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The extraocular muscles are responsible for executing eye movements and are innervated by three cranial nerves. These nerves control the contractions of the muscles, which produce movement of the eyes within the orbit. The oculomotor nerve (CN III) is the primary nerve that innervates the majority of the extraocular muscles, including the superior rectus, inferior rectus, medial rectus, and inferior oblique. The trochlear nerve (CN IV) specifically innervates the superior oblique muscle, while the abducens nerve (CN VI) innervates the lateral rectus muscle. Damage to these nerves can result in paralysis of the respective muscles, affecting eye movement and orientation.
| Characteristics | Values |
|---|---|
| Number of nerves innervating the eye muscles | 3 |
| Names of the nerves | Oculomotor nerve (CN III), Trochlear nerve (CN IV), Abducens nerve (CN VI) |
| Nerve controlling horizontal eye movements | Lateral rectus |
| Nerve controlling vertical eye movements | Medial rectus |
| Nerve controlling elevation of the upper eyelid | Levator palpebrae superioris (innervated by oculomotor nerve) |
| Nerve controlling elevation of the eyeball | Superior rectus (innervated by oculomotor nerve) |
| Nerve controlling depression of the eyeball | Inferior rectus (innervated by oculomotor nerve) |
| Nerve controlling depression, abduction, and medial rotation of the eyeball | Superior oblique (innervated by trochlear nerve) |
| Nerve controlling abduction of the eyeball | Lateral rectus (innervated by abducens nerve) |
| Effect of nerve damage | Paralysis of respective muscles, altered resting gaze, double vision, and head tilt |
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What You'll Learn
The oculomotor nerve (CN III)
The oculomotor nerve has both somatic and autonomic nerve fibres. Somatic nerve fibres are bundled deep inside the nerve, and they control the muscles that are involved in voluntary movements. On the other hand, autonomic nerve fibres surround the somatic fibres on the outside of the nerve and are responsible for involuntary movements.
The superior division of the oculomotor nerve innervates the superior rectus and levator palpebrae superioris muscles. The superior rectus muscle is responsible for upward gaze, as its contraction elevates, adducts, and medially rotates the eye. The levator palpebrae superioris is the only muscle involved in raising the superior eyelid.
The inferior division of the oculomotor nerve innervates the medial rectus, inferior rectus, and inferior oblique muscles. The inferior rectus muscle is responsible for downward gaze, as its contraction depresses, adducts, and laterally rotates the eye. The inferior oblique muscle passes beneath the inferior rectus and is the only muscle that must be retrieved from the tenons, not directly from the scleral insertion point.
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The trochlear nerve (CN IV)
The trochlear nerve, also known as cranial nerve 4 or CN IV, is a motor nerve that controls eye movement. It is one of 12 sets of cranial nerves and is responsible for supplying movement information to the superior oblique muscle, which is connected to the eye near the top. This nerve allows the eye to look down and towards or away from the nose.
The trochlear nerve gets its name from the Latin word "trochleae," which means "pulley." This name is fitting because the superior oblique muscle goes through a sling of connective tissue that acts as a pulley. The trochlear nerve is the smallest cranial nerve by the number of axons, yet it has the longest intracranial course. It is the only nerve to exit from the posterior midbrain and have a dorsal exit from the brainstem.
The trochlear nerve starts in the brainstem and passes through four areas before reaching the superior oblique muscle near the top of the eyeball. These areas include the trochlear nucleus, which is the part of the nerve closest to the brain. The nerve fibres from the trochlear nucleus cross in the midbrain before they exit, innervating the contralateral superior oblique.
Damage to the trochlear nerve can result in trochlear nerve palsy, commonly presenting with vertical diplopia (double vision) and exacerbated when looking downwards and inwards. Patients may also develop a head tilt away from the affected side. Trochlear nerve palsy can be caused by microvascular damage from diabetes mellitus, hypertensive disease, congenital malformation, thrombophlebitis of the cavernous sinus, or raised intracranial pressure.
