Post-tetanic hyperpolarization and electrogenic Na pump in stretch receptor neurone of crayfish

Hyperpolarization of a barnacle photoreceptor membrane following illumination

Membrane potential changes following illumination of a photoreceptor cell in the lateral ocellus of a barnacle (Balanus eburneus) were studied by means of intracellular recording and polarization techniques. Illumination produces a depolarizing response. When the illumination is terminated, the membrane potential temporarily becomes more negative than the resting potential prior to illumination. Although the amplitude of this postillumination hyperpolarization depends upon the intensity as well as the duration of the light pulse, the time course is fairly constant. The hyperpolarization is not associated with any significant membrane conductance increase and is abolished by 10(-5)M ouabain. It diminishes when the external Na or K ions are removed. An intracellular injection of Na ions produces a hyperpolarization similar to that following illumination. It is suggested that the postillumination hyperpolarization is produced by an electrogenic Na pump which is activated by the Na influx during illumination.

Prediction of repetitive firing behaviour from voltage clamp data on an isolated neurone soma

1. Membrane parameters of an isolated neural cell body have been determined by voltage clamp analysis. Data are expressed as membrane ion-specific conductances, leak conductance, and capacitance.2. Three ionic currents are present: Inward, I(I); and two operationally distinct outward currents, I(K) and I(A). Both outward currents are apparently carried by potassium ions.3. Hodgkin-Huxley-like equations were solved for the discharge of two sequential action potentials in response to a constant stimulus current. The digital computer solutions are compared with action potential data recorded from the investigated cell.4. The computed and experimentally measured relationships between firing frequency and stimulus current intensity are compared and are linear over the same portion of the total frequency range.5. Cell behaviour in the latter part of the interspike interval is dominated by the conductance g(A) while g(Na) and g(K) largely determine the character of the action potential and the initial portion of the interspike interval.6. Prehyperpolarization of the membrane activates g(A) and the membrane response to depolarizing current differs markedly from the response elicited when no prehyperpolarization is imposed.

Inhibition of impulse activity in a sensory neuron by an electrogenic pump

THE CRAYFISH TONIC STRETCH RECEPTOR NEURON MANIFESTS THREE PHENOMENA: (a) Impulse frequency in response to a depolarizing current decays exponentially to half the initial rate with a time constant of about 4 sec. (b) One or more extra impulses superimposed on steady activity result in a lengthening of the interspike interval immediately following the last extra impulse which is proportional to the number of extra impulses. However, above a "threshold' number of impulses the proportionality constant becomes abruptly larger. (c) Following trains of impulses, the resting potential of the cell is hyperpolarized by an amount proportional to impulse number. Such posttetanic hyperpolarization (PTH) decays approximately exponentially with a time constant of 11 sec, but this varies with membrane potential. These effects are attributed to the incremental increase of an inhibitory (hyperpolarizing) current with a long (relative to interspike interval) decay constant. We suggest that this inhibitory current is the result of increased electrogenic Na pumping stimulated by Na entering with each impulse. Evidence is presented that the three effects are reversibly inhibited by conditions which depress active Na transport: (a) Li substituted for Na in the bath; (b) application of strophanthidin; (c) K removal; (d) treatment with cyanide; (e) cooling. We conclude that a single process is responsible for the three responses described above and identify that process as electrogenic Na pumping. Our observations also indicate that electrogenic pumping contributes to this neuron's resting potential.

Effects of removing the external potassium on the smooth muscle of guinea-pig taenia coli

