Structure and Development of the Optic Chiasm - Optic nerves of left and right eye meet ventral to the brain - Left optic nerve crosses over the right one without fusing - Optic nerves merge in the optic chiasm in vertebrates with large overlap of visual fields - Partial decussation allows for binocular depth perception - Optic nerves become optic tracts, which insert on the optic tectum and lateral geniculate body - Optic nerve crossing guided by cues such as netrin, slit, semaphorin, and ephrin - Neuronal growth cone responds to cues and induces changes in cytoskeleton - Retinal ganglion cell axons are blocked by Slit2 and Sema5A inhibition - RGC axons change from retinotopic to flat sheet-like orientation near the chiasm - Number of axons that do not cross the midline depends on degree of binocular vision

Function and Evolution of the Optic Chiasm in Different Animals - Mammals possess optic chiasm, but its evolution is unknown - Theories proposed for the function of the optic chiasm - Siamese cats with certain genotypes of albino gene have disrupted wiring - Optic tracts in cephalopods and insects do not cross the body midline - Each side of the brain processes the ipsilateral eye in cephalopods and insects

History and Origins of the Optic Chiasm - Persian physician Esmail Jorjani first identified the crossing of nerve fibers and its impact on vision - Optic chiasm likely evolved as a consequence of a twist in the early embryo - Siamese cats with disrupted wiring may compensate with strabismus - Function of optic chiasm in vertebrates still not fully understood

Studies and Findings Related to the Optic Chiasm - Semaphorin6D is presented in the optic chiasm - Plexin-A1 and Nr-CAM are involved in the presentation - The presentation promotes retinal axon midline crossing - An ancestral axial twist explains the presence of the optic chiasm in vertebrates - The axial twist hypothesis predicts opposite asymmetries in different body parts - Albino cats exhibit abnormal retinotopic organization in the dorsal lateral geniculate nucleus - The abnormal organization is linked to the absence of tyrosinase - White tigers have genetic abnormalities in their visual pathways - The abnormalities affect the visual pathways in the brain - The cranial nerves have a long history of naming and classification - The naming of the cranial nerves has evolved over time

Additional Images and Visualizations of the Optic Chiasm - Scheme showing central connections of optic nerves and tracts - Brain seen from below with optic chiasm labeled - Transformations of visual field toward visual map on primary visual cortex - Brain and brainstem seen from below - Cerebrum, inferior view, deep dissection