Optic chiasm là gì

Visual processing of the text begins in the retina and progresses by way of the optic nerve to the optic chiasm and then the optic tract.

A 5-mm hole was drilled in the skull either directly over bregma for optic chiasm recordings or 1.5 mm lateral to bregma for optic tract recordings.

Electrodes were advanced into the brain either 0.5 mm anterior to bregma [optic chiasm] or 1.5 mm lateral to bregma [optic tract] through a protective guide tube.

Pathfinding at the mammalian optic chiasm.

In eutherian mammals, axons originating from temporal retina travel intermingled with the main body of the optic nerve up to the midline of the optic chiasm where decussation takes place.

For electrical stimulation of the optic chiasm, a low-impedance bipolar electrode was implanted and fixed to the skull with dental acrylic cement.

During development, axons from the temporal retina make an abrupt turn within the chiasm.

Stereoperception in cats following section of the corpus callosum and 0or the optic chiasm.

Immunoreactivity is strong on the interfascicular glia throughout the optic nerve, but is poor in the central chiasm.

These monocular regions received transneuronal input from the contralateral eye, indicating that a small population of temporal ganglion cells erroneously decussated at the optic chiasm.

The underlying optic chiasm was then cut in the midsagittal plane.

This shows that the time necessary for the terminalisation of chiasms and separation of homologous chromosomes can be greatly shortened.

Postsynaptic potentials recorded in neurons of the cat's lateral geniculate nucleus following electrical stimulation of the optic chiasm.

However, such dual innervation is rare, and we did not observe dual optic chiasm latencies for any of the geniculate cells in this study.

Domains of regulatory gene expression and the developing optic chiasm: correspondence with retinal axon paths and candidate signaling cells.

Optic nerves, chiasm, optic tract, and central optic pathway showed no significant alterations.

First, orthodromic latencies to electrical pulse stimulation of the optic chiasm was measured.

Cryostat sections at 12 mm were taken through the optic nerve and chiasm and collected onto 5% gelatinized slides.

At the optic chiasm the two optic nerves fuse, and fibers from each eye cross the midline or turn back and remain uncrossed. Having adopted their pathways the fibers separate to form the two optic tracts. Research into the architecture and development of the chiasm has become an area of increasing interest. Many of its mature features are complex and vary between different animal types. It is probable that numerous factors sculpt its development. The separate ganglion cell classes cross the midline at different locations along the length of the chiasm, reflecting their distinct periods of production as the chiasm develops in a caudo-rostral direction. In some mammals, uncrossed axons are mixed with crossed axons in each hemi-chiasm, whereas in others they remain segregated. These configurations are the product of different developmental mechanisms. The morphology of the chiasm changes significantly during development. Neurons, glia, and the signals they produce play a role in pathway selection. In some animals fiber-fiber interactions are also critical, but only where crossed and uncrossed pathways are mixed in each hemi-chiasm. The importance of the temporal dimension in chiasm development is emphasized by the fact that in some animals uncrossed ganglion cells are generated abnormally early in relation to their retinal location. Furthermore, in albinos, where many cells do not exit the cell cycle at normal times, there are systematic chiasmatic abnormalities in ganglion cell projections.

I. INTRODUCTION

At the optic chiasm, axons of retinal ganglion cells either cross the midline and innervate the contralateral hemisphere or remain uncrossed and project ipsilaterally. There is renewed interest in the chiasm because the binary decision made by optic axons may cast light on fundamental mechanisms of axon guidance in the central nervous system [CNS]. However, surprisingly little is known about the mature organization of the chiasm, how this varies between mammals, or the developmental mechanisms that sculpt its architecture. This review focuses on the chiasm in animals with partial patterns of decussation, where fibers from each eye give rise to both crossed and uncrossed hemispheric pathways. In these animals chiasmatic organization is more complex, and from a developmental point of view probably more interesting, because each fiber is required to make a choice as to whether it crosses the midline or not.

It is thought that optic fibers project across the midline because early in evolution they innervated motor neurons on the contralateral side of the body in primitive bilaterally symmetrical animals. Any change in illumination, such as the movement of a shadow, was interpreted as indicating the potential presence of a predator. Hence, the crossed fibers provided a simple reflexive pathway for turning away from the potentially threatened side []. Although this assumes that the crossed chiasmatic pathway predates the evolution of the uncrossed projection, ipsilaterally projecting fibers are not the preserve of mammals, but can be found, to a greater or lesser extent, in adult members of all vertebrate classes including the agnathans [jawless fish], amphibians, and birds []. Hence, it can be assumed that partial patterns of decussation have a long ancestral history.

