The major route of information flow from photoreceptors to the optic nerve consists of a 3-neuron chain—photoreceptor cell to bipolar cell to ganglion cell. Horizontal cells and amacrine cells are interneurons that regulate the flow of information. Glial cells and vascular elements support the neuronal components.
Retinal bipolar cells receive neural signals from photoreceptors (discussed earlier in the chapter) and convey them to the inner retina. Separate bipolar cells exist for cones and rods. Morphologically, there are 9–12 different kinds of cone bipolar cells but only 1 type of rod bipolar cell. Functionally, in the cone pathway there are on-bipolar and off-bipolar cells (Fig 12-6). On-bipolar cells are optimized to detect increases in light intensity, and off-bipolar cells detect decreases in light intensity. When light hyperpolarizes a cone, the on-bipolar cell is excited (turned on), and the off-bipolar cell is inhibited (turned off). When a shadow depolarizes the cones, the opposite occurs (see Fig 12-3).
Some cone bipolar cells synapse only with L cones and others only with M cones (see the section “Trivariant color vision”), a differentiation that is necessary for color vision. In the fovea, some cone bipolar cells (midget bipolar cells) synapse with a single L or M cone, which allows the highest spatial acuity. This cone selectivity is preserved throughout the ganglion cell layer. Selectivity for L- or M-cone inputs is transmitted by a tonic responding system of small ganglion cells. Separate L- and M-cone on-bipolar cells and off-bipolar cells transmit a faster, phasic signal to a parallel system of larger ganglion cells. Rods and probably S cones have only on-bipolar cells. Thus, neither rods nor S cones are involved in high spatial resolution. S cones are involved in color vision; rods, in dim light (night vision).
Figure 12-5 Schematic of the 3 major classes of retinal cells: glial cells (Müller cells, astrocytes, and microglia); neurons (photoreceptor, bipolar, horizontal, amacrine, and ganglion cells); and vascular cells (pericytes and endothelial cells).
(Reproduced with permission from Gardner TW, Antonetti DA, Barber AJ, LaNoue KF, Levison SW. Diabetic retinopathy: more than meets the eye.
2002;47(Suppl 2):S253–S262. Fig 1.)
Figure 12-6 Basic circuitry of the cones. Separate on- and off-bipolar cells contact each cone. In the fovea, a cone has midget bipolar cells contacting only a single cone, and usually a single ganglion cell, for high spatial acuity. Horizontal cells are antagonistic neurons between cones. Absorbing light hyperpolarizes the cone; this, in turn, hyperpolarizes the horizontal cell, which resembles an off-bipolar cell.
(Courtesy of Peter Gouras, MD.)
Horizontal cells are antagonistic interneurons that provide negative feedback to photoreceptors (see Figs 12-5, 12-6). The dendrites of horizontal cells synapse with cones. One type of horizontal cell modulates L and M cones; another type modulates mainly S cones. The dendrites of horizontal cells receive glutamate from cones and rods and release GABA back onto them. This process provides negative feedback. When light causes the cone to hyperpolarize and stop its transmitter release, the neighboring horizontal cells are also hyperpolarized (turned off). This effect stops the release of GABA from the horizontal cell onto the cone, consequently depolarizing the cone. This feedback inhibition allows visualization of low-contrast details against background luminance.
Like horizontal cells, amacrine cells are inhibitory interneurons. Cone amacrine cells mediate antagonistic interactions among on-bipolar, off-bipolar, and ganglion cells. Rod bipolar cells do not usually synapse directly with ganglion cells but rather send their signal to amacrine cells, which then deliver the signal to on- and off-bipolar ganglion cells. Thus, rod signals undergo additional synaptic delays before they reach the ganglion cell output.
The functional division of the cone pathway into on and off channels begins at the first synapse (between the cones and the on-bipolar or off-bipolar cells). This division is preserved across the pathway to the higher visual centers. On-bipolar cells synapse with on-ganglion cells and off-bipolar cells with off-ganglion cells. Midget ganglion cells, a special type of ganglion cell with small dendritic trees, are dominant in the central macula. They have a 1:1:1 ratio with cones and midget bipolar cells, allowing high spatial resolution.
Ganglion cells can be divided into 3 subgroups: (1) tonic cells driven by L or M cones; (2) tonic cells driven by S cones; and (3) phasic cells. The tonic system transmits signals from the cones that are relatively well maintained for the duration of the light or dark stimulus. The phasic system transmits signals at the beginning or end of a light stimulus, producing a brief or transient response.
Tonic cells driven by either L or M cones include small cells concentrated in the fovea (responsible for high acuity) and other cells located extrafoveally. They mediate both high spatial resolution and color vision. Tonic cells driven by S cones are designed to detect successive color contrast, for example, blue-yellow or gray-brown borders. These ganglion cells are excited by short waves entering and long waves leaving their receptive fields. Phasic cells are larger, less concentrated in the fovea, and faster conducting than the other ganglion cells. Phasic cells may be important in movement detection.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.