Hey folks! Welcome to another video in anatomy. In this section of the anatomy, we are going to discuss about the second cranial nerve, the optic nerve. Introduction. Vision is an important special sense in humans.
The visual pathway transmits the sense of vision from retina of the eyeball to the visual cortex of the brain. The components of the visual pathway include the following structures. The retina, optic nerve, optic chiasma, optic tract, lateral geniculate body, pre-tactile area, superior colliculi, optic radiation, and visual cortex. The retina receives visual impulses from the receptor cells, which includes the rods and cones, and transmit them to bipolar cells and later to ganglionic cells.
The axons of the ganglionic cells collectively form the optic nerve which initiates from the optic disc. The optic nerve is not a true nerve and is actually a part of the central nervous system, the diencephalon, which is myelinated by oligodendrocytes. Optic nerve is unique as it is covered by a layer of dura matter and leaves the orbit by passing through the optic canal. The components of the optic nerve are special somatic afferent fibers for vision, which arise from the retina, passes to the brain, and mediates the afferent limb of the pupillary light reflex, whereas parasympathetic fibers in the oculomotor nerve mediate the efferent limb.
At the optic chiasma, Fibers of each optic nerve decussate partially to form optic chiasma. The fibers from the nasal half of the retina cross to the opposite side, but those of the temporal halves do not cross. After decussation, the nerve fibers are called as optic tract.
Optic tract. Each optic tract contains fibers from the nasal half of the opposite side, called the contralateral nasal hemiretina and fibers from the temporal half of the same site or ipsilateral temporal hemiretina. The optic tract projects into the ipsilateral lateral geniculate body, pre-tactile area, and superior colliculi. The lateral geniculate body is a relay station of the optic tract and is structurally made of six-layered nucleus. Layer 1, 4 and 6 receive crossed fibers, while layers 2, 3 and 5 receive uncrossed fibers.
It receives fibers from the ipsilateral temporal hemiretina and the contralateral nasal hemiretina. It also receives input from layer 6 of the striate cortex, which goes to the Brodmann's area 17. It projects through the optic radiation or the geniculocalcorane tract. to layer 4 of the primary visual cortex located in the Brodmann's area 17. The pre-tactile area is present in the midbrain, in front of the tectum.
Superior colliculi are a pair of elevations in the dorsal aspect of the midbrain. Optic radiation arises from the geniculocalcarine tract. The visual cortex is situated in the occipital lobe and is located on the banks of the calcarine fissure.
The area concerned with the integration of vision is the Brodmann's area 17 around the visual cortex. The neighboring areas are 18 and 19 in our visual association areas. Here is the schematic of the visual pathway.
The axons of the ganglionic cells of the retina transmit the information to the optic nerve through the optic chiasma via the optic tracts and the information is relayed to the pre-tactile area, the lateral geniculate body and the superior colliculi. The lateral geniculate body is relayed via the optic radiation to the visual cortex, the area 17, 18 and 19. Common lesions associated with visual pathway. Lesions involving central part of the optic chiasma result in destruction of fibers from both the nasal halves of Retina causing bitemporal hemianalpia.
This may result due to compression of the optic chiasma by a pituitary tumor. Lesions involving lateral parts of the optic chiasma causes binasal hemianopia as the fibers from temporal halves of the retina are affected. Destruction of one optic nerve causes blindness of the affected eye.
A common example is retinobulbar neuritis. Lesions in the optic tract result in homonymous hemianopia, which means loss of both the right halves or left halves of the vision.