NEUROANATOMY OF THE VISUAL PATHWAYS Magrane Basic Science Course June 2018 **It is important for me to acknowledge the extensive input from Dr. Lola Hudson and Dr. Karen Munana to both these notes and the Power Point presentation.** CENTRAL NERVOUS SYSTEM All 5 divisions of the brain are involved with ocular function and/or adnexa in either conscious pathways or reflex pathways. The 5 divisions are the telencephalon (cerebral hemispheres), diencephalon (thalamus, hypothalamus), mesencephalon (midbrain), metencephalon (cerebellum and pons) and myelencephalon (medulla oblongata). The telencephalon and diencephalon together can be referred to as the prosencephalon (forebrain), and the metencephalon and myelencephalon can be referred to as the rhombencephalon (hindbrain). These 3 larger divisions (prosencephalon, mesencephalon and rhombencephalon) relate to the embryology of the brain from three original neural tube vesicles. The more cranial spinal cord is also involved with ocular function through sensory innervation into the cervical spinal cord and through sympathetic autonomic function particularly in the cranial thoracic spinal cord. PERIPHERAL NERVOUS SYSTEM The majority of the 12 pairs of cranial nerves (CN) have some involvement with ocular function: CNs II, III, IV, V (ophthalmic and maxillary branches), VI, VII and VIII including appropriate sensory and autonomic ganglia of these CNs. TELENCEPHALON (cerebrum=cerebral hemispheres=cerebral cortex) Telencephalon is the site of awareness, initiation of voluntary movements and perception of stimuli. The cerebrum functions in perception and integration of vision as well as voluntary control of eye/eyelid movements. The occipital lobes and the motor cortex of the frontal/parietal lobes are the primary regions involved in ocular and eyelid function. The occipital lobe occupies the caudal one-third of the cerebral hemispheres. It borders the parietal lobe dorsorostrally and the temporal lobe laterally. Medially, the right and left occipital lobes meet between the cerebral hemispheres across the longitudinal fissure. Caudally, the occipital lobes are adjacent to the osseous tentorium and the tentorium cerebelli, which both lie in the transverse fissure (between the cerebrum and cerebellum). Unlike the border between frontal and parietal lobes, there is not a definite line or sulcus to demarcate the borders of the occipital lobe and different sources will include greater or lesser areas. However, everyone includes the visual cortex in the occipital lobe. In domestic animals (dog), the occipital lobe includes parts of the marginal, ectomarginal, caudal suprasylvian, and caudal composite gyri. It also includes the splenial and occipital gyri medially. Endomarginal and ectomarginal gyri and sulci are part of the parietal lobe (rostral 2/3s) and the occipital lobe (caudal 1/3). The primary visual area, the main recipient of dorsal lateral geniculate nucleus (DLGN) output, occupies the region known as Brodmann’s area 17. This area is a histologically defined region in the occipital lobe. In all mammalian species mapped so far, this area lies in the posterior pole of the occipital lobe. In the cat, it occupies the posteromedial portion of the cortex, extending from the crown of the lateral gyrus on the dorsal surface to the superior bank of the splenial sulcus on the medial surface. In the dog, it is located at the junction of the marginal and endomarginal gyri. This area has also been called the striate cortex. Adjacent to the striate cortex is the parastriate cortex and then next is the peristriate cortex. Brodmann’s area 17 = striate = visual I Brodmann’s area 18 = parastriate = visual II Brodmann’s area 19 = peristriate = visual III Areas 18 and 19 together may be referred to as the extrastriate cortex or as visual association area. The extent of visual resolution in a species is reflected not only in the surface area of the striate cortex and the magnification factor (a term to quantify the disproportionate amount of cortical area devoted to processing visual information from the area centralis) but also in the number of visually responsive areas. Multiple visual areas have been discovered in almost every species studied. Three cortical areas have been identified in the hedgehog (a primitive insectivore) and four such areas in the mouse. In the cat, there are over a dozen visual areas. Three of them, visual I, visual II, and visual III (corresponding to Brodmann’s 17, 18 and 19) occupy most of the feline occipital lobe. Each of these contains one representation of the visual hemifield. More than 30 such areas have been identified in nonhuman primates on the basis of behavioral studies. It is assumed that these extraoccipital areas deal with “higher” visual processing such as shape and location discrimination as well as facial recognition. The point of central vision is located in the striate cortex. The point of central vision in the striate cortex varies in position between species and perhaps between breeds. The stereotaxic coordinates for the representation of the area centralis in the feline cortex