Showing papers in "The Journal of Physiology in 1980"

Electrophysiological properties of in vitro purkinje cell dendrites in mammalian cerebellar slices

Abstract: 1. Intradendritic recordings from Purkinje cells in vitro indicate that white matter stimulation produces large synaptic responses by the activation of the climbing fibre afferent, but antidromic potentials do not actively invade the dendritic tree. 2. Climbing fibre responses may be reversed in a manner similar to that observed at the somatic level. However, the reversal does not show the biphasicity often seen at somatic level. 3. Input resistance of these dendrites was found to range from 15 to 30 M omega. The non-linear properties seen at the somatic level for depolarizing currents are also encountered here. However, there seems to be less anomalous rectification. 4. Detailed analysis of repetitive firing of Purkinje cells elicited by outward DC current shows that, as in the case of the antidromic invasion, the fast somatic potentials (s.s.) do not invade the dendrite actively. However, the dendritic spike bursts (d.s.b.s) interposed between the s.s. potentials are most prominent at dendritic level. 5. Two types of voltage-dependent Ca responses were observed. At low stimulus level a plateau-like depolarization is accompanied by a prominent conductance change; further depolarization produces large dendritic action potentials. These two classes of response are TTX-resistant but are blocked by Cd, Co, Mn or D600, or by the removal of extracellular Ca. 6. Following blockage of the Ca conductance, plateau potentials produced by a non-inactivating Na conductance are observed mainly near the soma indicating that this voltage-dependent conductance is probably associated with the somatic membrane. 7. Spontaneous firing in Purkinje cell dendrites is very similar to that observed at the soma. However, the amplitude of these bursts is larger at dendritic level. It is further concluded that these TTX-insensitive spikes are generated at multiple sites along the dendritic tree. 8. Six ionic conductances seem to be involved in Purkinje cell electroresponsiveness: (a) an inactivating and (b) a non-inactivating Na conductance at or near the soma, (c) a spike- and (d) a plateau-generating Ca conductance, and (e) voltage-dependent and (f) Ca-dependent K currents. 9. The possible role of these conductances in Purkinje cell integration is discussed.

The variance of sodium current fluctuations at the node of Ranvier

Abstract: 1. Single myelinated nerve fibres 12-17 mum in diameter from Rana temporaria and Rana pipiens were voltage clamped at 2-5 degrees C. Potassium currents were blocked by internal Cs(+) and external tetraethylammonium ion. Series resistance compensation was employed.2. Sets of 80-512 identical, 20 ms depolarizations were applied, with the pulses repeated at intervals of 300-600 ms. The resulting membrane current records, filtered at 5 kHz, showed record-to-record variations of the current on the order of 1%. From each set of records the time course of the mean current and the time course of the variance were calculated.3. The variance was assumed to arise primarily from two independent sources of current fluctuations: the stochastic gating of sodium channels and the thermal noise background in the voltage clamp. Measurement of the passive properties of the nerve preparation allowed the thermal noise variance to be estimated, and these estimates accounted for the variance observed in the presence of tetrodotoxin and at the reversal potential.4. After the variance sigma(2) was corrected for the contribution from the background, its relationship to the mean current I could be fitted by the function sigma(2) = iI-I(2)/N expected for N independent channels having one non-zero conductance level. The single channel currents i corresponded to a single-channel chord conductance gamma = 6.4 +/- 0.9 pS (S.D.; n = 14). No significant difference in gamma was observed between the two species of frogs. The size of the total population of channels ranged from 20,000 to 46,000.5. The voltage dependence of i corresponded closely to the form of the instantaneous current-voltage relationship of the sodium conductance, except at the smallest depolarizations. The small values of i at small depolarizations may have resulted from the filtering of high-frequency components of the fluctuations.6. It is concluded that sodium channels have only two primary levels of conductance, corresponding to ;open' and ;closed' states of the channel.7. The fraction p(max) of channels open at the time of the peak conductance was found to be 0.59 +/- 0.08 (S.D.; n = 5) and 0.9 +/- 0.1 (S.D.; n = 3) for depolarizations to -5 and +125 mV, respectively. (50 ms hyperpolarizations to -105 mV preceded the depolarizations in each case.) These values are similar to those predicted by Hodgkin-Huxley kinetics.8. Fluctuations in the firing threshold of neurones are expected from the stochastic gating of sodium channels. A prediction of the size of these fluctuations based on the measured properties of the channels gives a value of about 1% for the relative spread, which agrees with experimental values in the literature.

Visual pigments of rods and cones in a human retina.

