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Neuroanatomy © NEUROANATOMY 2016 NEUROANATOMY © LC Hudson, DVM, PhD Professor Emerita of Anatomy North Carolina State University College of Veterinary Medicine Raleigh NC 27607 [email protected] I. CENTRAL NERVOUS SYSTEM All 5 divisions of the brain are involved with the function of the eyes and/or adnexa - in either conscious/response pathways or reflex pathways. The divisions are the telencephalon (cerebral hemispheres), diencephalon (thalamus, hypothalamus, metathalamus (lateral geniculate nuclei)), mesencephalon (midbrain (pretectal, oculomotor, trochlear nuclei, parasym. nucleus of CN III)), metencephalon (cerebellum and pons (vestibular nuclei, spinal tract of CN V)) and myelencephalon (medulla oblongata (abducens, facial, vestibular nuclei)). The more cranial spinal cord is also involved with function of the eyelids through sensory innervation into the cervical spinal cord, and through sympathetic autonomic function (T1-T3 segments). The spinal cord caudal to T3 is not involved with eye functions. II. PERIPHERAL NERVOUS SYSTEM The majority of the 12 pr. of cranial nerves have some function with the eyes/adnexa- CNs II, III, IV, V (ophthalmic and maxillary branches), VI, VII, and VIII including appropriate sensory and/or autonomic ganglia of these cranial nerves. Retrograde tracing studies in cats showed that some cervical spinal nerves have sensory projections from the eyelids even though apparently physically distant. Autonomic sympathetic fibers are projected via Tl-T3 spinal nerves into the sympathetic trunk, traveling through the neck in the vagosympathetic trunk and synapsing in the cranial cervical ganglion. Postganglionic fibers then travel along blood vessels and with other cranial nerves to reach the globe. III. NOMENCLATURE Veterinary ophthalmologists tend to use human/zoological nomenclature including eponyms such as Meibomian gland, Descemet's membrane, and canal of Schlemm; probably because of the intense use of human literature. Veterinary anatomists try to follow the Nomina Anatomica Veterinaria (NAV) <http://www.wava- amav.org/Downloads/nav_2012.pdf> (then click on organa sensuum on the left side list for the eye), which specifically forbids eponyms. Except for the eye, the use of directional terms superior, inferior, anterior and posterior are also not used, but with the globe and adnexa such directional terms are acceptable. Human/zoo Superior Inferior Anterior Posterior Alternate term for Dorsal Ventral Rostral Caudal eye - NAV NEUROANATOMY 2016 IV. TELENCEPHALON = cerebrum = cerebral hemispheres ≅ cerebral cortex. 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. The occipital lobe occupies the caudal aspect of the cerebral hemispheres. It borders with the parietal lobe dorsorostrally and the temporal lobe laterally. Medially, the left and right occipital lobes meet across the longitudinal fissure between the hemispheres. Caudally the occipital lobes lie against the osseous tentorium and the tentorium cerebelli which lie in the transverse fissure (between the cerebrum and cerebellum). Unlike some borders between lobes, there is not a definite landmark to demarcate all the borders of the occipital lobe and different neuroanatomists will include greater or lesser areas. However, despite this dichotomy, everyone includes the visual cortex in the occipital lobe. Gyri included in the human occipital lobe laterally are the cuneus, lateral occipital gyrus, superior and inferior occipital gyri, descending gyrus, superior and inferior polar gyri. Medially, more of the cuneus, parts of the lingual gyrus, and medial occipitotemporal gyrus are included. For domestic animals (dog), the occipital lobe includes parts of the marginal, ectomarginal, caudal suprasylvian. It also includes the splenial, occipital, and caudal composite gyri ventrally and medially. Within the visual cortex is the so-called striate cortex. It has the stripe (or line) of Gennari- cytoarchitectural layer of myelinated fibers that can be seen on unstained and stained sections. Adjacent to the striate cortex is the parastriate cortex and then next is the peristriate cortex. The point of central vision is located in the striate cortex. The same general region may be referred to as Brodman's area 17 for different species (Brodman's areas are based on cytoarchitecture) area 17 = striate = visual I This has 6 neuronal layers; layer IV has subgroups and is the termination of fibers from the lateral geniculate body. area 18 = parastriate = visual II area 19 = peristriate = visual III Areas 18 and 19 together may be referred to as the extrastriate cortex or as visual association area. The point of central