The Eyes – Anatomy, Nerve and Blood Supply, Functions

The eyes are a set of sensory organs that play a crucial role in the visual system. The eyes are responsible for detecting light that enters the eyes. Then, the light gets converted into an image in the brain. The sensory and motor innervation of the eyes originate from six paired cranial nerves. These nerves work in sync to manifest movements, reflexes, and vision.

Structure and Function

Six cranial nerves innervate motor, sensory, and autonomic structures in the eyes. The six cranial nerves are the optic nerve (CN II), oculomotor nerve (CN III), trochlear nerve (CN IV), trigeminal nerve (CN V), abducens nerve (CN VI), and facial nerve (CN VII). The oculomotor nerve and the trochlear nerve originate in the midbrain. As for the trigeminal nerve, abducens nerve, and facial nerve, they originate from the pons. Interestingly, the optic nerve arises from the optic disc in the eye then enters the brain as opposed to the other cranial nerve that originates in the brain and then exits peripherally.

• Optic Nerve (CN II) – The function of the optic nerve is purely sensory in the eyes. The optic nerve senses the incoming light and image displayed on the retina. The optic nerve then transmits this image into the cerebral cortex. The optic nerve also works in sync with the oculomotor nerve to change pupil size. The optic nerve conducts the afferent impulse to the brain. Then the oculomotor nerve will constrict the pupils if the light is bright, and it will allow pupil dilation when light is dim.[rx]
• Oculomotor Nerve (CN III) – The oculomotor nerve is responsible for innervating the major of the extraocular muscles of the eyes. The oculomotor nerve provides motor innervation to the superior rectus muscle, medial rectus muscle, inferior rectus muscle, inferior oblique muscle, levator palpebrae superioris muscle, ciliary muscle, and the sphincter muscle. The action of the superior rectus muscle is to rotate the eye superiorly. The medial rectus muscle functions to adduct the eye. The inferior rectus muscle rotates the eye inferiorly when it contracts. The inferior oblique muscle’s role is unique because it causes torques the eye downward and laterally when it contracts. The levator palpebrae superioris muscle is responsible for elevating the eyelid during eye-opening. The iris sphincter muscle is part of the autonomic nervous system, but specifically the parasympathetic nervous system. The action of the iris sphincter muscle is to constrict the pupils (miosis). Lastly, the contraction of the ciliary muscle allows for the accommodation of the eyes.[rx]
• Trochlear Nerve (CN IV) – The trochlear nerve also contributes to the motor function of the eyes, but it only innervates one muscle that attaches to the eyes. The muscle that receives its motor innervation from the trochlear nerve is the superior oblique muscle. This muscle’s action is unique due to its attachment. When the superior oblique muscle contracts alone, it causes the eye to torque inward and inferiorly.[rx]
• Trigeminal Nerve (CN V) – The trigeminal nerve has three main branches. The three branches are the ophthalmic branch (CN V1), maxillary branch (CN V2), and mandibular branch (CN V3). Only the ophthalmic branch of the trigeminal nerve innervates the eye. The ophthalmic branch of the trigeminal nerve provides sensory innervation to the eye. The ophthalmic branch of the trigeminal nerve works as the afferent part of the corneal and lacrimation reflex. The facial nerve is the efferent part of the corneal and lacrimation reflex.[rx]
• Abducens Nerve (CN VI) – The abducens nerve innerves only one muscle in the eye. This muscle is the lateral rectus muscle. When this muscle contracts, it causes the eye to abduct.[rx]
• Facial Nerve (CN VII) – The facial nerve provides innervation to the muscles of facial expression, salvation, the taste of the two-thirds anterior portion of the tongue, and auditory volume modulation. But in the eyes, the facial nerve is responsible for eye closure and blinking by the motor innervation of the orbicularis oculi muscle. The corneal and lacrimation reflex is a result of the sensory afferent input from the ophthalmic branch of the trigeminal nerve and the efferent output from the facial nerve.[rx]
• Sympathetic Nervous System – The action of pupillary dilation is called mydriasis and occurs via the sympathetic nervous system. The sympathetic nervous system pathway starts at the first neuron in the hypothalamus. The first neuron spans from the hypothalamus to the ciliospinal center Budge (C8-T2). Then the second neuron exits at the level of T1 and travels to the superior cervical ganglion. The path of the second neuron travels along the sympathetic chain near the apex of the lung and the subclavian vessels. Then the third neuron travels in the sympathetic plexus along the internal carotid artery. This third neuron travels through the cavernous sinus and arrives in the orbit and becomes the long ciliary nerve. The long ciliary nerve innervates the pupillary dilator muscles. These sympathetic nerve fibers also have a minor function in eyelid retraction and sweat glands in the forehead.[rx]
• Parasympathetic nervous system – The parasympathetic nervous system causes pupillary constriction through a process called miosis. The parasympathetic nerve fibers start at the first neuron called the Edinger-Westphal nucleus. This neuron’s nerve fibers span to the ciliary ganglion by traveling on the oculomotor nerve. After the ciliary ganglion, the second neuron is from the short ciliary nerves to the sphincter pupillae muscles. When the sphincter pupillae muscle contract, it will cause the constriction of the pupils.[rx]

Blood Supply and Lymphatics

Arterial Blood Supply

The blood supply to the nerves that innervate the eyes can subdivide into the intracranial and extracranial blood supply.

