Oscillatory potentials in the electroretinogram.

This paper presents an analog circuit model of the electroretinogram b-wave and subsequent oscillations postulated to result from the depolarization of the retinal glial cells produced by potassium. The system is described by a second-order equation which oscillates under conditions of light adaptation. With dark adaptation the oscillations undergo attenuation. Light falling on the retina ultimately liberates potassium ions and this ionic current is considered to be the system input. A first approximation is an exponentially decaying current which varies depending upon light background, flash intensity and various drug manipulations. This model is a reasonable fit to the electroretinogram over considerable light background levels and is suggested as a useful tool in the study of various pharmacological effects on retinal electrophysiology.     

Factors determining the decay of K+ potentials and focal potentials in the central nervous system.

Post-tetanic undershoots in extracellular focal potential (deltaV) and K+ potential (EK) can be recorded in the cuneate nucleus and dorsal horn of Dial-anaesthetized or decerebrate cat. They are seen best at depths where the largest deltaEK and deltaV are recorded and they increase with the frequency and duration of stimulation. The very different time courses of undershoots of deltaV and deltaEK indicate two hyperpolarizing influences: first, electrogenic pumping, and later, a reduced external K+ concentration. The importance of active K+ removal in determining the amplitude and duration of deltaEK and deltaV is illustrated by their marked potentiation (as well as the disappearance of post-tetanic undershoots) induced by a lowering of blood pressure or local application of strophanthidin.     

Membrane potentials in the Donnan equilibrium. II

Measurements were made of electromotive force in the Donnan equilibrium of systems containing dilute solutions of protein and acid. Removal of the membrane produced a decrease of no more than 2 to 4 mv. in electromotive force, while the membrane potentials, as estimated by the usual arbitrary assumption, were of the order of 12 to 34 mv. Ion ratios, as calculated from analyses for total chloride, were definitely greater than those calculated from the electromotive force of cells with salt bridges, as if there had been combination of some of the chloride ion with protein.     

Brainstem auditory evoked potentials in the rabbit.

Eight white New Zealand rabbits were submitted to auditory stimulation in order to obtain normative BAEP parameters. A monaural alternating 0.1 ms click stimulation at 20 Hz, 90 dB was used. Two series of 1000 responses were averaged (10 ms time-base, 160-3000 Hz band-pass) and highly reproducible peaks were obtained. Peaks P1, P2, P3, P4 were obtained in all ipsilateral recordings, whereas peak P5 was detectable in only 6 animals. In contralateral recordings P1 was absent and the following peaks were similar to those of ipsilateral recordings. Normative values of absolute and interpeak latencies, peak amplitudes and amplitude ratios were obtained. The procedure was repeated 24 hours after basal recordings and measures of test-retest variability were obtained.     

Auditory evoked potentials in the Japanese monkey.

Auditory sensitivity based on auditory brain stem response (ABR), whole nerve action potential (AP), and cochlear microphonics (CM) to tone bursts of 0.5-8 kHz were compared with behavioral audiometry in the Japanese monkeys. Although sensitivity loss at 4-6 kHz was observed in these potentials, an increase in sensitivity at 8 kHz was obtained only in the ABR. Thus the sensitivity loss at 4-6 kHz originates at the peripheral system and the increased sensitivity at 8 kHz originates at the central.     

Voltage-sensitive dye recording of action potentials and synaptic potentials from sympathetic microcultures.

