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D A Baylor

Publications and source records attributed to D A Baylor.

81 records · Page 5Linked to original sources
PubMed1969

Changes in extracellular potassium concentration produced by neuronal activity in the central nervous system of the leech.

1. Experiments were made on single neurones and glial cells in the central nervous system of the leech to study the accumulation of K that occurs in the extracellular spaces around neurones as a result of impulse activity.2. The resting potential of a neurone is too insensitive to be used for the estimation of small changes in K concentration. The undershoot of the action potential, however, provided a reliable indicator of the K accumulation that occurs around a neurone during activity.3. After a single impluse the amplitude of the undershoot of a second action potential was decreased; the effect corresponded to a peak increase in K concentration of about 0.8 mM/l. immediately after the spike and declined exponentially with a time constant of about 100 msec. With trains of impulses the K concentration increased exponentially, again with a time constant of about 100 msec. The final value of K depended on the frequency and could build up to about double the normal concentration of 4 mM/l.4. The build-up of K was markedly reduced when the extracellular space surrounding a neurone was enlarged by removing its glial investment.5. Synchronous, repetitive activation of groups of neurones caused a slow depolarization of neighbouring glial cells in the C.N.S. of the leech, similar to that observed in amphibia and mammals. The change in glial membrane potential was also used to estimate the changes in K concentration and these values agreed with measurements derived from the undershoot.6. Increases of K concentration in the bathing fluid of the same order as those caused by neural firing markedly affected the frequency of ;spontaneous' neuronal discharges and synaptic potentials occurring within certain neurones in the C.N.S.7. The possible effects of physiologically occurring increases of K concentration on integration are discussed.

Action Potentials↗
PubMed1969

After-effects of nerve impulses on signalling in the central nervous system of the leech.

A long-lasting hyperpolarization has been observed in sensory neurones of the C.N.S. of the leech following natural stimulation applied to their receptive fields. The underlying mechanism of this hyperpolarization and its effects on signalling were investigated by intracellular recording.1. The amplitude and duration of the hyperpolarization depended on the number of action potentials. Whereas a brief burst of impulses had little effect, a prolonged sensory discharge was followed by a hyperpolarization of up to 30 mV that gradually declined over a period of several minutes.2. The hyperpolarization was abolished by ouabain, reversibly inhibited by strophanthidin or cooling, unaffected by Mg and accompanied by an increase in membrane resistance. These observations suggest that the hyperpolarization is the result of membrane current generated by an electrogenic pump.3. A hyperpolarization similar to that recorded in the cell body also occurred in the neuronal processes within the neuropile, where synaptic contacts are made. This led to significant changes in integrative activity, such as an increase in the amplitude of excitatory synaptic potentials, a reversal of inhibitory synaptic potentials or a conduction block in parts of the neurones. All of these effects could be mimicked by injecting hyperpolarizing currents into resting cells, or abolished by injecting a depolarizing current into a cell hyperpolarized by previous impulses.4. During neuronal hyperpolarization the sensitivity of the membrane potential to small increments of external K was enhanced by a factor of about three. This effect varied with the magnitude of the hyperpolarization produced by preceding impulses and could not be mimicked by applying K to a neurone hyperpolarized by the injection of current into its cell body.5. These findings are discussed in relation to possible effects that changes in the intracellular and extracellular concentrations of ions produced by activity might have on integration in the C.N.S.

Action Potentials↗
PubMed1969

Chemical and electrical synaptic connexions between cutaneous mechanoreceptor neurones in the central nervous system of the leech.