Optic Nerve

Intracranial

• Anterior cerebral artery
• Anterior communicating artery

• Internal carotid artery

  • Superior hypophyseal artery
  • Anterior choroidal artery

• Posterior cerebral artery

  • Anterior choroidal artery
  • Calcarine artery
  • Posterior temporal artery
  • Parieto-occipital arteries

• Middle cerebral artery

  • Occipital artery
• Basilar artery
• Posterior communicating artery

Extracranial

• Central retinal artery

• Ophthalmic artery

  • Long and Short posterior ciliary arteries

Oculomotor Nerve

Intracranial

• Posterior cerebral artery

  • Thalamoperforating arteries
  • Collicular artery
  • Mesencephalic perforating arteries
  • Diencephalic perforating arteries

• Posterior communicating artery

  • Thalamoperforating arteries
• Superior cerebellar artery
• Basilar artery

• Internal carotid artery

  • Inferolateral trunk
  • Meningohypophyseal trunk

Extracranial

• Internal carotid artery
• Ophthalmic artery

Trochlear Nerve

Intracranial 

• Superior cerebellar artery

  • Vermian artery
  • Paravermian artery

• Internal carotid artery

  • Inferolateral trunk
  • Meningohypophyseal trunk

• Posterior cerebral artery

  • Collicular artery

Extracranial  

• Internal carotid artery

  • Posterior ethmoidal artery
• Ophthalmic artery

Ophthalmic Branch of Trigeminal Nerve

Intracranial

• Basilar artery

  • Superior cerebellar artery
  • Posterolateral artery
  • Superolateral artery
  • Inferolateral artery
  • Anterior inferior cerebellar artery
  • Trigeminocerebellar artery

• Internal carotid artery

  • Inferolateral trunk
  • Meningohypophyseal trunk

• External carotid artery

  • Middle meningeal artery
  • Ascending pharyngeal artery

Extracranial

• Internal carotid artery

  • Anteromedial branch of inferolateral trunk

• Ophthalmic artery

  • Lacrimal artery
  • Supraorbital artery
  • Ethmoidal arteries

Abducens Nerve

Intracranial

• Basilar artery

  • Anterolateral arteries
  • Pontomedullary artery
  • Anterior inferior cerebellar artery

• Internal carotid artery

  • Inferolateral trunk
  • Meningohypophyseal trunk

Extracranial

• Internal carotid artery
• Ophthalmic artery

Facial Nerve

Intracranial

• Basilar artery

  • Anterior inferior cerebellar artery
  • Labyrinthine artery

• External carotid artery

  • Petrosal branch of the middle meningeal artery
  • Stylomastoid artery
  • Tympanic arteries

Extracranial 

• External carotid artery

  • Posterior auricular artery
  • Occipital artery
  • Superficial temporal artery
  • Facial artery
  • Maxillary artery

Venous Blood Drainage

The venous blood drainage of the structures in the brain, such as the cranial nerves, drains into the dural sinuses. The dural sinuses form from the folding of the meninges. These dural sinuses are similar to veins, but they lack valves. The venous blood in the dural sinuses drains towards the internal jugular veins. Once the venous blood reaches the internal jugular veins, it will then converge with the subclavian vein to form the brachiocephalic vein. The brachiocephalic vein from both sides of the body will join into the superior vena cava. Finally, the venous blood in the superior vena cava will drain back to the heart.[8]

Lymphatics

The brain was once thought to lack a lymphatic system, but recent studies have shown that there is a lymphatic system in the brain. The lymphatic drain for the brain is embedded in the dura mater layer of the meninges. The lymph fluid from the dura mater drains into the cervical lymph nodes and then back to the central circulation via the right lymphatic duct (right side of the head) and the thoracic duct (left side of the head).

Nerves

There are six cranial nerves and nerve fibers from the autonomic nervous system.

• Optic nerve

• Oculomotor nerve

  • Short ciliary nerve (parasympathetic nervous system)
• Trochlear nerve
• Trigeminal nerve (ophthalmic branch)
• Abducens nerve
• Facial nerve
• Long ciliary nerve (sympathetic nervous system)

Muscles

The muscles that act on and around the eyes receive innervation from four cranial nerves and the sympathetic nervous system.

Oculomotor Nerve 

• Superior rectus muscle
• Inferior rectus muscle
• Medial rectus muscle
• Inferior oblique muscle
• Levator palpebrae superioris muscle
• Sphincter pupillae muscle
• Ciliary muscle

Trochlear Nerve 

• Superior oblique muscle

Abducens Nerve 

• Lateral rectus muscle

Facial Nerve 

• Orbicularis Oculi muscle

Sympathetic Nervous System

• Pupillary dilator muscles

• https://www.ncbi.nlm.nih.gov/books/NBK382/
• https://www.ncbi.nlm.nih.gov/books/NBK556051/
• https://www.ncbi.nlm.nih.gov/books/NBK482152/
• https://www.ncbi.nlm.nih.gov/books/NBK470353/
• https://www.ncbi.nlm.nih.gov/books/NBK55977/
• https://pubmed.ncbi.nlm.nih.gov/24272785/
• https://www.ncbi.nlm.nih.gov/books/NBK507907/
• https://ncbi.nlm.nih.gov/books/NBK549919/
• https://en.wikipedia.org/wiki/Olfactory_nerve
• https://en.wikipedia.org/wiki/Cranial_nerves