The oculomotor nerve is the III. Cranial nerve is called. It controls numerous eye movements.
What is the oculomotor nerve?
The oculomotor nerve (eye movement nerve) is one of the twelve paired cranial nerves. It forms the III. Cranial Nerve and is responsible for the innervation of four of the six external eye muscles. In addition, it moves two inner eye muscles and the eyelid elevator. Its work is primarily motor. However, there are also some parasympathetic parts in it. These become noticeable during accommodation. During this process, the control of the ciliary muscle takes place. Together with the abducens and trochlear nerves, the oculomotor nerve also moves the eyeball.
Anatomy and structure
The oculomotor nerve originates in the anterior segment of the midbrain. It exits this region of the body through the interpeduncular fossa. In doing so, it crosses the dura mater (hard meninges) at the sella turcica, also called the turcic saddle, and runs in the ventral direction along the lateral wall of the cavernous sinus. Through the superior orbital fissure, the oculomotor nerve enters the orbit. After crossing the annulus tendineus communis, which marks the origin of the eye muscles, the cranial nerve branches off into three branches. These are the inferior ramus, the superior somatomotor ramus, and the ciliary ganglion, which forms a general visceromotor branch. The inferior ramus supplies the inferior rectus muscle (straight inferior eye muscle), the medial rectus muscle (straight inferior eye muscle), and the inferior oblique muscle (oblique inferior eye muscle). The innervation area of the superior ramus is formed by the rectus superior muscle (straight superior eye muscle) and the levator palpebrae muscle. At the branch in the ganglion ciliare there is a connection to the postganglionic neuron. It takes care of the supply of the sphincter pupillae muscle and the ciliaris muscle (ciliary muscle). The oculomotor nerve is equipped with cranial nerve nuclei named nucleus nervi oculomotorii and nucleus accessorius nervi oculomotorii and nucleus Edinger-Westphal, respectively. The nucleus nervi oculomotorii forms the nucleus of the somatomotor fibers, whereas this is the case for the nucleus Edinger-Westphal for the general visceromotor fibers. The somatomotor fiber nucleus is found in the tegmentum of the midbrain (mesencephalon) at the colliculi superiores level. Each muscle supplied by the oculomotor nerve has its own subnucleus. However, the subnucleus of the levator palpebrae muscle is unpaired. For this reason, it is considered difficult to keep the other eye open when one eye is closed. On the posterior side of the nucleus nervi oculomotorii is the nucleus acessorius nervi oculomotorii.
Function and tasks
The tasks of the oculomotor nerve include supplying the eye muscles, which are important for the mobility of the eyeball. Thus, they allow the eyeball to be rotated in different directions. The muscle work is so precise that the image of the left and right eye are exactly superimposed. Regardless of the angle from which vision takes place, the same image is always fixed, which in turn ensures spatial vision. The eye muscles and thus the oculomotor nerve are also important for accommodation, i.e. the change between near and far vision. During accommodation, the parasympathetic part of the oculomotor nerve becomes active, controlling the ciliary muscle. Furthermore, it constricts the iris of the pupil through the sphincter muscle. This process is called miosis. The unpaired nucleus perlia nervus ocolumotorii is responsible for the special innervation of the ciliary muscle, which in turn enables accommodation of the eye.
Diseases
The oculomotor nerve can sometimes be affected by damage. One of the most common conditions is oculomotor nerve palsy, which is a paralysis of the eye movement nerve. This refers to a cranial nerve disorder that affects men and women to the same extent. Doctors distinguish between external and internal oculomotor palsy. Both unilateral and bilateral paralysis is possible. Likewise, other ocular paralyses can start at the same time at the eye muscles.Paralysis of the oculomotor nerve is caused by various factors. In most cases, it is due to circulatory disorders, aneurysms or tumors within the brain stem. In some cases, oculomotor nerve palsy is also a concomitant of other diseases. These include primarily Benedict’s syndrome, Weber’s syndrome, or Nothnagel’s syndrome. Furthermore, combined paralysis with the abducens nerve or trochlear nerve is possible. It is not uncommon for diabetics to suffer from paralysis of the oculomotor nerve. A significant symptom of oculomotor palsy is absolute pupillary rigidity. In addition, patients frequently squint and suffer from restricted eye movement or perceive double vision. Furthermore, the accommodation of the eye is restricted. If internal isolated oculomotor palsy occurs without involvement of the external eye muscles, physicians refer to it as ophthalmoplegia interna. Another typical sign of oculomotor nerve palsy is the low position of the eye on which the palsy occurs. There is a slight outward rotation of the eye. Some patients also adopt a head-restraint posture in order to maintain binocular single vision in this way. Treatment of oculomotor nerve damage is performed by a neurologist. The chances of recovery depend on the cause of the disease. For example, the prognosis is considered more favorable if the patient suffers from circulatory disorders. In contrast, aneurysms or tumors are expected to have an unfavorable course. In some cases, strabismus surgery is performed.