are P3-L5 (3 mm posterior to the interaural plane and 5 mm lateral to the midline). Anatomically, it is located on the crown of the lateral gyrus, near the junction of the lateral and posterior lateral gyri. Another source listed this area in the cat as the junction of marginal and endomarginal gyri. On average, the projection of the canine area centralis is 13.4 mm anterior to the interaural plane and 8.4 mm lateral to the midline. Beagle: 11.3 mm rostral to the interaural line and 8.3 mm lateral to the midline; Greyhound: 15.6 mm rostral to the interaural plane and 8.5 mm lateral to the midline. Knowledge of these coordinates is important when recording visual evoked potentials. Cells of the primary visual cortex, visual I, are arranged in 6 layers, which are defined on the basis of cytoarchitecture and myelination patterns of the cells. Layer 4 is heavily myelinated. It is in this layer that incoming LGN axons synapse with cortical neurons. Magnocellular (stereopsis, movement, directionality and contrast sensitivity) projections synapse in layer 4Ca and parvocellular (spatial resolution and color sensitivity) projections synapse in layer 4Cb (maintains the segregation and dual processing of visual information that has characterized the visual system up to this point). Layers 2 and 3 contain excitatory neurons that project to other cortical areas, while layers 1 and 2 receive feedback input from these same extrastriate visual areas. Similar descending feedback loops project from layers 5 and 6 back to the DLGN. Most of the neurons are GABAnergic, inhibitory neurons that do not project outside of V I and are devoted to processing of the signal in the striate cortex. Only a minority of the cells are excitatory, spiny (stellate or pyramidal) neurons that project outside of area 17. The basic cortical unit that processes an incoming signal is termed a column, which descends through all six layers of the cortex. As in the DLGN, vertical penetration through six cortical layers of the column will result in passage through cells with approximately identical receptive fields. Therefore, adjacent retinal receptive fields project onto adjacent columns in visual I. There is no difference in size between columns serving central and peripheral retina; rather, more columns are used to process visual input from the central retina. Area visual I receives input from the DLGN. This input consists of the entire contralateral visual hemifield as projected on both retinas. Again, this visual hemifield is mapped on the surface of the cortex in a retinotopic manner (adjacent loci of the contralateral visual hemifield are projected onto adjacent loci of the cortex in a simple, point to point manner). Each visual hemifield projects onto the cortical surface. The vertical meridian is a vertical line of demarcation that passes through the area centralis (or fovea) and divides the retina into a nasal hemifield (projected to the contralateral cortex) and a temporal hemifield (projected to the ipsilateral cortex). Medial movement (away from the lateral gyrus) on the surface of the striate cortex represents peripheral movement (away from the area centralis) in the visual field. Large cortical areas are devoted to processing signals originating from the area centralis. Magnification factor is a term to quantify the disproportionate amount of cortical area devoted to processing visual information from the area centralis. In the retina, each degree of the visual field is projected onto a similar-sized retinal area, regardless of whether it is peripheral or central. The increased resolution and processing achieved by the central retina is obtained by increasing the density of ganglion cell or photoreceptor population. In the cortex, the density of neurons serving the peripheral or central fields is identical. Increased cortical visual discrimination from the area centralis is a result of the increased cortical area devoted to representing the area centralis. This larger area results in magnification of its representation. In the hedgehog, the surface area of the striate cortex is 20 mm2 and ½ of this area is devoted to representing the central 35 degrees of the contralateral visual hemifield. In the cat, the surface area is 380 mm2 and ½ of this area is devoted to the central 20 degrees of the visual field. In other words, ½ of visual I is devoted to the central 20 degrees of the visual field and ½ is devoted to the rest of the visual field. Afferent connections to visual areas come from the lateral geniculate nucleus via optic radiation of internal capsule (main white matter connection between hemisphere and rest of brain). There are also reciprocal connections with other lobes and with parastriate/peristriate areas. Efferent connections from visual areas include the association areas (long and short association fibers connect visual cortex with other lobes of the same hemisphere such as motor cortex at frontal/parietal lobe), the opposite hemisphere via corpus callosum and the brain stem (to lateral geniculate nucleus and rostral colliculus, pontine nuclei and reticular formation).