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R L Chappell

Publications and source records attributed to R L Chappell.

27 records · Page 2Linked to original sources
PubMed1981

Synaptic feedback onto photoreceptors in the ocellar retina.

Intracellular studies from photoreceptors and second order neurons in the dragonfly ocellar retina suggest that the hyperpolarizing OFF oscillation in the photoreceptor reflects synaptic feedback from second order neurons onto receptor terminals. The receptor OFF response was normally observed when recording more proximally, closer to the nuclear and synaptic region, but it was not seen when recording more distally, closer to the rhabdomeric end of the cell. Both the hyperpolarizing OFF response in the receptor and the depolarizing OFF response in the second order cell are apparently generated in the ocellar plexiform layer because they were not eliminated when the second order processes were isolated from the brain. In both intact and cut nerve preparations, the receptor OFF response was blocked by curare and enhanced by picrotoxin, the same drugs that were reported to selectively modify the response of the second order cell. In addition, a normal-appearing OFF response was recorded intracellularly from the dark-adapted photoreceptor in response to the application of brief hyperpolarizing current pulses to the ocellar nerve. These results support a model of sign-conserving feedback from second order neurons onto receptor terminals and are consistent with the hypothesis that the receptor transmitter may be acetylcholine and the feedback transmitter could be GABA.

Animals↗
PubMed1981

Changes in ERG b-wave and Müller cell structure induced by alpha-aminoadipic acid.

The role of the Müller cell in the generation of the ERG b-wave was studied in the skate eye by examining the effects of a gliotoxic agent (alpha-aminoadipic acid; alpha-AAA) on retinal structure and function. Superfusing the eyecup for 1 h in 50 mM alpha-AAA resulted in the loss of the b-wave and extensive damage to glial cells, i.e. disruption of the cells' plasma membranes, and a marked loss of cytoplasmic substance. Of the other retinal elements, only the horizontal cells showed some signs of injury in alpha-AAA. On returning the retina to a normal Ringer solution, the widespread loss of cytoplasmic electron density persisted, but the Müller cell membranes appeared to have undergone repair, and the b-wave recovered fully its normal amplitude and waveform.

Adipates↗
PubMed1978

Lateral ocellar nerve projections in the dragonfly brain.

The central projections of the lateral ocellar neurons of the dragonfly were examined using whole nerve cobalt iontophoresis, supplemented by sectioning of the nerve and brain for inspection in the light and electron microscopes. At E.M. level the presence of cobalt in filled axon profiles and cell bodies was confirmed by analysis of X-ray energy spectra in the microscope. The pathways, cell body sites and terminal arborizations of four large (7--25 micrometer diameter) lateral ocellar neurons are described. Two of these fibers arborize in the ipsilateral posterior neuropil of the protocerebrum and two cross the brain and arborize in the contralateral posterior neuropil. Within each half of the posterior neuropil, two spatially separated regions of ocellar input have been identified. These regions receive median ocellar input plus input from either the ipsi- or contralateral ocellus, but not both. The arborizations of the contralateral fibers are more extensive than those of the ipsilateral fibers. One of the contralateral neurons crosses the brain in the region of the protocerebral bridge giving off a collateral in that region before descending to the posterior neuropil. This collateral arborizes almost immediately in a region receiving input from arborizations of a number of small ocellar neurons (those less than 5 micrometer in diameter) from the ipsilateral ocellar nerve, together with small neurons from the median ocellar nerve, forming a region in each half of the brain which receives input from all three ocelli. The small lateral ocellar neurons associated with these arborizations have cell bodies adjacent to the lateral ocellar tracts.

Animals↗
PubMed1978

Feedback synaptic interaction in the dragonfly ocellar retina.

The intracellular response of the ocellar nerve dendrite, the second order neuron in the retina of the dragonfly ocellus, has been modified by application of various drugs and a model developed to explain certain features of that response. Curare blocked the response completely. Both picrotoxin and bicuculline eliminated the "off" overshoot. Bicuculline also decreased the size of response and the sensitivity. gamma-Aminobutyric acid (GABA), however, increased the size of response. The evidence indicates the possibility that the receptor transmitter is acetylcholine and is inhibitory to the ocellar nerve dendrite whereas the feedback transmitter from the ocellar nerve dendrite may be GABA and is facilitory to receptor transmitter release. The model of synaptic feedback interaction developed to be consistent with these results has certain important features. It suggests that the feedback transmitter is released in the dark to increase input sensitivity from receptors in response to dim light. This implies that the dark potential of the ocellar nerve dendrite may be determined by a dynamic equilibrium established by synaptic interaction between it and the receptor terminals. Such a system is also well suited to signalling phasic information about changes in level of illumination over a wide range of intensities, a characteristic which appears to be a significant feature of the dragonfly median ocellar response.

