Big Fat Nerves

Recurrent Connections

feedback loops common in retina returning signals to presynaptic cells

Sympathetic Nervous System

“fight or flight”; uses norepinephrine; primary nucleus = superior cervical ganglion

Parasympathetic Nervous System

uses acetylcholine; ganglia close to target tissue (ciliary, pterygopalatine)

Autonomic Targets in Eye

iris & ciliary muscles, tarsal muscles, choroidal vessels, lacrimal and sweat glands

Optic Nerve (CN II)

main sensory nerve of eye, ~1 million RGC axons

Optic Chiasm

site where nasal retinal fibers cross and temporal fibers remain ipsilateral

Contralateral Projection

nasal hemiretinal fibers crossing to opposite brain side

Ipsilateral Projection

temporal hemiretinal fibers staying on same side

Foveal Representation

bilateral projection to both optic tracts, high in LGN (house of representatives), low in superior colliculus (senate)

Optic Tracts

post-chiasmal pathways maintaining rough retinotopy

Retinotopic Projection

neighboring retinal regions map to neighboring areas in LGN and SC

LGN (Lateral Geniculate Nucleus)

main relay from retina to cortex; six-layered structure

LGN Contralateral Inputs

layers 1, 4, 6 (1, 4 and 6 cross the river styx), layer 1= magnocellular (parasol, periphery), layer 4+6= parvocellular (midgets, central)

LGN Ipsilateral Inputs

layers 2, 3, 5 (layers 2, 3 and 5 stay alive), layer 2= magnocellular (parasols, periphery), layer 3+5= parvocellular (midgets, central)

Left LGN Input

left temporal retina + right nasal retina (right visual field)

Right LGN Input

right temporal retina + left nasal retina (left visual field)

Optic Radiations

axon pathway from LGN to primary visual cortex

LGN Retinotopy

neighboring RGCs synapse in neighboring LGN regions

LGN Foveal Magnification

large representation of macular fibers due to high RGC density

Scotoma

visual field defect corresponding to damaged retinotopic area

Non-Visual Retinal Targets

Superior colliculus (eye movement, underrepresented fovea), Pretectal nuclei (pupillary reflex), Accessory optic nuclei (head-eye coordination), Suprachiasmatic nucleus (circadian rhythm)

Hemianopia

blindness in half of each visual field from optic tract lesions

Primary vs Secondary Pathway Lesions

primary pathway lesions affect vision; secondary do not cause field loss but affect reflexes/movement

Trigeminal Nerve (CN V)

main sensory nerve of face; cell bodies in gasserian ganglion, forms opthalmic, mandibular and maxillary branches

Ophthalmic Division (V1)

sensory from eye and upper face, forms nasociliary, lacrimal and frontal divisions

Nasociliary Nerve

only V1 branch entering globe; receives long and short ciliary input (forms ethmoids, infratrochlear, long + short ciliary nerves)

Frontal Nerve

supratrochlear + supraorbital branches to forehead and upper lid

Lacrimal Nerve

carries parasympathetic fibers from pterygopalatine ganglion to lacrimal gland

Maxillary Division (V2)

forms zygomatic nerve, sensory for middle of the face

Zygomatic Nerve

branch of V2 entering orbit; skin of lower lid and cheek, carries lacrimal gland, facial and temporal branches

Mandibular Division (V3)

sensory & motor to lower jaw

Long Ciliary Nerves

sensory, mostly corneal fibers; join nasociliary nerve

Short Ciliary Nerves

mixed sensory, sympathetic, parasympathetic fibers passing through ciliary ganglion

Corneal Nociceptors

pain and touch receptors in corneal epithelium

Corneal Reflex

blink + lacrimation from corneal stimulation

Retrobulbar Block

injection behind eye producing anesthesia

HSV/VZV Trigeminal Infection

can cause severe corneal damage

Oculomotor Nerve (III)

innervates SR, MR, IR, IO, levator; parasympathetic fibers to ciliary ganglion

Trochlear Nerve (IV)

innervates superior oblique; smallest + longest CN → trauma-prone

Abducens Nerve (VI)

innervates lateral rectus; vulnerable to lateral head trauma

Superior Rectus Innervation

crossed; contralateral oculomotor nucleus

Oculomotor Branching

superior division (levator, SR) and inferior division (MR, IR, IO)

Motor Root of Ciliary Ganglion

parasympathetic fibers to ciliary ganglion

Edinger-Westphal Nucleus

preganglionic parasympathetic source for sphincter & ciliary muscles

Cavernous Sinus

venous sinus traversed by CN III, IV, V1, V2, VI and carotid artery; optic nerve excluded

Orbicularis Oculi

muscle closing eyelids; innervated by facial nerve (VII)

Blepharospasm

excessive orbicularis activity from CN VII irritation

Ectropion

eversion of lower lid from CN VII dysfunction

Levator Palpebrae

upper lid elevation; CN III

Tarsal Muscles

smooth muscle of eyelids; sympathetic innervation

Ptosis

drooping lid from CN III or sympathetic lesion

Sympathetic Pathway to Eye

superior cervical ganglion → internal carotid plexus → sympathetic root → short ciliary nerves

Iris Dilator Muscle

sympathetic; contraction causes mydriasis

Uveal Vasoconstriction

sympathetically controlled to regulate choroidal blood flow

Horner’s Syndrome

sympathetic lesion → ptosis, miosis, anhidrosis, possible low IOP and conjunctival vasodilatio

Parasympathetic Pathway to Eye

Edinger-Westphal → CN III → ciliary ganglion → short ciliary nerves

Rami Oculares

postganglionic fibers from pterygopalatine ganglion to orbit

Pupillary Light Reflex

direct & consensual constriction via pretectal → EW nuclei → sphincter muscle

Argyll Robertson Pupil

constricted pupils lacking light reflex but able to accommodate; lesion between pretectal and EW nuclei

Lacrimal Gland Parasympathetic Pathway

pterygopalatine ganglion → zygomatic nerve → zygomaticotemporal → communicating branch → lacrimal nerve

Lacrimal Stimulation

triggers from pain, emotion, bright light

Dry Eye from Efferent Lesion

loss of basal parasympathetic lacrimal stimulation

Growth Cones

motile structures guiding developing axons/dendrites

Neuroglial Guidance

glial “adhesive” paths directing axon growth

Tropic Factors

molecular signals directing axons to final targets

Critical Periods

developmental windows when neural circuits are highly modifiable

Ocular Albinism

pathfinding error causing abnormal optic chiasm crossing

Amblyopia

reduced acuity without refractive or pathological cause; synaptic development issue

Interstitial Growth

postnatal neuronal growth by expansion, not mitosis

Neuronal Regeneration

PNS neurons regenerate; CNS neurons do not

Corneal Nerve Regeneration

occurs due to limited pathway complexity; damaged fibers regrow

Transneuronal Atrophy

degeneration of neurons due to loss of input source