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J H Belgum

Publications and source records attributed to J H Belgum.

10 recordsLinked to original sources
PubMed1988

Synaptic transfer of rod signals to horizontal and bipolar cells in the retina of the toad (Bufo marinus).

1. Simultaneous intracellular recordings of responses to light flashes were obtained from rod-horizontal cell and rod-hyperpolarizing bipolar cell pairs in isolated retinae of the toad. The gain and temporal filtering of synaptic transfer were characterized throughout the rods' range of light responses. 2. Paired rod-horizontal cell and rod-bipolar cell responses to dim flashes (less than 0.4 Rh*, where Rh* denotes effective photoisomerizations per rod per flash) exhibited nearly the same time course. Analysis of the onset of the horizontal cell responses revealed a temporal lag equivalent to a single stage of low-pass filtering (tau f = 75-200 ms). No filtering was discerned in the transfer of dim-flash responses from rods to bipolars. On average, horizontal cells were five times as sensitive (mV/Rh*) and hyperpolarizing bipolar cells 10.7 times as sensitive as their paired rods. 3. For brighter flashes, up to 1600 Rh*, the rising and return phases of bipolar responses appeared to be simple scaled versions of the rod responses. The scaling factor was equal to the ratio of flash sensitivities for dim flashes. Rod responses greater than about 2 mV produced a saturation of the bipolar cell response. 4. The return phases of the horizontal cell responses were kinetically similar, scaled versions of the rod responses for rod potentials less than about 5 mV. However, the rising phases lagged significantly behind those of the rod. The effective time constant of the lag increased proportionally with flash intensity. For the brighter flashes, the horizontal cell response peaked as much as a second after the rod response. 5. The linear scaling, minimal temporal filtering and saturation of the bipolar cell responses were satisfactorily reproduced by a model of synaptic transfer that assumed that the rate of transmitter release followed the rod voltage exponentially and that the postsynaptic conductance followed Michaelis-Menten saturation (Falk & Fatt, 1972). 6. The progressively longer lag in the horizontal cell responses to brighter flashes was satisfactorily simulated by a kinetically limited Falk and Fatt model which postulated that the effective electrical time constant of the horizontal cell membrane strongly depended on synaptic or voltage-modulated conductances. 7. Satisfactory model simulations of all postsynaptic responses required that an e-fold change in the release rate of transmitter from the rod be obtained with a 2 mV change in the rod potential.

Action Potentials↗
PubMed1987

Bubble pressure measurement of micropipet tip outer diameter.

The bubble pressure of a micropipet tip is the minimum transmural pressure necessary to expel bubbles into a liquid in which the tip is submerged. Bubble pressure measurement is a simple, accurate and non-destructive technique for determining the tip outer diameter of micropipets used in microinjection, intracellular recording and patch-clamp experiments. For micropipets pulled from thin-wall glass capillary tubing, with tip outer diameters between 0.1 and 1.5 micrograms, observed bubble pressures are close to those predicted by considering surface tension at the pipet tip. Also, as predicted, neither glass composition nor shank geometry influences the relationship between bubble pressure and tip outer diameter. Tip outer diameter for a given bubble pressure is larger for micropipets pulled from thick-wall glass capillary tubing than it is for pipets pulled from thin-wall glass. However, for a given wall thickness, the outer diameter vs bubble pressure behavior is sufficiently reproducible to allow accurate size determination from bubble pressure measurements. The size of micropipet tips modified by fire-polishing or beveling can also be estimated by bubble pressure measurement, although with less accuracy. Thus, bubble pressure measurement can be a useful technique for following the progress of micropipet tip modification.

Calibration↗
PubMed1987

Push-pull effect of surround illumination on excitatory and inhibitory inputs to mudpuppy retinal ganglion cells.

