The conductive pathway of the visual analyzer

Light falling on the retina first passes through transparent light-refracting media of the eyeball: the cornea, aqueous humor of the anterior and posterior chambers, the lens, and the vitreous body. The pupil is in the path of the light beam. Under the influence of the muscles of the iris, the pupil sometimes narrows, sometimes expands. Light-refracting media (cornea, lens, etc.) direct the light beam to the most sensitive place of the retina, the place of best vision - the spot with its central pit. An important role in this is played by the lens, which, with the help of the ciliary muscle, can increase or decrease its curvature when seeing at a close or distant distance. This ability of the lens to change its curvature (accommodation) ensures that the light beam is always directed to the central pit of the retina, which is in line with the observed object. The direction of the eyeballs towards the object being viewed is ensured by the oculomotor muscles, which set the visual axes of the right and left eyes parallel when looking into the distance or bring them closer together (convergence) when looking at an object at close range.

Light that hits the retina penetrates into its deep layers and causes complex photochemical transformations of visual pigments there. As a result, a nerve impulse arises in the light-sensitive cells (rods and cones). The nerve impulse is then transmitted to the next neurons of the retina - bipolar cells (neurons), and from them - to the neurons of the ganglion layer, ganglion neurons. The processes of the ganglion neurons are directed towards the disk and form the optic nerve. The optic nerve, enveloped in its own sheath, exits the orbital cavity through the optic canal into the cranial cavity and forms the optic chiasm on the lower surface of the brain. Not all the fibers of the optic nerve cross, but only those that follow from the medial part of the retina facing the nose. Thus, the optic tract following the chiasm consists of nerve fibers of the ganglion cells of the lateral (temporal) part of the retina of the eyeball on its side and the medial (nasal) part of the retina of the eyeball on the other side. This is why, when the chiasm is damaged, the functions of conducting impulses from the medial parts of the retina of both eyes are lost, and when the optic tract is damaged, from the lateral part of the retina of the eye on the same side and the medial part of the other.

The nerve fibers in the optic tract go to the subcortical visual centers: the lateral geniculate body and the superior colliculus of the midbrain roof. In the lateral geniculate body, the fibers of the third neuron (ganglion cells) of the optic tract end and come into contact with the cells of the next neuron. The axons of these cells pass through the sublenticular part of the internal capsule, form the optic radiation (radiatio optica) and reach the area of the occipital lobe of the cortex near the calcarine groove, where the highest analysis of visual perceptions is carried out. Some of the axons of the ganglion cells do not end in the lateral geniculate body, but pass through it in transit and, as part of the handle, reach the superior colliculus. From the gray layer of the superior colliculus, impulses enter the nucleus of the oculomotor nerve and its accessory nucleus (Yakubovich's nucleus), from where the innervation of the oculomotor muscles, as well as the muscle that constricts the pupil and the ciliary muscle, is carried out. Along these fibers, in response to light stimulation, the pupil constricts (pupillary, pupillary reflex), and the eyeballs turn in the desired direction.