Optic Chiasm (or: optic commissure) is a flattened string made of neural fibers that form an X-shape in the forebrain.
Located in the visual cortex of the brain, it serves as a crossing point of the neural fibers so that the visual field can be distinguished and processed in the corresponding areas of the visual cortex.
Location and Structure of the Optic Chiasm
The Optic Chiasm is located at the base of the human brain, in front of the hypothalamus. Its neural fibers stem from the retina and extend to the chiasm through optic nerves, creating a brain structure in the form of the letter ‘X’.
Where in the Skull Is the Optic Chiasm Located?
Within the middle cranial fossa, i.e. the butterfly-shaped hollowing at the base of the skull, the optic nerves that stem from each eye join in an optic chiasm.
What Is the Function of the Optic Chiasm?
The crossing of the optic fibers at the optic chiasm enables the visual cortex of the brain to receive the same field of vision from the retinas of both eyes.
Hence, the right part of the brain processes the visual stimuli received in the left visual hemisphere, and vice versa – the left part of the brain processes the visual stimuli received in the right visual hemisphere.
How Is the Visual Pathway Created?
At the optic chiasm, neuron fibers from the cells that create the tissue of the middle part of each retina cross over to the opposite side of the optic tract, while the neuron fibers from the tissue located on the sides of the retina remain at the same side.
This is how the left and right visual paths are created. Both of these visual tracts continue to spread toward the corresponding hemisphere of the cerebral cortex, in order to get to the lateral geniculate nucleus.
What Is the Lateral Geniculate Nucleus of the Brain?
The lateral geniculate nucleus is the major brain center that receives the visual data from the sensory stimuli obtained by the retina and transmits them to the visual cortex to be translated and processed. This is the main point where the fibers create their synapses and transmit the stimuli signals further.
Furthermore, the axons of the neuron cells constituting the lateral geniculate nucleus transmit their information via a neural pathway called optic radiation. This neural pathway further splits into two distinct neural paths for transmission of sensory stimuli:
- Upper optic radiation – travels from the parietal lobe to the visual cortex of the brain.
- Lower optic radiation – travels from the temporal lobe to the visual cortex of the brain.
Via these two neural pathways, the visual stimuli received from the sensory organ of sight are transmitted to the primary visual cortex, as well as to the other dedicated areas of the brain where the information is translated and processed.
How Do We See Things?
We see the world that surrounds us in all shapes, colors, textures, and sizes. But, this is actually the image our brain creates in the visual cortex from the light received through the retina of the eyes.
What Is the Function of the Cornea?
At first, the objects which are in the center of our attention reflect light. It enters through the cornea – the transparent front part of our eyes, enveloping the pupil, the iris, and the back of the eye. The cornea is, actually, the window of our eyes through which we receive visual information. It refracts the rays of light that pass through the pupil.
What Is the Function of the Pupil in the Formation of Vision?
The pupil, i.e. the black spot in the middle of the colored part of the eye (iris), has the ability to regulate light and control how much of it enters the eyes. When there is a lot of light, it closes and becomes smaller, and vice versa – it opens and becomes big when the light is low.
The iris is a flat, round, and colored membrane of the eye, located behind the cornea. It’s made of smooth muscle fibers and connective tissue. The iris regulates the size of the pupil, controlling the amount of light that enters the retina. Also, it protects the eye structures from microorganisms and supports the process of color detection.
What Is the Function of the Lens?
From here, the light passes through the lens, a disk-shaped structure that focuses and reflects them onto the retina with the help of the ciliary muscle.
The focus of the lens is not a brain command, but a reflex response to light. Due to the fact it’s curved, the lens can manipulate its shape, thus changing the focal distance of the eye in order to focus on objects or people that are nearby or in the distance.
What Is the Function of the Retina in the Formation of Vision?
Located near the optic nerve in the inside and back part of the eye, the retina receives the light as a sensory stimulus and translates it into impulses.
The retina is the thin tissue at the back part of the eye, consisting of light-sensitive (or photo-sensitive) cells. According to their shape, these light-sensitive cells are labeled rods and cones.
The cone-shaped cells create the center of the retina, i.e. the macula. They are responsible for our clear vision, colors, and the fine details we see on objects. These cells are not sensitive to light.
