Orbital Nerves: Cranial Nerves II, III, IV and VI

Learning Objectives

  • Understand the visual field and the different components of CN II

  • Isolate the cranial nerves that innervate the skeletal and smooth muscles of the orbit

  • Apply this information in a clinical context

Review

Eye muscles

  • Extrinsic muscles: Extraocular muscles

    • Involved in movement of the eyeball or raising the upper eyelid

    • Levator palpebrae superioris m.: Raise eyelid

    • Superior oblique m.: Depression; ABduction

    • Lateral rectus m.: ABduction

    • Superior rectus m.: Elevation; ADduction

    • Inferior oblique m.: Elevation: ABduction

    • Inferior rectus m.: Depression; ADduction

    • Medial rectus m.: ADduction

      • All muscles with “rectus” originate from the common tendinous ring

  • Intrinsic muscles: Muscles within the eyeball

    • Control lens shape and pupil size

Orbital Nerves

CN

Name

Fiber

Function/Innervation

CN II

Optic n.

SSA (Special sensory afferent)

Special sense of vision

CN III

Oculomotor n.

Somatomotor (GSE) and Parasympathetic (GVE)

GSE: Levator palpebrae superioris; Superior rectus; Inferior rectus; Medial rectus; Inferior oblique

GVE: Sphincter pupillae; Ciliary m.

CN IV

Trochlear n.

Somatomotor (GSE)

Superior oblique

CN V1

Ophthalmic division of CN V

Somatosensory (GSA)

Skin around the ey; eyelid; lacrimal gland (sensory only); conjunctiva

CN VI

Abducent n.

Somatomotor GSE

Lateral rectus

  • N. that run along the lateral wall of cavernous sinus (inferior → superior): Ophthalmic n (V1), Trochlear n. (IV), oculomotor n. (III)

  • N. that travel through cavernous sinus: Abducens n. (VI)

CN II - Optic n.

  • SSA: Special sense of sight

  • Function: transmits info from the world → brain about brightness color, and contrast (visual acuity)

  • Pathway: Originate from retina and move distally into brain proper

    • Retina → L and R optic n. → Optic canal → R and L optic n. join at optic chiasm (in chiasnatic groove) → Optic tract → Midbrain (synapse) → Visual cortex (in occipital lobe)

  • Covered by pia, arachnoid, and dura mater

    • Central retinal a. (from opthalmic ← branch of ICA) pops through dura and follows optic nerve to pierce the back of the eye

  • Optic n. technically a myelinated tract from the CNS (not a true CN)

  • Limited regeneration → Damage results in blindness

    • Permanent regeneration

  • Reflexes (response to light)

    • Pupillary light reflex: Change how much light enters eye

    • Accommodation reflex: Changes focus with distance

Optic chiasm

  • Optic n. cross each other

    • In humans small part stays on ipsilateral side

      • Visual field covered by both eye = binocular depth perception

    • Remember…. LEFT goes to right and RIGHT goes to left

Visual field

  • Info from the RIGHT half of the visual field of both eyes are carried in the left optic tract

    Info from the LEFT half of the visual field of both eyes are carried in the right optic tract

  • Nasal component: closest to nose

    • Path of light from LEFT nasal field: Light captured by R side of R eyeball → R optic nerve (stays on R side) → R Lateral geniculate body continue down the R optic tract → Occipital lobe

    • Path of light from RIGHT nasal field: Light captured by L side of L eyeball → L optic nerve (stays on L side) → L lateral geniculate body continue down the L optic tract → Occipital lobe

  • Temporal component: closest to temple

    • Path of light from the LEFT temporal field: Light captured by R side of L eyeball → Cross over at optic chiasm (to R side) → R lateral geniculate body continue down the R optic tract → Occipital lobe

    • Path of light from the RIGHT temporal field: Light captured by L side of R eyeball → Cross over at optic chiasm (to L side) → L lateral geniculate body continue down the L optic tract → Occipital lobe

Clinical considerations

  • Prechiasmatic lesion: Retina or optic n. damage

    • If the lesion to the nerve is incomplete, blind spot in that eye’s visual field results

    • Ex/ damage to R optic n. means no info from R eyeball

  • Chiasmatic lesion: Optic chiasm

    • Temporal visual fields loss (tunnel vision)

    • Info from temporal visual fields cannot cross to opposing optic tract

  • Postchiasmatic lesion: Optic tract

    • Loss of input from the contralateral visual field from both eyes

    • Ex/ damage to R optic tract means that you lose vision from L temporal and nasal visual fields

Ophthalmic Division of Trigeminal (V1)

  • Branches of V1 (superior → inferior)

    • Lacrimal n. → Frontal n. → Nasociliary n.

