Introduction

The human body is controlled in movement and sensation by an advanced network system of nerves. There are twelve pairs of nerves known as the cranial nerves, which originate directly from the brain and brainstem regions. These cranial nerves serve a special purpose in sight, hearing, gustation and facial and other muscular movements. It is important to study them to understand the interactions between the brain and the rest of the body.¹

In particular, the oculomotor nerve (Cranial Nerve III) is of significance cranialas the majority of the eye muscles are controlled by it. This nerve enables individuals to look in various directions and is vulnerable to being affected by multiple diseases and disorders. Examples of these include inflammatory or infectious diseases, vascular disorders and congenital malformations.² Additionally, it is essential for the pupillary reflex - the process where the pupil size changes as a reaction to the light intensity. 

Anatomical structure of the oculomotor nerve 

As previously mentioned, the oculomotor nerve (Cranial Nerve III) is responsible for eye movements and the response to light stimuli. To understand this function, it is helpful to look at the location and where the oculomotor nerve travels in the body. 

Origins and pathways 

The oculomotor nucleus, which regulates muscular action, and the Edinger-Westphal nucleus, which regulates automatic (parasympathetic) processes including pupil size, are two significant locations deep within the brain. They both reside in the midbrain section, where the oculomotor nerve starts its journey.³ The cavernous sinus, a blood-filled region encircled by other cranial nerves, is where the nerve passes forward from the midbrain.⁴ It then passes through an opening named the superior orbital fissure to enter the orbit, the bony chamber that contains the eye.⁵ This is located in the superolateral area of the cavernous sinus and it is the largest and most superior compared to the cranial nerves.² 

Target muscles controlled by the oculomotor nerve

Specifically, the oculomotor nerve stimulates these seven muscles mentioned below: 

• Levator palpebrae superioris muscle – responsible for upper eyelid elevation 
• Superior rectus muscle – elevates and slightly intorts the globe, facilitating upward gaze
• Medial rectus muscle – adducts the eye and then directs the eye medially toward the nose
• Inferior rectus muscle – enables the downward gaze by rotating the eye forwards
• Inferior oblique muscle – elevates and extorts the globe when the eye is in an adducted position
• Ciliary muscle – changes the curvature shape of the lens to adjust to near vision objects
• Sphincter pupillae muscle – constricts the pupil, reducing pupillary diameter in response to light or near-focus⁶ 

Functions of the oculomotor nerve 

Eye Movements

The majority of the muscles that enable us to move our eyes in various directions are controlled by the oculomotor nerve.  In particular, it regulates the muscles that move the eye upward (the superior rectus), downward (the inferior rectus), inward (the medial rectus) towards the nose, and upward (the inferior oblique) to help rotate the eye. Together, these muscles enable us to gaze around with ease and accuracy, whether individuals are looking sideways, reading a page or seeing a moving object.

Additionally, there is strong coordination between the oculomotor nerve and two other cranial nerves: Cranial Nerve VI named the abducens and Cranial Nerve IV named the trochlear. The movement of both eyes towards the same direction (the conjugate gaze), is conducted successfully by the cooperation of these three cranial nerves.⁷ Clear eyesight and depth perception depend on this cooperation. Double vision or trouble focusing may result from misaligned eye movements caused by injury to the oculomotor nerve,⁸ which may significantly impair normal everyday activities such as reading and driving.

Pupillary Reflexes

In addition to assisting with the eye movement, the oculomotor nerve is essential for the parasympathetic processes that preserve and enhance eyesight. Axons of the parasympathetic neurons send these electrical signals along the oculomotor nerve to the post ganglion nerve fibres of the ciliary ganglion. Afterwards, ciliary nerves stimulate the action of the pupillary sphincter muscle which then results in pupillary constriction. This is the light reflex.⁹ Reducing the quantity of the light reaching the sensitive part of the eye named the retina, this protects the eye from harm and increases clarity in brightly illuminated environments. An example of this would be stepping outside on a sunny day from a dark room, this reaction is essential in adapting in changing light conditions. 