Abstract: 1. Microspectrophotometric measurements have been made of the photopigments of individual rods and cones from the retina of a man. The measuring beam was passed transversely through the isolated outer segments. 2. The mean absorbance spectrum for rods (n = 11) had a peak at 497.6 +/- 3.3 nm and the mean transverse absorbance was 0.035 +/- 0.007. 3. Three classes of cones were identified. The long-wave cones ('red' cones) had a lambda max of 562.8 +/- 4.7 nm (n = 19) with a mean transverse absorbance of 0.027 +/- 0.005. The middle-wave cones ('green' cones) had a lambda max of 533.8 +/- 3.7 nm (n = 11) with a mean transverse absorbance of 0.032 +/- 0.007. The short-wave cones ('blue' cones) had a lambda max of 420.3 +/- 4.7 nm (n = 3) with a mean transverse absorbance of 0.037 +/- 0.011. 4. If assumptions are made about the length of cones and about pre-receptoral absorption, it is possible to derive psychophysical sensitivities for the cones that closely resemble the appropriate pi mechanisms of W. S. Stiles. 5. If assumptions are made about the length of rods and about pre-receptoral absorption, however, the psychophysical sensitivity derived for the rods is considerably broader than the C.I.E. scotopic sensitivity function.

Two different responses of hippocampal pyramidal cells to application of gamma-amino butyric acid.

Abstract: 1. Extra- and intracellular recordings were made from CA1 cells in hippocampal slices in vitro. The effects of ionophoretically applied GABA on somatic and dendritic regions were studied. 2. Ionophoresis of GABA at dendritic sites gave a reciprocal effect by inhibiting the effect of excitatory synapses close to the dendritic application, while facilitating those lying further away. For example, GABA delivered to the mid-radiatum dendritic region reduced the population spike generated by a radiatum volley, while facilitating the population spike evoked by oriens fibre stimulation. Similarly, when single cells were recorded from, mid-apical dendritic delivery of GABA abolished the synaptically driven discharges evoked by fibres terminating at this part of the dendritic tree, but facilitated the responses to input from fibres terminating on the basal dendrites of the same cell. 3. With intracellular recording two effects were observed. Applied near the soma, GABA induced a hyperpolarization associated with an increased membrane conductance. When applied to dendrites, GABA caused a depolarization also associated with an increased membrane conductance. Both types of GABA applications could inhibit cell discharges, although in some cases the depolarizing response could facilitate other excitatory influences or cause cell firing by itself. 4. Both the hyperpolarizing and depolarizing GABA responses persisted after blockade of synaptic transmission by applying a low calcium high magnesium solution, indicating mediation via a direct effect upon the cell membrane. 5. The reversal potential for the hyperpolarizing GABA effect was similar to the equilibrium potential for the i.p.s.p. evoked from alveus or orthodromically, and was 10-12 mV more negative than the resting potential. The size of the depolarizing response was also dependent upon the membrane potential. By extrapolation an estimated equilibrium potential was calculated as about -40 mV. 6. Our results support the idea that the hyperpolarizing basket cell inhibition at the soma is mediated by the release of GABA. This hyperpolarizing response causes a general inhibition of firing. The dendritic effects of GABA, however, seem to represent another type of inhibition, which by shunting synaptic currents makes possible a selective inhibitory influence on afferents synapsing locally while facilitating more remotely placed excitatory synapses. We propose the term discriminative inhibition for this postulated new type of control of pyramidal cell discharges.

Two components of electrical dark noise in toad retinal rod outer segments.

Abstract: 1. Physiological noise in the visual transduction mechanism was studied by recording membrane current from single rod outer segments in pieces of isolated toad retina. 2. The inward current in darkness showed spontaneous fluctuations which disappeared during the response to bright light. 3. The dark noise consisted of two components, a continuous fluctuation of rms amplitude about 0.2 pA and occasional discrete events about 1 pA in size. 4. Intervals between discrete events followed the exponential distribution expected of a Poisson process with a mean rate of about one event per 50 sec (20 degrees C). 5. The amplitude and power spectrum of the discrete events resembled those of single photon effects in the same rod, suggesting that discrete events may arise from spontaneous activation of single rhodopsin molecules. 6. The temperature dependence of the mean frequency of occurrence of discrete events gave an activation energy of 22 kcal mole-1, probably characteristic of thermal isomerization of rhodopsin. 7. The variance of the continuous component of the dark noise rose linearly with the length of the outer segment drawn into the suction electrode, indicating that this component is generated in the outer segment. 8. The power spectrum of a rod's continuous noise was usually fitted by the square of a Lorentzian with the same time constant as that of the four first-order delays in the cell's single photon response. The shot effects composing the continuous component thus appear to be shaped by two of four sequential processes in transduction. 9. The variance and spectrum of the continuous noise are interpreted to reflect shot effects about 1/400 the size of a single photon effect occurring at a frequency of 6 x 10(3) sec-1. 10. The rod's flash sensitivity was halved by a steady light to giving about 8 photoisomerizations sec-1. The much lower mean rate of discrete events indicates that Io in increment sensitivity experiments on individual receptors is not set by thermal activation of rhodopsin. 11. Values of sensitivity and time-to-peak flash response collected from many cells in darkness were correlated by the same power law relation obtaining in the presence of backgrounds. The correlation observed would be explained if a single variable controlled both the gain and time scale of several stages of the transduction mechanism in background light and in darkness.

