The visual cortex corresponds to approximately 55% of the entire cortical area of the primate brain. For comparison purposes, 3% is associated with the auditory system and 11% with the somatosensory system.1,2 Given that structure and function are correlated, this suggests that over 50% of all the information stored in the brain is directly or indirectly related to vision. Unlike other organs and systems, the anatomy of the central nervous system remains poorly understood. In the past two decades, however, there has been a massive increase in research in neurosciences, which may lead to more accurate and sophisticated methods to detect and treat disorders of the visual system. This brief review provides a basic description of the main structural components of the visual system.
The first neurons of the visual pathway are the retinal ganglion cells (RGC) which are located in the innermost retinal layers (Fig. 1). First, light needs to cross all inner layers before it reaches the photoreceptors, which are neurons specialized on the reception and conduction of visual stimuli. The two types of photoreceptors, rods and cones, contain a photopigment which is composed of opsin, a membrane protein and 11-cis-retinal, a chromophore. A photon can cause conformational changes in this photopigment, leading to a cascade of chemical reactions that convert electromagnetic energy into an electrical stimulus. This stimulus travels to other retinal layers through neurotransmitters. From the photoreceptors the impulse is transmitted to the bipolar cells and then reaches the RGCs. Their axons reach the retinal nerve fiber layer (RNFL) where they converge to the optic nerve head.3
The arterial blood supply to the outer 1/3 of the retina comes from the posterior ciliary arteries, whereas that to the inner 2/3 comes from the central retinal artery, both branches of the ophthalmic artery.4
The optic nerve head is composed of four main regions (Fig. 1):
* Nerve fiber layer
* Prelaminar region
* Lamina cribrosa
* Retrolaminar region
After passing through the lamina cribrosa, the optic nerve becomes invested with meninges (pia- and dura-mater) and also becomes myelinated by oligodendrocytes.2,3 The cerebrospinal fluid (CSF) fills the subarachnoid space. Within the intraocular portion of the optic nerve (3–4 mm), the compartment between the subarachnoid space and the scleral tissue form a virtual space that produces a pressure gradient between the intraocular compartment and the CSF space (Fig. 2). The arterial circle of Zinn-Haller, which is composed of anastomotic branches of the posterior ciliary arteries, the pial arteriole plexus, and the peripapillary choroid, provides blood supply to the optic nerve head.4
Outside the globe, the optic nerve is divided into three portions: intracranial (25 mm), intracanalicular (within the optic canal and inside the lesser wing of the sphenoid bone, 6–7 mm), and intracranial portions (18–20 mm) before it reaches the optic chiasm. The distal and proximal portions of the optic nerve are supplied by branches of the ophthalmic artery and the circle of Willis.5
The two optic nerves travel toward, and meet at the optic chiasm, which lies in the subarachnoid space of the suprasellar cistern, above the sella turcica and the pituitary gland. Above the optic chiasm is the hypothalamus and behind it lies the infundibulum. Another important anatomical relation is the cavernous sinus, which lies below the chiasm and around the pituitary gland. The following structures cross through the cavernous sinus: internal carotid artery, oculomotor nerve (CN III), trochlear nerve (CN IV), abducens nerve (CN VI), ophthalmic nerve (the V1 branch of the trigeminal nerve, CN V), and the maxillary nerve (the V2 branch of CN V).3,6
In the chiasm, the nerve fibers originating in the nasal retina of each eye decussate to join the temporal fibers of the fellow eye (Fig. 3). The distribution of nerve fibers within the optic nerve and chiasm maintains a retinotopic organization. The blood supply to the optic chiasm comes from anastomotic arteries comprising the circle of Willis.2,3
From the optic chiasm, the same axons that originate in the RGC layer continue through the optic tract until they synapse with neurons situated in the lateral geniculate nucleus (LGN). A retinotopic organization is also maintained in the tract. Its blood supply is variable but typically arises from anastomotic branches of the posterior communicating and internal carotid artery.2,3
LATERAL GENICULATE NUCLEUS
Most axons from the optic tract synapse in the LGN, the cells of which are the second neurons of the visual pathway, and are distributed into 6 layers. Ganglion cell axons from the ipsilateral eye (temporal retina) synapse in layers 2, 3, and 5, while axons from the contralateral eye (nasal retina) synapse in layers 1, 4, and 6. Two main types of neurons can be identified in the LGN: large neurons located in layers 1 and 2 (magnocellular layers) and small neurons in layers 3, 4, 5, and 6 (parvocellular layers) (Fig. 4). There is a third cell layer distributed irregularly between the parvocellular and magnocellular layers called the koniocellular layer.2,3
Notably, only 5%–10% of the synapses in the LGN come from RGC axons. The majority comes from up-down modulating connections from the thalamic reticular nucleus, pulvinar nucleus, and the visual cortex.7 The retinotopic organization of the LGN is such that the central portion (hilum) receives macular fibers, whereas the lateral and medial horns receive fibers from the inferior retina and superior retina, respectively.8 The blood supply to the LGN arrives via branches of the internal carotid artery and posterior cerebral arteries.2,3
SUPERIOR COLLICULI, PRETECTAL NUCLEI, AND SUPRACHIASMATIC NUCLEUS
Not all fibers from the optic tract synapse in the LGN. Some fibers connect with other nuclei located in the midbrain and which are related to autonomic functions.
