The Optic Nerve




(1)
University of Sydney, Sydney, Australia

 




Overview (Fig. 12.1)




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Fig. 12.1
The optic nerve. (a) Structure. (b) Divisions





  • The optic nerve is a central nervous system (CNS) white matter tract that transmits visual information from the eye to the brain.


  • The optic nerve consists of:

    (a)

    Retinal ganglion cell (RGC) axons

     

    (b)

    Supportive glial tissue

     

    (c)

    Vascular tissue

     




  • It is surrounded by three layers of meningeal tissue (pia, arachnoid, and dura).


  • The RGC axons:

    (a)

    Course along the retinal nerve fiber layer (RNFL) to enter the optic disc

     

    (b)

    Continue through the intraorbital, intracanalicular, and intracranial portions of the optic nerve

     

    (c)

    Pass through the optic chiasm and optic tract toward the CNS

     




  • The optic nerve has a limited capacity for regeneration after significant damage, resulting in irreversible visual loss.


Optic Nerve Divisions (Fig. 12.1b)




1.

Retinal nerve fiber layer



  • The eye contains on average approximately 1.2 million retinal ganglion cells (RGC) [1].


  • Each RGC sends one axon toward the optic nerve in the RNFL [2, 3].

 

2.

Optic nerve head (Fig. 12.2a) [4]

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Fig. 12.2
(a) The optic nerve head. (b) Topographical organization of the retinal nerve fiber layer




  • The optic nerve head (optic disc) is located within the eye, consisting of a rim and a cup.


  • RGC axons from the RNFL turn 90° to dive into the disc to form the neuroretinal rim.


  • The RGC axons exit the eye through pores of the lamina cribrosa, a perforated portion of sclera that provides structural support to the optic nerve head.

 

3.

Intraorbital portion



  • The intraorbital portion courses through the orbit from the lamina cribrosa toward the optic canal.


  • It has redundancy in length to prevent globe tethering on eye movement and proptosis.


  • However, severe proptosis may result in optic nerve stretch and injury [5].

 

4.

Intracanalicular portion [5]



  • The optic nerve passes through the optic canal in the sphenoid bone and enters the cranial cavity.

 

5.

Intracranial portion



  • The intracranial optic nerve travels upward, posteriorly and medially.


  • It meets the contralateral nerve at the optic chiasm.

 

6.

Optic chiasm



  • At the optic chiasm, RGC axons from the temporal retina remain ipsilateral.


  • Those from the nasal retina cross the chiasm and course contralaterally (Fig. 12.3) [6]

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    Fig. 12.3
    Central nervous system targets of optic nerve projections

 


Topographic Organization of the Optic Nerve




1.

Retinal nerve fiber layer [7] (Fig. 12.2b)



  • Superior and inferior fibers are segregated by the horizontal raphe (H) which divides the inferior and superior visual fields.


  • The blind spot is a physiological temporal scotoma that corresponds to the optic disc [6].


  • Temporal macular fibers (T) course around the fovea (F) to enter the disc superiorly or inferiorly.


  • Nasal macular fibers travel in the papillomacular bundle (P) to enter the temporal aspect of the optic disc.


  • Fibers nasal to the optic disc enter nasally.

 

2.

Optic nerve head [8]



  • In the optic nerve head, peripheral retinal fibers are found peripherally and macular fibers centrally.

 

3.

Intraorbital, intracanalicular, and intracranial portions



  • From the intraorbital portion, the organization of fibers changes:

    (a)

    Temporal fibers gather temporally.

     

    (b)

    Nasal fibers gather in the nasal portion in preparation to cross at the chiasm.

     

 


Meningeal Layers Covering the Optic Nerve [5, 9]






  • The optic nerve is covered (from outer to inner) by dura, arachnoid, and pia matter (Fig. 12.1).


  • The dura is a thick, tough fibrovascular tissue continuous with the CNS dura.


  • The arachnoid is a loose, thin, and vascular connective tissue.


  • The subarachnoid space is continuous with that around the CNS, containing subarachnoid fluid [10].