1. Changes in membrane potential and conductance were measured in on-centre and off-centre ganglion cells during the responses to illumination of different portions of the receptive field. 2. In on-centre ganglion cells the sustained depolarizing response to steady illumination of the receptive field centre was associated with a net increase in conductance. In the presence of centre illumination, stimulation of the surround with an annulus of light caused a hyperpolarization and a net decrease in conductance, and the reversal potential of the light-evoked response was shifted in a negative direction. In the absence of centre illumination the same annular stimulus caused a hyperpolarization and a net increase in conductance. 3. In off-centre ganglion cells the sustained hyperpolarizing response to centre illumination was associated with a net increase in conductance. In the presence of centre illumination, stimulation of the surround with an annulus caused a depolarization and a net decrease in conductance, and the reversal potential of the light-evoked response was shifted in a positive direction. In the absence of centre illumination the same annulus caused a depolarization and a net increase in conductance. 4. The results indicate that illumination of the receptive field surround can affect both the excitatory and inhibitory sustained inputs to a given ganglion cell in a 'push-pull' manner, by decreasing the synaptic input that was increased by centre illumination and increasing the synaptic input of opposite sign. The relative effect of a given surround illumination on these two inputs, and hence the sign and magnitude of the net conductance change, varied with the amount of centre illumination.

Action Potentials↗
PubMed1984

Strychnine blocks transient but not sustained inhibition in mudpuppy retinal ganglion cells.

Transient and sustained inhibitory synaptic inputs to on-centre, off-centre, and on-off ganglion cells in the mudpuppy retina were studied using intracellular recording in the superfused eye-cup preparation. When chemical transmission was blocked with 4 mM-Co2+, application of either glycine or gamma-aminobutyric acid (GABA) caused a hyperpolarization and conductance increase in all ganglion cells. For both amino acids, the responses were dose dependent in the range 0.05-10 mM, with a half-maximal response at about 0.7 mM. Glycine and GABA sensitivities were very similar in all three types of ganglion cells. The response to applied glycine was selectively antagonized by 10(-5) M-strychnine and the response to applied GABA was selectively antagonized by 10(-5) M-picrotoxin. In all ganglion cells, 10(-5) M-strychnine eliminated the transient inhibitory events which occur at the onset and termination of a light stimulus. The block of transient inhibition was associated with a relative depolarization of membrane potential and decrease in conductance at these times. Strychnine had no effect on membrane potential or conductance in darkness or during sustained inhibitory responses to light. Picrotoxin (10(-5) M) did not block transient inhibitory events in any ganglion cells, but did affect other components of their responses. The results suggest that in all three classes of ganglion cells transient inhibition, but not sustained inhibition, may be mediated by glycine or a closely related substance.

Animals↗
PubMed1983

Sustained and transient synaptic inputs to on-off ganglion cells in the mudpuppy retina.

Synaptic inputs to on-off ganglion cells in mudpuppy retina were studied by measuring current-voltage relations in darkness, during different phases of the response to light, and in the presence of 4 mM-Co2+. The addition of Co2+ to the bathing medium usually caused a hyperpolarization of the membrane potential in darkness and an increase in input resistance, indicating that on-off ganglion cells receive tonic excitatory synaptic input in darkness. Other results suggest that an additional synaptic input, with a reversal potential near the dark potential, may also be active in darkness. At the onset of a light stimulus in the receptive field centre all on-off ganglion cells responded with transient excitatory and inhibitory synaptic events, both of which were due to increases in conductance. Similar transient excitatory and inhibitory events occurred at the termination of the light stimulus. In about one-half of the on-off ganglion cells studied the synaptic activity during steady illumination was the same as in darkness. In the remaining cells steady illumination caused an increase in sustained inhibition.

Action Potentials↗
PubMed1982

Sustained synaptic input to ganglion cells of mudpuppy retina.

