anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having i ...

a chiasm is the spot where two structures cross, forming an X-shape (). This can be: * A ''tendinous chiasm'' is the spot where two

tendons A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its abilit ...

cross. For example, the tendon of the

flexor digitorum superficialis muscle Flexor digitorum superficialis (''flexor digitorum sublimis'') is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints. It is in the anterior compartment of the forearm. It is sometimes considered to be the deepest pa ...

, and the tendon of the flexor digitorum longus muscle which even forms two chiasms. * In

neuroanatomy Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defi ...

, a chiasm is the crossing of fibres of a

nerve A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...

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 The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divid ...

) without fusing, e.g., the

trochlear nerve The trochlear nerve (), ( lit. ''pulley-like'' nerve) also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a cranial nerve that innervates just one muscle: the superior oblique muscle of the eye, which operates through the pu ...

(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 Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...

such as

insects Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...

and

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, a ...

. * 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 The central nervous system (CNS) is the part of the nervous system consisting primarily of the brain and spinal cord. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all p ...

. A chiasm also differs from a

ganglion A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympathe ...

in that axons run through it without making any synapses. A chiasm is thus not a nervous processing centre.

Optic chiasm in vertebrates

By far the most widely known chiasm is the optic chiasm in vertebrate animals, including ourselves. See

Examples

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

and in

. In

vertebrates Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

three of the

cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

show a chiasm. *The Optic chiasm of the

optic tract In neuroanatomy, the optic tract () is a part of the visual system in the brain. It is a continuation of the optic nerve that relays information from the optic chiasm to the ipsilateral lateral geniculate nucleus (LGN), pretectal nuclei, and su ...

(type I or II) *The chiasm of the

Trochlear nerve The trochlear nerve (), ( lit. ''pulley-like'' nerve) also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a cranial nerve that innervates just one muscle: the superior oblique muscle of the eye, which operates through the pu ...

(type I) *The decussation of part of the

Oculomotor nerve The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of ...

(type I) The optic chiasm of vertebrates involves the optic tract. The trochlear nerve is a

motor nerve A motor nerve is a nerve that transmits motor signals from the central nervous system (CNS) to the muscles of the body. This is different from the motor neuron, which includes a cell body and branching of dendrites, while the nerve is made up of ...

that innervates one of the muscles that move the contralateral eye (i.e., the

superior oblique muscle The superior oblique muscle, or obliquus oculi superior, is a fusiform muscle originating in the upper, medial side of the orbit (i.e. from beside the nose) which abducts, depresses and internally rotates the eye. It is the only extraocular mu ...

). It emerges from the dorsal aspect of the ventral

midbrain The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', ...

, leaves the brain on the dorsal side where it crosses to the opposite side. The

oculomotor nerve The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of ...

originates from the third nerve

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

at the level of the

superior colliculus In neuroanatomy, the superior colliculus () is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form '' tectal'' is common ...

(in non-mammalian vertebrates this is the optic tectum) in the midbrain. The

rostral Rostral may refer to: Anatomy * Rostral (anatomical term), situated toward the oral or nasal region * Rostral bone, in ceratopsian dinosaurs * Rostral organ, of certain fish * Rostral scale, in snakes and scaled reptiles Other uses * 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 In neuroanatomy, the optic chiasm, or optic chiasma (; , ), is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrat ...

of vertebrates is the best known. The optic nerve runs from the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...

towards the

ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

midline of the brain and crosses to the opposite side to continue as the optic tract which inserts to the optic tectum (=superior colliculus)) on the dorsal midbrain (as well as branching off to the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direct ...

amniotes Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are dis ...

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

mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...

and

birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

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 A branch, sometimes called a ramus in botany, is a woody structural member connected to the central trunk of a tree (or sometimes a shrub). Large branches are known as boughs and small branches are known as twigs. The term '' twig'' usuall ...

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 posterior visual field. Since there are two chiasms, the retinotopic map is not affected. Cephalopods ( squids and

octopuses An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefis ...

) 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 Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and musical set theory * ...

of the image on the

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 In neuroanatomy, the optic radiation (also known as the geniculocalcarine tract, the geniculostriate pathway, and posterior thalamic radiation) are axons from the neurons in the lateral geniculate nucleus to the primary visual cortex. The op ...

which rotates the retinal map on the visual cortex by 180° (see Figure 3).

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 is of type III.

invertebrates

The lens eye inverts the visual image that is projected on the retina due to the

camera obscura A camera obscura (; ) is a darkened room with a small hole or lens at one side through which an image is projected onto a wall or table opposite the hole. ''Camera obscura'' can also refer to analogous constructions such as a box or tent in w ...

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 (

hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...

and

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

), 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.

References

External links

* (human) * (non-human) {{Authority control Neuroanatomy Optic nerve Nerves Peripheral nervous system Soft tissue