Neuroanatomy Suzanne Stensaas April 8, 2010, 10:00-12:00 p.m. Reading: Waxman Ch. 15, Computer Resources: HyperBrain Ch 7 THE VISUAL PATHWAY Objectives: 1. Describe the pathway of visual information from the retina to the visual cortex. 2. Draw the expected visual fields seen in classic lesions of the nerve, chiasm, thalamus, optic radiations and visual cortex. 3. Describe the blood vessels that when occluded could lead to visual problems, as well as the expected field loss. I. OPTIC CHIASM PARTIAL DECUSSATION Photo: Suzanne Stensaas 1
II. OPTIC TRACT Ganglion cell axons diverge A. 90% go to Lateral geniculate nucleus (LGN) of thalamus (the retinogeniculo-calcarine path ) B. 10% go to Superior colliculus and pretectum (the retinocollicular path for reflexes) C. The hypothalamus for circadian rhythms (not to be discussed) III. THALAMIC RELAY NUCLEUS -- the LATERAL GENICULATE NUCLEUS OR BODY A. Specific retinotopic projection. B. Six layers. Three layers get input from from each eye. Thalamus LGN Red Nucleus LGN 2
The optic tract projects to the LGN Crainial Nerves, Wilson-Pauwels et al., 1988 IV. OPTIC RADIATIONS A. Retinotopic organization from the LGN neurons to the cortex. B. Axons of neurons in the lateral geniculate form the optic radiations = geniculocalcarine tract. The retinotopic organization is maintained. 1. Some loop forward over inferior (or temporal) horn of lateral ventricle = Meyer's Loop 2. Other axons take a more direct posterior course through the deep parietal white matter. 3. All fibers run lateral to the lateral ventricle. 3
Crainial Nerves, Wilson-Pauwels et al., 1988 Fig. 4
V. PRIMARY VISUAL CORTEX = CALCARINE OR STRIATE CORTEX. ALSO KNOWN AS BRODMANN'S AREA 17 A. Organization of cerebral cortex into six layers (I -VI). B. Stripe or line of Gennari - massive termination of myelinated thalamocortical axons in layer IV = striate cortex. C. Retinotopy of optic radiation axons as they project into cortex. Inferior (lower) visual field projects dorsal to calcarine fissure. Superior (upper) field projects ventral to fissure. Macular field projects to posterior area. D. Because of retinotopy, many brain lesions result in predictable visual field lesions. These lesions can remove all or part of either or both visual fields. Primary visual cortex Calcarine sulcus 5
VI. PRINCIPAL VISUAL FIELD DEFECTS A. Lesions of the visual pathway and resultant visual field losses (Circles represent visual field of each eye tested separately and viewed as if physician is standing behind the subject). Basic Clinical Neuroscience, Young, Young, and Tolbert, Fig. 14-8. 6
VII. VASCULAR SUPPLY TO THE VISUAL PATHWAY A. Ophthalmic Artery - the first branch off the internal carotid as it emerges from the cavernous sinus. 1. Central retinal artery - ganglion cells, bipolars, inner part of receptors. Sole supply of retina inner surface. 2. Ciliary arteries - outer segments of receptors. B. Middle cerebral artery - deep branches vascularize optic radiation in parietal lobe. C. Posterior cerebral artery (PCA) branches and forms calcarine artery. The PCA is easily compressed during herniation of the medial temporal lobe over the free edge of the tentorium (to be discussed later) Clinical Neurology (5 th edition), Greenberg et al., p. 130 7
VIII. EXTRASTRIATE CORTEX (not to be tested) There are over 50 different visual representations in cortex in primates. A. Area 18, (V2 and V3) Visual association areas, with separate retinotopic parallel processing channels for form, color, motion, depth, location, objects. Lesions in V1, V2, and V3 all produce identical visual field defects. B. Angular and supramarginal gyri of occipitaoparietal area processes position and motion ( where pathway). Lesions result in hemispatial neglect but do not disturb visual sensation. C. Fusiform or occipitotemporal gyrus identifies objects, symbols, colors ( what pathway). Lesions in this area result in visual agnosia and alexia (on left side) and prosopagnosia (on right side). 8