Eyelid Elevation

The accommodation reflex is essential for clear near vision, allowing the eyes to adjust focus when viewing close objects. This process involves coordinated muscle movements: the medial rectus muscles contract while the lateral recti relax, enabling both eyes to turn inward and maintain focus. At the same time, the pupils constrict to improve depth of field, while the ciliary muscles contract to thicken the lens for sharper vision. These adjustments are controlled by the oculomotor and Edinger-Westphal nuclei, which process signals from the optic nerve. Through the ciliary ganglion and short ciliary nerves, the lens changes shape for close-up tasks.¹⁰

Oculomotor nerve palsy

Common oculomotor nerve disorders impact eye function and vision significantly. Oculomotor nerve palsy is one of the more well known disorders associated with Cranial nerve III dysfunction. The effect of this disorder can vary, withthe condition thought to be caused by pressure being applied on the nerve or when the nerve does not receive much blood. The term ‘palsy’ refers to paralysis. Due to this inadequate blood flow this results in paralysis.  Serious conditions may occur due to the compression of the nerve such as aneurysms and the herniation of the brain. In small blood vessels which supply the nerve or the larger blood vessels in the brain stem, experiencing inadequate blood flow is more common. This could be caused by high blood pressure, disorders which affect any blood vessels and diabetes . These disorders include strokes, transient ischemic attack and vasculitis. Individuals may enter a coma and potential brain death can occur.¹¹

The symptoms and diagnosis

Those with oculomotor nerve palsy may experience double vision when glancing in a certain direction, eyelid drooping and pupil dilation. The affected eye rests downward and outward i.e. the eye cannot move upwards or downwards. Whereas, the unaffected eye remains with normal function which contributes to the double vision. As the oculomotor nerve affects the elevation of the eyelids in normal function, this causes droopy eyelids in oculomotor nerve palsy. A normal response to light would not occur as pupil may not narrow in bright light as effectively. The diagnosis of the third Cranial Nerve disorders can be determined by the neurological examination, magnetic resonance imaging and computed tomography.¹¹

Management 

The management, recovery and rehabilitation of the oculomotor nerve palsy would rely on determining the cause of the disorder (etiology). Treatment would be directed towards certain risk factors such as hypertension and diabetes. Emergency procedures such as coiling and clipping can be conducted for the treatment of aneurysms. Completely affected palsies and posttraumatic palsies have a lower likelihood of improved recovery.  Diplopia (double vision) treatment may include specifically customized contact lens or spectacles. Surgical intervention is typically needed when the diplopia becomes static and or does not improve after six months of appropriate treatment.¹²

Summary 

In conclusion, the oculomotor nerve (Cranial Nerve III) is a nerve responsible for the eye movements, adjusting to light as a pupillary reflex and lifting the upper eyelid. It is one of twelve cranial nerves which originate from the brain and the brainstem. Additionally, Cranial Nerve III originates in the midbrain region. Specifically, the oculomotor nucleus and the Edinger-Westphal nucleus. The oculomotor nucleus is responsible for motor control and the Edinger-westphal nucleus is responsible for parasympathetic control. This nerve travels through the cavernous sinus and into the eye socket where this connects to various muscles for movement. Furthermore, cranial nerves IV and VI work together to perform coordinated movements to ensure both eyes would move together. It is also responsible for accommodation reflexes which help the eye focus on near objects.

Serious disorders such as oculomotor nerve palsy, may arise as a result of inadequate blood flow to cranial nerve III. One cause of this is trauma. Symptoms include double vision, drooping eyelids and abnormal pupil response to light brightness. Diagnosis involves neurological examinations and imaging. Treatments depend upon the causes, the severity and any underlying conditions. Management may include specific spectacles or if severe, surgery would be recommended to fix diplopia.