Experiments have been made to study the synaptic connexions between sensory cells in the C.N.S. of the leech. Each segmental ganglion contains six neurones that respond specifically to light touch applied to the skin; each of these ;touch cells' innervates a discrete area on the surface of the body and has a characteristic set of properties by which it can be recognized. Using intracellular electrodes it has been shown that these sensory cells interact with one another through chemical and electrical synapses by way of a stereotyped set of pathways.1. Action potentials occurring in one touch cell gave rise to synaptic potentials in the five other touch cells in the same ganglion and also in the three ipsilateral touch cells in the adjacent ganglia. Thus, synaptic interactions took place between sensory cells whose receptive fields lay within the same segment and on the same side of adjacent segments.2. The post-synaptic potentials consisted of a short-latency coupling potential, followed by an excitatory potential and a number of inhibitory potentials. These delayed synaptic potentials occurred inconsistently and with a variable latency; they could also be recorded in the cell which had been stimulated. All of the touch cells appeared to be equally effective in initiating synaptic potentials.3. The short-latency coupling potential was shown to be mediated through an electrical synapse by observing a voltage change in one touch cell when current was injected into its neighbour. It was not abolished by high concentrations of Mg in the bathing fluid, which blocked chemical synapses in this ganglion. This electrical synapse displayed remarkable rectification; a depolarization could spread from cell to cell in both directions, while a hyperpolarization could spread in neither.4. The inhibitory potentials were reversed by injecting Cl into the cell. In Cl-free Ringer solution this effect was so marked that the reversed IPSPs caused long trains of impulses in touch cells, which tended to excite each other by a process of positive feed-back.5. Synaptic potentials evoked by activation of a touch cell did not usually reach threshold since excitation and inhibition tended to cancel. The connexions between touch cells that mediated the delayed excitatory and inhibitory potentials are polysynaptic; the interneurones have not yet been found but some of their connexions could be inferred from electrical recordings.6. Action potentials in sensory cells of a different modality (responding to pressure) also initiated synaptic potentials in the same family of touch cells.7. The possible significance for integration of these synaptic interactions between sensory cells is discussed.

Action Potentials↗
PubMedDate not supplied

Spectral sensitivity of single cones in the retina of Macaca fascicularis.

Colour vision depends on the wavelength-dependent absorptions of three different photolabile pigments each located in a particular type of retinal cone. The spectral absorption of primate cones has been examined by microspectrophotometry, but this method gives information only at wavelengths where the absorption is relatively strong. Here we have analysed the absorption of two of the cones over a wider range of wavelengths by recording their electrical responses to monochromatic light. The observations were made on the retina of the monkey Macaca fascicularis, an animal thought to have cone pigments like those of man. The measured spectral sensitivities of the red-sensitive ('red') and green-sensitive ('green') cones agreed well with estimates of the cone pigment absorptions derived from colour-matching experiments in humans. At long wavelengths the sensitivity of the red cones was found to decline more rapidly than that of the green. This behaviour, attributable to the cone pigment molecules themselves, explains the paradoxical hue shift, in which a light of very long wavelength is perceived to be identical to a light of shorter wavelength.

Action Potentials↗
PubMedDate not supplied

Cyclic GMP-sensitive conductance of retinal rods consists of aqueous pores.

The surface membrane of retinal rod and cone outer segments contains a cation-selective conductance which is activated by 3',5'-cyclic guanosine monophosphate (cGMP). Reduction of this conductance by a light-induced decrease in the cytoplasmic concentration of cGMP appears to generate the electrical response to light, but little is known about the molecular nature of the conductance. The estimated unitary conductance is so small that ion transport might occur via either a carrier or a pore mechanism. Here we report recordings of cGMP-activated single-channel currents from excised rod outer segment patches bathed in solutions low in divalent cations. Two elementary conductances, of approximately 24 and 8 pS, were observed. These conductances are too large to be accounted for by carrier transport, indicating that the cGMP-activated conductance consists of aqueous pores. The dependence of the channel activation on the concentration of cGMP suggests that opening of the pore is triggered by cooperative binding of at least three cGMP molecules.

Animals↗
PubMedDate not supplied

Spectral sensitivity of human cone photoreceptors.

The brain computes visual colour by analysing the relative excitations of three types of retinal cones. Each cone excitation is governed by a spectral sensitivity function which relates the amplitude of the neural response to wavelength at constant light intensity. The spectral sensitivities of human cones are not well characterized. We report measuring the sensitivities by recording electrical responses of human cones to stimuli of different wavelengths. Spectral sensitivities of 'green' and 'red' cones, determined over the entire visible region, show peaks near 530 and 560 nm respectively, and are remarkably similar to those of the old-world monkey Macaca fascicularis. They satisfactorily predict the photopic luminosity function, a measure of the sensitivity of cone-mediated human vision to light of different wavelengths. The kinetics of the light responses of human cones also appeared similar to those of macaque cones: the time to peak response to a dim flash was 50-100 ms and there was a characteristic undershoot during recovery.

Color Perception↗