Visual pathway and visual cortical areas

Diagram of visual pathway

The visual pathway is a multisynaptic, four-neuron sensitive pathway, belonging to the sensory pathways . The first three neurons of the visual pathway are located in the nervous part of the retina .

The first neurons are transformed into special light-sensitive cells (photoreceptors) and are of two types: rods and cones .

The second neurons are collectively referred to as the ganglion retinae.

The third neurons are collectively referred to as the optic ganglion , they have long axons that run through the optic nerve and on to the lateral geniculate corpus of the thalamus .

The fourth neurons are located in the lateral geniculate body and their axons run as the geniculocortical tract to the cortex of the occipital lobe.

The axons of some third neurons disconnect along the path and form additional connections and branches, controlling, for example, miosis and mydriasis , oculomotor functions, search movements, and others.

The main function of the visual pathway is the conversion of the image captured by the photoreceptor cells, which is made possible by its retinotopic arrangement in all its parts.

Course of the visual pathway

• The first neurons are special photosensitive cells , whose dendrites are transformed into photosensitive processes that convert light stimuli into nerve signals; they are located in the outermost layer of the retina (meaning away from the corpus vitreum) and their short axons point inward, where they connect to the dendrites of bipolar neurons.
• The second neurons are bipolar neurons, collectively called ganglion retinae . Their dendrites are connected to the axons of the photoreceptor cells, and the axons lead to the dendrites of the ganglion cells.
• The third neurons are ganglion cells, collectively known as the optic ganglion . Three types of ganglion cells have been described – parasol cells , midget cells and bistratified cells . Their axons run along the inner periphery of the bulb and converge in the optic disc , where they penetrate the bulb wall. After leaving the eye, they acquire a myelin sheath and form the optic nerve , which is covered on the surface by the meninges . The myelin sheath is formed by oligodendroglia (according to some authors, the optic nerve is not a peripheral nerve in the true sense of the word due to its myelin sheath). After passing through the optic canal, the two optic nerves join to form the optic chiasma , in which axons originating from the medial parts of the retina and some axons from the macula lutea cross. The axons that emerge from the lower nasal (medial) quadrants of the retina form the so-called Willebrand's elbow in the optic chiasm, because when they cross they slightly run into the contralateral optic nerve (therefore, in the case of a complete lesion of the optic nerve just before the chiasm, not only amaurosis of the ipsilateral eye occurs, but also a scotoma of the upper temporal quadrant of the contralateral visual field). The right and left optic tracts continue from the optic chiasm , the fibers of which lead to the corpus geniculatum laterale . There they end in six layers of gray matter, marked with numbers 1–6. Layers 1 and 2 form the so-called magnocellular cells , the remaining layers parvocellular cells . Between these layers are interlaminar spaces, collectively marked as the koniocellular system. Parasol ganglion cells (see above) terminate their axons in the magnocellular system, midget cells in the parvocellular system, and bistratified cells in the koniocellular system. Layers 2, 3, 5 receive ipsilateral fibers, layers 1, 4, 6 fibers of the contralateral side.Towards the lateral geniculate body, the optic tract divides into a thicker lateral root , which enters directly into the lateral geniculate body , and a weaker medial root , which goes into the superior colliculus and some of its fibers end in the superior tectal colliculus . Other fibers divide into the mesencephalic optic root , which enters the pretectal area , and the hypothalamic optic root , which ends in the supraoptic and suprachiasmatic nuclei of the hypothalamus.

• The fourth neurons are the cells of the corpus geniculatum laterale . Their axons form the tractus geniculocorticalis Gratioleti (radiatio optica). During its course, it is divided into an upper and lower part, with the lower part running in the temporal lobe and at the beginning of its course forming the so-called Meyer's loop. The upper part of the fibers runs in the parietal lobe. All the fibers therefore run backwards and medially to the cortex of the occipital lobe, where they end in area 17, whose neurons create the so-called cortical image of the external world . Some fibers also end in area 18 and area 19, in the 2nd, 3rd and especially 4th layers of the cerebral cortex. Information transmitted by the magnocellular system answers the question where − localization of the object and detection of movement, the parvocellular system is responsible for the fact what − structural analysis of the image.

