Cranial Nerve 2 : The Optic Nerve

The optic nerve is the second cranial nerve, which is also written at CN II or cranial nerve II. It connects the brain to the human eye. During an eye exam, an ophthalmologist can use a special lens to view the optic nerve’s entry into the eye. The optic nerve is considered to be part of your nervous system.

Optic Nerve Function

The function of the optic nerve is to carry visual information that it receives from the eye to the brain. This allows your brain to form images. There are many different types of visual information that the optic nerve is responsible for transmitting to the brain. This includes brightness perception, color vision, visual acuity or visual accuracy, and the ability to discern contrast or contrast sensitivity. Therefore, if someone has damage to their optic nerve, they will also have difficulties with their vision. Depending on the type of damage, the vision loss from optic nerve damage can be related to peripheral vision, central vision, color vision, contrast sensitivity, or complete vision loss.

The optic nerve is also responsible for how your pupil reacts in light and dark and also when you are trying to read fine print. In bright lights, the optic nerve receives a signal that it sends to the brain telling the pupil to constrict or get smaller. When you are reading fine print, the pupil and lens of the eye change to help you read, and this process is called accommodation. These two neurological reflexes are called the light reflex and the accommodation reflex. Certain disease states such as syphilis may affect one of these reflexes but not both.

Blind Spot In The Eye

The blind spot in the eye is real. This means there is an area of your visual field that you can not see because there are no photoreceptors there. The blind spot in the eye is caused by the entry of the optic nerve into the eyeball.

Optic Nerve Damage

Damage to the optic nerve and optic nerve function can be determined by several tests which an eye doctor can do in their office. Examining the optic nerve may reveal that one nerve is different in color than the other. Ophthalmologists refer to this as optic nerve pallor. Optic nerve pallor may need testing beyond what a general eye doctor can do and you may need to be referred to a Neuro Ophthalmologist.

Optic Nerve Function Test

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There are a few different ways to test for optic nerve function. The first is a pupillary exam. An eye doctor will evaluate one pupil first and then the second pupil. They will then also perform a swinging flashlight test where they quickly move the light from side to side to see how the pupils react. Another test is a visual field test which helps to assess if any parts of your visual field have been compromised. Another optic nerve function test is a color vision test. In patients who have optic nerve damage, their color vision may also be compromised. Finally, an OCT or optical coherence tomography test can test the retinal nerve fiber layer and see if there is any thinning of the optic nerve.

Cranial Nerve 2: Take Home Points

Cranial nerve II is also called the optic nerve and it is responsible for carrying visual information from the eyeball to the brain. Damage to the optic nerve will likely cause visual problems.


Selhorst JB, Chen Y. The optic nerve. Semin Neurol. 2009 Feb;29(1):29-35. doi: 10.1055/s-0028-1124020. Epub 2009 Feb 12. PMID: 19214930.

Miller NR. The optic nerve. Curr Opin Neurol. 1996 Feb;9(1):5-15. doi: 10.1097/00019052-199602000-00003. PMID: 8722657.

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