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The eyelids are controlled by several muscles, including the orbicularis oculi, which has three distinct parts: palpebral, lacrimal, and orbital. The eyelids are also connected to the levator palpebrae superioris muscle, which is the only muscle involved in raising the superior eyelid. The oculomotor nerve (CNIII) innervates the levator palpebrae superioris, and the inferior division of CNIII innervates the inferior rectus muscle, causing lower eyelid retraction in downgaze. The inferior oblique muscle is also innervated by the inferior division of the oculomotor nerve. The maxillary nerve (V2) and its infraorbital and zygomaticofacial branches supply the lower eyelid. The zygomaticofacial nerve supplies the lateral lower eyelid, while the infraorbital nerve supplies the lower eyelid proper.
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What You'll Learn
The oculomotor nerve (CNIII)
The oculomotor nerve, also known as CNIII, is the third cranial nerve. It provides motor and parasympathetic innervation to some of the structures within the bony orbit. The oculomotor nerve exits the brainstem from between the midbrain and pons, passing between the posterior cerebral and superior cerebellar arteries. The nerve then pierces the dura mater and enters the lateral aspect of the cavernous sinus, where it receives sympathetic branches from the internal carotid plexus.
The oculomotor nerve innervates the main upper eyelid retractor, the levator palpebrae superiorus, via its superior branch. The inferior division of CNIII also innervates the inferior rectus muscle, which by extension via the capsulopalpebral fascia causes lower eyelid retraction in downgaze. The somatomotor fibres of the oculomotor nerve supply the medial, superior, and inferior rectus muscles, and the inferior oblique muscle. The nerve also carries parasympathetic fibres that supply the intrinsic muscles of the eye and transmits proprioception from the extraocular muscles.
The oculomotor nerve is involved in the pupillary light reflex, which can be tested by shining a bright light into one eye and observing for pupil constriction in the stimulated eye and the contralateral eye. This reflex provides information about oculomotor nerve involvement.
Congenital oculomotor palsy is a rare condition caused by intracranial pressure at the junction between the posterior communicating artery and the internal carotid artery, resulting in local compression of the oculomotor nerve.
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The orbicularis oculi muscle
The orbital portion of the orbicularis oculi primarily facilitates the voluntary, forceful closure of the eyelids. Medially, this section attaches to the anterior limb of the medial canthal tendon and the surrounding periosteum. Laterally, this part of the muscle connects to the lateral palpebral raphe. The orbital segment of the orbicularis oculi interdigitates with neighbouring muscles along its periphery, including the frontalis, making it integral to facial expression. The frontalis muscle, which runs from the upper forehead, attaches to the eyebrow skin. It is the antagonist of the orbicularis oculi and is used in looking up and increasing vision in low light or when objects are far away.
The palpebral part of the orbicularis oculi muscle is crucial for voluntary and involuntary blinking and is subdivided into the preseptal, pretarsal, and ciliary sections. The preseptal section of the orbicularis oculi muscle is split into deep and superficial heads. The deep head attaches to the fascia around the lacrimal sac and the posterior lacrimal crest, playing a role in the lacrimal drainage system. The superficial head attaches to the anterior limb of the medial canthal ligament. Laterally, the preseptal section connects to the Whitnall lateral orbital tubercle, situated deep to the lateral palpebral raphe. The pretarsal section is similar to the preseptal section, also dividing into a deep and a superficial head. The deep head, often referred to as the Horner muscle, attaches posteriorly to the medial canthal tendon and the posterior lacrimal crest. The pretarsal orbicularis is thought to be responsible for the spontaneous blink.
The marginal portion of the palpebral orbicularis oculi, also known as the muscle of Riolan or pars ciliaris, is adjacent to the eyelid margin. This muscle segment appears as a distinct coloured line along the length of the upper and lower eyelid margins. This feature, known as the grey line, is a critical surface anatomical landmark for the surgical repair of eyelid margin lacerations. The orbicularis oculi muscle interacts dynamically with the nasolacrimal duct system to facilitate tear drainage. The preseptal and pretarsal parts of the palpebral orbicularis oculi contract during blinking or eyelid closure, compressing the small channels that drain tears from the eye's surface to the lacrimal sac.
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The levator palpebrae superioris
Damage to the levator palpebrae superioris or its innervation can cause ptosis, or drooping of the eyelid. This can be more pronounced than ptosis caused by paralysis of the superior tarsal muscle. Lesions in CN III can also lead to ptosis as the levator palpebrae superioris requires stimulation from the oculomotor nerve to oppose the force of gravity.
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The superior tarsal muscle
The function of the superior tarsal muscle is not entirely clear, but it may act as an accessory muscle to raise the upper eyelid. Damage to certain elements of the sympathetic nervous system can inhibit the muscle, resulting in a drooping eyelid, a condition known as partial ptosis or blepharoptosis.
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The zygomaticofacial nerve
The nerve innervates the skin on the prominence of the cheek and the lateral lower eyelid. It penetrates the orbicularis oculi muscle, which has three distinct parts: palpebral, lacrimal, and orbital. The orbicularis oculi muscle plays a crucial role in blinking, as it contracts when the levator is temporarily inhibited during the blinking process.
The facial nerve, also known as the nerve of facial expression, is closely associated with the orbicularis oculi muscle. It supplies somatomotor innervation to this muscle, enabling the muscle contractions that result in various facial expressions. The facial nerve also contributes to the innervation of the lower eyelid, working in conjunction with the oculomotor nerve and sympathetic fibres to maintain the function and position of the eyelid.
In summary, the zygomaticofacial nerve is a vital component of the sensory and motor systems involved in facial expression and eyelid function. Its role in innervating the skin of the cheek and the lateral lower eyelid demonstrates the intricate network of nerves that contribute to our sensory perception and ability to convey emotion through facial movements.
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Frequently asked questions
The oculomotor nerve (CNIII) and the maxillary nerve (V2) are the nerves that innervate the lower eyelid.
The oculomotor nerve innervates the inferior rectus muscle, which causes lower eyelid retraction in downgaze. It also carries parasympathetic fibres that supply the intrinsic muscles of the eye.
The maxillary nerve supplies the lower eyelid as the zygomaticofacial and infraorbital nerves. The zygomaticofacial nerve supplies the lateral lower eyelid, while the infraorbital nerve supplies the lower eyelid proper.
The orbicularis oculi is the muscle that closes the eyelids. It is innervated by the facial nerve (CNVII).