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The abducens nerve (CN VI)
The abducens nerve arises from the abducens nucleus in the pons of the brainstem. It exits the brainstem at the junction of the pons and the medulla. It then enters the subarachnoid space and pierces the dura mater to travel in an area known as Dorello’s canal. At the tip of the petrous temporal bone, the abducens nerve leaves Dorello’s canal and enters the cavernous sinus. It travels through the cavernous sinus and enters the bony orbit via the superior orbital fissure. Within the bony orbit, the abducens nerve terminates by innervating the lateral rectus muscle.
The abducens nerve is examined in conjunction with the oculomotor and trochlear nerves by testing the movements of the eye. The patient is asked to follow a point with their eyes without moving their head. The target is moved in an ‘H-shape’ and the patient is asked to report any blurring of vision or diplopia (double vision).
A common condition that affects CN VI is abducens nerve palsy or sixth nerve palsy. This is nerve damage that weakens the lateral rectus muscle and leads to muscle paralysis. This can occur due to diabetes, high blood pressure, head injuries, stroke, or many other causes. It is important to see a healthcare provider right away to determine the cause. Nerve damage is also common because this nerve stretches from the back to the front of your brain.
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The sympathetic nervous system
The SNS has a thoracolumbar outflow. Sympathetic nerves arise from the intermediolateral nucleus of the lateral grey column, beginning at the first thoracic vertebra of the vertebral column and extending to the second or third lumbar vertebra. Axons of these nerves leave the spinal cord through the anterior root and pass near the spinal ganglion, entering the anterior rami of the spinal nerves. However, they quickly separate through white rami connectors that connect to either the paravertebral or prevertebral ganglia extending alongside the spinal column. The SNS is composed of many pathways that perform a variety of functions on various organ systems. The preganglionic neurons of the SNS arise from the thoracic and lumbar regions of the spinal cord (T1 to L2), with cell bodies distributed in four regions of the gray matter in the spinal cord bilaterally and symmetrically.
Fibers from the SNS innervate tissues in almost every organ system, providing at least some regulation of functions such as pupil diameter, gut motility, and urinary system output and function. The SNS is perhaps best known for mediating the neuronal and hormonal stress response, commonly known as the fight-or-flight response, or the sympatho-adrenal response. In response to stress, the preganglionic sympathetic fibers that end in the adrenal medulla secrete acetylcholine, which activates the secretion of adrenaline (epinephrine) and, to a lesser extent, noradrenaline (norepinephrine). This response acts primarily on the cardiovascular system, increasing heart rate, force of contraction, and rate of conduction, resulting in increased cardiac output.
The clinical significance of the SNS is vast, as it affects many organ systems. For example, in the eye, sympathetic activation causes the radial muscle of the iris to contract, leading to mydriasis (pupil dilation) and allowing more light to enter. In the lungs, sympathetic activation causes bronchodilation and decreased pulmonary secretions, increasing airflow. In the stomach and intestines, sympathetic activation decreases motility and causes sphincter contraction, slowing digestion. Techniques to reduce SNS activity include exercise training, acupuncture, and reducing caffeine intake.
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The parasympathetic nervous system
The oculomotor nerve (CN III), which is one of the cranial nerves involved in the parasympathetic nervous system, plays a role in innervating the levator palpebrae superioris muscle, which is responsible for raising the superior eyelid.
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Frequently asked questions
Extraocular muscles are the muscles that execute eye movements. There are six muscles that work together to control eye position and movement.
The oculomotor nerve (CN III) innervates the majority of extraocular muscles.
The superior oblique muscle is innervated by the Trochlear nerve (CN IV).
The lateral rectus muscle is innervated by the Abducens nerve (CN VI).
Damage to the nerve that innervates an extraocular muscle will cause paralysis of that muscle, altering the resting gaze of the affected eye.