1. Removal of the external K ions depolarized the membrane of the guinea-pig taenia coli by about 5 mV, and increased the frequency of the spontaneous spike activity. After about 20 min in K-free solution, the frequency gradually decreased and spontaneous activity stopped after about 60 min.2. Readmission of K produced a transient hyperpolarization by 10-15 mV for 10-20 min, and lowered the excitability. The membrane resistance decreased during the hyperpolarization. When the external K concentration was returned to normal before the spontaneous activity had disappeared in K-free solution, there was a complete suppression of the spontaneous electrical and mechanical activities, accompanied by hyperpolarization of the membrane. The duration of this suppression of electrical and mechanical activities depended on the exposure time to K-free solution, the recovery being delayed by longer exposure.3. The effects of removing external K were essentially the same in the presence of tetrodotoxin (10(-7) g/ml.), of atropine (10(-6) g/ml.), and also in low Cl (2-7 mM).4. The effects of ouabain (10(-6) g/ml.) were similar to those of zero K but appeared more slowly. After treatment with ouabain, the membrane was hyperpolarized by removing the external K and there was no hyperpolarization nor suppression of the mechanical activity on readmission of external K.5. Rb substitution for K produced little effect and addition of 5.9 mM-Rb after K-removal produced a typical suppression of activity as the readmission of K. When K was replaced with an equimolar concentration of Cs, the effects were similar to those produced by K-free solution.6. The pattern of the response to a change in the external K concentration was different below 23 degrees C. The spontaneous activity was suppressed by removal of K, and increased by returning the external K.7. It is concluded that, in the taenia, the membrane potential can be modified by an electrogenic pump, the activity of which is inhibited by removing K, by ouabain, and by lowering the temperature to about 23 degrees C. During the recovery from the exposure to K-free solution or to ouabain, the pump activity is potentiated, causing hyperpolarization of the membrane by 10-20 mV. Rb can, Cs cannot, replace K in activating the pump at the same concentration as K.

Cyclic adenosine monophosphate and norepinephrine: effects on transmembrane properties of cerebellar Purkinje cells

Electrical properties of Purkinje cells were recorded by intracellular microelectrode during extracellular electrophoretic application of gamma aminobutyrate, norepinephrine, cyclic adenosine monophosphate, and dibutyryl cyclic adenosine monophosphate. All these substances hyperpolarized Purkinje cells. Transmembrane resistance decreased during gamma aminobutyrate hyperpolarization. In contrast, norepinephrine and the cyclic nucleotides generally elevated resistance. These results show that cyclic nucleotides mimic the unique effects of norepinephrine on the bioelectric properties of neuronal membranes.

Contributions of the sodium pump and ionic gradients to the membrane potential of a molluscan neurone

1. The membrane potential of the gastro-oesophageal giant neurone of the marine mollusc, Anisodoris nobilis, was examined during changes of temperature and of the ionic medium.2. The response of the membrane potential to rapid changes in the external K concentration was prompt, stable, and reversible up to 200 mM-K, and was independent of the external Cl concentration.3. Warming the cell produced a prompt hyperpolarization that was approximately 10 times greater than predicted by the Nernst or constant field equations. Electrogenic activity of the Na-K exchange pump was shown to be responsible for this effect.4. At temperatures below 5 degrees C, the relationship between the membrane potential and the external K concentration could be predicted by a constant field equation.5. At temperatures above 5 degrees C, the membrane potential could not be predicted by the constant field equation except after inhibition of the electrogenic Na pump with ouabain or the reduction of internal Na.6. Inhibition of the electrogenic Na pump by low external K concentrations was dependent upon the external Na concentration.7. It is concluded that the membrane potential is the sum of ionic and metabolic components, and that the behaviour of the ionic component can be predicted by a constant field type equation.

Post-tetanic hyperpolarization produced by an electrogenic pump in dorsal spinocerebellar tract neurones of the cat

1. Hyperpolarization following single and repetitive excitation of dorsal spinocerebellar tract (DSCT) neurones of the cat was studied by intracellular recording.2. Hyperpolarization following an antidromic action potential consisted of an initial, brief phase (undershoot) and a late, prolonged phase. The latter hyperpolarization was independent of the membrane potential, whereas the former was reversed in polarity by hyperpolarizing pulses applied across the DSCT cell membrane.3. DSCT neurones showed a prolonged hyperpolarization after a train of antidromic action potentials. The amplitude and duration of the hyperpolarization were dependent on the number and the frequency of action potentials. A similar hyperpolarization was observed following a train of impulses evoked by depolarizing pulses applied through the intracellular electrode.4. There was no detectable conductance change during the post-tetanic hyperpolarization. The latter showed no reversal potential when the membrane potential was altered.5. The half-decay time of the post-tetanic hyperpolarization was lengthened when the cord temperature was lowered. The temperature coefficient (Q(10)) was 2.4 within the range of 31-40 degrees C.6. The amplitude of the undershoot following each action potential was assumed to provide a criterion for the accumulation of the extracellular K(+). Alterations in the amplitude of undershoots during repetitive excitation suggested that the duration of post-tetanic hyperpolarization depends on the accumulation of extracellular K(+) as well as of intracellular Na(+) associated with a train of impulses.7. It is suggested that post-tetanic hyperpolarization is produced by an electrogenic sodium pump. A possible significance of such a hyperpolarization in impulse coding is discussed.

