Striate Cortex

Brodmann area 17 (or V1, primary, calcarine, or striate cortex) is the end organ of the afferent visual system and is situated in the occipital lobe. The upper bank of striate cortex lies superior to the calcarine fissure, and the lower bank lies below the fissure. Other boundaries of striate cortex include the splenium of the corpus callosum anteriorly, interhemispheric fissure medially, and occipital pole posteriorly. Laterally, some striate cortex may be visible on the posterolateral outer surface of the occipital lobes.

Neuronal input arrives in a retinotopic fashion from fibers of the LGN and synapses within striate cortex, which is divided into six layers. The thick, light-colored layer IV is visible to the naked eye and has been termed the striae

of Gennari. The parvocellular neurons synapse within the layer IVB, whereas the magnocellular cells synapse within layers II and III. [10] Fibers from the medial aspect of the LGN, carrying information from superior retina, project to the upper bank of the calcarine cortex, whereas those from the lateral aspect, carrying information from the inferior retina, project to the lower bank. The inferior visual fields are thus represented within the upper bank, and the superior visual fields are represented within the lower bank. Left and right visual fields are represented within right and left occipital lobes, respectively. Macular projections synapse in the occipital pole and occupy about one half the entire surface area of striate cortex. The occipital tip is devoted to foveal vision.

The majority of the blood supply to striate cortex derives from branches of the PCA: the calcarine artery, mostly, with lesser contributions from the posterior temporal and parieto-occipital arteries. In most cases, small penetrating branches from the calcarine artery supply both the upper and lower banks of calcarine cortex. In up to one third of cases, one major branch to each bank may be seen. At the occipital pole, there is an anastomosis between PCA vessels and the superior temporo-occipital sylvian artery from the MCA. This dual blood supply to the area responsible for central vision is one vascular explanation for macular sparing in the setting of PCA occlusion.

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