Lecture 13 - Optic Nerve Physiology

What cranial nerve is the optic nerve?

Cranial nerve II

What is the optic nerve considered part of?

The central nervous system

What forms the optic nerve?

Axons of retinal ganglion cells

Approximately how many fibers are in the optic nerve?

About 1.2 million fibers

What is the approximate length of the optic nerve?

5–6 cm

What is the average optic disc diameter?

Approximately 1.7–1.8 mm

What is the function of the optic nerve?

To transmit all visual information from the retina to the brain

Where do 90% of optic nerve fibers terminate?

Lateral geniculate nucleus (LGN)

Where do the remaining 10% of optic nerve fibers terminate?

Superior colliculus, pretectal nucleus, and hypothalamus

What are the four segments of the optic nerve?

Intraocular, intraorbital, intracanalicular, and intracranial

What is the length of the intraocular segment of the optic nerve?

Approximately 0.7–1 mm

Is the intraocular optic nerve myelinated or unmyelinated?

Unmyelinated

What structures are included in the intraocular optic nerve segment?

Peripapillary border tissues, pre-laminar region, and laminar region

What is the length of the intraorbital optic nerve segment?

Approximately 30 mm

What is another name for the intraorbital segment?

Retrolaminar segment

Which optic nerve segment is the longest?

The intraorbital segment

Is the intraorbital optic nerve myelinated?

Yes

What major artery enters the optic nerve sheath in the intraorbital segment?

Central retinal artery

What is the length of the intracanalicular optic nerve segment?

Approximately 6–10 mm

What artery enters the sheath in the intracanalicular segment?

Ophthalmic artery

What is the length of the intracranial optic nerve segment?

Approximately 10–16 mm

Where is the intracranial optic nerve located relative to the cavernous sinus?

Superior to the cavernous sinus

What is the lamina cribrosa?

A fenestrated network of interwoven collagen fibrils

What is the function of the lamina cribrosa?

Provides structural and nutritional support to retinal ganglion cell axons

What structures are present within the lamina cribrosa?

Capillaries and bundled axons

What extracellular matrix components are found in the lamina cribrosa?

Collagen, elastin, and fibronectin

What is the lamina cribrosa considered structurally in the sclera?

The weakest area of the sclera

How thick is the lamina cribrosa compared to the sclera?

Approximately one-third the thickness of the sclera

What pressures act on the lamina cribrosa?

Intraocular pressure (IOP) on one side and cerebrospinal fluid (CSF) pressure on the other

What happens when pressures across the lamina cribrosa are unbalanced?

Unequal forces cause mechanical stress

What structural changes can occur in the lamina cribrosa due to pressure imbalance?

Deformation, bowing, and tissue remodeling

What is the primary source of blood supply to the optic nerve?

Branches of the ophthalmic artery

Which arteries supply the optic nerve?

Central retinal artery and short and long posterior ciliary arteries

What is a key feature of optic nerve vasculature?

It exhibits autoregulation similar to retinal vessels

What is autoregulation in the optic nerve?

Maintenance of stable blood flow under varying conditions without external input

Why is autoregulation important in the optic nerve?

To maintain stable oxygen levels across different intraocular pressures

What are pericytes?

Contractile cells that help regulate blood flow in capillaries

What role do endothelial-derived vasoactive substances play?

They regulate vascular tone

What effect does nitric oxide have on blood vessels?

Vasodilation

What effect does endothelin-1 have on blood vessels?

Vasoconstriction

What is the optic nerve derived from?

Neuroectoderm

When do the optic grooves form during development?

Around weeks 3–5

What structures do optic grooves form?

Optic vesicles

What do optic vesicles eventually form?

The optic stalk

What structure is the precursor to the optic nerve?

The optic stalk

What does the optic stalk connect?

The optic vesicle to the forebrain

What part of the brain is the optic stalk an extension of?

The diencephalon

What occurs with closure of the fetal fissure?

Formation of two layers of the optic stalk

What does the outer layer of the optic stalk become?

