Visual pathway
11.1
Central nervous system
The visual pathway consists of four neurons which transmit visual impulses from the retina via the metathalamus and then subsequently to the primary visual area of the occipital lobe. Perikarya of the first, second, and third order visual neurons are situated in the retina. The main function of the visual pathway is to transmit and process visual information from photoreceptors to the cerebral cortex. Collaterals from the visual pathway to subcortical structures enable control of the pupillary light reflexes (mydriasis and miosis), reflex and coordinated eye movements, and also full-body motor control. The hypothalamic collateral has an important effect on autonomic functions and circadian rhythms. Neurons of the visual pathway
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1. First order neuron: rods and cones in the retina 2. Second order neuron: bipolar cells in the retina (cell bodies form the retinal ganglion) 3. Third order neuron: retinal ganglion cells in the retina (cell bodies form the optic ganglion) 4. Fourth order neuron: located in the lateral geniculate nuclei
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4
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Course of the visual pathway
• 1 The first, second, and third order neurons are situated in the optic part of the retina • 2 The visual pathway extends from the retina as the optic nerve • 3 The right and left optic nerve fibers are partially crossed after passing via the optic canal;
the optic chiasma is formed by a decussation of axons from the medial parts of the retina (temporal parts of the visual field) and subdivision of axons from the macula 4 The pathway continues as the optic tract (tractus opticus) 5 A majority of the visual pathway enters the lateral geniculate body of the metathalamus – lateral root (radix lateralis) 6 The pathway is interpolated at the fourth order neuron 7 Passes as the optic radiation and terminates in the primary visual area (BA 17)
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Visual pathway collaterals On the way to the lateral geniculate body the optic tract ramifies into: 8 Medial root (radix medialis) passes via the brachium of the superior colliculus into the layers of the superior colliculus (of the tectum of the midbrain) – axons terminate in the pretectal nuclei responsible for pupillary reflex controlling 9 Mesencephalic optic root (radix optica mesencephalica) – enters the pretectal nuclei (in the pretectal area) 10 Hypothalamic optic root (radix optica hypothalamica) – enters the suprachiasmatic and paraventricular nucleus of the hypothalamus
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1 2 3 4 6 7
8
Course of the tract
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Partial crossing of the left and right visual tract
The blind spot (optic disc) of the retina is clinically termed as the optic nerve papilla (papilla nervi optici) and represents a part of the retina which is insensitive to light. It is a site where the retinal nerve fibers leave the eye and become a part of the optic nerve. The first, second, and third order neurons are located in the retina. They are interconnected by interneurons termed amacrine and horizontal cells. These cells are involved in pre-processing of visual information at the level of the retina. The lens, from the physical point of view, is a converging lens which inverts all objects in the visual field. This means that the medial part of the retina represents a reflection of the temporal/lateral half of the visual field and conversely, the lateral part of the retina is a reflection of the nasal/medial half of the visual field. The collateral of the visual pathway into the hypothalamus affects the suprachiasmatic and paraventricular nucleus, so-called “biological clock”. Intensity of daylight stimulates activity of the hypothalamic nuclei, and in this way the production and release of hormones, the autonomic nervous system and production of melatonin are stimulated or inhibited. Consensual pupillary response is caused by the decussation of the pupillary reflex tracts. Light directed into one eye elicits a pupil constriction of the contralateral eye. The optic radiation of Gratiolet (fibrae geniculocalcarinae) also contains fibers passing from the cerebral cortex to the lateral geniculate nucleus. This feedback enables to filter out impulses coming from the retina. The optic radiation splits into two parts: the inferior part (loop of Meyer-Archambault) contains fibers from the inferior part of the retina (thus from the superior part of the visual field). This loop passes through the temporal lobe forming a loop around the inferior horn of the lateral ventricle. The superior part (loop of Baum) contains fibers from the superior part of the retina (thus from the inferior part of the visual field) and passes straight through the parietal lobe into the occipital lobe.