors and other mechanical devices. The lens of the eye helps us to focus but sometimes needs some additional help in order to focus clearly. Glasses, contact lenses, and artificial lenses all help us to see more clearly. 


What are the parts of the eyes?

Even though eyes seem pretty small organ of the body, but it has many parts which consists of the following

·         Choroid
The layer containing blood vessels that line the back of the eye and is located between the retina (the inner light-sensitive layer) and the sclera (the outer white eye wall).  
·         Ciliary Body
Structure containing muscle and is located behind the iris, which focuses the lens.
·         Cornea
The clear front window of the eye which transmits and focuses (i.e., sharpness or clarity) light into the eye. Corrective laser surgery reshapes the cornea, changing the focus.
·         Fovea
The center of the macula which provides a sharp vision.
·         Iris
The colored part of the eye which helps regulate the amount of light entering the eye. When there is a bright light, the iris closes the pupil to let in less light. And when there is low light, the iris opens up the pupil to let in more light.
·         Lens
Focuses light rays onto the retina. The lens is transparent and can be replaced if necessary. Our lens deteriorates as we age, resulting in the need for reading glasses. Intraocular lenses are used to replace lenses clouded by cataracts.
·         Macula
The area in the retina that contains special light-sensitive cells. In the macula these light-sensitive cells allow us to see fine details clearly in the center of our visual field. The deterioration of the macula is a common condition as we get older (age-related macular degeneration or ARMD).


·         Optic Nerve
A bundle of more than a million nerve fibers carrying visual messages from the retina to the brain. (In order to see, we must have light and our eyes must be connected to the brain.) Your brain actually controls what you see, since it combines images. The retina sees images upside down but the brain turns images right side up. This reversal of the images that we see is much like a mirror in a camera. Glaucoma is one of the most common eye conditions related to optic nerve damage.
·         Pupil
The dark center opening in the middle of the iris. The pupil changes size to adjust for the amount of light available (smaller for bright light and larger for low light). This opening and closing of light into the eye are much like the aperture in most 35 mm cameras which lets in more or less light depending upon the conditions.
·         Retina
The nerve layer lining the back of the eye. The retina senses light and creates electrical impulses that are sent through the optic nerve to the brain.
·         Sclera
The white outer coat of the eye, surrounding the iris.
·         Vitreous Humour
the, clear, gelatinous substance filling the central cavity of the eye.





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