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Optic nerve and the visual pathway.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on the optic nerve. During this tutorial, we'll go over all of the structures seen in this image which are involved in the visual pathway. This image here is a superior transverse section of the brain and we can see structures such as the globe of the eye, the optic nerve and the optic chiasm here. The visual pathway transmits information interpreted from visual light from the eyes to the visual cortex in the brain. At the end of this tutorial, we'll go over some clinical notes relevant to the optic nerve and the visual pathway.
But before we have a look at those structures, let me first give you a bit of an overview of the main visual pathway. The visual pathway begins with light entering the ocular bulb and being processed by the retina. Information is then passed on from the retina by the optic nerve which travels to the optic chiasm. From the optic chiasm, the axons are now called the optic tract. The axons then synapse at the lateral geniculate nucleus. Axons from the lateral geniculate nucleus travel via the optic radiation to finally reach the visual cortex.
It is important to note that only ninety percent of the retinal axons synapse directly at the lateral geniculate nucleus. The other ten percent project to other subcortical nuclei mainly the superior colliculus. We will have a look at the superior colliculus later on but first let's look at the main pathway in more detail.
The first structure we're going to look at is the ocular bulbs. The ocular bulbs or globe of the eye is considered to be the eyeball without any structures within it such as the lens or the pupil. The ocular bulb consists of a wall enclosing a cavity of fluid. The wall has three layers – the sclera, the choroid and the retina. In this image, these colored areas in front of the ocular bulbs can be referred to as the visual field. The blue part can be considered the left visual field and the red part the right visual field.
The retina can also be divided into two with the lateral part closest to the temples called the temporal retina and the medial part closest to the nose called the nasal retina. So now let's have a look at which side of the visual field will travel to which part of the retina.
Let's start with the left eye. Light from the left visual field will travel to the nasal retina and light from the right visual field will travel to the temporal retina. Now looking at the right eye, we can see that light from the left visual field travels to the temporal retina and light from the right visual field travels to the nasal retina. The retina then processes this information into action potentials which then travel down axons to the brain.
In the illustrations of this tutorial, axons which carry information from the left visual field have been color-coded blue and axons from the right visual field have been color-coded red so that we can follow their pathway. The axons that project from the retina travel within the optic nerve. The optic nerve is a paired nerve that is also known as cranial nerve II. The optic nerve is ensheathed by three meningeal layers – the dura, the arachnoid and the pia mater. The optic nerve travels posteromedially towards the optic chiasm which is this crossover point here. So let's look at the optic chiasm in more detail.
The optic chiasm is the place where part of the optic nerves cross. As we can see in this image, the optic nerve axons from the nasal retina cross over to the other side so that all the information from the left visual field is now on the right side of the brain and all the information from the right visual field is now on the left side of the brain. After some of the axons cross at the optic chiasm, the uncrossed and crossed fibers continue as the optic tract. Each optic tract conveys information from the contralateral visual field. This simply means that the left optic tract conveys information from the right visual field and the right optic tract conveys information from the left visual field.
The retinal axons which have now traveled within the optic nerve, the optic chiasm and the optic tract synapse at the lateral geniculate nucleus. The lateral geniculate nucleus can also sometimes be referred to as the lateral geniculate body. This structure is a small oval projection from the thalamus on either side of the brain. We must remember that only 90% of retinal axons terminate in the lateral geniculate nucleus with the other ten percent terminating in other subcortical areas. If we look at this image of the midbrain, we can see the thalami here and the lateral geniculate nuclei here. The lateral geniculate nuclei are located lateral to the medial geniculate nuclei which are these structures seen here.
If we go back to the previous view, we can see the medial geniculate nucleus medial to the lateral geniculate nucleus here. However, the medial geniculate nucleus is not involved in the optic or visual pathway but is instead involved in the auditory pathway. Once the retinal axons have synapsed with the lateral geniculate nuclei axons, the lateral geniculate nuclei axons project to the primary visual cortex via the optic radiation. As we can see, the optic radiation curves from the lateral geniculate nucleus dorsomedially towards the primary visual cortex on either side of the brain. The primary visual cortex is located within the medial part of the occipital lobe of the brain and is also known as Brodmann area 17.
Like we mentioned earlier, the left primary visual cortex processes information from the right visual field whereas the right primary visual cortex processes information from the left visual field. Therefore, we can say that each primary visual cortex processes information from the contralateral visual field. We can also see this by the color coding in this image with the left visual field and the right primary visual cortex being blue and the right visual field and the left primary visual cortex being red. So now let's move on to look at the superior colliculus.
As I mentioned earlier, only ninety percent of the retinal axons synapse directly with the lateral geniculate nuclei axons. Most of the other retinal axons project to the superior colliculus instead. The two superior colliculi sit inferior to the thalamus and they are also part of the midbrain. The function of the superior colliculus is to assess in controlling eye movements. We can also see the superior colliculus here in this dorsal view of the midbrain. We can see that it sits medial to the lateral geniculate nuclei, the medial geniculate nuclei and inferior to the thalamus. It is also located superior to the inferior colliculus which is this structure here.
If we go back to the previous view, we can see the two inferior colliculi with the brainstem beneath it. It's important to note that the inferior colliculus is not involved in the visual pathway but instead is involved in the auditory pathway like the medial geniculate nucleus.
Now that we've had a look at the optic nerve and other structures involved in the visual pathway, we can go over some clinical notes relevant to these structures. Inflammation of the optic nerve is referred to as optic neuritis. Optic neuritis can cause blurry vision, a partial or complete loss of vision and pain when moving the eye. In children, there is usually bilateral involvement whereas in adults usually only one eye is affected. The most common aetiology of optic neuritis is multiple sclerosis or MS. It can also be caused by infection, autoimmune diseases and certain medications.
Optic nerve lesions usually cause permanent and potentially severe vision loss as well as an abnormal pupillary reflex. Damage to the optic nerve can be caused by strokes, trauma, infection and surgical complications. The part of the visual field that is lost depends on what part of the optic nerve has been damaged. If the optic nerve is damaged before the optic chiasm, vision is lost from the eye on the same side of the lesion as none of the fibers have crossed over yet. For example, if the right optic nerve is damage before the chiasm, the right eye will be affected as it will prevent these fibers here which are coming from the right eye from being transmitted to the brain.
However, if the optic nerve is damaged after the optic chiasm, visual field loss occurs on the opposite side of the lesion. For example, if the right optic nerve is damaged after the chiasm, the left visual field will be lost in both eyes. The blue fibers which represent the left visual field on both eyes are being prevented from being transmitted to the right side of the brain. It's important to remember that vision is lost in the opposite visual field and not the opposite eye. This specific loss of vision is called homonymous hemianopsia.
Damage at the optic chiasm typically causes visual loss laterally on each visual field. As we can see, these blue fibers here which represent the left visual field of the left eye are prevented from being transmitted. The red fibers here which represent the right visual field of the right eye are also prevented from being transmitted. This specific type of visual loss is called bitemporal hemianopsia. Bitemporal hemianopsia can also be caused by pituitary adenoma as it will compress on the optic chiasm due to its close proximity.
So that concludes our tutorial on the optic nerve. I hope you enjoyed it and thank you for listening.
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Now, good luck everyone, and I will see you next time.