Thermoregulatory and rhythm-generating mechanisms governing the sudomotor and vasoconstrictor outflow in human cutaneous nerves.

Abstract: 1. Recordings of multiunit sympathetic activity were made from human nerve fascicles supplying hairy and glabrous skin of the extremities in healthy subjects exposed to different ambient temperatures. Sudomotor and vasomotor events accompanying the neural activity were monitored by simultaneous recordings of electrodermal and pulse plethysmographic events (Pleth) in the neural innervation zones. 2. By exposing the subject to warm (43 degrees C) or cold (15 degrees C) environments, it was possible to obtain a selective activation of either the sudomotor or the vasoconstrictor neural system, respectively, with suppression of spontaneous activity in the other system. 3. Bursts of both vasoconstrictor and sudomotor nerve activity were found to occur at certain preferred intervals which were integer multiples of a period of about 0 . 6 sec (100 cycles/min). With high sudomotor or vasoconstrictor tone the 100 cycles/min rhythm was prominent but with decreasing tone slower subharmonic rhythms prevailed. Respiratory rhythms were also discerned as well as slower rhythms attributable to oscillatory tendencies in thermoregulatory servos. 4. Vasoconstrictor bursts had longer mean duration than sudomotor bursts, a finding attributed to a slower conduction velocity of vasoconstrictor as compared to sudomotor impulses. 5. With increasing incidence of bursts transient electrodermal or plethysmographic responses following individual bursts merged, and thus the fast neural rhythms were not discernible in either the electrodermal or Pleth traces. Given increments in firing rate of nerves produced less additional vasoconstriction at high than at low firing rates. The rhythm generating mechanisms may help to restrict rates of individual fibres to the low range which provides high gain in the neuroeffector transfer functions.

Selective depression of excitatory amino acid induced depolarizations by magnesium ions in isolated spinal cord preparations.

Abstract: 1. The depressant actions of Mg2+ and a range of other divalent ions on synaptic excitation and on responses produced by excitatory amino acids and other putative transmitters have been investigated in hemisected isolated spinal cords of frogs and neonatal rats. Some comparative studies were also made using the rat isolated superior cervical ganglion. 2. At concentrations above 10 microM, Mg2+ selectively antagonized N-methyl-D-aspartate (NMDA)-induced motoneurone depolarization as recorded from ventral roots of tetrodotoxin-blocked spinal cords. Depolarization evoked by quisqualate (unaffected by 20 mM-Mg2+) was resistant to the depressant action of these ions, while depolarizations evoked by other excitant amino acids were depressed to intermediate degrees. 3. Mn2+, Co2+ and Ni2+ had qualitatively similar actions to Mg2+; Mn2+ was somewhat less potent and Co2+ and Ni2+ more potent than Mg2+. The alkaline earth metal ions, Ca2+, Sr2+ and Ba2+, had very weak Mg2+-like actions. Ca2+ and Mg2+ acted additively in depressing amino acid-induced responses. 4. Mg2+ also depressed motoneurone responses evoked by noradrenaline, substance P and carbachol in the neonatal rat isolated spinal cord. However, none of these effects were as marked as the depression of NMDA-induced responses by Mg2+ in this preparation. Mg2+ did not depress motoneurone depolarization produced by 5-HT in the rat spinal cord or the depolarizing action of GABA on primary afferent terminals of the isolated frog spinal cord. 5. At concentrations producing marked depression of NMDA-induced responses, Mg2+ also depressed synaptic transmission in spinal cords in the absence of an effect on ganglionic transmission. At the same concentrations, Mn2+, Co2+ and Ni2+ depressed synaptic transmission in both preparations. 6. From the similarity in action between Mg2+ and the D-alpha-aminoadipate group of NMDA antagonists, it is suggested that the central depressant action of low concentrations of Mg2+ involves predominantly a postsynaptically mediated interference with the action of an excitatory amino acid transmitter.

Electrical properties of individual cells isolated from adult rat ventricular myocardium.

