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), cell body location

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 (communicating branch), facial and temporal branches

Mandibular Division (V3)

sensory & motor to lower jaw

Long Ciliary Nerves

sensory, mostly corneal fibers (also some iris and CB); join nasociliary nerve, activation leads to blink reflex via CN VII and tearing

Short Ciliary Nerves

mixed sensory (info from uvea), sympathetic, parasympathetic fibers passing through ciliary ganglion, 10-15 nerves down to 1 collection point (sensory root of ciliary ganglion)

Corneal Reflex

blink + lacrimation from corneal stimulation

Retrobulbar Block

injection behind eye producing anesthesia

Oculomotor Nerve (III)

innervates SR, MR, IR, IO, levator; nuclei= ipsilateral except SR, all nerves ipsilateral, also carries parasympathetic fibers to ciliary ganglion

Trochlear Nerve (IV)

innervates superior oblique; smallest + longest CN → trauma-prone, nuclei contralateral, nerves ipsilateral

Abducens Nerve (VI)

innervates lateral rectus; vulnerable to lateral head trauma, nerves and nuclei ipsilateral

Oculomotor Branches

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

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 innervation and function

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 of ciliary ganglion (NO SYNAPSE) → short ciliary nerves → choroid to branch to targets

Iris Dilator Muscle

sympathetic; contraction causes mydriasis

Uveal Vasoconstriction

sympathetically controlled to regulate choroidal blood flow (constant blood flow while arterial pressure increases, inversely proportional to resistance)

Horner’s Syndrome

sympathetic lesion → ptosis (sympathetic tarsal muscle innervation), miosis, anhidrosis, possible low IOP and conjunctival vasodilation

Parasympathetic Pathway to Eye

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

Rami Oculares

EW → 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

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

Lacrimal Stimulation

triggers from pain, emotion, bright light

Growth Cones

motile structures guiding developing axons/dendrites

Neuroglial Guidance

glial “adhesive” paths directing axon growth

Laminin

trophic signal directing axons to final targets

Ocular Albinism

pathfinding error causing abnormal optic chiasm crossing

Amblyopia

reduced acuity without refractive or pathological cause; synaptic development issue

Neuronal Regeneration

PNS neurons regenerate (but not the same as before damage); 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