Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

and ganglion cells are slightly denser in the superonasal retina than the inferotemporal retina. This asymmetric distribution reflects the asymmetry of the visual field and accounts in part for the relatively shortened nasal field.

Dark and Light Adaptation

The sensitivity of the photoreceptors adapts dramatically over a wide range of light levels. The cones dark-adapt more rapidly than the rods and reach maximal light sensitivity after 10–12 min. The rods dark-adapt at a slower rate but reach a plateau of much higher light sensitivity after 30–40 min (Fig. 1.23). Under scotopic conditions, the rods are 100to 1000-fold more sensitive to light than the cones, and a dim flash stimulus activates only rods.

In a lit environment, the sensitivity of the photoreceptors falls rapidly but full light adaptation is not reached until after 15–20 min of light exposure. Under photopic conditions, the cones are very responsive and the rods are suppressed, and a flash stimulus activates only cones. At least 10 min of light adaptation are needed to stabilize photopic ERG responses (2).

Photoreceptor Structure and Renewal

Each photoreceptor cell consists of an inner and an outer segment. The inner segments of the photoreceptors contain the cell nuclei and make up the outer nuclear layer of the retina. The outer segments are cellular extensions contacting the retinal pigment epithelium. The outer segments contain intracellular membranes where initial light activation of the retina takes place. The intracellular membranes of the rods are disc-shaped and are separate from the plasma membrane while the internal membranes of the cones are formed by infoldings of the plasma membrane. The outer segments are continuously regenerated, shed, and phagocytized by the retinal pigment epithelium. The greatest rate of shedding occurs when the photoreceptors are diurnally the least active—at about one to three hours following onset of daylight for rods and in the early darkness hours for cones.

Figure 1.23 A schematic diagram of a dark adaptation curve from a normal subject. The shape of the dark adaptation curve varies with the retinal region tested. In the Goldmann–Weekers dark adaptometer, the light threshold is repeatedly tested in darkness by a 11 circular white test light of gradual-increasing intensity presented at 7 above the fovea. The difference between the dark-adaptive properties of the rods and cones explains the biphasic nature of the curve. The first 10 min of the curve is dominated by the more rapid dark adaptation of the cones which reach their maximal light sensitivity after about 10–12 min. The rods dark-adapt at a slower rate but reach a much lower final light threshold after about 40 min in darkness. The fully dark-adapted retina is in the range of 1000-fold more sensitive to light than the light-adapted retina.

Initial Photoreceptor Response to Light

Light activates the light-sensitive visual pigments in the outer segment of the photoreceptors and trigger events leading to the ERG response. The rod system is better understood because rods are present in all mammals as well as many non-

mammals and are therefore easier to study while primates are the only mammals known to have three types of cones. Cone visual pigments are also much less abundant and less stable than the rod visual pigment.

All human rod and cone visual pigments consist of opsin, an intergral intracellular membrane protein of the photoreceptor outer segment, attached to 11-cis-retinaldehyde (11- cis-retinal), a light-sensitive chromophore molecule derived from vitamin A. The amino acid sequences of rod opsin and the three types of cone opsins are similar, and the chromophore is the same for all four types of visual pigments. The rod visual pigment is called rhodopsin. Light initiates visual excitation by isomerizing the chromophore from 11- cis-retinal to 11-trans-retinal producing an unfolding of the attached opsin. During this light bleaching process, rhodopsin transforms into several transient intermediaries such as metarhodopsin I and metarhodopsin II before fully activated. A single rod photoreceptor cell contains 108–109 rhodopsin molecules and the likelihood of passing photons being detected is high.

The Visual Cycle

The recycling or regeneration of the chromophore is called the visual cycle (87). This process involves the photoreceptors and the retinal pigment epithelium (Fig. 1.24). Genetic mutations of several proteins of the visual cycle are associated with retinal dystrophies. For example, heterozygous recessive mutations of the gene encoding adenosine triphosphate (ATP)-binding transporter protein called ABCA4 are associated with Stargardt macular dystrophy. Recessive mutations of RPE65 protein cause Leber congenital amaurosis and retinitis pigmentosa.

Phototransduction

The phototransduction cascade is the series of biochemical reactions initiated by the light-activated visual pigment that decreases sodium and calcium ion permeability of the photoreceptor plasma membrane leading to a lower rate of

52 Chapter 1

release of the photoreceptor neurotransmitter, glutamate (Fig. 1.25).

In darkness, the sodium ion (Naþ) and calcium ion (Ca2þ) channels of the outer segment of the rod are open, allowing Naþ and Ca2þ into the cell. The Naþ=Ca2þ–Kþ exchange pumps at the outer segment cellular membrane and a compensatory extrusion of potassium ion (Kþ) at the inner segment maintain the intracellular and extracellular cation concentration. This circulating dark current maintains the rod in a relatively depolarized state, and the release of its neurotransmitter, glutamate, continuous at a relatively high rate.

With light, phototransduction causes the closure of the outer segment Naþ and Ca2þ channels, and the release of glutamate is diminished (Fig. 1.25). Light-activated rhodopsin activates transducin that in turn activates phosphodiesterase which subsequently hydrolyzes cyclic guanosine monophosphate (cGMP). Amplification occurs so that one activated rhodopsin produces more than 100 activated transducin molecules and one activated phosphodiesterase can hydrolyze thousands of cGMP. Therefore, one activated rhodopsin can lead to the hydrolysis of 100,000 cGMP. With a decrease in intracellular cGMP, the cellular membrane Naþ and Ca2þ channels close and the rate of release of glutamate is decreased. This hyperpolarization of the photoreceptor causes an increase of predominantly extracellular Naþ as well as Ca2þ. This relative increase in outer retina positivity is measured indirectly at the cornea as the initial negative portion of the ERG a-wave. The modulation of the phototransduction process is complex and involves Ca2þ as well as Ca2þ-binding proteins.

