How do eyes move together?
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Since nobody is directly answering your question OP, unfortunately this is difficult to answer as a true ELI5 because it is so complex.
The extraocular eye movements are controlled by 6 muscles and 3 nerves. For the purposes of this answer, let’s simplify it into 4 muscles and 2 nerves (since the superior and inferior oblique muscles and the function of the trochlear nerve are more complex, but feel free to read more about these if you wish).
**For vertical eye movements:**
The superior rectus is responsible to bringing the eye up. The inferior rectus for bringing it down. Both of these are innervated by the 3rd cranial nerve, or the oculomotor nerve, on the same side. When you want to move your eyes up or down, signals will be generated in the left and right frontal eye fields of the frontal lobes. These areas project to the cerebellum, thalamus, and superior colliculus, and eventually these signals make it to the oculomotor nuclei. Because the signals go to both the right and left nuclei, the right and left nerves will fire and the eyes will move in the same direction (up or down).
**For horizontal eye movements:**
These are more complicated. The medial rectus brings the eye towards the nose, and is also controlled by the 3rd cranial nerve. The lateral rectus brings the eye away from the nose or towards the temple, and is controlled by the 6th cranial nerve, or the abducens nerve. So, to look to the right for example, you need your right eye to look to the temple (cranial nerve 6 on the right to fire), and you need your left eye to look to the nose (cranial nerve 3 on the left to fire). So, you decide to look to the right. From the left frontal eye field, the signal travels again to all the places we discussed for horizontal but then goes to the right PPRF (another spot in the brainstem). From there, it goes to the right cranial nerve 6 nucleus and this will make the right eye go right (towards the temple). It also will from there go to the left cranial nerve 3 nucleus, making the left eye go right (towards the nose)
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