On the other hand, the rod-shaped cells are located on the outer part of the retina and aid our ability to see at night and in dim light, as well as to detect motion with our eyes. These cells are not sensitive to color.
Furthermore, rods and cons transform the visual signals into electrical impulses. They are further transmitted via neurons in order to be translated and processed in the visual cortex of the brain.
What Is The Function of the Optic Nerve in the Formation of Vision?
The optic nerve (or second cranial nerve) is located in the visual cortex at the back of the skull and connects with the retina of each eye. Its task is to transmit the visual information received from the retina of the eyes to the primary visual cortex for further processing.
The optic nerve stems from the optic vesicle, i.e. an outward growth of the forebrain. It is formed by the crossing of the axons which stem from the retinal ganglion cells and which receive signals from the rods and cons of the eyes, i.e. the photoreceptors.
When the image is clear and focused in the retina, the nerve impulses transmit the information on that image via optic nerves.
Due to the curved shape of the lens, the light bends and thus creates an upside-down image of what we see. Later, this image becomes rectified in the correct position by the brain.
What Happens if the Optic Chiasm Is Damaged?
The loss of vision after damage to the optic chiasm, retina, nerve fibers, or the brain unfortunately leads to incurable medical conditions. In particular, damage inflicted to the optic chiasm or the optical nerve can lead to various medical conditions, such as:
- Chiasmal Syndromes – Defects of the visual field mainly caused by a tumor, multiple sclerosis, or neurofibromatosis.
- Bitemporal Hemianopia – Partial blindness that occurs when the vision is blocked in the other half of both visual fields.
- Pituitary Adenoma – A tumorous growth on the pituitary gland affecting the function of the optic chiasm.
- Optic Atrophy – A deterioration of the optic nerve, which results in the dimming and reduction of the field of vision.
In addition, some conditions can cause damage to the optic nerve. This list includes:
- Optic Neuritis – Damage of the optic nerve caused by swelling.
- Multiple Sclerosis – A central nervous system condition that also affects vision.
- Optic Neuropathy – General damage of the optic nerve that can be the result of a myriad of causes.
- Glaucoma – Increased pressure in the eyeball which gradually leads to loss of sight.
- Anterior Ischemic – An injury of the optic nerve caused by a reduced flow of blood in the front part of the nerve.
- Optic Nerve Head Drusen – A medical condition of the optic nerve caused by deposited proteins under the retina of the eyes.
- Optic Nerve Hypoplasia – An underdeveloped optic nerve.
In the end, mental stress has been labeled as the number one cause of vision loss, due to its influence on the structures and functions of the brain.
To Sum Up
The optic nerve connects the eyes to the visual cortex of the brain. The part of the brain where the optic nerves from each retina cross while the fibers from the other part of the retina branch to the same brain hemisphere is called an optic chiasm.
Due to the way the optic nerves are constructed, both the left and the right hemisphere of the brain receive the visual stimuli from both eyes.
Since our eyes see different sides of an object, the crossing of the optic fibers at the optic chiasm enables the visual cortex to integrate different information on the same object, thus helping us to see depth in the visual representation.
Fun Facts
Did you know?
- The crossed nerve fibers form the optic chiasm in the 7th week of the gestation period.
- The name of this tiny part of the brain comes from the Greek word ‘χίασμα, meaning ‘crossing’ or ‘to mark with X’.
- During surgery performed on an anterior circulation aneurysm, the optic chiasm serves as a very important landmark in the brain.
- An optic nerve consists of approximately 770.000 to 1.7 million nerve fibers. Those nerve fibers are, in fact, axons of the retinal ganglion cells contained in only one of the retinas.
- Damage to the optic nerve is irreversible because the nerve fibers that create it do not have the ability to renew their structures on their own.
- Green leafy vegetables, carrots, tuna, salmon, eggs, nuts, beans, as well as citrus fruits, are foods that help us maintain a clear and good vision.
- Green color rests the eyes. This is one of the reasons why taking long walks in nature is beneficial for our overall health.
- The master-gland of the endocrine system, the pituitary gland, is located right underneath the optic chiasm. It’s a tiny pea-sized gland that is responsible for our general wellbeing, as well as some of the most vital functions of our body.
- The Circle of Willis is the junction point of several arteries that supply the optic chiasm with blood.
- The image that is perceived by the retina is actually upside down, not the way we see it after the visual information is translated and processed in the visual cortex of the brain.