Type IV : A torsion or loop by 180 degrees of a nerve can also reverse the order of the functional map. This type is usually not referred to as chiasm.
Chiasms are found in vertebrates but also in invertebrates. The optic chiasm in vertebrates can be of type I or II. However, an optic chiasm of type III is found in many insects and in cephalopods. In vertebrates three of the cranial nerves show a chiasm.
The optic chiasm of vertebrates involves the optic tract. The trochlear nerve is a motor nerve that innervates one of the muscles that move the contralateral eye. It emerges from the dorsal aspect of the ventralmidbrain, leaves the brain on the dorsal side where it crosses to the opposite side. The oculomotor nerve originates from the third nerve nucleus at the level of the superior colliculus in the midbrain. The rostral part of the nerve crosses the midline to merge with the part of the contralateral nerve that does not cross. Since the midline crossing occurs inside the brain, it is not strictly a chiasm but rather a decussation.
Structure
As stated above, very different kinds of nerve crossings are known as chiasm. The optic chiasm of vertebrates is the best known. The optic nerve runs from the retina towards the ventral midline of the brain and crosses to the opposite side to continue as the optic tract which inserts to the optic tectum ) on the dorsal midbrain.
Type I: Passing in the sagittal plane
In many vertebrates, the left-eye optic nerve crosses over the right-eye one, without blending.
Type II: Fusion in the sagittal plane
In mammals and birds and other vertebrates with frontal eyes, the optic nerves do blend in the optic chiasm, and only part of the nerve fibres cross the midline. The drawings of Cajal suggest that the axons of the optic nerve may branch in the optic chiasm, and thus give off a branch both in the ipsi- and contralateral optic tract. Note, however, that such branching is not neural processing as occurs in a ganglion.
Type III: Systematic crossing of fibres inside a nerve
The optic tract of various clades of insects shows two chiasms, the first and second optic chiasm. In contrast to those in vertebrates, the insect chiasms do not cross the body midline. Rather, the first and second chiasm invert the anterior and posteriorvisual field. Since there are two chiasms, the retinotopic map is not affected. Cephalopods possess highly developed lens eyes. The optic tract of cephalopods, such as the squid Loligo, chiasmates without midline crossing. This chiasm is distributed along the optic tract and effectively compensates the inversion of the image on the retina.
Type IV: Torsion or looping of a nerve
This type is usually not called chiasm. Such a looping occurs, for example, in the optic tract between the optic chiasm and the optic tectum. Another example is the optic radiation which rotates the retinal map on the visual cortex by 180°.
Theories and evolution
Vertebrates
A number of theories have been proposed to explain the existence of the optic chiasm in vertebrates. The first is these theories was the Visual map theory by Ramón y Cajal. The axial twist hypothesis also explains the chiasm of the trochlear nerve. The hypothesis of Cajal might be valid for the optic chiasm of cephlopods, although in a different manner, because Cajal designed his idea for a chiasm of type II but the cephalopod chiasm os of type III.
invertebrates
The lens eye inverts the visual image that is projected on the retina due to the camera obscura effect. The chiasm in the optic tract of cephalopods corrects this inversion. In insects, the optic chiasms seem to have evolved gradually, since primitive groups have no chiasm, whereas later evolved groups have one or two optic chiasms along the optic lobe.
Exceptions
In jawless vertebrates, the optic tracts do cross in the midline, but only after entering the ventral side of the central nervous system. After crossing the tracts insert on the dorsal optic tectum as in all other vertebrates. Therefore, given the obvious and undisputed homology, the optic chiasm is called chiasm also in these clades, even though the crossing is technically a decussation.
