Visual pathway

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A part of the neural nerve tract of the optical system from the optic chiasm to the brain , anatomically called the optic tract, is called the visual pathway in a narrow sense . In a broad sense, the term “visual pathway” or visual conduction is understood to be the chain of interconnected neurons of the visual system from the eye to the primary visual cortex in the cerebrum .

The Sehleitung comprises in this sense - primary addition to the (1) and (2) secondary afferent neurons of the retina - in addition to the nerve fibers of the (3rd) retinal ganglion cells which in their course from the eye outlet to the optical chiasma as optic nerve ( nervus opticus ) and then as the visual pathway sensu stricto or Sehstrang ( optic tract are referred to), and that of (4) neurons in the lateral geniculate body ( lateral geniculate nucleus ) in metathalamus the diencephalon outgoing fibers of the optic radiation ( optic radiation ), which in the ( 5.) Neurons of the (primary) visual cortex ( area striata ) of the endbrain end.

Greatly reduced scheme of the visual pathway

The rod and cone outer links of the photoreceptors in the retina are parts of these receptor cells that represent the beginning of the neuron chain. The cell bodies of these at the same time 1st neurons are located in the outer granular layer ( stratum nucleare externum ). From here the excitation is passed on to the inner granule cells of the retina. The bipolar nerve cells are the 2nd afferent neurons. Their neurites move to the multipolar nerve cells in the ganglion cell layer ( stratum ganglionare ) of the retina, which represent the 3rd neuronal level. The first two switchings take place within the retina. Additional lateral processing takes place through the amacrine cells and the horizontal cells of the inner granular layer ( Stratum nucleare internum ).

The long axons of the retinal ganglion cells initially run in the nerve fiber layer ( stratum neurofibrarum ) on the inner surface of the retina, collect and leave the eye in the optic disc ( discus nervi optici ). Together they now myelinated form the optic nerve ( Nervus opticus ) of an eye. This pulls from the eye socket in the optic canal through the skull to the optic nerve junction ( chiasma opticum ) at the base of the brain near the pituitary gland . In all vertebrates, the fibers change from the nasal (medial) half of the retina to the other side of the body; the proportion for the remaining of the temporal (lateral) retinal halves varies depending on the species. In amphibians such as frogs , these also cross completely, so that 100% of the fibers of an optic nerve can then be found on the opposite side ( contralateral ). In most bird species almost all fibers cross, in ungulates around 90%, in owls less than 70%, in cats mostly around 60%, in primates like humans it is around 53%, which is optimal for their binocular spatial perception .

The optic pathway continues from the optic chiasm as the optic cord to different brain regions. After entering the brain base, the majority of the fibers reach the corpus geniculatum laterale (CGL, lateral knee hump) of the diencephalon . This is where the first interconnection of the visual pathway outside the retina takes place on 4th neurons. From here axons move from projection paths to equilateral areas of the cerebral cortex . As the geniculocalcarinus tract, they form the ascending part of the visual radiation Radiatio optica ( also called Gratiolet visual radiation after Louis Pierre Gratiolet ). The fibers, which terminate above the calcarin sulcus, pull through the retrolentiform part of the internal capsule . The fibers, which end under the calcarin sulcus, pull forward and then down around the inferior cornu of the lateral ventricle (Meyer loop). They then pull backwards through the sublentiform part of the internal capsule into the occipital lobe . The fibers mainly end at 5th neurons in the primary visual cortex (area V1 // Brodmann area 17) in lamina IV (lamina granularis interna) of the isocortex .

About a tenth of the optic nerve fibers are not used for vision, but for unconscious processes. These fibers of the non-visual system of photosensitivity start from the optic tract and run directly, without prior interconnection in the corpus geniculatum laterale, as the retinohypothalamic tract on each side to the suprachiasmatic nucleus in the hypothalamus or adjacent (via the superior brachium colliculi ) to the prethalamic nucleus in the epithalamus of the diencephalon as well as to the neighboring superior colliculi in the roof of the midbrain ( tectum mesencephali ). These components have essential functions for the circadian rhythm synchronized by timers and also for accommodation and pupillary reflexes as well as adaptive movements of the eyes and head.

Individual evidence

  1. Benninghoff: Macroscopic and microscopic anatomy of humans, Vol. 3. Nervous system, skin and sensory organs . Urban and Schwarzenberg, Munich 1985, ISBN 3-541-00264-6 , p. 495ff.
  2. Patestas, Maria A .; Gartner, Leslie P .: A Textbook of Neuroanatomy . Second ed. Hoboken, New Jersey 2016, ISBN 978-1-118-67722-3 , pp. 360 .

See also