Detours from the visual pathway

Pathway of the pupillary reflex

The pupillary reflex pathway continues to the mesencephalic optic root , which goes to the area pretectalis , where the pretectal nuclei are located, and is divided into the pathway for miosis and the pathway for mydriasis . The pathway for miosis continues to the parasympathetic nucleus oculomotorius accesorius (Edinger-Westphal nucleus). From there, it goes along the oculomotor nerve to the ciliary ganglion in the orbit and from there, after a switch, to the sphincter pupillae muscle . The pathway for mydriasis is led to the RF mesencephalon , from where it descends via the reticulospinal pathways to the lateral horns of the spinal segment C8 where the sympathetic ciliospinal center is located . From there, the fibers lead to the superior cervical sympathetic ganglion and, after a switch, further along the arteries, to the dilator pupillae muscle .

Path of accommodation

The accommodation pathway of the eye follows the same path as the pupillary reflex pathway. The connection from the visual pathway occurs in the nucleus interstitialis (Cajal's nucleus). From there it continues to the nucleus oculomotorius accesorius and further to the m. ciliaris .

Path of eye convergence

The eye convergence pathway is connected from the visual pathway in the nucleus interstitialis . It is further connected via the fasciculus longitudinalis medialis system to the nuclei of the oculomotor muscles , which ensure convergence.

Tectal visual circuit

The tectal visual circuit refers to the pathways that enable motor responses to visual stimuli. They branch off from the visual pathway into the superior tectal colliculus , where visual stimuli are processed and connected to the descending motor pathways: the tectospinal tract, the tectoreticulospinal tract, the tectonuclear tract, and the tectoreticulonuclear tract. They also connect to the tectocerebella tract for visual and proprioceptive coordination.

Functions of the visual pathway

The main function of the visual pathway is to transfer the image of the external world, captured by photoreceptor cells, to the cerebral cortex, which is made possible by a precise retinotopic arrangement along its entire length. Branches from the visual pathway then enable the control of reflexes such as miosis and mydriasis, and various eye movements and motor functions of the whole body. The branch to the hypothalamus affects vegetative functions and the control of circadian rhythms.

Visual pathway disorders

Scotoma

A scotoma is a loss of vision in the field of vision. The cause may be glaucoma, macular degeneration, optic neuritis, ischemia of the retina (retinopathy), or even disruption of the visual pathway itself in the brain.

Unilateral anaurosis (blindness)

Unilateral blindness can occur, for example, due to an extensive retinal lesion (embolism) or damage to most of the optic nerve fibers for a given eye. Amaurosis fugax is a term used to describe temporary blindness caused by vascular causes.

Visual agnosia

The cause of visual agnosia is damage to the visual association centers. The condition in which the patient does not recognize faces is called prosopagnosia , the condition in which they do not recognize objects is then called object visual agnosia.

Irritating phenomena

Irritating phenomena include, for example, phosphenes (sparkles), dysmorphopsia (distorted shapes of objects) or pseudohallucinations (ants in Parkinson's).

Links

Related articles

• Eye muscles
• Eye (histology)
• Eye (biophysics)

External links

• JANČÁLEK, Radim and Petr DUBOVÝ.  Fundamentals of Neuroscience in Dentistry  [online]. MEFANET, ©2011. Last revised 27.10.2011, [cited 26.11.2011]. < http://portal.med.muni.cz/clanek-560-zaklady-neuroved-v-zubnim-lekarstvi.html >.

References

• ČIHÁK, Radomír.  Anatomy 3.  2nd edition. Prague: Grada Publishing, 2004. 692 pp.  ISBN 978-80-247-1132-4 .
• AMBLER, Zdeněk.  Clinical Neurology.  2nd edition. Prague: Triton, 2008. 976 pp.  ISBN 978-80-7387-157-4 .