Chiasm (anatomy)

In anatomy a chiasm is the spot where two structures cross, forming an X-shape (). Examples of chiasms are: A tendinous chiasm, the spot where two tendons cross. For example, the tendon of the flexor digitorum superficialis muscle, and the tendon of the flexor digitorum longus muscle which even forms two chiasms. In neuroanatomy, the crossing of fibres of a nerve or the crossing of two nerves. Different types of crossings of nerves are referred to as chiasm: Type I: Two nerves can cross one over the other (sagittal plane) without fusing, e.g., the trochlear nerve (see figure). Type II: Two nerves can merge while at least part of the fibres cross the midline (see figure 2). Type III: The fibres within a single nerve cross, such that the order of the functional map is reversed, e.g., the optic chiasms of various invertebrates such as insects and cephalopods. 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. Note that in the third type there is no crossing of the mid sagittal plane. Only in the first type, the crossing is complete. There are other kinds of crossings of nerve fibres. The chiasm is distinguished from a decussation, which is a crossing of nerve fibres inside the central nervous system. A chiasm also differs from a ganglion in that axons run through it without making any synapses. A chiasm is thus not a nervous processing centre. optic chiasm By far the most widely known chiasm is the optic chiasm in vertebrate animals, including humans. 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 the optic tract (type I or II) The chiasm of the Trochlear nerve (type I) The decussation of part of the Oculomotor nerve (type I) The optic chiasm of vertebrates involves the optic tract.