      • Frontal n → Supraorbital n. and Supratrochlear n.

      • Nasociliary n. → Infrotrochlear n.

Oculomotor n. (CN III) - SM and Parasymp/Pre

  • Upper division: Innervates levator palpebrae superioris and superior rectus

  • Lower division: innervates inferior rectus and inferior oblique

Trochlear n. (CN IV) - SM ONLY

  • Innervates superior oblique m.

    • Trochlear swing before turning onto eye

Abducent n. (CN VI) - SM ONLY

  • Innervates lateral rectus

Sympathetic and Parasympathetic Pathways

Sympathetics

  • Synapse at superior cervical ganglion (right next to ICA)

    • Post-synaptic sympathetics carried by the ICA into the head via the carotid plexus

  • Sympathetics to vessels within the eye

    • ICA → Nasociliary n. (V1) → Ciliary ganglion → Short ciliary nn.

    • ICA → Plexus branch → Ciliary ganglion → Short ciliary nn.

  • Sympathetics from carotid plexus innervate dilator pupillae m.

    • Dilates pupil to bring in more light

    • ICA → Nasociliary n. (V1) → Long ciliary n. → Sclera → Dilator pupillary

      • Bypass the ciliary ganglion

Parasympathetics

  • Parasymoathetics innervate….

    • Sphinctor pupillae: Constricts pupil

    • Ciliary m.: Controls lens thickness for accomodation/focus

      • Zonular fibers attach lens from ciliary m.

      • Relaxed ciliary m. = flatten lens (unaccommodated)

      • Contracting ciliary m. = thickened lens (accommodated)

Clinical Correlations

Pupillary Light Reflex

  • Afferent limb: CN II from stimulation to retinal ganglion cells

  • Efferent limb: CN III parasympathetics via short ciliary nn. to sphincter pupillae

  • Clinically test by shining bright light and observing if the pupil changes size

    • Constrict in bright light

    • Dilate in dim light

Accommodation Reflex

  • Afferent limb: CN II from stimulation of retinal ganglion cells

  • Efferent limb: CN III parasympathetics via short ciliary nn. to ciliary body (m.) and sphincter pupillae

  • Clinically test by asking patient to converge eyes (bring object closer)

    • Observe convergence, pupil constriction, lens gets thicker

      • Aid in near focus

Horner’s Syndrome

  • Neurologic disorder that affects the sympathetics

    • More specifically, the sympathetics to the head

    • Lesion in sympathetic ganglion/sympathetic chain

  • Clinical manifestations: Ptosis, miosis, and anhidrosis

    • Ptosis: drooping eyelid

      • Result from loss of control of superior tarsal muscle (smooth muscle under autonomic control) therefore can’t assist levator palperae superioris

    • Miosis: constricting pupil

      • Result from loss of control of pupillary dilator m.

    • Anhidrosis: lack of sweating on the face

      • Result from loss of control of the smooth muscle within the aa. of the forehead (supraorbital and supratrochlear aa.)

Clinical Testing (“H Test”)

  • Goal: Isolate actions to isolate specific nerves

  • Ex/ Testing trochlear n.

    • Innervates: Superior oblique

    • Actions of SO: Depression, ABduction

      • Depression is primary action → look medially (ADduction) and downwards

  • Ex/ Testing oculomotor n.

    • Innervates: Superior rectus; Inferior rectus; Medial rectus; Inferior oblique

    • Actions of SR: Elevation, ADduction

      • Elevation is primary action → look laterally (ABduction) and upwards

    • Actions of IR: Depression, ADduction

      • Look laterally and downwards

    • Actions of MR: ADduction

      • Look medially (ADduction)

    • Actions of IO: Elevation, ABduction

      • Look medially (ADduction) and upwards