2.1.1 Ganglion cells
The optic nerve is a conduit for ganglion cell axons carrying partially processed visual information from the eye to the brain. The optic nerve is really a tract of white matter o f the central nervous system (CNS) that retains many CNS features such as being myelinated by oligodendrocytes, covered by meninges, and having astrocytes and microglia. Anatomically the optic nerve can be subdivided into four parts, intraocular {papilla, optic disc or optic nerve head), intraorbital, intracanalicular and intracranial. The optic nerve ends in the optic chiasm but ganglion cell axons continue as the optic tract to synapse in the lateral geniculate body. The intraocular portion of the optic nerve - the optic nerve head - is taken to be the site of end-organ damage in glaucoma and many o f the signs o f the disease are seen here.
The bodies of ganglion cells are in the ganglion cell layer of the retina. Here, visual information is received from rod and cone photoreceptors through synapses with bipolar cells, the second order neurones of the visual pathway, and intervening amacrine and horizontal cells. Ganglion cells are morphologically diverse and have been classified by their common histological and physiological features as seen in the retinae of primates, cats and rabbits.
Classification, Polyak (1941) classified ganglion cells as two functionally significant classes. Midget cells had small soma and single apical processes that gave off dendrites in the inner plexiform layer, and are now thought to correspond to a physiological class of cells called P-cells (Rodieck et al 1985; Shapley and
Perry, 1986; Kaplan et al, 1990). P-cells process visual information by receptive fields having a small concentric centre-surround organisation (order of magnitude of a single cone) and respond optimally to colour stimuli in the red-green spectrum, but have low contrast sensitivity. Speed of axonal conduction in P-cell fibres is considered slow to medium, and their axons project to the parvocellular layer o f the lateral geniculate nucleus.
Polyak described parasol cells as having larger bodies with several apical processes, giving off dendrites with more extensive arborisation than midget cells. Two physiological classes of cells are associated with parasol cells. The first has concentric centre-surround receptive fields and responds optimally to blue stimuli. The second has receptive fields that are sensitive to luminous contrast, motion, respond to a broad band of spectral sensitivity, and have high scotopic sensitivity. These cells are termed M-cells, have fast axonal conduction velocities and project to the magnocellular layer o f the lateral geniculate nucleus (Rodieck et al 1985; Shapley and Perry 1981; Kaplan et al, 1990). Additionally, existence of a third retinogeniculate pathway, called the koniocellular or K-cell pathway, which is anatomically distinct from M- and P-cells and mediates blue-yellow opponent information, has recently been reported (Martin et al, 1997).
About 90% of our million or so ganglion cells are classified as P-cells, with M-cells making up most of the remaining 10%. P- and M-cell physiological systems are called the ‘parallel pathways’ o f visual processing. Separate studies report the mean diameters of ganglion cell axons in the optic nerve as 0.72pm and 0.96pm (Repka and Quigley, 1989; Mikelberg et al, 1989). Ganglion cell axons transmit electrical nerve impulses, but they also transport essential neurotrophins and subcellular constituents to and from the brain.