Neuroglial sheath surrounding the optic nerve and parts of the lamina cribrosa

What happens to the inner layer of the optic stalk?

Forms glial cells or undergoes apoptosis to create passageways for RGC axons

What are axons primarily formed from?

Cytoskeletal proteins

What structures make up axons?

Microtubules

What proteins compose microtubules?

Alpha-tubulin and beta-tubulin

What is the polarity of microtubules in axons?

Polarized

Which end of the microtubule is oriented toward the cell body?

Negative (-) end

Which end of the microtubule is oriented toward the target tissue?

Positive (+) end

What is the growth cone?

The growing end of an axon

How are microtubules arranged in axons?

Discontinuous but overlapping to form a compact structure

What directs axon pathfinding during development?

Other neurons, glial cells, and proteoglycans

How do proteoglycans influence axon guidance in the central retina?

They repel axons

What attracts axons to the optic disc?

Netrin guidance proteins

What guidance proteins repel ipsilateral-projecting neurons at the optic chiasm?

Slit proteins

How does optic nerve myelination differ from most neurons?

It occurs in the opposite direction

When does optic nerve myelination begin?

Around month 5 of development

What triggers the start of myelination?

When ganglion cell axons reach the LGN

In what direction does myelination proceed in the optic nerve?

Anteriorly toward the optic chiasm and lamina cribrosa

When do optic tract and intracranial axons begin myelination?

Around 32 weeks

When do axons nearest the globe begin myelination?

From birth to approximately 7 months

How long can axons continue to thicken?

Up to 2 years or longer

How does the number of retinal ganglion cells change during development?

It decreases

How many RGCs are present around month 2?

Approximately 2.6 million

How many RGCs are present around month 8?

Approximately 1.1 million

What is physiological cupping?

A large cup-to-disc ratio that is not due to pathology

What determines optic nerve diameter more strongly than RGC number?

Structural variation rather than RGC count

Why can patients with large optic discs have large cups without pathology?

They may still have a normal number of RGCs

Which part of vision develops late?

The fovea and central vision

At what age does visual acuity reach adult levels?

Around 5–6 years

What factors are required for development of normal visual acuity?

Proper myelination and proper axon targeting to the LGN and striate cortex

What can result from abnormal myelination or axon targeting?

Amblyopia

Approximately what percentage of cells at the optic nerve head (ONH) are glial cells?

About 50%

What is the overall role of glial cells in the optic nerve?

Maintenance of health and function of neural tissue

What types of conditions do glial cells respond to?

Inflammation, injury, and ischemia

What major barrier is associated with glial cell function in the optic nerve?

Blood-brain barrier (BBB)

What are the main types of glial cells in the optic nerve?

Oligodendrocytes, astrocytes, and microglia

What is the primary function of oligodendrocytes?

Myelination of axons

How does myelination affect action potentials?

It improves conductance and efficiency

What is saltatory conduction?

Rapid propagation of action potentials along myelinated axons where signals jump between nodes of Ranvier

What are nodes of Ranvier?

Gaps in the myelin sheath that contain many Na+ and K+ channels

Where does ion exchange occur along myelinated axons?

Only at the nodes of Ranvier

What happens when Na+ enters at the nodes of Ranvier?

It pushes ions within the axon to the next node

What is the role of myelin in electrical conduction?

Prevents leakage of current and preserves signal strength

How does myelin improve signal transmission?

Allows rapid conductance and regeneration of the action potential

What metabolic role do oligodendrocytes provide?

Supply lactate for retinal ganglion cell energy

What are astrocytes?

The most abundant glial cells in the CNS

What is the function of astrocytes as a syncytium?

Maintain an ideal extracellular environment and extracellular matrix (ECM)

What is buffering in astrocyte function?

Regulation to prevent excess ions and neurotransmitters

What factors do astrocytes release to support neurons?

Neurotrophic factors

What conditions activate astrocytes?

Mechanical strain and hypoxia

What is the primary function of microglia?

Immune function within the CNS

What do microglia secrete?

Cytokines and trophic factors