vision in striate cortex varies in position between species and perhaps between breeds. dog Beagles: 11.3 mm rostral to interaural line and 8.3 mm lateral to midline; Greyhound: 15.6 mm rostral and 8.5 mm lateral cat junction of marginal and endomarginal gyri other domestic species: unable to find specific information Afferent connections to visual areas: From the lateral geniculate body via optic radiation of internal capsule (main white matter connection between hemisphere and rest of brain). [This may also be referred to as the geniculocalcarine tract as it originates in the lateral geniculate body and projects to the cortex surrounding the calcarine sulcus of human brains.] It is located in the caudal aspect of internal capsule, and the fibers sweep rostrally, then laterally, then caudally in the hemisphere. There are also reciprocal connections with other lobes and with parastriate/peristriate areas via association fibers. NEUROANATOMY 2016 Efferent connections from visual areas: 1. 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. 2. opposite hemisphere via corpus callosum 3. brain stem- to lateral geniculate body and rostral colliculus, pontine nuclei and reticular formation. V. DIENCEPHALON Structures of the diencephalon involved with vision are the optic chiasm, optic tract, lateral geniculate body, internal capsule, hypothalamus, and thalamus. The optic chiasm is located at the rostroventral surface of the brain stem and demarcates the rostral level of the diencephalon. It is closely associated with the 3rd ventricle, hypothalamus, and to some extent the pituitary gland. The percentage of optic nerve fibers crossing the midline in different species varies widely. most birds and fish: 100% cat: ~65% dog: ~75% large animals: ~80-90% It is the existence of uncrossed fibers that give the ability for binocular vision. [There is a letter in Nature from several years ago about an inherited, congenital lack of an optic chiasm in Belgian sheep dogs. In these animals all fibers project ipsilaterally, but project to the lateral geniculate body and to the correct layers of the body.] The optic tract is located lateral to internal capsule of diencephalon; it begins ventrally, and then travels laterally, caudally, and finally, dorsally. In doing so, it stays on the surface of the diencephalon. If you follow the optic tract it will lead you to the lateral geniculate body (LGB) where most of the fibers terminate. In the whole brain, most of the optic tract is covered by the overlying cerebrum. There is conflicting evidence about segregation of fiber size within the tract although there is no doubt that different sizes exist, W, X, Y. There is a difference in conduction speeds and projections of those different classes of axons - in general, fastest fibers (Y) project to the lateral geniculate body, intermediate (X) to the pretectum, and slowest (W) to the rostral colliculus. In the lateral geniculate body, larger fibers synapse in the dorsal laminae while smaller fibers synapse in the ventral laminae. More specifically there are connections from the optic tract to: 1. lateral geniculate body (actually more dorsal in position in domestic animals). It is estimated that 80% of optic tract fibers terminate in the LGB. 2. via brachium of rostral colliculus to pretectal nuclei 3. via brachium of rostral colliculus to rostral colliculus 4. hypothalamus (controversial for carnivores) 5. accessory nucleus of optic tract Lateral Geniculate Body- This structure has topographic (retinotopic) organization- dorsal or main nucleus has become more complex with advent of binocular vision. The ventral nucleus may also be called the pregeniculate nucleus. The LGB is part of the diencephalon which has grown caudally to overhang the mesencephalon. Its major function is as a complex relay nucleus on the conscious vision pathway. NEUROANATOMY 2016 The dorsal nucleus (pars dorsalis) has a sigmoid curved laminar structure - number of cell layers varies in species - 3 in cat and 6-7 in primates. The optic tract fibers enter on the concave surface. The laminae are usually referred to as A, AI, B or 1-6 (layer 1 is the deepest). These have intervening fiber layers. The dorsal nucleus receives large diameter optic tract fibers. Specifically the A and B layers or layers 1, 4, 6 receive crossed fibers whereas the AI layer or layers 2,3,5 receives uncrossed fibers. Layers 1 and 2 are also called the magnocellular subdivision; 3-6 are parvocellular subdivision. Only the dorsal nucleus of the LGB projects to the visual cortex. Ventral nucleus or pars ventralis is wedged between entering branches of the optic tract
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