Anatomy - Nerves of the eye

Nerves of the eye
There are six main cranial nerves that innervate the eye and its accessory structures. Most of these nerves enter the eye through the orbital cone while others reach the accessory structure directly.


Figure 1: Lateral view of the nerves of the eyeball. Nerves are in the green boxes.
Source: Evans H.E. and Lahunta A. (2010) Guide to the dissection of the dog, 7th edn., Missouri: Saunders Elsevier.
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Figure 2: Dorsal view of nerves of the eye displaying oculomotor, trochlear, trigeminal (ophthalmic branch) & abducent nerves. These nerves are in the green boxes. 
Source: Evans H.E. and Lahunta A. (2012) Miller's anatomy of the dog, 4th edn., Missouri: Saunders Elsevier.


Figure 3: Dorsal view of the superficial distribution of nerves in the eye of a dog. The main nerves are in the green boxes while the those in yellow boxes are ganglions.
Source: Evans H.E. and Lahunta A. (2012) Miller's anatomy of the dog, 4th edn., Missouri: Saunders Elsevier.

Main nerves
1.     Optic nerve (II)
2.     Oculomotor nerve (III)
3.     Trochlear nerve (IV)
4.     Trigeminal nerve (V) 
1.     Ophthalmic nerve
2.     Maxillary nerve
5.     Abducent nerve (VI)
6.     Auriculo-palpebral branch of facial nerve (VII)

 
Figure 4: Cranial nerve supply of the eye. Direction of signal transmission is indicated by arrows.
Source: Maggs D.J., Miller P.E. and Ofri R. (2013) Slatter's fundamentals of veterinary ophthalmology, 5th edn., Missouri: Elsevier.

Table 1: Summary of optic nerve, oculomotor nerve and trochlear nerve supply to the eye and adnexa. 
Nerve
Optic n.
Oculomotor n.
Trochlear n.
Anatomy
·      Cranial nerve II
·      Axons of retinal ganglion cells (RGCs)
·      RGC cell bodies lie in ganglion cell layer of inner retina
·     Cranial nerve III
·     Cranial nerve IV
Origin
·      Midbrain
·     Nucleus lies in brainstem
·     Leaves brainstem on ventromedial surface & passes ventral to optic tracts
·     Through cavernous sinus
·     Leaves brainstem on dorsal surface
·     Runs lateral to tentorium cerebelli
Exit from skull
·      Optic foramen towards optic chiasm
·     Enters orbit via orbital fissure

Branching

·     Dorsal  rami
·     Ventral rami
·     Ciliary ganglion: preganglion parasympathetic fibers synapse

Innervation

·      Dorsal rectus m.
·      Medial rectus m.
·      Ventral rectus m.
·      Ventral oblique m.
·      Levator palpebral m.
·      Part of retractor bulbi
·      Parasympathetic innervation :
·         Iris sphincter
·         Ciliary m.
·     Dorsal oblique m.
Functions
·      Relays visual signal from retina to central nervous system (CNS)Sensory function
·      Innervate extraocular muscles
·      Innervate  dorsal oblique m.



































Table 2: Summary of trigeminal nerve, abducent nerve and facial nerve supply to the eye and adnexa
Nerve
Trigeminal n.
Abducent n.
Facial n.
(auriculo-palbebral branch)
Anatomy
·   Cranial nerve V
·     Cranial nerve VI
·     Cranial nerve VII
·     Contains somatic motor & parasympathetic fibers
Origin
·   Trigeminal ganglion
·     Ventral surface of medulla oblongata
·     Passes through the wall of cavernous sinus
·     Leaves brainstem lateral to trapezoid bodies
Exit from skull
·   Ophthalmic n. via orbital fissure
·   Maxillary n. via round foramen
·     Orbital fissure
·     Stylomastoid foramen
Branching
·   Ophthalmic n.
·   Supraorbital n.
·   Lacrimal n.
·   Nasociliary n.
·    Ethmoidal n.
·    infrathrochlear n.
·    long ciliary n.
·   Maxillary n.
·     Zygomatic n.
·    Zygomaticotemporal n.
·    Zygomaticofacial n.
·   Mandibular n.



Innervation
·          Opthalmic n.
·          Skin of upper lid
·          Forehead
·          Medial canthus
·          Third eyelid
·          Lacrimal gland
·          Nasal mucosa
·          Cornea, iris & ciliary body
·          Extraocular muscles
·          Maxillary n.
·          Upper & lower eyelid
·          Facial area
·          Lacrimal gland
·     Retractor bulbi
·     Lateral rectus m.
·     Orbicularis oculi
·     Retractor anguli m.
Functions
·          Received sensory input from eye & adnexa, including cornea, conjunctive, orbit, lacrimal gland & periocular skin
·     Innervates extraocular muscles
·     Blink response
·     Lacrimal gland
































Other nerves

1.     Sympathetic nerve fibers:
o    arises from cranial cervical ganglion
o    innervates orbital muscle & dilator of the pupil
2.     Parasympathetic nerve fibers:
o    enter orbit within oculomotor nerve
o    synapse in ciliary ganglion
o    post synaptic fibers (=short ciliary nerves) innervate ciliary muscle & constrictor of pupil
o    control accomodation of lens & pupillary contraction (miosis) response to light

References
  1. Dyce K.M., Sack W.O. and Wensing C.J.G. (2010) Textbook of veterinary anatomy, 4th edn., Missouri: Saunders Elsevier.
  2. Evans H.E. and Lahunta A. (2012) Miller's anatomy of the dog, 4th edn., Missouri: Saunders Elsevier.
  3. Evans H.E. and Lahunta A. (2010) Guide to the dissection of the dog, 7th edn., Missouri: Saunders Elsevier.
  4. Gelatt K.N. (2007) Veterinary opthalmology, 4th edn., Oxford: Blackwell Publishing.
  5. Maggs D.J., Miller P.E. and Ofri R. (2013) Slatter's fundamentals of veterinary ophthalmology, 5th edn., Missouri: Elsevier.
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