Innervation of the Eye and Orbit Part 1: The Optic Nerve and Innervation of the Eye and Orbit Part 1: The Optic Nerve and its Projections VS112

Introduction We often think of the eye only as a sensing organ under complex but voluntary control. In fact, there are many functions of the eye that are automatic. As a practicing optometrist you will be required to assess the function of both the both these autonomic or automatic functions as well as the movement and control of the voluntary muscles.

A sensory organ with complex mobility Performing the basic neurological exam of the eye isn’t difficult, but to accurately diagnose a problem requires knowledge of the wiring and connections that underlie the observable behaviors. There are a lot of terms, anatomy and pathways you’ll need to know.

Organization Optic Nerve - pathways, lesions. Cranial Nerves - who’s on first. Parasympathetic & Sympathetic. Nerve Damage, Growth and Development.

Background: Neurons What is a nerve cell? Anatomy of a cell. (Fig 5.1) What is their job and how to they do it? Anatomy of a cell. (Fig 5.1) Gather information (synaptic input) encode information Deliver information (Fig 5.2)

Different morphologies for different specialization of nerve function.

Nerve Action Membrane potential Excitability Receptor potential Synapses Inhibition Action potentials

Synapses are communication sites

Synapses: Cell to cell connections: Electrical vs. Chemical Fig 5-1

Lecture 1 Optic Nerve and Optic Pathways Parsing out the pathways. Central Destinations. Lesions and Visual Fields. Remember the goal is to get all this information out to the brain for processing The shortest route (along the interior surface of a sphere) is an arc directly to exits sign. But in the interest of optical clarity the fovea is considered a “no fly” or no fiber zone.

Optic Nerve (sensory output) Forming the nerve - GANGLION Cells(5.3) Economy of movement or flow Geographical segregation Organizing the flow of information Step 1: Nasal/Temporal Visual Fields Left Brain Right Right Brain (5.5)

Forming the nerve - GANGLION Cells(5.3) Temporal fibers coalesce at the chiasm

Visual Fields (5.4 draw on board) Nasal field = temporal retina Temporal (lateral) field = ?? Superior field (sky) = inferior retina E.g.: mouse or rabbit blue cone density Inferior field (earth) = ??

Primary Optic Pathway is to the LGN and on to striate cortex Contralateral visual field Through ipsilateral eye’s nasal retina, And contralateral eye’s temporal retina

Decussation at the Chiasm Information sorting begins here (Fig. 5.5) Partial cross-over of ganglion cell fibers organizes the visual world into left vs. right visual fields Temporal - tight (short- non crossing) Nasal - meandering (crosses over)

Decussation follows a pattern related to the visual fields. Fig 5-5 draw on overhead

Lesions of the Optic Pathway Optic nerve => one entire eye (5.12) Split chiasm => loss of crossing fibers (nasal retina=temporal visual fields) Optic tract =>Loss of contralateral visual field Partial chiasm - unpredictable due to meandering fibers (5.13)

Visual field deficits predict location of the lesions

Projections of the optic nerve (5.7) Lateral Geniculate Nucleas (main relay to cortex) Superior Colliculus (eye movement) Pretectal Nuclei (pupillary light reflex) Accessory optic system Biological clock (suprachiasmatic n.)

Brainstem and Midbrain (Fig5-7)

Lateral Geniculate Nuclei - paired structure. 90% of G-cell projection (fig 5.8-10) Distinctly layered structure (6 layers - monkey) Layer specific to eye (1,4,6 contralateral) Layers 2,3,5 ispsilateral eye Retinotopic mapping

Lateral Geniculate nucleas is part of the thalamus- a paired structure, important for relaying information to cortex

The Big Picture

LGN is a layered structure

Retino-topic mapping An exaggeration of the foveal input Fig 5.9 Central 5° of visual field (3% of retina) covers 50% of LGN and visual cortex.

Multiple maps overlie one another in the layers of LGN Retinotopicmaps overlie, meaning that common points in visual fields stack in the layers. Layer 1,4,6 contralateral Layer 2,3,5 ipsilateral

LGN - organization Summary Left LGN - right visual field From left eye temporal retina Right eye nasal retina Retinotopic mapping (6 -copies)5.8-5.9 Layer 1 & 2 Magnocellular (large cells) Layers 3-6 parvocellular Maps are in register in layers 5.10 2,3,5 ipsilateral 1,4,6 contralateral

Superior colliculus Largest axon bundle for non-visual projections Axon collaterals create retinotopic map Contralateral visual field (diagram on board) Binocular-non-segregated input Under-represented fovea Input from many sites (eye movement - related)

Projections to superior colliculi X Left eye ipsilateral visual field has no blind spot Fig 5-11

Pretectal nuclei - Grab bag of 5 nuclei receiving G-cell inputs Olivary (projects to Edinger-Westphal nuclei) and involve in pupillary light reflex Nucleus of the optic tract - eye movement controls Anterior, posterior and medial pretectal nuclei (specific functions unknown).

Accessory Optic System Three pairs of nuclei receive small inputs from ganglion cells Output to vestibular nuclei and cerebellum Thought to be involved in coordinating head/eye movements

Suprachiasmatic Nucleus Part of the hypothalamic pathway Degeneration studies show direct input from the retina Thought to be part of circadian clock (jet lag. S.A.D.)

Summary Optic Nerve LGN SCN, AON, Sup Coll, Pretectal Formation Crossing at chiasm Projections LGN Visual field segregation Contra - ipsi-lateral eye separation Functional G-cell type separation Parvo and magno pathways SCN, AON, Sup Coll, Pretectal