Chiasmal Syndrome - Anatomy


The optic chiasm is formed by the union of the two optic nerves. The nasal fibers of each optic nerve decussate (cross) across the chiasm to the opposite side while the temporal fibers course posteriorly to form the optic tract on the same side. This arrangement allows the left half of the visual field to end up on the right side of the brain and the right half of the visual field to locate to the left side.

The optic nerves consist of the axons from the retinal ganglion of each eye. At the chiasm, 53% of the axons from the nasal retina cross the midline to join the uncrossed temporal fibers. These nasal fibers carry information from the temporal visual field. Similarly, the temporal fibers transmit images from the nasal field. The two optic tracts, representing the right and left visual fields, emerge posteriorly from the posterior chiasm. Most of these fibers synapse in the lateral geniculate nucleus or the pretectal nucleus.

The crossing of the nasal half of macular fibers of central vision occurs posteriorly in the chiasm. The inferior and superior fibers remain inferior or superior, respectively. However, the inferonasal fibers pass more anteriorly in the chiasm while the superonasal fibers pass more posteriorly. Classical teaching was that, once crossed, the inferonasal fibers briefly loop back into the contralateral optic nerve sheath, before returning to the chiasm. This bend into the contralateral optic nerve had been called Wilbrand's knee. However, today there is significant evidence that Wilbrand’s knee is simply an artifact (error). Optic nerve axons from one eye can only be selectively studied in the human after enucleation of the contralateral eye and thus degeneration of the axons on one side. After several years, the occurring optic nerve atrophy results in artifactual looping of the axons into the atrophic nerve. This looping was initially described by Wilbrand, who studied subjects with severe optic nerve atrophy after enucleation, and who then appears to have exaggerated the importance of this looping in later drawings1. Clinically, no optic field deficits have been observed in a small series of optic nerve sections at the optic nerve-chiasm junction2.

Several important structures are located adjacent to the optic chiasm. The supraclinoid branches of the internal carotid artery flank the chiasm. The cavernous sinuses are lateral and inferior to the chiasm. The frontal lobe of the brain lies above. The pituitary gland sits below in the sella turcica. The sella turcica is bound in front by the tuberculum sellae and behind by the dorsum sellae. Behind the chiasm lies the floor of the third ventricle.

Read more about this topic:  Chiasmal Syndrome

Other articles related to "anatomy":

Anatomy - Other Branches
... Comparative anatomy relates to the comparison of anatomical structures (both gross and microscopic) in different animals ... Anthropological anatomy or physical anthropology relates to the comparison of the anatomy of different races of humans ... Artistic anatomy relates to anatomic studies for artistic reasons ...
Guy De Chauliac - Works - Chirurgia Magna - Emphasis On Anatomy
... can clearly be seen in the latter's belief that surgeons should have a thorough understanding of anatomy ... He wrote, "A surgeon who does not know his anatomy is like a blind man carving a log" ...
Somatosensory System - Anatomy - Brain
... A relationship between the somatosensory cortical areas and their projection of the body was discovered by recording electrical activity in the human cortex after mechanosensory stimulation of different body parts during neurosurgical procedures ... These data led to the construction of somatotopic maps in which a Somatotopic arrangement was generated ...
Pale-throated Sloth - Description - Anatomy
... The mouth is lined by a black colored mucosa, although the large and heavy tongue is pink ... The palate is wrinkled in texture, and the tongue is lined with numerous grooves, apparently adaptations to the sloth's diet ...
... their careers as prosectors working for lecturers and demonstrators in anatomy and pathology ... professionally prepared dissection prepared by a prosector – a person who is well versed in anatomy and who therefore prepares a specimen so that others may study and learn anatomy from it ...

Famous quotes containing the word anatomy:

    But a man must keep an eye on his servants, if he would not have them rule him. Man is a shrewd inventor, and is ever taking the hint of a new machine from his own structure, adapting some secret of his own anatomy in iron, wood, and leather, to some required function in the work of the world. But it is found that the machine unmans the user. What he gains in making cloth, he loses in general power.
    Ralph Waldo Emerson (1803–1882)

    I love to see, when leaves depart,
    The clear anatomy arrive,
    Roy Campbell (1902–1957)

    Man is a shrewd inventor, and is ever taking the hint of a new machine from his own structure, adapting some secret of his own anatomy in iron, wood, and leather, to some required function in the work of the world.
    Ralph Waldo Emerson (1803–1882)