Article

Two Different Ionic Mechanisms Generating the Spike "Positive" Afterpotential in Molluscan Neurons

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Abstract

The ionic bases of the "positive" afterpotential (ap) have been examined in the so-called DInhi neurons of the central nervous system of Cryptomphallus aspersa. In these cells E(K) has been determined and its value compared with the equilibrium, potential of the ap (E(ap)). It has been found that in half of the studied cells the E(K) value is very close to E(ap) whereas in another half, the difference (E(K) - E(ap)) is large and amounts to circa -10 mv. The effects of changes in the concentration gradients of K(+), Cl(-), and Na(+) were assayed in both groups of cells. When the [K(i)/[K](o) ratio is reduced in both groups of neurons, the ap amplitude and the E(ap) diminished. In cells displaying a large (E(K) - E(ap)), Cl-free Ringer's solution diminished the ap amplitude and E(ap), but produced no effect in the neurons with a reduced (E(K) - E(ap)). A similar effect was observed if [Cl], was increased by intracellular injection of NaCl. Changes in both [Na](o) and [Na](i) were ineffective. It is concluded that K(+) is the only ion involved in the origin of the ap in the groups of cells with a low value for (E(K) - E(ap)). On the contrary, the ap of the neurons presenting large (E(K) - E(ap)) is produced by a simultaneous increase in the fluxes of both K(+) and Cl(-).

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... Thus with snail neurones, Kerkut & Thomas (1965) showed that the injection of Na salts (by allowing them to leak into the cell from low resistance micro-electrodes) caused an increase in the membrane potential (i.e. an increase in internal negativity), which was blocked by removal of external K and by the application of ouabain. They concluded that the hyperpolarization caused by Na injection into snail neurones, which has also been reported by Chiarandini & Stefani (1967), was due to the stimulation of an electrogenic Na pump. ...
Article
1. Sodium was injected into an identified snail neurone by passing current between two intracellular micro-electrodes, the membrane potential being recorded with a third micro-electrode.2. The injection of about 25 p-equiv Na, but not the injection of similar quantities of K or Li, caused a hyperpolarization of up to 20 mV. This response to Na injection was blocked by application of ouabain or removal of external K, indicating that it was due to the stimulation of an electrogenic pump.3. To measure the current produced by the sodium pump the output of a feed-back amplifier was fed into the cell via a fourth intracellular micro-electrode so as to keep the average membrane potential constant. The pump current, measured in this way, rose at a constant rate during, and declined exponentially after, an injection of Na, the decline having an average time constant of 4.4 min. The total charge transferred by the pump was between a third and a quarter of the charge passed to inject sodium.4. An intracellular Na-sensitive glass micro-electrode was used to follow changes in the concentration of intracellular Na ions. The results showed that both the pump current and the rate of Na extrusion were proportional to the concentration of intracellular Na ions above the normal level.5. It was concluded that about two thirds of the Na extruded was coupled to the active transport of other ions, probably to the uptake of K, the uncoupled third producing the electrogenic effect.
... Low-resistance microelectrodes rilled with i-oM-NaCl were used for sodium injection experiments (cf. Kerkut & Thomas, 1965;Chiarandini & Stefani, 1967;Moreton, 1969). ...
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Chapter
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Article
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Article
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Article
Tauc and Gerschenfeld1,2 have shown that in Aplysia depilans and Helix pomatia acetylcholine (ACh) possibly mediates synaptic excitation on neurones conventionally named D cells and also synaptic inhibition in other neurones called H cells. D neurones are recognized because they have only one type of input which is excitatory in character and cholinergic in nature. The analysis of synaptic activity in intracellular recording in the isolated nervous ganglia of the Argentine land-snail, Cryptom-phallus aspersa, reveals also the presence of typical H and D cells. Besides them the perio-oesophagic ring of this snail contains also other types of neurone that in addition to a cholinergic excitatory input as in D cells, show a direct inhibitory synaptic input. These cells will be conventionally named here DInhi (D cells with Inhibition) neurones.
Article
1. The concentrations of sodium, potassium, calcium, magnesium, chloride as well as pH and osmolality were measured in the hemolymph of the land-snail .2. Potassium, calcium, and magnesium concentrations show great variations.3. A saline for Pulmonata conveniently replacing hemolymph is described.
Article
1. 1. If sodium ions are injected at the rate of 4·4 mM/min into a snail neurone, there is an increase in the membrane potential by about 30 mV in 10 min. 2. 2. This marked hyperpolarization is not brought about if potassium ions are injected instead. 3. 3. The hyperpolarization is inhibited by ouabain, parachloromercuribenzoate or reduction in the potassium concentration. 4. 4. It is concluded that the hyperpolarization is due to the stimulation of an electrogenic sodium pump in the nerve cell.
A non-cholinergic inhibition in the central nervous system of a Mollusc
GERSCHENFELD, H. M. 1964. A non-cholinergic inhibition in the central nervous system of a Mollusc. Nature. 203:415.