Characteristics of electrogenic sodium pumping in rat myometrium

Sodium-rich myometrium, obtained from the uteri of pregnant rats, rapidly hyperpolarized when 4.6-120 mM potassium was added to the bathing medium at 37 degrees C. Hyperpolarization was due to sodium pumping since the process was markedly temperature dependent, was abolished by ouabain, and required both intracellular sodium and extracellular potassium. The observed membrane potential exceeded the calculated potassium equilibrium potential during hyperpolarization providing evidence that sodium pumping was electrogenic. Hyperpolarization was reduced in the presence of chloride. The rate of sodium pumping may influence potassium permeability since potassium apparently did not short-circuit the pump during hyperpolarization.

Heterosynaptic facilitation and post-tetanic potentiation in Aplysia nervous system

1. Heterosynaptic facilitation was defined as an increase of amplitude of a test excitatory post-synaptic potential (EPSP) after the activation of a pathway (heterosynaptic pathway) different from that which produced the test EPSP. This phenomenon has been studied in Aplysia central nervous system under conditions which excluded the participation of post-tetanic potentiation.2. A unitary test was produced in the left and right giant cells, by indirect stimulation of an interneurone located in the peri-oesophageal ring.3. During heterosynaptic stimulation, orthodromic and antidromic activation of the test interneurone was prevented by (1) isolating the synaptic afferent region of the test interneurone from the tested synapse on the right giant cell by a sucrose block applied on the left pleurovisceral connective, and (2) using physiological stimulation of a piece of skin as a heterosynaptic stimulus. Under these conditions which prevented any firing in the test interneurone, heterosynaptic facilitation is observed as a 200% increase of amplitude of the test EPSP in the right cell which lasted more than 15 min. When instead of the physiological stimulus a supramaximal electrical stimulation of the nerves afferent to the abdominal ganglion was used, the increase of amplitude of the test EPSP could reach as much as 500% of its original amplitude. The effectiveness of such heterosynaptic stimulus was smaller when it was applied in the absence of a block of the left pleuro-visceral connective.4. It was possible to produce heterosynaptic facilitation when the preparation was cooled to 7-9 degrees C or if Li(+) replaced Na(+) in the medium. Both of these changes suppressed post-tetanic potentiation.5. It was concluded that heterosynaptic facilitation is a phenomenon different from post-tetanic potentiation. Heterosynaptic facilitation is similar to heterosynaptic inhibition seen in other cells in the same preparation, except for the polarity of action. Both phenomena seem to result from comparable mechanisms, probably acting on the quantity of transmitter released.

The membrane potential of Acetabularia mediterranea

The cytoplasm of an Acetabularia cell is normally at a potential of about -170 mv relative to the external solution; the vacuole is also at this potential. Although there is strict flux equilibrium for all ions, the potential is more negative than the Nernst potentials of any of the permeating ions. Darkness, CCCP, low temperature, and reducing [Cl(-)](o) by a factor of 25 all rapidly depolarize the membrane and inhibit Cl(-) influx. Some of these treatments do not inhibit the effluxes of K(+) and Na(+). Increasing [K(+)](o) also depolarizes the membrane both under normal conditions and at low temperature; in the latter case the membrane is partially depolarized in normal seawater (low [K(+)](o)) and in high [K(+)](o) positive potentials of up to +15 mv are attained. It is concluded that the membrane potential is controlled by the electrogenic influx of Cl(-), and also, at least in some circumstances, by the diffusion of K(+). In addition, it is suggested that electrogenic efflux of H(+) may be important in transient nonequilibrium situations. An Appendix deals with the interpretation of simple nonsteady-state tracer kinetic data.