Although there are extensive data in a range of vertebrates on the existence of crossed and uncrossed projections, few consider the organization of fibers through the chiasm and how they develop. The literature on this subject is marked by a limited number of detailed studies undertaken in a few specific animal models, which with a few exceptions are almost exclusively confined to mammals. These studies do not always tell a consistent story, and there is growing evidence that even within mammals, the organization of the chiasm is variable, and as such, the factors that influence its development are probably not universal. Despite this, considerable advances are being made in our understanding of the mechanisms of chiasmatic pathway selection in the mouse. The advantage of using the mouse is the relative ease with which methods of molecular genetics can be applied to addressing the problems of CNS development.

This review considers the fiber organization of the chiasm and how this might arise. This cannot be undertaken without reviewing the organization of the optic nerve and tract as well as patterns of retinal ganglion cell production. The role of fiber-fiber interactions and fiber-glia relationships in the chiasm is also reviewed. Although there is an ever-expanding literature on chemical signals/markers in development, these are not considered extensively, but only where they are directly relevant to the chiasm. Likewise, the literature on differential neural adhesion chemicals are not extensively reviewed because these appear to play a smaller role in pathway selection than was originally thought. Consideration is given to abnormalities of the chiasm where these cast light on mechanisms of normal development.

A.  Chiasmatic Pathways and the Naso-temporal Division in the Retina

Optic decussation was first postulated by Albert the Great, who in the 13th century noted the loss of contralateral visual fields after unilateral damage to the back of the head []. The partial pattern of decussation at the chiasm was first proposed by Newton [], but was formulated more explicitly by Taylor almost 100 years later [, ]. Such a pattern forms the basis of binocular vision, such that in humans, fibers originating from the temporal retina remain uncrossed at the chiasm, whereas those originating from the nasal retina cross. This pattern of partial decussation results in the continuity of the visual field representation in central visual structures in the brain. The line that divides these two populations of ganglion cells in the retina passes vertically through the fovea. The projections from each eye are in conjugate register in the lateral geniculate nucleus, which projects a map of the visual hemifield to the primary visual cortex [, , ].

The retinal line that segregates ipsilaterally from contralaterally projecting cells is termed the naso-temporal division and has varying degrees of precision depending on the species examined. In all cases the crossed projection is larger than the uncrossed. Generally, in mammals, the majority of fibers that give rise to the uncrossed chiasmatic pathway are located in the temporal retina, but complete spatial segregation of cells giving rise to crossed and uncrossed pathways is primarily the preserve of primates []. In most mammals other than primates, the crossed projection arises from the entire retina, whereas the uncrossed projection is mainly confined to the region temporal to the area of maximum ganglion cell density, the area centralis [, , ,]. In these animals the temporal retina contains a mixed population of cells in terms of their chiasmatic pathways. The relative proportion of cells within the temporal retina that project ipsilaterally varies with the degree of retinal specialization and the location of the eyes in the head, and hence the size of their binocular visual field. In the cat, which has frontally placed eyes, cells with an uncrossed pathway form a greater proportion of the total ganglion cell population in the temporal retina than they do in rodents, who have a less specialized retina than that commonly found in carnivores and laterally placed eyes. Rabbits have specialized retinae, but very laterally placed eyes, and hence a small uncrossed pathway [, , , ,].

Cells giving rise to the uncrossed projection terminate in the main visual nuclei that receive retinal input. These include the lateral geniculate nucleus [LGN], the superior colliculus [SC], and pretectal nuclei []. They also project to the superachiasmatic nucleus and the accessory optic system []. In primates, retinal projections to both the LGN and SC respect the sharp naso-temporal division, in that only cells located nasal to the fovea project to the contralateral LGN and SC [,]. Although the projection to the LGN respects the naso-temporal division in carnivores, the projection to the SC does not. Hence, although cells projecting ipsilaterally to the SC are confined to the temporal retina, cells projecting to the contralateral SC are found distributed across the entire retina []. The only exception to this is the projection pattern found in large fruit-eating bats, which have a retinal projection to the SC with a vertical hemi-decussation of the kind commonly only found in primates []. This pattern is not found in other bats and has given rise to considerable speculation about the evolutionary relationship between mega bats and primates [].

In the cat, the line that segregates the uncrossed cells projecting to the LGN varies depending on the subtypes of ganglion cell considered. In this animal, as in most mammals, ganglion cells can be grouped roughly into three classes on the basis of morphology and physiological properties []. β-Cells form almost half of the ganglion cell population and have brisk sustained responses and medium-sized cell bodies. They project primarily to the LGN [, ]. These cells have a relatively sharp naso-temporal division as the line segregating those with different chiasmatic routes is