Given the appropriate multicell electrophysiological techniques, small networks of cultured neurons (microcultures) are well suited to long-term studies of synaptic plasticity. To this end, we have developed an apparatus for optical recording from cultured vertebrate neurons using voltage-sensitive fluorescent dyes (Chien, C.-B., and J. Pine. 1991. J. Neurosci. Methods. 38:93-105). We evaluate here the usefulness of this technique for recording action potentials and synaptic potentials in microcultures of neurons from the rat superior cervical ganglion (SCG). After extensive dye screening and optimization of conditions, we chose the styryl dye RH423, which gave fast linear fluorescence changes of approximately 1%/100 mV for typical recordings. The root mean square noise of the apparatus (limited by shot noise) was typically 0.03%, equivalent to 3 mV of membrane potential. Illumination for at least 100 flashes of 100 ms each caused no noticeable photodynamic damage. Our results show that voltage-sensitive dyes can be used to record from microcultures of vertebrate neurons with high sensitivity. Dye signals were detected from both cell bodies and neurites. Signals from presumptive dendrites showed hyperpolarizations and action potentials simultaneous with those in the cell body, while those from presumptive axons showed delayed propagating action potentials. Subthreshold synaptic potentials in the cell body were occasionally detectable optically; however, they were usually masked by signals from axons passing through the same pixel. This is due to the complex anatomy of SCG microcultures, which have many crisscrossing neurites that often pass over cell bodies. Given a simpler microculture system with fewer neurites, it should be possible to use dye recording to routinely measure subthreshold synaptic strengths.     

Midpoint potentials of the mitochondrial cytochromes of Crithidia fasciculata.

The midpoint potentials of the mitochondrial respiratory chain cytochromes of the protozoan Crithidia fasciculata at pH 7.2, Em7.2, show great similarity to those measured in higher organisms. Values of Em7.2 for cytochromes a and a3 are +165 and +340 mV. Both c cytochromes have Em7.2 = +230 mV. There are two b cytochromes with the same spectral characteristics with Em7.2 = -20 and -135 mV. These values are compatible with two sites of energy conservation for oxidative phosphorylation in these mitochondria. All cytochrome components show potentiometric titrations with n = 1. There is a fluorescent flavoprotein in these mitochondria with Em7.2 = -40 mV and n =2, whose function is not known.     

Evoked potentials in patients with Huntington's disease and their offspring. I. Somatosensory evoked potentials

Spinal and cortical somatosensory evoked potentials (SEPs) were recorded in a large sample of patients with Huntington's disease (n = 37) and subjects at risk (first order offspring, n = 43). The SEPs were elicited by stimulation of the median and tibial nerves and recorded at Erb's point, the cervical level (C2) and at the corresponding scalp areas.The most striking finding in patients with Huntington's disease was a drastic diminution of the amplitude of the early cortical components, especially N20/P25 for the median nerve and N33/P40 for the tibial nerve. The latencies (Erb's point, C2, cortical) were only slightly prolonged in comparison to the normal values.Forty-three per cent of the persons at risk exhibited pathological results with a clear reduction in amplitude of the early cortical responses or with a pathological side difference between the amplitudes. Fifty-three per cent of the persons at risk exhibited a normal result. Two persons at risk (=6%) could not be classified unambiguously. Some patients with benign and symptomatic chorea were investigated. These showed normal results with one exception.The diagnostic and predictive value of the investigation of SEPS in Huntington's disease is discussed.     

Long-lasting synaptic potentials and the modulation of synaptic transmission.

Long-lasting postsynaptic potentials (PSPs) generated by decreases in membrane conductance (permeability) have been reported in many types of neurons. We investigated the possible role of such long-lasting decreases in membrane conductance in the modulation of synaptic transmission in the sympathetic ganglion of the bullfrog. The molecular basis by which such conductance-decrease PSPs are generated was also investigated. Synaptic activation of muscarinic cholinergic receptors on these sympathetic neurons results in the generation of a slow EPSP (excitatory postsynaptic potential), which is accompanied by a decrease in membrane conductance. We found that the conventional "fast" EPSPs were increased in amplitude and duration during the iontophoretic application of methacholine, which activates the muscarinic postsynaptic receptors. A similar result was obtained when a noncholinergic conductance-decrease PSP--the late-slow EPSP--was elicited by stimulation of a separate synaptic pathway. The enhancement of fast EPSP amplitude increased the probability of postsynaptic action potential generation, thus increasing the efficacy of impulse transmission across the synapse. Stimulation of one synaptic pathway is therefore capable of increasing the efficacy of synaptic transmission in a second synaptic pathway by a postsynaptic mechanism. Furthermore, this enhancement of synaptic efficacy is long-lasting by virtue of the long duration of the slow PSP. Biochemical and electrophysiological techniques were used to investigate whether cyclic nucleotides are intracellular second messengers mediating the membrane permeability changes underlying slow-PSP generation. Stimulation of the synaptic inputs, which lead to the generation of the slow-PSPs, increased the ganglionic content of both cyclic AMP and cyclic GMP. However, electrophysiological analysis of the actions of these cyclic nucleotides and the actions of agents that affect their metabolism does not provide support for such a second messenger role for either cyclic nucleotide.     

