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Biomedical subjects

D A Burkhardt

Publications and source records attributed to D A Burkhardt.

At least 37 records · Page 2Linked to original sources
PubMed1980

Electrical responses and photopigments of twin cones in the retina of the walleye.

1. The properties of twin and single cones in the retina of the walleye (Stizostedion vitreum vitreum) were studied by intracellular recording, dye injection and microspectrophotometry. 2. Twin cones generate hyperpolarizing responses to central illumination, can receive depolarizing influences (feed-back) from the receptive field surround, and show no detectable dye coupling when injected with Procion yellow. In seventeen of eighteen dye-injected cones, fluorescence was intense in the inner segment and undetectable or weak in the cone pedicle. 3. Both members of the twin cone contain the same photopigment in their outer segments. It absorbs maximally at about 605 nm. 4. A 533 nm green-sensitive photopigment was found in single cones. No blue-sensitive cones have been found. 5. With the exception of a modest discrepancy in the violet, the absorptance spectrum of the 605 nm photopigment of twin cones agrees closely with the action spectrum measured by intracellular recording. 6. The spectral properties established by the twin cone's photopigment are not detectably altered by the hyperpolarizing influences arising from nearby cones or by the depolarizing influences arising from the receptive field surround. 7. The twin cones of the walleye retina are thus "identical twins', both photochemically and physiologically, and seem designed to function as long-wave, spectrally univariant receptor units for colour vision. 8. The available evidence suggests that identical twin cones differ functionally from double cones and non-identical twin cones. 9. Although they outnumber single cones by about three to one in adults, identifiable twin cones were rarely observed in the cone population of retinas examined 3-5 days after birth. If walleye twin cones develop by fusion of single cones this process apparently occurs only for cones containing the 605 nm photopigment.

Animals↗
PubMed1979

Synaptic connections linking cones and horizontal cells in the retina of the pikeperch (Stizostedion vitreum).

Cones and horizontal cells of the pikeperch retina were studied with morphological and physiological techniques. Gap junctions were observed between cone pedicles and basal processes emitted by neighboring cones. Intracellular recordings showed that the light-evoked hyperpolarizing cone response was enhanced by light falling upon neighboring receptors within a radius of 50 microns. We suggest that the network of gap junctions between cones mediates the summative lateral interaction described. Three sub-classes of horizontal cells (H1, H2, H3) send dendrites to cones; H1 and H2 cells appear to contact twin cones, exclusively or preferentially, whereas H3 cells appear to synapse only with single cones. Horizontal cells of the same sub-class are joined by gap junctions between dendrites or at the lateral faces of perikarya. These unions extend over several micron 2 and as seen in transmission electron microscopy consist of patches of close apposition alternating with areas of membrane separation, folding and occasional zonulae adherents. Freeze-fracture profiles of horizontal gap junctions show localized areas of dense particle aggregation on the P-face and pits on the E-face flanked by regions of unspecialized membrane. These morphological findings provide support for the known spatial and color-coding properties of pikeperch horizontal cells.

Animals↗
PubMed1978

Influences of cones upon chromatic- and luminosity-type horizontal cells in pikeperch retinas.

1. The spectral sensitivity and spatial organization of cones and horizontal cells have been analysed by intracellular recording in pikeperch retinas. 2. The vast majority of cone recordings were obtained from orange-sensitive cones. They have an action spectrum which peaks at about 605 nm. Recordings from several green-sensitive cones have also been obtained. 3. The results of action spectrum measurements and spectral screening tests indicate that the vast majority of luminosity-type horizontal cells receive predominant input from the orange-sensitive cones. 4. Chromatic-type horizontal cells were recorded at more proximal levels of the retina than luminosity-type cells and were the classic red-depolarizing, green hyperpolarizing (R/G) type. 5. The action spectra of the depolarizing and hyperpolarizing responses of chromatic horizontal cells peak at about 650 and 530 nm, respectively. When the depolarizing mechanism is selectively depressed by a red background field, the action spectrum of the hyperpolarizing mechanism shows an enhanced sensitivity, peaks at 530--540 nm, and may approximate the action spectrum of the green-sensitive cones. 6. Small red fields evoke depolarizing responses from chromatic-type horizontal cells but do not seem to significantly activate the depolarizing surround mechanism of cones. 7. These and other results suggest that the colour-opponent properties of the chromatic-type horizontal cells are not fundamentally dependent upon feed-back to cones but rather originate from antagonistic interactions generated in post-receptor networks.