Abstract: 1. Individual cells were isolated from adult rats ventricular myocardium by a collagenase digestion procedure. 2. Steady membrane potentials recorded with conventional intracellular glass micro-electrodes from cells in a modified Krebs solution containing 3 . 8 mM-KCl and 0 . 5 mM-CaCl2 were less negative than -40 mV in most cells (-25 . 3 +/- 10 . 9 mV, mean +/- S.D., 211 cells). 3. After addition of the potassium selective ionophore valinomycin (60 nM) to the bathing solution all recorded membrane potentials were more negative than -60 mV (-74 . 8 +/- 7 . 0 mV, sixty-three cells). 4. The internal concentration of potassium in the cells was determined as 120 . 8 +/- 1 . 7 mM (+/- S.E., n = 24) by flame emission spectrometry after centrifugation through silicone oil, using tritiated water and D-[1-14C] mannitol to estimate total and extracellular water in the pellet. 5. In the majority of cells in the standard solution the membrane potential recorded within a few msec of penetration was more negative than -70 mV (-78 . 4 +/- 9 . 7 mV, seventy-three cells). In sixty-six cells penetration initiated an action potential which overshot zero by 31 . 3 +/- 7 . 1 mV. This overshoot was abolished by reducing the external sodium to 0 . 1 of the normal value, and reduced or abolished by addition of tetrodotoxin (30 microM). 6. Modifications of the standard bathing solution which increased the number of cells with steady recorded membrane potentials more negative than -60 mV were: isosmotic substitution of sucrose for NaCl; replacement of NaCl and KCl by sodium isethionate and potassium methyl sulphate; addition of 5 or 10 mM-CaCl2; addition of 10 mM-MnCl2. 7. For cells in solution containing 2 . 5 or 5 . 5 mM-CaCl2, input resistances estimated from the amplitude of hyperpolarizations evoked by 200 msec current pulses were approximately 40 M omega at a resting potential close to -80 mV and became much greater as cells were depolarized. Time constants measured at the resting potential were approximately 8 msec. 8. In certain conditions, repeated spontaneous action potentials were recorded from contracting cells, and in quiescent cells evoked action potentials could be initiated by applying brief depolarizing pulses through the micro-electrode. Action potentials were coincident with contractions. 9. It is concluded that the resting potential of these isolated cells is normally more negative than -70 mV, and that the cells retain the ionic mechanisms necessary for the generation of active currents.

Possible mechanisms for long-lasting potentiation of synaptic transmission in hippocampal slices from guinea-pigs.

Abstract: 1. Long-lasting potentiation of synaptic transmission was studied in the CA1 region of guinea-pig hippocampal slices maintained in vitro. 2. Stimulating pulses were delivered alternately to two independent afferent pathways, stratum radiatum and stratum oriens. The presynaptic volleys and field e.p.s.p.s. were recorded from the same two layers, while an electrode in the pyramidal cell body layer recorded the population spike or in other experiments the extra- or intracellular potentials from a single pyramidal cell. 3. A short tetanus to either of the two input pathways produced a long-lasting enhancement of the field e.p.s.p. as well as an increased size and a reduced latency of the population spike. This long-lasting potentiation was observed for up to 110 min after tetanization. Extracellular unit recordings showed that this potentiation is accompanied by an increased probability of firing and a reduced firing latency. Intracellular recordings showed an increased e.p.s.p., through the increase was smaller and less regular than for the extracellular field e.p.s.p. 4. No corresponding changes were seen in the field potential responses to stimulation of the untetanized input path, or in the intracellularly measured soma membrane potential, resistance, or excitability. The latter two properties were measured by intracellular injection of current pulses. It is concluded that long-lasting potentiation is specific to the pathway which has received the tetanization. 5. Following tetanization there was also a short-lasting (usually 2-4 min) depression, most often seen for the control pathway but sometimes visible on the tetanized side as well, superimposed on the potentiation. It is concluded that the short-lasting depression is not confined to any particular pathway but is a generalized (unspecific) phenomenon.

Paired-pulse and frequency facilitation in the CA1 region of the in vitro rat hippocampus.

Abstract: 1. Several types of facilitation of evoked synaptic responses were investigated in the CA1 region of the in vitro rat hippocampus. Homosynaptic paired-pulse facilitation and heterosynaptic frequency facilitation were characterized and found to be differentiable processes on the basis of several characteristics. 2. Paired-pulse facilitation, which occurs when the same input is stimulated twice in rapid succession, is manifested as an increase in both the extracellularly recorded population spike and the field e.p.s.p., and is specific to the set of afferents excited by the first impulse. Responses to other excitatory afferents show no facilitation by a heterosynaptic conditioning pulse. 3. At intervals less than 200 msec, the degree of facilitation produced by a preceding impulse appears to decline as a first order exponential function of time. Facilitation is increased by lowering calcium or raising magnesium concentrations in the bathing medium, with no apparent change in the time constant of the decay process. 4. The phenomenon that has sometimes been termed frequency facilitation, and which occurs during the early phase of repetitive stimulation, appears to be an extension of paired-pulse facilitation. It is seen as an increase in amplitude of both the e.p.s.p. and population spike in response to stimulation of homosynaptic inputs, can be predicted with fair accuracy by assuming that the residual paired-pulse facilitation produced by each impulse adds linearly with that from previous impulses, and is affected by calcium and magnesium ions in the same manner as is paired-pulse facilitation. These two types of facilitation, which apparently share a common mechanism, are termed synaptic or primary facilitation. 5. Another type of facilitation, which we suggest might more properly be called frequency facilitation, develops slowly during the course of repetitive stimulation. It is the result of an increase in cell firing in response to any excitatory input, either homo- or heterosynaptic, at time points at which the field e.p.s.p. is typically depressed. 6. Increases in the potassium concentration of the perfusion medium produce effects similar to those observed with frequency facilitation; stimulation-evoked increases in the extracellular concentration of this ion are hypothesized to underlie this type of generalized facilitation.