Penicillin as epileptogenic agent: its effect on an isolated neuron

Penicillin induces partial depolarization and increased excitability of the neuronal membrane of crayfish stretch receptor. Such effects suggest that the epileptic focus created by the topical application of penicillin to the mammalian cerebral cortex may result from the lowering of the threshold for impluse initiation by excitatory synaptic action within the neuron population.

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.

Membrane current and intracellular sodium changes in a snail neurone during extrusion of injected sodium

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.

Synaptic activation of an electrogenic sodium pump

An identified molluscan interneuron mediates different cholinergic synaptic actions by increasing the conductance of its follower cells to different ions. We have now found that this interneuron also mediates a new class of synaptic actions which does not involve a conductance change but the activation of an electrogenic sodium pump. This synaptic action results in a prolonged inhibitory synaptic potential which is dependent on metabolism and is selectively blocked by cooling and ouabain. In cells which have this synaptic potential, part of the resting membrane potential is also maintained by an electrogenic sodium pump. The same transmitter, acetylcholine, can independently stimulate both a chloride ion conductance and a sodium pump mechanism in the same follower cell by acting on two different postsynaptic receptors.

Membrane properties of the stretch receptor neurones of crayfish with particular reference to mechanisms of sensory adaptation

1. Membrane properties of neurones of the two morphologically different types of stretch receptor of crayfish, the slowly adapting (RM(1)) and the rapidly adapting (RM(2)) receptors, were investigated with two microelectrodes inserted into the same neurone.2. The action potential was usually larger in the slowly adapting than in the rapidly adapting neurone. But the distributions of the height were not sharply delimited, and there was an overlap from the two groups of neurone.3. There were no marked differences in the current-voltage relationship between the two types.4. Under voltage clamp, depolarizations evoked a large delayed outward current, which slowly diminished during maintained depolarization (K-inactivation). Under a moderate depolarization, development of the K-permeability increase was very slow.5. When stimulated by intracellularly applied constant currents, the slowly adapting neurone always adapted slowly, and gave rise to long-lasting trains of spikes, whereas the rapidly adapting neurone never produced maintained repetitive discharges.6. The same marked differences in the adaptation behaviour of spike discharge between the two types were also observed when the neurones were stimulated by constant currents applied through external electrodes.7. When the stimulating point was shifted along the axon of the slowly adapting neurones, the ability to produce long-lasting repetitive discharges was found to be confined to the axonal region near the soma, where the diameter was very small, and where impulses were first initiated.8. Possible ionic mechanisms of the adaptation of the spike generating membrane were discussed. The importance of slowly occurring changes in the Na- and K-permeabilities and changes in the electromotive force of the membrane due to electrogenic pump was emphasized.

Adaptation of the generator potential in the crayfish stretch receptors under constant length and constant tension

1. Generator potentials were investigated in stretch receptors of crayfish after abolishing spike potentials with tetrodotoxin.2. The time courses of the decline of generator potential (generator adaptation) were almost the same in the slowly and rapidly adapting receptors.3. The time courses of the tension changes after suddenly stretching the receptor muscles did not differ much between the two receptor types.4. The amplitudes of generator potential per unit stress or per unit strain in the receptor muscle were roughly the same in the two receptor types.5. By comparing generator adaptation under length-clamp and tension-clamp in the slowly adapting receptors, it was suggested that roughly 70% of the generator adaptation could be explained by a simple visco-elastic property of the receptor muscle, when observed for 1 sec after the beginning of the stretch.6. It was concluded that the marked differences in the receptor adaptation between the two receptor types were attributable to the differences in the properties of spike generating membrane rather than to the properties of the generator potentials.7. In each type of receptor, both the generator adaptation and the adaptation of spike generating mechanisms contributed to determining the whole rates of receptor adaptation. In the slowly adapting receptor, however, the generator adaptation seemed more important, while in the rapidly adapting receptor the spike generating mechanisms seemed more important.

