OPTIC NERVE HEAD; OPTIC DISK

 

• The optic disk or the Optic Nerve Head (ONH) is the distal portion of the optic nerve.

• It extends from the retinal surface to the beginning of the myelinated portion of the optic nerve posterior to the lamina cribrosa (LC).
• The ONH can be regarded as a round or oval plughole containing a sieve-like sheet known as the LC through which the RGC axons pass through.
• There are no rods or cones overlying the optic disc, and so it corresponds to the blind spot in each eye.
• The LC inserts and anchors itself into the parapapillary scleral connective tissue which provides substantial support to counteract IOP.
• The LC comprises of multiple ‘‘plates’’ of connective tissue which includes several types of interstitial collagen, proteoglycans and elastin.
• The posterior or scleral portion of the LC contains significantly higher amounts of collagen compared to the anterior or choroidal part of the LC. However, the anterior portion of the LC contains large numbers of astrocyte processes rich in glial fibrillary acidic protein (GFAP). These processes surround the bundles of axons passing through the LC.
• The fibrous astrocyte processes contain bundles of intermediate filaments and form the borders of the canals in the LC. The intermediate filaments provide tensile strength to the canals.
• The canals vary in diameter from 25-250µm. These canals are further subdivided into smaller compartments measuring 2.5-5.0µm by webs of smaller astrocytic processes.
• The LC region can be regarded as a specialized extracellular matrix with fenestrated sheets of connective tissue and occasional elastic fibers. The extracellular matrix components of the LC, such as collagen types I-VI, laminin and fibronectin, have characteristics which are different from those in the sclera. This could be a significant biomechanical factor in the development of GOA.
• The retinal nerve fibers pass through the LC, become aggregated to form the optic nerve behind the eyeball and travel towards the brain.
• The retinal nerve fibers from all over the retina converge towards the ONH.
• Thus, nearly 1-2 million afferent nerve fibers pass out through the LC posteriorly and form the optic nerve.

• The central retinal artery and vein also occupy this space.
• The optic disc is placed 3 to 4 mm to the nasal side of the fovea. It is a vertical oval, with average dimensions of 1.76mm horizontally by 1.92mm vertically.
• The center of the ONH has a depression of variable size, known as the optic cup.