The afferent visual fibers exit the chiasm posteriorly and diverge to form the left and right optic tracts, each of which is made up of ipsilateral temporal fibers and contralateral nasal fibers. The optic tracts sweep around and above the infundibulum, below the third ventricle, and superomedially to the uncal gyri. They then turn posterolaterally to the interpenduncular cistern, just ventral to the rostral midbrain and cerebral peduncles.
Most of the fibers synapse within the ipsilateral lateral geniculate nucleus; however, a few axons depart from the optic tract to complete the afferent limb of the pupillary light reflex. These fibers pass ventral to the medial geniculate nucleus; then they continue through the brachium of the superior colliculi before reaching the pretectal nuclei, where they synapse (Fig. 8-4 (Figure Not Available) ). In turn, these nuclei connect bilaterally to the Edinger-Westphal nuclei in the oculomotor complex. Parasympathetic pupillary fibers leave the brain stem within the third nerve, then synapse in the
Figure 8-4 (Figure Not Available) Pupillary light reflex-parasympathetic pathway. Light entering one (syraight black arrow, bottom right) stimulates the retinal photoreceptors (RET), resulting in excitation of ganglion cells, whose axons travel within the optic nerve (ON), partially decussate in the chiasm (CHI), then leave the optic tract (OT) (before the lateral geniculate nucleus [LGN]) and pass through the brachium of the superior colliculus (SC) before synapsing at the mesencephalic pretectal nucleus (PTN). This structure connects bilaterally within the oculomotor nuclear complex at the Edinger-Westphal (E-W) nuclei, which issues parasympathetic fibers that travel within the third nerve (inferior division) and terminate at the ciliary ganglion (CG) in the orbit. Postsynaptic cells innervate the pupillary sphincter, resulting in miosis. Note light in one eye causes bilateral pupillary constrictf/Tom Liu GT: Disorders of the eyes and eyelids. In Samuels MA, Feske S [eds]: The Office Practice of Neurology. New York, Churchill-Livingstone, 1996, p 61. Adapted from Slamovits TL, Glaser JS: The pupils and accommodation. In Glaser JS [ed]. Neuro-ophthalmology, 2nd ed. Philadelphia, J.B. Lippincott, 1990, p 460.)
ciliary ganglion in the orbit. Postganglionic fibers mediate pupillary constriction. Details regarding the Edinger-Westphal nuclei in oculomotor complex are contained in the chapter on cranial nerves III, IV, and VI (see Chapter.9 ).
The lateral geniculate nucleus (LGN), which is situated within the lateral recess of the choroidal fissure, above the ambient cistern, is considered part of the thalamus. Coronally, the LGN has six neuronal layers, and the input into each is monocular and retinotopically organized. Visual information from the ipsilateral eye synapses within laminae 2, 3, and 5, whereas that from the contralateral eye synapses within laminae 1, 4, and 6. Layers one and two contain large neurons (magnocellular LGN layers), whereas layers three through six contain smaller neurons (parvocellular LGN layers). Studies suggest the existence of at least two types of retinal ganglion cells (M and P, respectively) that project preferentially to each group, and that each layer may have distinct projections within the striate cortex as well.
The blood supply of the optic tract is variable but typically comes from an anastomotic network made up of the anterior thalamic perforators (from the posterior cerebral artery) and the anterior choroidal artery (from the internal carotid artery). The LGN also has a rich anastomotic blood supply made up of the anterior and posterior choroidal arteries. The lateral wedge of the LGN is supplied by the anterior choroidal artery, the medial wedge by the posterior choroidal artery, and the middle wedge (hilum) by both vascular supplies. In about 50 percent of cases, small portions of the LGN receive blood from other small posterior cerebral artery
(PCA) branches. Ischemic lesions to the posterior portion of the optic tract and the LGN are considered rare because of their rich anastomotic blood supply. y , [6 Optic Radiations
The optic radiations (geniculocalcarine fibers) exit dorsally from the LGN, then spread into two major bundles. The group of fibers containing contralateral superior quadrant visual information (inferior fascicle) curves in an anteroinferior direction into the anterior pole of the temporal lobe, forming Meyer's loop. The superior fascicle lies deep within the parietal lobe and subserves visual information from the contralateral inferior quadrant. The temporal and parietal fascicles project in a retinotopic fashion to the lower and upper banks of calcarine cortex, respectively.
The temporal portion of the optic radiations receives its blood supply from the anterior choroidal artery and other middle cerebral artery (MCA) branches within the sylvian fissure, including the lenticulostriate and inferior temporo-occipital artery. The distal branches of the MCA, including the angular and posterior temporal arteries, supply the more superiorly situated parietal fascicles. The most posterior portions of the optic radiations, just before their entry into the occipital lobe, are supplied by the superior temporo-occipital sylvian artery branch of the MCA and the anterior temporal and calcarine arteries of the posterior cerebral artery (PCA). [5 , y
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