A role for the sodium pump in photoreception in Limulus

The membranes of photoreceptor cells in Limulus have an electrogenic sodium pump which contributes directly to membrane potential and whose activity is changed by light. These light-induced changes in pump activity underlie the receptor potential.

Effects of some organic cations on generator potential of crayfish stretch receptor

The generator potential of both slowly and rapidly adapting crayfish stretch receptor cells can still be elicited by mechanical stimuli when all the Na of the bathing medium is replaced by various organic cations. In the presence of tris(hydroxymethyl)aminomethane (Tris), the generator potential is particularly large, about 30-50 % of that in the control saline, while spike electrogenesis of the cell is abolished. Persistence of the generator response is not due to retention of Na by a diffusion barrier, and ionic contributions to the electrogenesis by Ca and Cl can also be excluded. Thus, whereas the electrogenesis of the generator membrane must be due to an increased permeability to monovalent cations, the active receptor membrane appears to be less selective for different monovalent cations than is the receptor component of some other cells, or the conductile component of the stretch receptor neuron.

A contribution of an electrogenic Na+ pump to membrane potential in Aplysia neurons

The resting membrane potential (RMP) of Aplysia neurons is very temperature-dependent, and in some cells increases with increasing temperature by as much as 2 mv/ degrees C. RMP at room temperature may significantly exceed the potassium equilibrium potential, which can be determined by measurement of the equilibrium point of the spike after potential. The hyperpolarization on warming is completely abolished by ouabain, replacement of external Na(+) by Li(+), removal of external K(+), and by prolonged exposure to high Ca(++), while it is independent of external chloride but is increased by cocaine (3 x 10(-3)M). In an identified cell that shows a marked temperature dependence of RMP, both the potassium equilibrium potential and the membrane resistance were found to be relatively independent of temperature. The hyperpolarization on warming, which may increase RMP by as much as 50%, can most reasonably be ascribed to the activity of an electrogenic Na(+) pump.

The influence of H+ on the membrane potential and ion fluxes of Nitella

The resting membrane potential of the Nitella cell is relatively insensitive to [K](o), but behaves like a hydrogen electrode. K(+) and Cl(-) effluxes from the cell were measured continuously, while the membrane potential was changed either by means of a negative feedback circuit or by external pH changes. The experiments indicate that P(K) and P(Cl) are independent of pH but are a function of membrane potential. Slope ion conductances, G(K), G(Cl), and G(Na) were calculated from efflux measurements, and their sum was found to be negligible compared to membrane conductance. The possibility that a boundary potential change might be responsible for the membrane potential change was considered but was ruled out by the fact that the peak of the action potential remained at a constant level regardless of pH changes in the external solution. The conductance for H(+) was estimated by measuring the membrane current change during an external pH change while the membrane potential was clamped at K(+) equilibrium potential. In the range of external pH 5 to 6, H(+) chord conductance was substantially equal to the membrane conductance. However, the [H](i) measured by various methods was not such as would be predicted from the [H](o) and the membrane potential using the Nernst equation. In artificial pond water containing DNP, the resting membrane potential decreased; this suggested that some energy-consuming mechanism maintains the membrane potential at the resting level. It is probable that there is a H(+) extrusion mechanism in the Nitella cell, because the potential difference between the resting potential and the H(+) equilibrium potential is always maintained notwithstanding a continuous H(+) inward current which should result from the potential difference.

On the electrogenic sodium pump in mammalian non-myelinated nerve fibres and its activation by various external cations

