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An excellent example is shown in the suction electrode recordings of monkey rods by Schneeweis and Schnapf Fig. Each dot in the figure below represents delivery of a very dim pulse of light containing only a few quanta. Voltage responses appear to come in 3 sizes: none, small, and large, representing the detection of 0, 1 or 2 quanta in each flash.
The granularity of response to dim light stimuli is evident. Photovoltages recorded in monkey rods. Rod sensitivity appears to be bought at a price, however, since rods are much slower to respond to light stimulation than cones. This is one reason why sporting events such as baseball become progressively more difficult as daylight fails. Ultrastructure of rod and cone synaptic endings. The job of the photoreceptor cell in the retina is to catch quanta of light in the visual pigment-containing membranes of the outer segment and pass a message, concerning numbers of quanta of light and sensitivities to the different wavelengths, to the next stage of integration and processing at the outer plexiform layer see Phototransduction.
The information transmitting end of the cone cell is known as the pedicle and of the rod cell as the spherule. They lie packed between and above the cone pedicles Fig.
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At their synapses to second-order neurons bipolar and horizontal cells , both rod spherules and cone pedicles exhibit dense structures known as synaptic ribbons pointing to the postsynaptic invaginated processes asterisks in Fig. In the rod spherule 2 ribbons are associated with 4 invaginated second-order neurites while the cone pedicle delivers information to over a hundred second-order neurons Fig.
In addition, other varieties of bipolar cell have dendrites making synaptic contacts on the under surface of the cone pedicle at what were first called flat contacts FBC Missotten, ; Dowling and Boycott, ; Kolb, Fig. Rods spherules have only two synaptic ribbons associated with two lateral elements that are horizontal cell axon terminals HC and two central invaginating dendrites of rod bipolar cells rb Missotten, ; Dowling and Boycott, ; Kolb, There are no basal junctions on rod spherules. Rod triad. Interphotoreceptor contacts at gap junctions. There also appears to be a pathway for crosstalk between cones and cones and cones and rods in the human retina.
Cone pedicles have small projections from their sides or bases that pass to neighboring rod spherules and cone pedicles. Where these projections, called telodendria, meet they have a specialized junction known to be typical of electrical synaptic transmission. These are minute gap junctions Fig. Gap junctions between photoreceptors. As many as gap junctions occur on a single rod spherule from neighboring cone telodendria, and a single cone pedicle can have as many as 10 contacts to neighboring rods. Pedicles of S-cones do not have as many telodendrial gap junctions with either neighboring rods or cones Ahnelt et al.
Direct interactions between different functional classes of photoreceptors were not anticipated based on any known or theoretical needs of the visual system. In fact such connections would appear to degrade spatial resolution, and, potentially, color perception by mixing signals from photoreceptors at different locations or with different photopigments. Nonetheless, in agreement with anatomical findings, mammalian cones appear to carry rod signals. The slow wave forms of rod signals are easily distinguished from the rapid wave forms of cone signals in voltage recordings from single cones.
Examples of voltage responses from monkey cones are illustrated in Figure 30 Schneeweis and Schnapf, Voltage records resulting from balanced red and green stimuli are illustrated. Voltage recording from monkey cone. Both records exhibit the same initial peak hyperpolarization in response to the brief stimulus, however the green stimulus solid trace also evokes a slower hyperpolarizing phase after the initial response which the red stimulus dotted trace does not Fig.
This latter electrical wave has the characteristics of a rod signal.
Such signals have also been observed in cat cones Nelson, One theory of the utility of this arrangement is that it allows rods to utilize neural pathways devoted to both cones and rods in sending visual information to the inner plexiform layer. Cone pathways may be tuned to faster temporal characteristics than rod pathways, and so by utilizing both pathways, rods may transmit a wider bandwidth of temporal information.
There is evidence for two rod pathways with different dynamic signatures in perceptual experiments Sharpe et al, Although the functional role of interreceptor junctions is still a matter of debate, they perhaps serve as a philosophical warning to studies of biological sensory systems: Not even the receptor cells themselves stand in isolation of the activity and influence of neighboring neurons.
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Retinol-binding in bovine interphotoreceptor matrix. Biochem Biophys Res Commun. Identification of a subtype of cone photoreceptor, likely to be blue sensitive, in the human retina. J Comp Neurol. Identification of pedicles of putative blue sensitive cones in human and primate retina. Ammermuller, J. Kolb Functional architecture of the turtle inner retina.
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The ‘Third’ Photoreceptor System of the Eye – Photosensitive Retinal Ganglion Cells
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