Transmission and Processing of Visual Signals from the Photoreceptors

Different characteristics of the light signal such as brightness and color are processed by the bipolar, amacrine, and horizontal cells of the inner retina before reaching the retinal ganglion cells. The cone and rod systems are not independent.

Figure 1.25 Schematic depiction of phototransduction of the rod photoreceptor. In darkness, the Naþ and Ca2þ channels of the outer segment of the rod are open, allowing Naþ and Ca2þ into the cell. With light, a series of biochemical reactions reduces the intracellular concentration of cGMP. The Naþ and Ca2þ channels close, and the rate of release of neurotransmitter glutamate by the photoreceptor is decreased.

The cones and rods are connected by electrical synapses, and a multitude of synapses connects the photoreceptors and to these inner retinal neuronal cells.

The optic nerve consists of approximately 1.2 million axons from the retinal ganglion cells and delivers the visual

signal as electrical action potentials to the brain. The axons are eventually separated into tracts that terminate in different areas such as the suprachiasmal nucleus, the lateral geniculate nucleus, the pretectum, the superior colliculus, and the accessory optic nuclei. Each of the 10–20 types of retinal ganglion cells carries specific information of the light stimulus (e.g., brightness, movement, etc.). Each ganglion cell type receives dendritic inputs from the entire retina and a light stimulus, regardless of its retinal location, is detected by all ganglion cell types. In this way, different aspects of the visual signal are initially processed together in parallel within the retina.

Receptive Fields and ONand OFF-Responses

The receptive field of a neuronal cell is the visual field area where a change in light will alter the cell’s activity. This physiologic concept is applicable to any cell at any level of visual processing. For example, each bipolar cell has its own receptive field, and a considerable overlapping of receptive fields occurs for bipolar cells that are near one another. The receptive field can be divided into center and surround regions, and the cellular response is different for a spot of light positioned at the center vs. surround regions. The specific cellular response is dependent on the size, shape, orientation, intensity, contrast, color, motion, direction, and duration of the light stimulus. In general, the cells can be divided into ‘‘on’’ and ‘‘off ’’ pathways. Neurons that depolarize in response to increased light in their receptive fields are called ON-cells, and neurons that depolarize in response to reduced light are called OFF-cells. Therefore, bipolar cells that depolarize to light are ON-bipolar cells and those that hyperpolarize to light are OFF-bipolar cells. This concept may be applied to other levels of visual processing, and ganglion cells may be classified as ONand OFF-ganglion cells. By definition, all photoreceptors are ‘‘OFF’’ cells, because they hyperpolarize with light and depolarize in darkness.

Processing of Photoreceptor Signals in

the Inner Retina

The bipolar cells are the primary receivers of the visual signals from the photoreceptors. Approximately 10 different types of mammalian bipolar cells can be differentiated anatomically based on shape and axonal position in the inner plexiform layer where the axons of a specific bipolar cell type terminate in a specific sub-layer. This stratification also applies to the dendrites of ganglion cells, and dendrites of OFF-ganglion cells generally originate from the outer half of the inner plexiform layer and those of ON-ganglion cells originate from the inner half of the layer (88,89). Of the 10 types of bipolar cells, nine have cone photoreceptor contacts while only one type has rod contacts. Physiologically, the cone bipolar cell types can be classified into ONand OFF-cells.

In the outer plexiform layer, the photoreceptors synapse with bipolar and horizontal cells. Dendrites of the rod-specific bipolar cell type make invaginated contacts with rod spherules, the structural endings of the rod photoreceptor. The axons of the rod-specific bipolar cells terminate in the inner region of the inner plexiform layer and synapse with a specific amacrine cell type called AII amacrine cells. The AII amacrine cells have narrow receptive fields and make inhibitory chemical synapses with OFF-cone bipolar and OFF-ganglion cells, and electrical synapse though gap junctions with ON-cone bipolar cells (Fig. 1.26) (90).

In contrast, the structural endings of the cone photore- ceptors—the cone pedicles—have three kinds of synaptic specializations (91). First, each pedicle has gap junctions for electrical contacts with other cone pedicles and rod spherules. Second, the pedicle has invaginations with synaptic arrangement called triad; each consisting of two lateral elements contacting horizontal cells and at least one central element contacting an ON-bipolar cell dendrite. Third, the cone pedicle makes flat contacts with OFF-bipolar cells. Therefore, the cone pedicles generally make invaginated contacts with ON-bipolar cells and flat contacts with OFF-bipolar cells. The number and characteristic of the connections are complex and dependent

Figure 1.26 Rod and cone photoreceptor ON and OFF connections. The rod-specific ON-bipolar cell uses an intermediary (AII amacrine cell) to connect to the ONand OFF-ganglion cells. The AII amacrine cells have inhibitory chemical synapses with OFFcone bipolar and OFF-ganglion cells, and electrical synapse though gap junctions with ON-cone bipolar cells.

on bipolar cell type and cone location. The number of cone contacts for a bipolar cell is highly variable. For instance, bipolar cells providing high spatial resolution information convey signals from only a few cones, whereas those bipolar cells with high contrast sensitivity receive input from many cones.

The inner retinal ERG signal is a summation of all of the cells of the inner retina with primary contributions from depolarizing ON-bipolar cells and hyperpolarizing OFF-bipolar cells (92). The predominantly depolarization process results in an outflow of intracellular potassium ion (Kþ) resulting in increased extracellular Kþ in the outer plexiform layer. This produces a depolarization of the Mu¨ller cells generating a transretinal potential that is measured as the corneal positive ERG b-wave.

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