Optic chiasm is the name given to a junction of the optic nerve. At this section, the nerve fibers of the nasal halves of the retina cross.
What is the optic chiasm?
The optic chiasm is also known as the optic nerve junction and forms an important part of the visual pathway. In it, the nerve fibers of the optic nerve (nervus opticus) of both eyes cross. Thus, the meeting of the optic nerve fibers of the medial fibers, which lie in the direction of the nose, occurs at this point, while the outer (lateral) fibers remain on their original side. In this way, visual impressions originating from the left side of the face can be processed on the right side of the brain. The same procedure occurs in reverse on the other side of the body. Whether a partial or even a complete change of the fibers takes place depends on the respective vertebrate genus. Thus, at the optic nerve junction of amphibians, a complete exchange of both optic nerves takes place. In humans and primates, on the other hand, the proportion of crossing fibers is about 50 percent. There is a correlation between eye position as well as human binocular vision.
Anatomy and structure
The optic chiasm is located in the anterior fossa of the skull. There it is located in the sulcus chiasmatis of the sphenoid bone (Os sphenoidale). In this area the meeting of the anterior wall as well as the floor of the 3rd cerebral ventricle occurs. Below the optic nerve junction is the so-called turcic saddle (sella turcica) together with the pituitary gland (hypophysis). On the dorsal side the pituitary stalk is located. The optic chiasm represents only a partial nerve fiber crossing. The axons (nerve cell processes), which originate from the left half of both retinas, run across the thalamus (visual mound) to the left half of the brain. Within the optic nerve junction, the nerve fibers from the half of the retina that lies on the nasal side of the right eye switch to the opposite side, that is, to the left. The nerve fibers located on the temporal side of the left eye remain on the left side. On the opposite side, it is exactly the opposite. This means that the axons originating from the right side of the retinas pass to the right half of the brain. Thus, within the optic chiasm, the change of nerve fibers from the retinal half of the left eye, which is on the nasal side, to the right side occurs. In contrast, the nerve fibers of the right eye, which are located toward the temple, remain in their ancestral position. The optic chiasm also forms the transition from the optic nerve to the optic tract (visual cord).
Function and tasks
The optic chiasm marks an important component of the visual pathway. Thus, because of the partial optic nerve crossing, the right hemisphere of the cerebrum processes only the optical impressions from the left half of the face. In contrast, the left cerebral hemisphere processes exclusively the optical stimuli originating from the right half of the visual field. In this process, the proportion of crossing nerve fibers optimally matches the human visual field. An important role is played by binocular vision, which on the one hand enables the plastic perception of objects and on the other hand provides for the estimation of spaces and distances. From the optic chiasm, the nerve cords, which are then called visual pathways, run toward the visual cortex of the cerebrum.
Diseases
The optic chiasm can be affected by a variety of diseases. Among the most common health problems affecting the optic chiasm is optic chiasm syndrome. In this case, three characteristics show up that are considered typical. Thus, the affected individuals suffer from bitemporal failures of the visual field. The visual impression is missing only on the outside, so that there is a vision like wearing blinkers. For this reason, chiasma syndrome is also called blinker syndrome. Furthermore, there is a decreased visual acuity, which is either only noticeable on one side of the eye or on both sides. Another characteristic of the chiasma syndrome is optic atrophy, in which the neurons of the optic nerve are destroyed. In most cases, chiasma syndrome is caused by space-occupying lesions, often resulting from tumors that form in the pituitary gland and exert pressure on the chiasm. More rarely, melingeoma, a tumor originating in the meninges, is responsible for the development of the syndrome.Another possible cause is an aneurysm. This is a vascular dilatation, mostly affecting the carotid artery, which compresses the optic nerve junction, causing discomfort. Occasionally, optic chiasm syndrome is also caused by space-occupying lesions of the optic nerve. Typical symptoms of optic chiasm syndrome include seeing double images, chronic headaches, and hormonal imbalances. The latter are triggered by tumors on the pituitary gland. If the tumor presses on the middle area of the optic chiasm, this results in bitemporal visual field loss. This primarily involves compression of the nasal retinal hemifibers. When diagnosing chiasma opticum syndrome, X-ray examinations often reveal changes in the sella turcica. If optic chiasm syndrome is caused by a tumor on the pituitary gland, surgery must be performed to remove it. The resulting relief causes recovery of visual field and visual acuity. However, long-term damage cannot always be ruled out. If the optic chiasm is transected at the median level, this results in loss of the temporal halves of the visual field in each eye, causing bitemporal hemianopsia. In the case of transection of the tractus opticus, homonymous hemianopsia results, leading to loss of the reciprocal visual field halves of the eyes.