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“Please examine this patient’s cranial nerves.”
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- Neurological 0%
The extraocular muscles that control the position of the globe within the orbit are the third, fourth and sixth cranial nerves – also known as the oculomotor, trochlear and abducens nerves respectively.
The oculomotor nerve controls by some margin the greatest number of extraocular muscles, and thus a palsy of this nerve is the most clinically apparent: The eye will be deviated downwards and outwards, and will exhibit a marked ptosis. It is likely that you (or the patient) will have to hold the eye open in order to appreciate the gaze deviation.
In addition to gaze deviation and ptosis, the pupil may also display dilatation which may cause the symptom of glare in bright light and difficulty in accommodation. This arises because parasympathetic innervation to the pupil joins the third nerve. As a consequence compressive lesions affecting the third nerve will cause pupillary dilatation whereas medical causes (such as a mononeuritis associated with diabetes) will not.
The path of the oculomotor nerve starts either side of the midbrain tegmentum before passing ventrally through the red nucleus towards the cerebral peduncle. The nerve then passes through the subarachnoid space. This is of pertinence because a Berry aneurysm at the junction of the internal carotid artery and the posterior communicating artery is an important cause of a third nerve palsy.
Subsequently the nerve runs through the lateral wall of the cavernous sinus – another space in which pathology may cause a lateralising sign in the eye. Finally the oculomotor nerve enters the orbit through the superior orbital fissure and branches to innervate the following muscles:
- Levator palpebrae;
- Superior rectus;
- Inferior rectus;
- Medial rectus;
- Inferior oblique;
- Ciliary muscle;
- Iris sphincter.
- Review / Skip
Question 1 of 4
1. Question1 point(s)Category: Neurological
What is the name of the IIIrd cranial nerve?
Question 2 of 4
2. Question1 point(s)Category: Neurological
Which of the following muscles does the oculomotor nerve supply? Select all that apply.CorrectIncorrect
Question 3 of 4
3. Question1 point(s)Category: Neurological
The IIIrd nerve passes through the subarachnoid space. True or false?CorrectIncorrect
Question 4 of 4
4. Question1 point(s)Category: Neurological
The IIIrd cranial nerve passes through the cavernous sinus. True or false?CorrectIncorrect