Voltage-clamp investigations of membrane currents underlying pace-maker activity in rabbit sino-atrial node.

Abstract: 1. Small preparations of spontaneously beating rabbit sino-atrial node have been investigated using the two micro-electrode voltage-clamp technique. 2. Hyperpolarizing clamp pulses given from holding potentials of about -45 mV reveal a time-dependent change of a membrane current, if, which is shown to overlap the pace-maker range (-65 mV to -45 mV) for these preparations. 3. The changes in if are shown to be quite distinct from the de-activation of the time-dependent outward current, iK. 4. The time-dependent changes of the if system increase during adrenaline application and therefore contribute to the chronotropic action of adrenaline on the heart. 5. Evidence for a link between slow inward current (iSi) and time-dependent outward current (iK) in rabbit sino-atrial node is presented and assessed.

Mechanical relaxation rate and metabolism studied in fatiguing muscle by phosphorus nuclear magnetic resonance

Abstract: 1. We have used phosphorus nuclear magnetic resonance (31P NMR) to study muscular fatigue in anaerobic amphibian muscle. In this paper the biochemical and energetic changes that result from a series of tetani are related to the decrease in rate constant (1/tau) for the final, exponential, phase of relaxation. 2. Using 31P NMR we have measured the concentrations of phosphocreatine (PCr), inorganic phosphate (Pi) and ATP as well as the internal pH. From our measurements we have calculated [creatine], [free ADP], the free-energy change (more precisely, the affinity A = -dG/d xi) for ATP hydrolysis and the rates of lactic acid production and of ATP hydrolysis. 3. We have found that 1/tau, the rate constant of relaxation, is correlated with each of the following, independently of the pattern of stimulation: isometric force production, all of the measured or calculated metabolite levels, pH and dG/d xi. 4. There is a clear dependence upon the pattern of stimulation of the relation between 1/tau and each of the following: total duration of the experiment, number of contractions, rate of lactic acid production and rate of ATP hydrolysis. 5. The rate of relaxation is linearly related to [PCr], [creatine], [Pi] and dG/d xi. It is nonlinearly related to isometric force, [ATP], [H+] and rate of ATP hydrolysis. 6. We conclude that the change in 1/tau, like that of isometric force, depends upon metabolic factors, and not upon any independent changes in the activation or deactivation of contraction. We suggest that 1/tau may depend upon the free-energy change for ATP hydrolysis which in turn may be related to the rate of Ca2+ uptake into the sarcoplasmic reticulum.

Physiological compensation for loss of afferent synapses in rat hippocampal granule cells during senescence.

Abstract: 1. The effects of senescence on the input-output characteristics of the perforant path projection to granule cells of the fascia dentata were studied in rats using extracellular techniques in vivo, and both extra- and intracellular recording in vitro. 2. Senescent animals exhibited a significant reduction in the perforant path excitatory synaptic field potential at all stimulus intensities tested. This was associated with a reduction in the size of the afferent fibre response, although there was no apparent change in the threshold for fibre activation. These data support the anatomical literature which indicates a loss of afferent synapses with advanced age. 3. For a given magnitude of afferent fibre response, however, the old animals exhibited a larger synaptic field potential, suggesting that the remaining synapses were in fact more powerful. Furthermore, the magnitude of the extracellular population spike, an index of the number of discharging granule cells, was greater in the old animals when plotted as a function of extracellular e.p.s.p. amplitude. 4. Intracellular recording from a total of 190 granule cells in the transverse hippocampal slice preparation revealed a 17% reduction in the voltage threshold for synaptically elicited granule cell discharge, and a 13% reduction in the latency of the action potential in old compared to young rats. Resting potentials, action potential amplitudes, whole neurone time constants, the relations between applied current and input resistance, and the discharge threshold following depolarizing current pulses, were not different between age groups. 5. These data indicate that granule cells could partly compensate for a loss of synapses during senescence by an increase in their electrical responsiveness to synaptic activation and possibly by an increase in synaptic efficacy.

The reflex responses of single motor units in human first dorsal interosseous muscle following cutaneous afferent stimulation.

Abstract: 1. Changes in the probability of firing of motor units active during voluntary muscle contraction have been studied in human first dorsal interosseous muscle in response to cutaneous afferent stimulation. 2. Electrical stimulation of the digital nerves of the index finger produces a reflex response consisting of three phases, early excitation followed by inhibition followed by late excitation. 3. Motor units recruited at low levels of voluntary contraction strength had larger early inhibitory and larger late excitatory responses than those recruited at higher levels of force. 4. Taking together the size of the short latency excitatory and inhibitory responses, units recruited at low levels of voluntary contraction strength and with slow twitch contraction times had predominantly inhibitory responses. In contrast, units in which the short latency excitatory responses predominated had fast twitch contraction times and were recruited at high levels of contraction strength. 5. It is concluded that for the cutaneous reflex pathway there are differences in the balance of excitatory and inhibitory sets of interneurones impinging on first dorsal interosseous motoneurones which are related to the kind of muscle unit innervated.