Functional significance to glaucoma and its testing. Ganglion cell loss underlies the reduced visual sensitivity seen in glaucoma, and this can be replicated in experimental primate glaucoma (Harweth et al, 1999). In ‘early’ glaucoma, visual deficits that are not detectable by white-on-white perimetry may be detected by selectively testing visual sensitivity to short spectral wavelengths (Johnson et al, 1995), motion (Silverman et al, 1990), phase reversal/flicker (Anderson and
O ’Brien, 1997), contrast (Falcao-Reis et al, 1990) and the spatial frequency doubling illusion (Maddess and Henry, 1992), all of which probe functions conventionally attributed to the M-cell pathway. Whether abnormality on such testing reflects a preferentially reduced M-cell population is not clear. M-cells are suggested to correspond to large-bodied retinal ganglion cells, which in turn are reported to be selectively lost in glaucoma (Dandona et al, 1991; Glovinsky et al, 1991; Quigley et al, 1987). But some have not found any selectivity in cell loss (Kalloniatis et al, 1993; Vickers et al, 1997). Some studies of the lateral geniculate nucleus in human and primate glaucomatous eyes have reported more loss in its magnocellular than parvocellular layers (Dandona et al, 1991; Chatuverdi et al, 1993; Weber et al, 2000) but others (Yucel et al, 2000; 2001) found in primate experimental glaucoma that while lateral geniculate nucleus cells were lost both in its parvo- and magnocellular layers, parvocellular loss was greater. It may also be that the apparently selective loss of large ganglion cells simply reflects cell body shrinkage (Morgan et al, 2000; Weber et al, 1998). Alternatively, it could be that while both M- and P-cell populations are similarly reduced in glaucoma, visual abnormalities from M-pathway dysfunction become manifest first because M-cells are fewer, more sparse and have less ‘redundancy’ than do P-cells (by a factor of nine) (Johnson, 1994).
Ganglion cell death. Because of embryological attrition by apoptosis, adult eyes have half as many ganglion cells as fetal eyes (Rakic and Riley, 1983; Provis et al,
1985). In adults, ganglion cells are still lost, but more slowly, at an estimated rate of 500-2000 per year (Repka and Quigley, 1989; Mikelberg et al, 1989; Balaszi et al
1984). By what process the physiological loss of adult ganglion cells occurs is not well described.
In glaucoma, the primary site of ganglion cell axonal damage leading to cell death is believed to be at the level o f the optic nerve head’s lamina cribrosa which lies in line with the sclera (Anderson and Hendrickson, 1974; Minckler et al, 1977; Quigley et al, 1981). Here, mechanical distortion, diminished blood supply or secondary neurotoxicity could lead to cell damage. At least some ganglion cell death in glaucoma occurs by apoptosis (Quigley et al, 1995; Kerrigan et al, 1997; Okisaka et al, 1997), postulated as due either to primary injury or secondary degeneration (Schwartz and Voles, 2000). Apoptosis may be a pathway by which
putative pathological mechanisms such as elevated hydrostatic pressure (Agar et al, 2000; 2001), glutamate neurotoxicity (Dreyer et al, 1996; Vorwerk et al, 2000), neurotrophic deprivation (Pease et al, 2000; Yuan and Yanker, 2000) and autoimmunity (Tezel and Wax, 2000) cause ganglion cell death. Nitric oxide (Neufeld, 1999) and intracellular calcium toxicity and ischaemia have also been reported to lead to ganglion cell death, probably by affecting the mitochondria.
2.1.2 The retinal nerve fibre layer
The retinal nerve fibre layer (RNFL) contains the axons of ganglion cells travelling from the ganglion cell layer of the retina to the optic nerve head, and is visible ophthalmoscopically as fine, bright striations. Processes of Muller cells bundle axons together into fascicles surrounded by astroglia and a rich capillary bed. Axons tend not to deviate from their own bundles and course radially toward the optic nerve head where they turn perpendicularly to form the neuroretinal rim. They then traverse the lamina cribrosa, beyond which they become myelinated to speedily and efficiently conduct electrical impulses by saltatory conduction.
Axons from the upper and lower halfs of the retina do not intermingle and are separated by the median raphe passing through the fovea. Nasal axons travel straight to the optic nerve head. Axons originating in the foveal and macular region travel in the papillomacular bundle and enter the optic nerve head temporally. Axons from superior, inferior and temporal to the macular region arch around the papillomacular bundle to form the arcuate bundles and enter the optic nerve head superiorly and inferiorly. The arcuate bundles are thicker than the papillomacular bundle, where the RNFL is thinnest. The RNFL is thickest at the edge of the optic nerve head where it becomes heaped up, hence the term papilla (Quigley and Addicks, 1982; Radius, 1980; Varma et al, 1996).
