The visual pathway refers to special-somatosensitive fibers that run from the retina of the eye to the visual cortex of the brain. The complex structure of the visual pathway makes human vision possible.
What is the visual pathway?
The visual pathway is a component of the brain. Thus, all components originate in this region of the body. This includes the optic nerve (nervus opticus), which is also a part of the visual pathway. The neuronal circuitry of the optical system takes place via the visual pathway. Special somatosensitive fibers are guided from the retina towards the brain. The first link of the visual pathway is formed by the photoreceptor cells of the retina, which receive the incoming light stimuli. The cell bodies of the photoreceptor cells are located in the outer eye granule layer. They are considered the first neuron (nerve cell). From them, nerve impulses travel via the second neuron in the inner eye granule layer toward multipolar retinal neurons within the stratum ganglionare. From these ganglion cells, the third nerve circuit layer is established. With their long processes they form the optic nerve. The first switching of incoming nerve impulses already takes place within the retina.
Anatomy and structure
The human visual pathway has a complex structure. For example, it extends from the posterior poles of the eyes to the cortex of the cerebrum. The retinal ganglion cells, which combine to form the optic nerve, reach their exit in the orbit (eye socket). The optic nerve is then composed of two distinct fiber bundle portions. In the right eye, the outer (lateral) retinal portion is on the right side, while the nasal portion is on the left side. In the left eye, it is the other way around. The fiber bundles of the retinal nerve cells of the respective eye attach to each other and cross. A little later, their union occurs in a different combination. The branching point is called the optic chiasm. At this point, the fibers of the nasal retinal segments cross. Following the crossing, the course of the fibers of the corresponding retinal segments takes place within the tractus opticus. While the right tractus opticus carries the fibers of the right retinal halves, the left tractus opticus does so with the left halves. The crossed fibers of the right eye as well as the uncrossed fibers of the left eye form a union in the left tractus opticus. This corresponds to the right half of the face. In contrast, the crossed fibers of the left eye as well as the uncrossed fibers of the right eye form their union within the right tractus opticus, which corresponds to the left half of the face. Through the retinal sections, the human visual fields are reflected in an opposite arrangement. This means that the absorption of the right visual field portion of the eyes occurs on the left side of the retina. In contrast, the right retinal sections reflect the left halves of the visual field. The switching of right and left tractus opticus takes place in the midbrain. From there, the so-called visual radiation goes towards the cerebral cortex. Its end is located within the meningeal lobe in the visual center on the inner sides of both cerebral hemispheres.
Function and tasks
The visual pathway performs the function of transmitting visual impressions and signals from the eye to the brain. In this way, the perception of sensory impressions is made possible. Without the transmission of electrical signals to the cerebrum, humans would not be able to register the impressions they see. Furthermore, there is a coupling between the visual pathway and the sense of balance as well as the position reflexes. In the case of a deviation of an eye impression from the equilibrium organ, a compensation takes place through the position reflexes. For example, if a person is standing on a swaying ship, the swaying is perceived by the eyes and the equilibrium organ. By activating the corresponding muscles, the person can continue to stand firmly. The visual pathway is divided into three functional systems. These are color and shape vision (parvocellular system), motion vision (magnocellular system), and optomotor (coniocellular system).
Diseases
The visual pathway can be affected by various damages or diseases. In most cases, this results in excessive pressure on the visual pathway or there is an insufficient blood supply.Possible causes are hemorrhages, degenerative processes, injuries, inflammations, tumors, reduced blood flow or interruption of blood flow. Another possible cause is aneurysms, in which an artery is bulging or dilated. Damage to the visual pathway can cause visual field loss in affected individuals, depending on which area of the visual pathway is affected. If there is a lesion of the optic nerve that leads to its interruption, this causes unilateral blindness. Physicians then speak of amaurosis. The most common causes of this damage are optic neuritis or papilledema. A bilateral half visual field loss on the outer side of the face is seen in chiasm syndrome, also known as blinker phenomenon. It is most often caused by tumors that exert pressure on the optic nerve junction. Other conceivable causes are syphilis or multiple sclerosis. With a quick surgery there is a possibility to regress the visual field defects. Otherwise, there is a risk of further visual disturbances. A lateral compression of the chiasm, which is called heteronymous binasal hemianopsia by physicians, causes an equilateral hemianopsia. The reason for this is damage to the uncrossed nerve fibers. Usually, sclerosis of the internal carotid artery or bilateral aneurysm are responsible. In the case of ophthalmic migraine, flicker scotomas are possible and may be accompanied by headache, dizziness, flashes of light, nausea and vomiting. In some cases, sufferers also experience paralysis of the eye muscles. This is due to temporary circulatory disturbances.