Animals↗
PubMed1975

Action spectra and chromatic mechanisms of cells in the median ocelli of dragonflies.

Spectral sensitivities were recorded intracellulary in median ocelli of Anax junius, Aeschnatuberculifera, and Libellulapulcella. All cells had peak sensitivities at 360 and 500 nm while UV-blue+green cells found only in Anax had a third peak sensitivity at 440 nm. Ratios of UV-to-green sensitivities varied from cell to cell in each ocellus, but no UV-only or green-only cells were recorded. Half of the cells tested had a reverse Purkinje shift: They were more sensitive in the green at low illuminations but more sensitive in the UV at high illuminations; their intensity-response curves at 370 and 520 nm crossed but became parallel for large responses. Wave-lengths 420 nm and shorter elicited a family of low intensity-response curves with one slope; wavelengths 440 nm and longer elicities a family of curves with another slope. Orange-adapting lights selectively adapted sensitivity in the green, but UV-adapting lights had little selective effect. Amounts of log-selective adaptation were proportional to log orange-adapting intensity. It is concluded that two spectral mechanisms can be recorded from each cell, possibly by coupling of UV and green cells or possibly because each cell contains two visual pigments. Selective chromatic adaptations may provide the ocellus with a kind of "authomatic color control," while the reverse Purkinje shift could extend the ocellus' sensitivity to prevailing skylight.

Animals↗
PubMed1972

Neural organization of the median ocellus of the dragonfly. I. Intracellular electrical activity.

Intracellular responses from receptors and postsynaptic units have been recorded in the median ocellus of the dragonfly. The receptors respond to light with a graded, depolarizing potential and a single, tetrodotoxin-sensitive impulse at "on." The postsynaptic units (ocellar nerve dendrites) hyperpolarize during illumination and show a transient, depolarizing response at "off." The light-evoked slow potential responses of the postsynaptic units are not altered by the application of tetrodotoxin to the ocellus. It appears, therefore, that the graded receptor potential, which survives the application of tetrodotoxin, is responsible for mediating synaptic transmission in the ocellus. Comparison of pre- and postsynaptic slow potential activity shows (a) longer latencies in postsynaptic units by 5-20 msec, (b) enhanced photosensitivity in postsynaptic units by 1-2 log units, and (c) more transient responses in postsynaptic units. It is suggested that enhanced photosensitivity of postsynaptic activity is a result of summation of many receptors onto the postsynaptic elements, and that transients in the postsynaptic responses are related to the complex synaptic arrangements in the ocellar plexus to be described in the following paper.

Animals↗
PubMed1972

Neural organization of the median ocellus of the dragonfly. II. Synaptic structure.

Two types of presumed synaptic contacts have been recognized by electron microscopy in the synaptic plexus of the median ocellus of the dragonfly. The first type is characterized by an electron-opaque, button-like organelle in the presynaptic cytoplasm, surrounded by a cluster of synaptic vesicles. Two postsynaptic elements are associated with these junctions, which we have termed button synapses. The second synaptic type is characterized by a dense cluster of synaptic vesicles adjacent to the presumed presynaptic membrane. One postsynaptic element is observed at these junctions. The overwhelming majority of synapses seen in the plexus are button synapses. They are found most commonly in the receptor cell axons where they synaptically contact ocellar nerve dendrites and adjacent receptor cell axons. Button synapses are also seen in the ocellar nerve dendrites where they appear to make synapses back onto receptor axon terminals as well as onto adjacent ocellar nerve dendrites. Reciprocal and serial synaptic arrangements between receptor cell axon terminals, and between receptor cell axon terminals and ocellar nerve dendrites are occasionally seen. It is suggested that the lateral and feedback synapses in the median ocellus of the dragonfly play a role in enhancing transients in the postsynaptic responses.

Animals↗