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The optic chiasm is found in all vertebrates, although in cyclostomes (lampreys and hagfishes), it is located within the brain. [2] [3] This article is about the optic chiasm of vertebrates, which is the best known nerve chiasm, but not every chiasm denotes a crossing of the body midline (e.g., in some invertebrates, see Chiasm (anatomy)).
Several important structures are located adjacent to the optic chiasm. The supraclinoid branches of the internal carotid artery flank the chiasm. The cavernous sinuses are lateral and inferior to the chiasm. The frontal lobe of the brain lies above. The pituitary gland sits below in the sella turcica.
The optic recess – marks the inferior end of the lamina terminalis, with the optic chiasm forming the immediately adjacent floor. The portion of the floor immediately posterior of the optic chiasm distends inferiorly, and slightly anteriorly, to form a funnel (the infundibulum); the recess leading to the funnel is known as the infundibular ...
Bitemporal hemianopsia most commonly occurs as a result of tumors located at the mid-optic chiasm. Since the adjacent structure is the pituitary gland, some common tumors causing compression are pituitary adenomas and craniopharyngiomas. Also, another relatively common neoplastic cause is meningiomas.
Damage to the pituitary stalk blocks the release of antidiuretic hormone, resulting in polydipsia (excessive water intake) and polyuria (excessive urination, central diabetes insipidus). The diameter of the pituitary stalk at the level of optic chiasm is 3.3 mm, and at the pituitary gland insertion site is measured at 1.9 mm. [3]
Chiasms are found in vertebrates but also in invertebrates. The optic chiasm in vertebrates can be of type I or II. [1] [4] However, an optic chiasm of type III is found in many insects [2] and in cephalopods. [3] In vertebrates, three of the cranial nerves show a chiasm. The optic chiasm of the optic tract (type I or II)
They showed this to be the case [8] by studying vasopressin secretion from the isolated posterior pituitary gland in vitro. They found that vasopressin secretion could be evoked by electrical stimulus pulses applied to the gland, and that much more hormone was released by a phasic pattern of stimulation than by a continuous pattern of stimulation.
Since the sella turcica forms a bony caudal border for the pituitary gland, a pituitary tumor usually extends upward in the rostral direction into the suprasellar region. This can result in compression of the optic chiasm , which lies on top of the pituitary, enveloping the pituitary stalk .