Action Potentials↗
PubMed1977

Responses and receptive-field organization of cones in perch retinas.

1. Cones in the retinas of two closely related species of perch, the walleye and sauger (S, vitreum vitreum and S. canadense), are remarkably large. This paper reports a first series of intracellular recordings obtained from 77 of these cones. 2. A small spot of light evokes a sustained hyperpolarizing response from perch cones which may exceed 10 mV in amplitude, is graded with stimulus intensity, and is markedly reduced when the spot is decentered. Most cones seem to be orange sensitive with peak sensitivity at about 600 nm. 3. Enlarging the stimulus diameter from 0.04 to 0.25 mm produces a modest increase in the hyperpolarizing response. However, larger stimuli which illuminate surrounding regions of the retina often evoke a delayed depolarizing potential which antagonizes the sustained phase of the cone's hyperpolarizing response to central illumination. 4. The outer diameter of the region of the antagonistic surround is at least 2.2 mm in extent. An annulus evokes a depolarizing response only if the central region of the receptive field is simultaneously activated. 5. The present results provide the first direct evidence that the receptive fields of cones in fish retinas have an antagonistic center-surround organization. Luminosity-type horizontal cells probably serve as the interneurons which mediate the depolarizing influence of the surround.

Animals↗
PubMed1974

Sensitization and centre-surround antagonism in Necturus retina.

1. The impulse discharge of ganglion cells and the proximal negative response were recorded with extracellular micro-electrodes in Necturus retina and spatial influences of background illumination studied.2. When a steady background of constant illuminance was extended in diameter from 0.25 to 1 mm, the response to a small concentrically placed test flash was enhanced (sensitization).3. This sensitization was tonic, showed marked spatial summation, was also found by surrounding a central background with an annulus, and when quantified by threshold measurements, closely resembled the sensitization effect found psychophysically in human vision.4. Sensitization was prominent in intracellular recordings from bipolar cells and usually apparent in horizontal cells. In bipolar cells, a background change which produced sensitization evoked a large but predominantly transient potential whose polarity was opposite to that evoked by a central flash.5. The tonicity and large spatial extent of the sensitization effect strongly suggest that it is mediated by horizontal cells. They might do this by either acting upon receptors, bipolar cells, or both.

Animals↗
PubMed1973

Intensity coding in the frog retina. Quantitative relations between impulse and graded activity.

The impulse discharge of single on-off neurons and a graded field potential, the proximal negative response (PNR), were simultaneously recorded with an extracellular microelectrode in the inner frog retina. Normalized amplitude-intensity functions for the on-response of the PNR and the neuron's post-stimulus time histogram (PSTH) were nearly coincident and typically showed a dynamic range spanning approximately 2 log units of intensity. Thus a nearly linear relation is found between the amplitude of the PNR and the neuron's PSTH. A neuron's PSTH amplitude and maximum instantaneous frequency of discharge were usually highly correlated, but occasional marked disparities indicate that temporal jitter of the first spike latency is an additional, relatively independent variable influencing PSTH amplitude. It typically changes by a factor of 20-30 over the intensity range. These and other findings have implications for the functional significance of the PNR and the PSTH, for a possible linear link between amacrine and on-off ganglion cells, and for a mechanism of intensity coding in which temporal jitter of latency exerts a major role.

Action Potentials↗