Voltage-clamp experiments in normal and denervated mammalian skeletal muscle fibres.

Abstract: 1. The vaseline-gap voltage-clamp method has been applied to the study of ionic currents in cut pieces from innervated and 5--7 days denervated rat skeletal muscle fibres. 2. Kinetic analysis of sodium currents in innervated rat muscle showed them to be similar to those in frog muscle, except that rat sodium channels are activated at slightly more negative potentials. Peak sodium conductances of 40--50 m-mho/cm2 were measured, corresponding to values of GNa of 100--120 m-mho/cm2. 3. The permeability sequence of the sodium channel to several organic and inorganic cations is Li+ greater than Na+ greater than hydroxylammonium greater than hydrazinium greater than guanidinium approximately ammonium greater than K+. TMA+ and Ca2+ were not measurably permeant. 4. Denervation appears to shift activation and inactivation parameters of sodium currents by approximately 10 mV to more negative potentials, but does not appreciably affect the maximum peak sodium conductance or the time constants for activation and inactivation. 5. Dose--response curves for block by tetrodotoxin in innervated fibres are fitted well by assuming binding of toxin to a single population of channels with a dissociation constant of about 5 nM. In denervated fibres there appears in addition a second population of channels with a dissociation constant in the micromolar range. These relatively toxin-insensitive channels respond less rapidly to potential changes, and can contribute up to 25--30% of the total sodium conductance. 6. The potassium currents of innervated rat muscle were similar to those of frog muscle in their voltage dependence of activation. 7. The time constant for inactivation of the sodium current, tau h, at -13 mV showed a temperature dependence measured between 10 and 20 degrees C equivalent to an average Q10 of 2 . 3. The Q10 for the time constant of activation of the potassium current tau n, averaged 2 . 5 between -40 mV and +40 mV, measured over the same temperature range.

The frequency selectivity of auditory nerve fibres and hair cells in the cochlea of the turtle

Abstract: 1. The electrical responses of single auditory nerve fibres or cochlear hair cells were recorded in the isolated half-head of the turtle Pseudemys scripta elegans. Responses to sound stimuli presented to the tympanum could be recorded for at least 4 hr after isolation. 2. Impulses were recorded extracellularly from single auditory nerve fibres. For tones of suprathreshold intensity the impulses occurred with a preferred phase relation (i.e. they were phase-locked) to the cycles of the sound stimulus. Nerve fibres had sharp tuning curves (Q10 db = 0·5-7·5) with single characteristic frequencies (c.f.) ranging from about 30 to 700 Hz. Best threshold sensitivities of fibres at their c.f. were in the region of 30-40 db sound pressure level with respect to 20 μPa. 3. Intracellular recordings were made from hair cells in the basilar papilla. Following injection of a fluorescent dye into a cell through the recording electrode, the dye was localized in a single hair cell in a transverse section of the cochlea. 4. Hair cells had resting potentials of about -50 mV, and, to low frequency tones, gave periodic responses graded with the intensity and frequency of the stimulus. Recordings were obtained from cells with characteristic frequencies between 70 and 670 Hz. 5. The voltage response to a pure tone at low sound pressure was sinusoidal for all frequencies of stimulation; at higher sound pressures a number of non-linearities were apparent in the response wave form. One of these was a steady depolarizing component, which, relative to the periodic component of the response, was most prominent at high frequencies. 6. The amplitude of the response evoked in a hair cell by a low intensity tone was linearly related to the sound pressure; for loud sounds, the response eventually reached a saturating amplitude, which in some cells was as great as 30-45 mV peak-to-peak. 7. The linear sensitivity of a hair cell is defined as the r.m.s. voltage for a linear response of the cell at its c.f. divided by the sound pressure at the tympanum. In the most sensitive cells this value was 30-90 mV/Pa. 8. If the frequency selectivity of a hair cell was expressed in terms of the sound pressure needed to produce a constant amplitude of response, the sharpness of this frequency selectivity was found to be virtually independent of the response criterion for responses between 1 and 10 mV; in the cells which gave the largest responses, the frequency selectivity expressed in this way was comparable to that of the nerve fibres. Cells with smaller maximum responses often had broader tuning curves. 9. Responses of hair cells to short low intensity tone bursts at the c.f. built up approximately exponentially during the tone, and decayed away exponentially when the tone was terminated. The terminal oscillations were at the c.f. of the cell, and independent of the frequency of stimulation. 10. From the time constant of the build up and decay of the linear response to a tone burst at the c.f. the sharpness of tuning of the cell was estimated and found to agree with that obtained from the responses of the cell to continuous tones. The most highly tuned cells had quality factors (Q3 db) in the range 5-10. 11. The c.f. of a hair cell was correlated with its position along the basilar membrane. Low frequency hair cells were located towards the apical or lagenar end and high frequency cells were found towards the basal or saccular end. On the assumption of an exponential distribution of c.f. with distance, each octave occupied about 94 μm along the membrane. 12. A hair cell's response to a click was a decaying oscillation at the characteristic frequency of the cell. From the initial polarity of the responses to condensation and rarefaction clicks it was concluded that the hair cell depolarized as a result of movements of the basilar membrane towards the scala vestibuli, and hyperpolarized for motion towards the scala tympani. 13. In the absence of deliberate sound stimulation, the hair cell voltage fluctuated continuously about its mean level. The principal frequency components in the noise were concentrated around the c.f. of the cell. The voltage noise in the hair cells showed no significant cross-correlation with sound pressure fluctuations at the tympanum.

