optic chiasma


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Related to optic chiasma: corpus callosum, optic tract

optic chiasma

n.
A structure in the brain formed by the partial intersection or crossing of the optic nerve fibers on the underside of the hypothalamus. Also called optic chiasm.

optic chiasma

The junction of the two OPTIC NERVES lying under the brain. In the chiasma optic nerve fibres from the inner half of each RETINA cross over. Those from the outer half do not. Thus fibres from the inner half of each retina run out of the chiasma in close association with fibres from the outer half of the retina on the other side. The two optic tracts so formed run into the brain. This arrangement ensures that input to both eyes from the right field of vision causes signals that pass to the left half of the brain, and vice versa.

optic chiasma

a point under the hypothalamus of the brain where the two optic nerves meet and cross over, so that stimuli from each eye are interpreted in the OPTIC LOBE of the opposite side of the brain.

chiasma, optic 

A structure located above the pituitary gland and formed by the junction and partial decussation (crossing-over) of the optic nerves. The fibres from the nasal half of the retina of the left eye cross over to join the fibres from the temporal half of the right retina to make up the right optic tract and vice versa. About 53% of the axons of the optic nerves cross to the opposite tract (Fig. C9). A lesion of the chiasma produces a typical field defect (heteronymous hemianopia). Note: also spelt chiasm. See circle of Willis; decussation; visual pathway; stereo-blindness; optic tracts.
Fig. C9 An illustration of the decussation of the optic nerve fibres occurring in the optic chiasmaenlarge picture
Fig. C9 An illustration of the decussation of the optic nerve fibres occurring in the optic chiasma

optic

of or pertaining to the eye.

optic chiasma
see optic chiasm.
optic cortex
see visual cortex.
optic cup activity
optic disk
the disk in the fundus of the eye marking the point at which the optic nerve enters; it is accompanied by blood vessels, is oval, light in color and the blind spot of the retina.
optic nerve
the second cranial nerve; it is purely sensory and is concerned with carrying impulses for the sense of sight. The rods and cones of the retina are connected with the optic nerve which leaves the eye slightly to the nasal side of the center of the retina. The point at which the optic nerve leaves the eye is called the blind spot because there are no rods and cones in this area. The optic nerve passes through the optic foramen of the skull and into the cranial cavity. It then passes backward and undergoes a division; those nerve fibers leading from the nasal side of the retina cross to the opposite side in the optic chiasma while those from the temporal side continue to the thalamus uncrossed. The nerve tracts proceeding backward from the optic chiasm, pass around the cerebral peduncle, and dividing into a lateral and medial root, which end in the superior colliculus and lateral geniculate body, respectively. After synapsing in the thalamus the neurons convey visual impulses to the occipital lobe of the brain.
Injury to the nerve leads to partial or complete loss of sight on the opposite side. Commonly bilateral.
optic nerve aplasia
an uncommon congenital anomaly, most frequently seen in Collie dogs; affected animals are blind from birth. Hypovitaminosis A and prenatal infection with bovine virus diarrhea are possible causes.
optic nerve inflammation
optic neuritis.
optic primordia
the eyes begin in the embryo as a pair of shallow optic grooves on each side of the developing forebrain. The grooves form optic vesicles which invaginate to form a double-walled optic cup.
optic radiation
fibers from the lateral geniculate body entering the occipital cortex.
optic stalk
the evagination from the neural tube of the developing embryo which develops the optic cup at its extremity; the stalk persists as the optic nerve.
optic sulcus
see optic groove.
optic vesicle
the initial evagination from the neural tube which gives rise to the optic cup and the optic stalk.
References in periodicals archive ?
MR imaging is preferred over CT for the diagnosis of pituitary adenomas because it provides multiplanar high contrast images, higher definition of small sellar lesions (Microadenomas) of the pituitary and further it improves anatomical definition of the pituitary, optic chiasma compression, cavernous sinus invasion before surgery.
Optic chiasma is a flattened band embedded in anterior third of ventricle between two thalami and projecting into the chiasmatic cistern.
Retinotopic distribution of axons continues in optic chiasma
In the optic tract fibres from optic chiasma are rearranged to correspond with the position of lateral geniculate body.
It may be accompanied by ventricular collapse and may include: Descent of Cerebellar tonsils, which maybe mistaken for an Arnold-Chiari malformation Type I, prepontine cistern effacement, an inferiorly displaced optic chiasma, obliteration or effacement of the prechaismatic cistern, and reduction of the subarachnoid cisterna.
In their case optic globe, nerves and optic chiasma was absent.
The re-calculation of the radiotherapy doses applied; showed that the tissues in the base of the skull, the brain and the optic chiasma would tolerate a maximum dose of further 45 Gy.