The Optic Pathway

The optic nerve, which is known as the second cranial nerve, plays an essential role in helping us see. It carries the information collected by the retina and delivers it to the primary visual cortex in the brain, where our brain makes sense of what we are looking at.

Outside the brain (extracranial):
Inside the retina, special light-sensitive cells called photoreceptors turn light into tiny electrical signals. These signals are passed on to cells called bipolar cells, which release a chemical messenger called glutamate. This chemical activates the retinal ganglion cells, which create action potentials—tiny electrical impulses that are the language of our nervous system.

The long extensions (axons) of the retinal ganglion cells come together to form the optic nerve. This nerve leaves the protective bony eye socket through a small passage called the optic canal and then enters the skull’s middle cranial fossa, a space near the pituitary gland.

Inside the brain (intracranial):
Once inside the middle cranial fossa, the two optic nerves from each eye meet and merge at a structure called the optic chiasm. This crossing point sits right above a part of the sphenoid bone called the sella turcica. At the optic chiasm, the nerve fibers from the inner (medial) part of each retina cross over to the opposite side of the brain, while the fibers from the outer (lateral) part stay on the same side.

This arrangement means that information from the left side of what you see ends up traveling to the right side of your brain, and information from the right side goes to the left side.

After crossing (or not crossing) at the optic chiasm, these nerve pathways are called optic tracts. Each optic tract carries the visual information to a relay center deep in the brain called the lateral geniculate nucleus, or LGN, which is part of the thalamus. From the LGN, the signal is passed to the next set of neurons that send it along special pathways called optic radiations.

The lower parts of what we see are sent along the upper optic radiations, while the upper parts of our view travel along the lower optic radiations. Both paths lead to the primary visual cortex in the occipital lobe at the back of the brain—where the brain finally pieces everything together and creates the clear, detailed images we see every day.

The Eye

Our eyes are one of the most remarkable parts of our body. They help us take in the world around us by detecting light and sending information to the brain through the optic nerve. The brain then pieces these signals together to create the images we see every day.

The bones that form the eye socket, or orbit, are quite intricate. They include seven different bones: the maxilla, zygomatic bone, frontal bone, ethmoid bone, lacrimal bone, sphenoid bone, and palatine bone.

Our eyelids, which are made of soft tissue, act as protective covers for the eye. The white part we see is called the sclera, and it’s lined by a thin, clear layer called the conjunctiva. The cornea is the clear outer layer at the front of the eye. It stays smooth and moist thanks to tears produced by the lacrimal gland. Right behind the cornea is a small space called the anterior chamber, which is filled with a fluid known as aqueous humor.

The colored part of the eye is called the iris, and it determines whether someone’s eyes look brown, blue, green, or hazel. Inside the iris are muscles that control the size of the pupil—the dark circle at the center. The pupil adjusts its size automatically to let in the right amount of light. When it’s bright, the pupil gets smaller so we aren’t overwhelmed by light. In a dark room, it widens to help us see better. These changes happen because of tiny muscles called the sphincter pupillae and the dilator pupillae, which work without us having to think about them.

When light enters the eye, it first passes through the cornea and the aqueous humor. Then, it goes through the lens, which focuses the light, and into a gel-like substance called the vitreous humor. This jelly helps the eye keep its round shape. After passing through all these layers, light finally reaches the retina, which is a special layer full of cells that react to light and turn it into signals for the brain.

In the retina, there are two main kinds of light-sensitive cells: rods and cones. Rod cells help us see in low light and are mostly found around the edges of the retina, giving us peripheral vision. Cone cells, which are concentrated in the center area called the macula, let us see colors like red, green, and blue. There are about 120 million rods and 6 million cones in each eye!

These photoreceptor cells turn light into tiny electrical messages. These messages travel along the optic nerve to the brain. There is one spot on the retina, called the optic disc, where the optic nerve leaves the eye—this spot doesn’t have any light-detecting cells, so it creates a small blind spot in our vision.

Here’s a simple way to find your own blind spot:

1. Close your left eye.
   
   

2. Hold your left arm straight out and point your left thumb up.
   
   

3. Hold your right thumb up next to your left thumb.
   
   

4. Focus your right eye on your left thumb.

5. Keeping your gaze on your left thumb, slowly move your right thumb to the right. When your thumbs are about six inches apart, your right thumb will disappear. You’ve just found your blind spot!