  • The pia is very thin and tightly adherent to the optic nerve.


  • The pia extends septae into the nerve parenchyma that contain supportive blood vessels.


Central Nervous System Targets of Optic Nerve Projections (Fig. 12.3)




1.

Lateral geniculate nucleus (LGN) (90 % of all RGC axons)



  • The LGN receives binocular input from the optic nerves via the optic tract and projects to the primary visual cortex [6, 11].


  • Optic nerve projections to the LGN are involved in conscious visual perception.

    (See Chap. 13, The Lateral Geniculate Nucleus)

 

2.

Pretectal nucleus



  • Each pretectal nucleus (in the dorsal midbrain) receives bilateral optic nerve projections.


  • Each projects bilaterally to the Edinger–Westphal nuclei.


  • They are involved in controlling the pupillary light reflex [12, 13].

    (See Chap. 6, The Iris and Pupil.)

 

3.

Superior colliculus



  • The superior colliculus is a midbrain structure, dorsal to the pretectal nuclei.


  • It integrates visual and auditory stimuli and is involved in generating saccadic eye movements (see Chap. 18, Neural Control of Eye Movements) [14, 15].


  • It also has a role in visual attention [16].

 

4.

Pulvinar nucleus



  • The pulvinar nucleus (posterior thalamus) receives optic nerve and superior colliculus projections.


  • It sends projections to the primary and extrastriate visual cortical areas [16].


  • The pulvinar pathway codes visual importance (salience) and may have a role in hand–eye coordination [17, 18].

 

5.

Hypothalamus



  • The hypothalamic suprachiasmatic nucleus receives RGC axons involved in the control of circadian rhythms [19].

 

6.

Accessory optic tract



  • This midbrain structure is involved in the optokinetic reflex (fixation on a moving target) [20].

    (See Chap. 18, Neural Control of Eye Movements)

 


Optic Nerve Parenchyma: Cellular Components






  • Glial cells provide structural and metabolic support for the RGC axons.


1.

Oligodendrocytes (Fig. 12.4)

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Fig. 12.4
Oligodendrocytes




  • Oligodendrocytes form a myelin sheath around axons posterior to the lamina cribrosa [21].


  • Myelin, a fatty multilaminated structure, provides electrochemical insulation to the axon [22].


  • Each oligodendrocyte has 20–30 processes that each myelinate a small portion of an axon.


  • Between each segment of myelin is the node of Ranvier.


  • The action potential jumps from one node to the other (saltatory conduction) to greatly increase the speed and efficiency of conduction [23].

 

2.

Oligodendrocyte origins and development



  • Oligodendrocytes are derived from oligodendrocyte precursor cells that migrate from the brain [24].


  • Their differentiation and renewal is controlled by neurotrophic factors including:

    (a)

    Platelet-derived growth factor (PDGF)

     

    (b)

    Basic fibroblast growth factor (bFGF) [25]

     




  • Myelination begins at 32 weeks gestation from the lateral geniculate nucleus.


  • Myelination progresses as far as the lamina cribrosa; the process is complete by 2 years of age [26].


  • Oligodendrocytes and ganglion cell axons interact to influence their growth and metabolic functions during development and throughout adulthood [27, 28].

 

3.

Astrocytes



  • Astrocytes are common CNS glial cells that express glial fibrillary acid protein [29].


  • They provide crucial supportive functions within the optic nerve, including:

    (i)

    Maintenance of water and electrolyte (especially K + ) homeostasis [30]



    • Electrolyte homeostasis is necessary for optimal electrical function of the axon.


    • K+ is released by axons on action potential repolarization.


    • Astrocytes absorb excess extracellular K+.

     

    (ii)

    Metabolic supply to axons [25]



    • Astrocytes store glycogen.


    • They can shuttle lactate to adjacent axons in ischemic or hypoglycemic states.

     

    (iii)

    Maintenance of neural tissue barriers [22]



    • Astrocytic foot processes maintain the blood–brain barrier at capillary basement membrane and pial surfaces [31].


    • They regulate levels of extracellular neurotransmitters and solutes at the nodes of Ranvier.