The superior colliculi are responsible for coordinating eye and head movements to sudden visual and other sensory stimuli, as well as saccadic gaze. Their neurons also receive input from other sensory organs (e.g.: labyrinth, somatosensory system) as well as the visual cortex.
The pretectal nuclei receive afferent input from the RGC, which travel via dual connections to each Edinger-Westphal nucleus. From the Edinger-Westphal nuclei, parasympathetic fibers travel through the oculomotor nerves to the ciliary ganglion and are responsible for the control of pupil size and consensual reflex.2
Some RGC containing melanopsin send axons to the suprachiasmatic nucleus at the base of the anterior hypothalamus. This center is sensitive to changes in ambient light and sends fibers to the pineal gland. It is responsible for the regulation of physiologic functions related to circadian rhythms.9
From the LGN, the second neurons send axons to the visual cortex through the optic radiations. These fibers initially project anteriorly and then turn posteriorly toward the occipital lobe (Fig. 5). The anterior portion of the optic radiations receives its blood supply from branches of the circle of Willis and middle cerebral artery, whereas distal (posterior) portions are supplied by anastomotic branches of the posterior cerebral artery.10
Axons from the six layers of the LGN travel along the optic radiations and synapse in the primary visual cortex, named V1 (Fig. 6). These axons make connections in the cortical layer IV (‘stripe of Gennari’). Axons from the parvocellular layers of the LGN synapse at layer IV-C-β, while those from the magnocellular layers synapse at layer IV-C-α. Notably, there is a 300-400-fold increase in the number of neurons in V1 as compared to the RGC layer.11 The corresponding representation of the vertical meridian of the visual field lies medially within the calcarine lips, whereas the horizontal meridian is represented deep within the calcarine fissure. Macular projections representing the central field synapse in the posterior pole of calcarine cortex. The macular representation is greatly magnified in the visual cortex retinotopic map. For instance, connections from 1 mm2 around the fovea, representing the central 10 degrees (which corresponds to approximately 2% of the total visual field) represent roughly 60% of the striate cortex.12,13
Most of the blood supply to the visual cortex comes from the posterior cerebral arteries and its branches. At the occipital pole, however, there is a dual blood supply to the area corresponding to the central vision, with anastomoses between branches of the posterior cerebral artery and branches of the middle cerebral artery (Fig. 7).14
FEEDBACK MECHANISMS AND THE HIGHER-ORDER VISUAL CORTEX
Despite reaching the visual cortex, stimuli from the retina still need regulation and processing before they can be perceived as images. Up-down connections between thalamic and higher-order cortical levels provide an accurate perceptual interpretation of light stimuli.15 Some of these loops were described above, such as the connections between V1 and the LGN and between the different mesencephalic nuclei.16,17 From V1 the information travels to extrastriate areas responsible for different features of vision, such as color, motion, depth, contrast, and memory. From V4 and V5, then, the information is conducted and/or stored in different areas which are related with other functions (e.g.: somatosensory, speech, and hearing systems), motor activity, and emotions.17–19
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