Cilia-forming potentials of endocrine organs originating from the foregut.

Cilia were found in intracellular localizations or on the surface of certain cells of frog thymus and chicken parathyroid gland and ultimobranchial body. The experimental observations suggest that ciliogenesis is probably a general property of branchiogenic epithelium, which either persists, or can be activated by certain influences after the functional differentiation of endocrine organs arising from the foregut.     

Simulated propagation of cardiac action potentials.

We have used numerical methods for solving cable equations, combined with previously published mathematical models for the membrane properties of ventricular and Purkinje cells, to simulate the propagation of cardiac action potentials along a unidimensional strand. Two types of inhomogeneities have been simulated and the results compared with experimentally observed disturbances in cardiac action potential propagation. Changes in the membrane model for regions of the strand were introduced to simulate regions of decreased excitability. Regional changes in the intercellular coupling were also studied. The results illustrate and help to explain the disturbances in propagation which have been reported to occur at regions of decreased excitability, regions with changing action potential duration, or regions with changing intercellular coupling. The propagational disturbances seen at all of these regions are discussed in terms of the changing electrical load imposed upon the propagating impulse.     

Atherogenic potentials of some Nigerian Meals.

The atherogenic potentials of peeled grated cocoyam (Xanthosoma maffafa scot) "ekpang nkukwo", pounded yam (Discorea spp) with plain soup "afia efere", and plantain porridge (Musa paradisiaca) "iwuk ukom" meals were investigated. The three meals were fed to three different groups of albino rats of Wistar strain for a period of twenty eight days. A fourth group which served as control was feed with normal rat pellet. The mean total plasma cholesterol level in the pounded yam with plain soup fed group was significantly lower [P < 0.05] when compared to the control and peeled grated cocoyam fed groups. The mean total plasma triglyceride (MTPTG) level in the pounded yam with plain soup fed group was significantly lower [P < 0.05] when compared to the control group. However the MTPTG level in the peeled grated cocoyam and plantain porridge fed groups were comparable to control. The mean HDL-cholesterol level in the peeled grated cocoyam and plantain fed groups were comparable control. The mean LDL-cholesterol level in the peeled grated cocoyam and plantain porridge fed groups was significantly lower [P < 0.05] than the control group. The LDL-cholesterol and VLDL-cholesterol in the pounded yam with plain soup fed group was significantly lower [P < 0.05] when compared to control. These findings suggest low atherogenic potentials of the pounded yam with plain soup meal compared to the peeled grated cocoyam and plantain porridge meals.     

Structural origins of redox potentials in Fe-S proteins: electrostatic potentials of crystal structures.

Redox potentials often differ dramatically for homologous proteins that have identical redox centers. For two types of iron-sulfur proteins, the rubredoxins and the high-potential iron-sulfur proteins (HiPIPs), no structural explanations for these differences have been found. We calculated the classical electrostatic potential at the redox site using static crystal structures of four rubredoxins and four HiPIPs to identify important structural determinants of their redox potentials. The contributions from just the backbone and polar side chains are shown to explain major features of the experimental redox potentials. For instance, in the rubredoxins, the presence of Val 44 versus Ala 44 causes a backbone shift that explains a approximately 50 mV lower redox potential in one of the four rubredoxins. This result is consistent with experimental redox potentials of five additional rubredoxins with known sequence. Also, we attribute the unusually lower redox potentials of two of the HiPIPs studied to a less positive electrostatic environment around their redox sites. Finally, molecular dynamics simulations of solvent around static rubredoxin crystal structures indicate that water alone is a major factor in dampening the contribution of charged side chains, in accord with experiments showing that mutations of surface charges produce relatively little effect on redox potentials.