78 Clinical Anatomy of the Visual System

Clinical Comment:

Electroretinogram­ (ERG)

An electroretinogram is a recording of the electrical response of the retina to light stimulus, which may be a flash of light or a light pattern. It can be measured in a clinical setting and can be useful diagnostically in differentiating certain retinal diseases.

NUMBER AND DISTRIBUTION OF NEURAL CELLS

In 1935, Østerberg94 estimated the cell count in the retina at 110 million to 125 million rods and 6.3 million to 6.8 million cones. More recent research indicates that there are 80 million to 110 million rods and 4 million to 5 million cones.95,96 The density of rods is greater than that of cones except in the macular region, where cones are concentrated; rods are absent from the foveola,15 the macular center (Figure 4-23). Rod density is greatest in an area concentric with the fovea, beginning approximately 3 mm (7 degrees) from it.52,60 The number of both types of photoreceptors diminishes toward the ora serrata.

There are approximately 35.68 million bipolar cells97 and 1.12 million to 2.22 million ganglion cells.98 The signals from numerous photoreceptors converge at one ganglion cell, indicating integration and refinement of the initial response of the photoreceptor cells.

PHYSIOLOGY OF THE NEURAL RETINA

The complex structure of the retina contains millions and millions of neurons and synapses, and has been extensively investigated in studies of cats, rabbits, and monkeys. Although most knowledge of the retinal circuitry is based on animal models, visual scientists have found much of the information to be applicable to the human retina.

Retinal Synapses

Information transmission between retinal neurons occurs by ion channel activity at gap junctions or by neurotransmitter release in chemical synapses. The gap junction is an electrical synapse, allowing current to pass directly between cells, ensuring a rapid rate of signal transmission; no chemical mediator is necessary. Gap junctions are found between photoreceptor and photoreceptor, between photoreceptor and horizontal cell, between horizontal cells, and between a bipolar axon and an amacrine process.8

Chemical synapses contain synaptic vesicles that release a neurotransmitter from the presynaptic terminal

FIGURE 4-23

Distribution of rods and cones in human retina.  Instead of retinal distances, Østerberg’s values84 for corresponding perimetric angles are given. Although approximate only, especially at higher angles, such values are more useful in

practice than are distances on the retina. Note that distribution of rods and cones on nasal side in and near the fovea, not given on this graph, would be approximately the same as distribution on temporal side of retina, which is seen to the left of vertical, passing through 0 degrees on the angle scale. (From Pirenne M: Vision and the eye, London, 1948, Pilot Press.)

into the synaptic cleft. The transmitter binds to specific sites on the postsynaptic membrane, eliciting an excitatory or inhibitory change in that neuron. In the OPL synapses occur either on the flat part of the pedicle or in invaginations in spherules and pedicles. The synapses in the invaginations are often ribbon synapses; ribbon synapses allow for fast and sustained neurotransmitter release. An electron-dense bar, surrounded by a large number of synaptic vesicles, extends into the cytoplasm perpendicular to the pre-synaptic membrane. The ribbonlike­ structure seems to guide the vesicles to a release site on the pre-synaptic membrane, causing sustained release. Calcium ion channels facilitate vesicle fusion with the membrane and promote high-speed release. Ten times more vesicles per second are released at a ribbon synapse than at a conventional synapse.27 Triads are ribbon junctions, located in the OPL, that have three postsynaptic processes; dyads are ribbon synapses found in the IPL with two postsynaptic processes. Although visual interpretation occurs in the striate cortex, there is significant organization and processing of neural signals in excitatory and inhibitory circuits within the retina. The process is extremely complex and most current understanding is based on animal studies.

Neurotransmitters

Glutamate is the excitatory neurotransmitter released by photoreceptors, bipolars, and ganglion cells. Glycine and GABA are inhibitory neurotransmitters released from amacrines; it is unclear what neurotransmitter horizontal cells secrete. In addition to neurotransmitters,