Chiasm (anatomy)

Chiasm (anatomy) is a term used in anatomy to describe a crossing or intersection of two tracts, such as nerves or ligaments, in the form of an X. The term is derived from the Greek letter 'Chi', which is shaped like an X. The most well-known example of a chiasm in human anatomy is the optic chiasm, where the optic nerves from the two eyes cross each other.

Optic Chiasm

The optic chiasm is an X-shaped structure formed by the crossing of the optic nerves in the brain. The optic nerve from each eye carries visual information in the form of nerve impulses from the retina to the brain. At the optic chiasm, information from the right visual field (which comes from both eyes) is sent to the left visual cortex, and information from the left visual field is sent to the right visual cortex.

The optic chiasm and other parts of the optic pathway.

Other Examples of Chiasms

Other examples of chiasms in the human body include the trochlear nerve (the fourth cranial nerve), which is the only cranial nerve to cross (decussate) completely, and the pyramidal tracts in the medulla oblongata of the brainstem, where motor fibers cross the midline.

Clinical Significance

Disorders of the chiasm can lead to various clinical conditions. For example, a tumor pressing on the optic chiasm can cause bitemporal hemianopsia, a condition characterized by loss of vision in the outer (temporal) half of the visual field in both eyes. This is because the fibers carrying information from the temporal visual fields cross at the optic chiasm.

See Also

• Decussation
• Nervous system
• Visual system