2.1.3 The optic nerve head
The optic nerve head has a superficial nerve fibre layer, and pre-laminar, laminar and post-laminar portions. The superficial nerve fibre layer of the optic nerve head is covered by the inner limiting membrane of Elschnig, a continuation of the inner
limiting membrane of the retina which is derived from astrocytes. Astroglia between bundles of axons in this region is sparse but increases progressively towards the retro-laminar region; it makes up 5% of the volume o f the superficial nerve fibre layer, 15% of the pre-laminar region, and 23% of the laminar region (Minckler et al, 1976a). Wang et al (2002) report that astroglia in the retina and optic nerve head are morphologically and functionally changed in glaucoma.
The pre-laminar optic nerve head comprises the neuroretinal rim, optic cup and central retinal vessels. The neuroretinal rim contains bundles o f unmyelinated axons lying within channels made from astrocytes. Astrocytes and capillaries are interspersed between axons. Apart from structural support, astrocytes have a role in repair, glycogen storage, electrolyte homeostasis, inactivating neurotransmitters and participating in immune responses. The lamina cribrosa can be seen in the floor of the cup with its sieve-like pores. The central retinal vessels are surrounded by connective tissue and lie centrally in this region.
The lamina cribrosa in the laminar portion of the optic nerve head is a dense strip o f connective tissue bridging the scleral foramen where the strong wall of the eye is deficient. The remarkably engineered cribriform plates of the lamina have two roles to balance: they provide structural and nutritional support to axon bundles traversing its pores, while maintaining the structural integrity of the eye. The development of laminar trabeculae is accompanied by ingrowth o f branches of the short ciliary arteries and circle of Zinn, and scleral connective tissue and glia (Anderson et al, 1967). It is thus not surprising that each trabeculum has its own collagen-surrounded capillary. About ten stacked cribriform plates alternate with sheets of glia and blend peripherally with sclera (Anderson, 1969; Radius and Gonzales, 1981).
The cribriform plates have pores arranged almost but not exactly in anterior- posterior register to form tunnels for traversing axons. The course of axons does not strictly adhere to the tunnel arrangement, however, as a few axons deviate from one tunnel to another (Morgan et al, 1998). Deviant axons may be particularly susceptible to injury were the cribriform plates to collapse into each other as is reported to happen in glaucoma (Quigley et al, 1983). Astrocytes make up 40% of the axonal bundle tissue mass (Minckler et al, 1976a). The several hundred pores in
each plate (Ogden et al, 1988; Dandona et al, 1990) are bounded by strong trabeculae and vary in size by region. Quigley and Addicks (1981) showed that pores in the superior and inferior laminar quadrants are bigger than pores in the nasal and temporal quadrants so that the lamina cribrosa is not uniform in structure, being denser temporally and nasally than superiorly and inferiorly. The nature of pore size variation differs between individuals (Radius and Gonzales, 1981). It is postulated that structural variation within the lamina cribrosa and between people influences axon susceptibility to mechanical damage from TOP and predisposition to glaucoma.
The cribriform plates have a core of longitudinally orientated elastin fibres (Hernandez et al, 1987; 1989; 1991; 1992; 1994). The plates insert into a specialised concentric region of sclera that is rich in circumferentially orientated elastin. By contrast, the sclera outside this insertion region has short, sparse and randomly orientated elastin fibres. The cribriform plates are more elastin-dense than surrounding sclera and expected to be more flexible. With age, elastic fibres thicken, lengthen and look more tubular, and collagen fibres change in composition and become more densely packed so that the lamina cribrosa probably becomes more rigid.
In the post-laminar region, axons are myelinated posterior to the lamina cribrosa; this is the intra-orbital portion of the optic nerve. The nerve increases in calibre because o f myelination, is bathed in cerebrospinal fluid, and invested in thick meninges. Myelination is by oligodendrocytes, and immunologically competent microglia here participate in phagocytosis.