Phase relationship between normal human respiration and baroreflex responsiveness.

Abstract: 1 We studied the influences of phase of respiration and breathing frequency upon human sinus node responses to arterial baroreceptor stimulation 2 Carotid baroreceptors were stimulated with brief (06 sec), moderate (30 mmHg) neck suction during early, mid, and late inspiration or expiratin at usual breathing rates, or, during early inspiration and expiration at breathing rates of 3, 6, 12, and 24 breaths/min 3 Baroreceptor stimuli applied during early and mid inspiration and late expiration provoked only minor sinus node inhibition; stimuli begun during late inspiration and early expiration provoked maximum sinus node inhibition 4 At breathing rates of 3, 6 and 12 breaths/min, expiratory baroreflex responses were significantly greater than inspiratory responses; at 24 breaths/min, however, inspiratory and expiratory baroreceptor stimuli produced comparable degrees of sinus node inhibition 5 Our results delineate an important central biological rhythm in normal man: human baroreflex responsiveness oscillates continuously during normal, quiet respiration The phase shift of baroreflex responsiveness on respiration suggests that this interaction cannot be ascribed simply to gating synchronous with central inspiratory neurone activity Regularization of heart rate during rapid breathing is associated with loss of the differential inspiratory-expiratory baroreflex responsiveness which is present at usual breathing rates

Release of calcium ions linked to the activation of potassium conductance in a caffeine‐treated sympathetic neurone.

Abstract: 1. The mechanism of spontaneous and rhythmic hyperpolarizations which occur in bullfrog sympathetic ganglion cells under the effect of caffeine (2--10 mM) were further analysed. 2. Intracellular injection of EGTA blocked generation of caffeine hyperpolarizations (C-hyperpolarizations): this confirmed the previous conclusion (Kuba & Nishi, 1976) that these hyperpolarizations are caused by rhythmic increases in the K+ conductance (GK) of the membrane as a result of rises in free intracellular Ca2+. 3. The amplitude and duration of a C-hyperpolarization induced by an action potential was a function of the time since the previous one; the longer the interval, greater the area. 4. The relationship between the product of the amplitude and duration of a C-hyperpolarization and the preceding interval depended on external Ca2+; when this was low the relationship shifted, so as to indicate an involvement of a Ca2+ accumulating process in the generation mechanism of C-hyperpolarizations. 5. A rapid lowering of temperature triggered the generation of a C-hyperpolarization before appearance of a rhythmic one. There seemed to be no threshold temperature for the effect of such a cold shock; cooling from any temperature within a certain range (10--25 degrees C) by more than a few degrees was effective. 6. The rapid cooling effect was observed even in a Ca2+-free Mg2+ solution. 7. Dantrolene Na increased the interval between rhythmic C-hyperpolarizations or blocked them, but affected less those triggered by an action potential or cold shock. 8. Intracellular injection of Ca2+ triggered the generation of a C-hyperpolarization before the appearance of a rhythmic one. 9. The latency of the generation of an action potential-evoked C-hyperpolarization was dependent on the preceding interval; the shorter the interval, the longer the latency. There was a refractory period for induction of an action potential-induced C-hyperpolarization. 10. The interval between rhythmic C-hyperpolarizations was increased by a small or moderate membrane hyperpolarization (5--20 mV) and decreased by a larger hyperpolarization; the refractory period of an action potential-induced C-hyperpolarization was similarly increased and then decreased by progressive membrane hyperpolarization. 11. These results suggest that rhythmic increases in the GK under the effect of caffeine are due to oscillations of the intracellular Ca2+ concentration and that there may be Ca storage sites in the bullfrog sympathetic ganglion cell which are comparable to the sarcoplasmic reticulum in the skeletal muscle fibre.

Search for a cardiac nociceptor: stimulation by bradykinin of sympathetic afferent nerve endings in the heart of the cat.

