Chiasm (anatomy)

rdf:langString Chiasm (anatomy)

rdf:langString Chiasm

rdf:langString chiasma

rdf:langString Chiasm

rdf:langString Schema of the optic chiasm and the trochlear chiasm in vertebrates.

rdf:langString In anatomy a chiasm is the spot where two structures cross, forming an X-shape (from Greek letter χ, Chi). This can be: * A tendinous chiasm is 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, a chiasm is the crossing of fibres of a nerve or the crossing of two nerves. Very 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.

rdf:langString Anatomical feature where two structures cross

rdf:langString chiasma