1. A study has been made of the hyperpolarization that follows a period of electrical activity (the post-tetanic hyperpolarization) in mammalian non-myelinated nerve fibres.2. Evidence is presented that under certain circumstances this postetanic hyperpolarization is a result of activity of an electrogenic sodium pump that normally is absolutely dependent on the external presence of potassium.3. When the external chloride is replaced by sulphate or by isethionate the post-tetanic hyperpolarization, which in normal Locke solution is only a few millivolts in amplitude, is increased usually to about 20 mV, and on occasion to 35 mV.4. This effect of removing the chloride takes several minutes to develop and is consistent with the idea that the increase in the post-tetanic response is the result of removing the short-circuiting effect of internal chloride ions (by their being washed out into the chloride-free bathing medium).5. Small anions, such as chloride, nitrate, iodide, bromide, and thiocyanate can short-circuit the electrogenic pump, whereas larger anions such as sulphate and isethionate cannot. The bicarbonate ion, which is larger than chloride, short-circuits the pump but less effectively.6. In Locke solution containing 5 mM potassium the post-tetanic hyperpolarization declines exponentially, with a time constant of about 1-3 min. The time constant is inversely related to the external potassium concentration.7. However, when the external potassium concentration is zero the hyperpolarization declines rapidly to a very small value. Subsequent addition of potassium to the bathing medium causes a marked redevelopment of the hyperpolarization.8. This potassium-activated response declines exponentially with a time constant that is inversely related to the potassium concentration. When the added potassium concentration is 5 mM, the time constant is 1.9 min.9. The amplitude of the potassium-activated response increases with increasing concentrations of potassium.10. Other cations can produce this activated response. Thus, thallium is more effective than, rubidium as effective as, caesium and ammonium about 1/10 as effective as, and lithium ions about 1/30 as effective as potassium in producing the activated response. Choline is quite ineffective.11. The size of the post-tetanic response is little affected by changes in the duration of the period of stimulation. However, increasing the duration definitely increases the time constant of recovery.12. Reducing the external sodium concentration increases the size of the post-tetanic hyperpolarization (by about 25%), but the effect is complex and requires further study.13. Reducing the calcium of the Locke-solution from 2.2 to 0.2 mM has no appreciable effect on the post-tetanic response, nor has increasing the pH of the Locke from 7.2 to 9.2.14. When the membrane potential is increased or decreased, by externally applied currents, there is relatively little change in the post-tetanic response.15. A mathematical model of the electrogenic pump, devised to mimic the experimental results, was analysed with an analogue computer. A satisfactory agreement between model and experiment was achieved by a model in which: (1) the rate of extrusion of sodium ions depends on the degree to which a pool of carrier molecules on the inside surface of the membrane is combined with sodium; (2) each carrier molecule transfers three sodium ions at a time; (3) the rate constant for extrusion of sodium ions also depends on the presence externally of potassium ions, which combine with some sites on the external surface of the membrane that are half-saturated when the external concentration of potassium is 2.8 mM.

Effects of tetrodotoxin on the slowly adapting stretch receptor neurone of lobster

1. A study has been made of the effects of tetrodotoxin on the impulse activity, resting membrane potential, input resistance, and the generator potential and its after-hyperpolarization of the slowly adapting stretch receptor neurone of the lobster.2. Tetrodotoxin was able to abolish completely within about 2 min the impulse activity in most cells, when given in a dose of 2 x 10(-8) g/ml., but in all cells, when administered in a dose of 4 x 10(-8) g/ml. After blockage by the toxin in concentrations as high as 4 x 10(-6) g/ml. for periods of up to 30 min the action potential was restored by washing the preparation in physiological solution for 1 hr.3. In a concentration of 4 x 10(-8) g/ml. tetrodotoxin produced within 1-2 min an average increase of 4.8 mV of the resting membrane potential and a simultaneous 47% reduction of the resting input resistance. These effects were reversed by washing the preparation in physiological solution for 1 hr.4. Tetrodotoxin administered in doses as high as 4 x 10(-6) g/ml. for periods of up to 30 min had no effect on the amplitude of the steady phase of the generator potential.5. In a concentration of 4 x 10(-8) g/ml. tetrodotoxin produced within 5 min a 65% reduction of the amplitude of the hyperpolarization following the generator potential. This effect was reversed by washing the preparation in physiological solution for 1 hr.6. The simultaneous increase in resting membrane potential and decrease in membrane resistance is suggested to be due to an elevated potassium permeability besides a reduced sodium conductance. The constancy in height of the generator potential in the presence of a decreased membrane resistance makes necessary the assumption of an augmented generator current. The decrease in amplitude of the hyperpolarization following the generator potential may be the result of an increase in potassium conductance and/or a reduction in acceleration of an electrogenic pump in consequence of a diminished sodium influx during the generator potential.