1. Intracellular responses were recorded from on-centre and off-centre ganglion cells in isolated eyecups of the mudpuppy, Necturus maculosus.2. Current-voltage relations were measured in darkness, during illumination of the receptive field centre, and after chemically mediated synaptic inputs were blocked by 4 mM-cobalt chloride.3. In on-centre cells the membrane potential in darkness was -56+/-6 mV (mean+/-S.D.). Addition of Co(2+) resulted in an average depolarization of 10 mV and an average decrease in conductance of 2.1 nS. These results suggest that in darkness on-centre cells are tonically inhibited by synaptic input which increases conductance and has a reversal potential more negative than the dark membrane potential. In off-centre cells the membrane potential in darkness was -46+/-5 mV. Addition of Co(2+) caused an average hyperpolarization of 6 mV and an average decrease in conductance of 1.5 nS. These results suggest that in darkness off-centre cells receive a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.4. In on-centre cells light causes a sustained depolarization. This response involves an increase in a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.5. In off-centre cells, light causes a sustained hyperpolarization. This response involves an increase in a sustained inhibitory input which increases conductance and has a reversal potential more negative than the dark membrane potential.6. The depolarizing off-response of off-centre cells is associated with an increase in an excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential. This response may be due to a temporary increase in the excitatory input which is tonically active in darkness or may reflect an additional excitatory input.7. It is suggested that in both on- and off-centre ganglion cells the balance of sustained excitatory and inhibitory synaptic inputs determines the resting potential in darkness. Centre illumination alters the balance of these inputs, by increasing one and decreasing the other, to produce the characteristic sustained light responses.8. The possible presynaptic sources of the sustained excitatory and inhibitory inputs are discussed.

Animals↗
PubMed1980

Case report: responses to stripe width changes and to complex gratings of a patient with pattern-sensitive epilepsy.

A pattern-sensitive subject's ability to see faint gratings of several different stripe widths (spatial frequencies) was essentially normal, with the greatest sensitivity occurring for gratings with 2 cycles of light and dark stripes per degree of visual arc (c/deg). Epileptiform discharges, however, occurred maximally at a narrower stripe width, 5 c/deg. The 50% bandwidth of the epileptic response was approximately one octave, which is similar to postulated 'spatial frequency channels' in the visual system. Hypothesized inhibitory interactions between the 5 c/deg grating and a 15 c/deg grating were not observed.

Adult↗
PubMed1980

Pattern-sensitive epilepsy. I: A demonstration of a spatial frequency selective epileptic response to gratings.

Rare individuals suffer epileptic seizures when they view certain images, particularly stripes. Contrast-threshold functions (the ability to see faint stripes of various widths) were determined for two pattern-sensitive brothers, and the epileptogenicity of various patterns was assessed for one of them. Sine wave grating contrast-detection thresholds for the two subjects were essentially normal, with lowest thresholds at approximately 2 cycles/degree (c/deg). Epileptiform discharges occurred maximally at 5 c/deg with a 1-octave 50% bandwidth. Pattern epileptogenicity was increased by the addition of a third harmonic sine wave grating to its fundamental, but was unaffected by the phase relation of the two gratings. The frequency selectivity of epileptic responsiveness was quantitatively similar to a "spatial frequency channel." Inhibitory interaction were not present. The findings suggest relations between the phenomena of pattern-sensitive epilepsy and hypothesized spatial frequency channels which merit further exploration.

Adolescent↗
PubMed1980

Pattern-sensitive epilepsy. II: Effects of pattern orientation and hemifield stimulation.

The majority of pattern-sensitive epileptics described in the literature have responded mainly to veritical or near-vertical gratings. In this study, orientation selectivity was reexamined in two patients and the effects of hemifield stimulation are also reported. Prior studies of our subjects by Chatrian et al. (1970) revealed that Case 1 was sensitive to all orientations and that Case 2 was responsive to vertical or near-vertical gratings only. Upon reexamination, Case 1 was distinctly less sensitive to oblique orientations and Case 2 was slightly responsive to horizontal gratings. At the time of testing, Case 1 was clinically improved and Case 2 was acutely worse compared to their conditions during the 1970 studies. With low-contrast gratings, Case 1 responded only to vertical orientations. However, when presented with a checkerboard pattern, he responded only when the edges were diagonal and not when they were vertical and horizontal. Upper hemifield gratings were epileptogenic, whereas lower hemifield presentations were almost ineffective. Lateral hemifield presentation produced discharges mainly in the contralateral hemisphere. The results suggest that orientation selectivity is not a fixed property of the pattern-sensitive condition. Instead, it is apparently influenced by clinical status and pattern properties. The checkerboard results suggest that the epileptogenic feature of the pattern is its Fourier fundamental component rather than individual lines and edges. The hemifield results may indicate a functional difference between the upper and lower visual fields.

Adolescent↗