Standing on all existing protocol is how I will be starting my post today. This should sit well with those who have been reading through my series on Cranial nerves, you know I do not know how to start a post, and I am still learning how to start one. If you haven't read through my post, you should take your time to read through it. Today, I will be discussing the fourth cranial nerve, which is the Trochlear Nerve. I will be looking at the originating site of the fourth cranial nerve, and the innovated structures.
If you read through the cranial nerve 3, then it will be easier to get the location of the fourth cranial nerve. The brain stem like I said in my previous post has the Cerebral Aqueduct in its middle, which is surrounded by Periaqueductal gray matter. Similar to the third cranial nerves, which are found at the sides of the Cerebral Aqueduct, the fourth cranial nerve is also found beside the Periaqueductal gray matter, but its nerve movement is different from the third cranial nerve. While the third Cranial Nerve will run between the Posterior Cerebral Artery and the Superior Cerebellar Artery, the fourth cranial nerve will move through the inferior colliculus crossing one another (which means that the nucleus of the right cranial nerve four will go to the left eye, and the nucleus of the left cranial nerve four will go the right eye). Running anteriorly through the sides of the mid-brain.
For easy reminder, the brief anatomical structure of the midbrain from my previous post.
The midbrain has in its center, the Cerebral Aqueduct which contains cerebrospinal fluid, superiorly connected to the third ventricle of the brain, and inferiorly connected to the fourth ventricle of the brain. Just beside the cerebral aqueduct, is the nucleus of the oculomotor nerve (cranial nerve III), as well as the nucleaus of the cranial nerve IV beside the periaqueductal gray matter. Away from the nerve nucleus, by the sides, are the Edinger-Westphal Nucleus, which are parasympathetic nucleus. Anterior to the cranial nerve III and IV is the Crux Cerebri which carries descending cortical spinal pathways, at the sides of the Edinger-Westphal Nucleus are the Medial Lemniscus which has the spinothalamic tract right above it (posteriorly). Posterior to the spinothalamic tract, are the superior colliculus (remember in my last post I mentioned that the Superior Colliculus is responsible for the control of reflexive eye movement). Interior to the medial lemniscus is the Red Nucleus. - source (but added a little more knowledge. Coudn't read through my post without addig to it).
It is important to know that the third cranial nerve moves through the Superior colliculus while the fourth cranial nerve moves through the inferior colliculus, exiting through the side of the midbrain under the posterior cerebral artery, and above the superior cerebellar artery. The nerve moves into the posterior aspect part of the Cavernous sinus, and goes inferiorly through the lateral wall of the Cavernous sinus, just like the third cranial nerve but below it.. Moving from the Cavernous sinus, the fourth cranial nerve moves into the superior orbital fissure and connects to the superior oblique and innervates the muscle which passes through the trochlea to the eyeball. The trochlea is a U-shaped cartilage which is attached to the nasal side of the orbital plate of the frontal bone., . The superior oblique muscle has its primary responsibility to be eyeball intorsion (internal rotation of the eyeball), other responsibility will be the depression and lateral rotation of the eyeball when it contracts, and abduction when it retracts. It is important to know that Cranial nerve 4 is the cranial nerve with the longest intracranial portion.
If there is a palsy or paralysis to the trochlea nerve, in cases of Trochlear nerve damage/Palsy which could be congenital or cranial injury, cases or aneurysm, Cavernous sinus thrombosis, it causes the superior oblique muscle stops to funtion. When the superior oblique isn't functioning properly, it causes weakness of depression, failure to laterally rotate the eyeball, and failure of intorsion. When these occur, it affects the binocular view of the eyes, since the eyes aren't aligned anymore when viewing objects, causing objects to be doubled. A condition known as Diplopia. To understand this, when a person has a troclear nerve palsy, and is asked to look downwards, the normal unaffected eyeball will look downwards, but the affected eyeball will remain upwards not going in the same direction with the unaffected eyeball, creating two visual fields. In cases where there isn't intorsion as a result of a damage 4th cranial nerve, the visuals in the affected visual field become tilted..
In a simplify mode of explanation, I hope you understand all the jargon I have been writing above with this. The troclear nerve nucleus is located at the level of the inferior colliculus, within the midbrain. Each nucleus crosses one another to feed the oppsite eyeballs (meaning that the left trochlear nerve necleus innervate the righ superior oblique, therby affecting the right eyeball, while the right trochlear nerve nucleus innervates the left superior oblique, affecting the left eyeball). Like i said, it supplies the superior oblique eye muscle, which depresses the eyeball and laterally rotates it when it contracts. The Trochlear nerve is a motor neuron which carries information from the brain to the eye muscles, using the general somatic effect. It is responsible for controlling eye movements which includes depression, lateral rotation and intorsion.
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