     

    (iv)

    Response to optic nerve injury



    • Astrocytes hypertrophy and extend cellular processes in response to parenchymal injury or loss, a process known as gliosis [32].

     

    (v)

    Axonal growth and development



    • Astrocytes act as a scaffold for axon growth in the developing optic nerve [33].


    • In the mature optic nerve, astrocytes inhibit axonal regrowth [34].

     

 

4.

Microglia [35]



  • These are bone marrow-derived phagocytic scavenger cells that resemble macrophages.


  • They move to sites of injury, proliferate, phagocytose, and degrade extracellular material.


  • They express cell surface antigen, stimulating T lymphocytes and activating the immune system.

 


Optic Nerve Axonal Physiology




1.

Retinal ganglion cell presynaptic input (see Chap. 8, The Retina).



  • Presynaptic input to RGCs is from bipolar (graded potentials) and amacrine cells (graded and action potentials).


  • Glutamate is the main excitatory neurotransmitter for RGC presynaptic input; it activates N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic receptors, and metabotropic receptors [36].


  • Retinal Müller cells control extracellular levels of glutamate by active uptake via glutamate transporters and conversion to glutamine [37].


  • This prevents excessive glutamate-related excitotoxicity.

 

2.

Axonal conduction: action potential (Fig. 12.5, Table 12.1)

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Fig. 12.5
The action potential



Table 12.1
Mechanisms of action potentials [21, 38–42]


































Component
Explanation

Overview
An action potential (AP) is an all-or-nothing phenomenon: the amount of voltage change (depolarization) is always the same

Intracellular ionic concentration
Axons have high K + and low Na + intracellular content compared to extracellular space

AP generation – chemical events
Synaptic input to the RGC cell results in membrane depolarization at the base of the axon, the axon hillock

AP generation – electrical events
Partial depolarization results in opening of voltage-sensitive Na + channels

An inflow of Na+ causes the membrane to depolarize

AP propagation
Increased potential rapidly affects adjacent sections of the axon

This causes adjacent Na + channels to open and the membrane to depolarize

This results in rapid transmission of the action potential down the axon

Membrane repolarization
This occurs due to opening of K + channels and gated closure of the Na + channels

The K + current dominates the membrane potential which hyperpolarizes

This occurs more slowly than depolarization

Restoration of ionic equilibrium
Transmembrane Na + /K + ATPase actively pumps Na+ out and K+ in. This restores ionic equilibrium and aids in restoration of resting membrane potential

Saltatory conduction
The myelin sheath increases the speed and efficiency of conduction

It insulates the axon from transmembrane leakage of charge

Only the exposed axonal membrane at the nodes of Ranvier depolarizes

The current passes from one node to another by saltatory conduction

This reduces the amount of ionic flux required to transmit action potentials




  • RGCs are the first visual pathway neurons that exclusively use action potentials (AP) [38].


  • RGC axons use glutamate as the main neurotransmitter to synapse on their CNS targets.


  • The frequency and distribution of APs code visual information [39].

 

3.

Axonal transport



  • This can be anterograde (cell body to axonal terminal) or retrograde (axon terminal to cell body).


  • Anterograde transport can be fast or slow.

    (i)

    Fast anterograde (90–350 mm/day)



    • Transport is highly energy dependent.


    • It is reliant on kinesin motor proteins and microtubule formation.


    • Neurotransmitter vesicles and organelles are transported to the axon terminal [43, 44].

     

    (ii)

    Slow anterograde (0.2–8 mm/day)



    • Transport for axonal structural proteins, e.g., cytoskeleton proteins, tubulin, actin and myosin [45, 46]

     

    (iii)

    Retrograde transport



    • This occurs at half the velocity of fast anterograde transport.


    • It involves the dynein/dynactin motor protein complex. [47, 48]


    • Damaged axonal products and endocytosed synaptic neurotransmitter and neurotrophins are transported to the cell body [21, 49, 50].

     

 


Optic Nerve Blood Vessels (See Chap. 11, Ocular Circulation)




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Oct 28, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on The Optic Nerve

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