Abstract: 1. We have examined the effect of bradykinin on impulse traffic in sympathetic afferent fibres from the heart, great vessels and pleura, and have attempted to identify cardiac nociceptors that on the basis of their functional characteristics might have a role in the initiation of cardiac pain. 2. In anaesthetized cats, we recorded afferent impulses from ‘single-fibre’ slips of the left 2nd--5th thoracic rami communicantes and associated chain, and selected fibres arising from endings in the heart, great vessels, pericardium and pleura. We applied bradykinin solution (0 . 1--1 . 0 microgram/ml.) locally to the site of the ending; we also injected bradykinin (0 . 3--1 . 0 microgram/kg) into the left atrium. 3. Afferent endings excited by bradykinin (159 of 191 tested) were of two types. The larger group (140) were primarily mechanoreceptors with A delta of C fibres (mean conduction velocity, 7 . 5 +/- 0 . 6 m/sec). They were very sensitive to light touch. Those located in the heart, great vessels or overlying pleura had a cardiac rhythm of discharge and were stimulated by an increase in blood pressure or cardiac volume. 4. Bradykinin increased mechanoreceptor firing from 0 . 7 +/- to 5 . 0 +/- 0 . 3 (mean +/- S.E. of mean) impulses/sec. Some endings appeared to be stimulated directly by bradykinin, others sensitized by it so that they responded more vigorously to the pulsatile mechanical stimulation associated with the cardiac cycle. 5. The smaller group of eighteen endings, of which ten were in the left ventricle, were primarily chemosensitive. Most had C fibres, a few had A delta fibres (mean conduction velocity, 2 . 3 +/- 0 . 7 m/sec). They were insensitive to light touch. With one exception they never fired with a cardiac rhythm, and even large increases in aortic or left ventricular pressure had little effect on impulse frequency. 6. Chemosensitive endings were stimulated by bradykinin, impulse activity increasing from 0 . 6 to 15 . 6 +/- 1 . 3 impulses/sec and remaining above the control level for 1-3 min. The evoked discharge, which was either continuous or occurred in irregular bursts, was not secondary to mechanical changes in the heart and great vessels. 7. Tachyphylaxis occurred when the interval between successive applications of bradykinin was 20 min or less. It was a feature of the response of both mechanosensitive and chemosensitive endings. 8. Because of their responsiveness to changes in pressure and their sensitivity to light touch, the mechanosensitive endings appear to be unlikely to subserve a primarily nociceptive function, although they may be responsible for evoking some of the components of the pseudoaffective response. By contrast, the chemosensitive endings appear well fitted to act as cardiac nociceptors.

Calcium-activated transient outward current in calf cardiac Purkinje fibres.

Abstract: 1. The possibility that the transient outward current of calf cardiac Purkinje fibres depends on intracellular calcium was investigated using a two micro-electrode voltage clamp. 2. Upon removal of Cao and replacement with Sr or Ba, the transient outward current was strongly suppressed. At the same time a large slow inward current was revealed. 3. Partial removal of Cao with replacement by Mg also diminished the transient outward current. The inhibition was not due to voltage shifts in the inactivation curve. 4. The kinetics of the peak transient outward current were compared with the kinetics of peak twitch force, an approximate measure of the level of Cai. The two signals were related in a linear manner during beat-dependent changes with trains of voltage clamp depolarizations. 5. Tension and transient outward current were also found to inactivate with a similar dependence on pre-potential and recover from inactivation along a similar time course. Both processes activated with membrane depolarization in a similar manner. 6. Intracellular injection of EGTA reduced the magnitude of the transient outward current and the twitch contraction. 7. The inhibition of outward current following EGTA injection was more pronounced for strong depolarizations. With pulses negative to - 10 mV, there was often little apparent change in the peak net outward current. 8. All lines of evidence support the hypothesis that the transient outward current is activated by intracellular Ca. 9. The functional significance of the transient outward current is discussed. Since a Ca-activated outward current would automatically offset slow inward Ca current, such a system may help prevent arrhythmogenic slow responses in the His-Purkinje network.

On the release of transmitter at normal, myasthenia gravis and myasthenic syndrome affected human end‐plates.

Abstract: 1. Transmitter release has been studied at normal, myasthenia gravis (m.g.) and myasthenic syndrome (m.s.) affected human end-plates. At normal and diseased end-plates evoked transmitter release is Poisson for a mean quantal content, m less than ten. 2. The relation between log m and log [Ca]o, at normal and m.g. end-plates is linear, with a slope of 3.3-3.4. The value of m at m.g. end-plates is about five times larger than normal, below Ca 0.7 mM (Mg, 2mM). This difference in m is reduced at higher Ca levels. 3. The slope of the relation between log m.e.p.p. frequency and log [K]o is similar at normal and m.g. end-plates. Over its linear portion the relationship has a slope of approximately 6. 4. Fluctuations in the latency of evoked transmitter release were compared at normal and m.g. nerve terminals. At normal end-plates the probability of release reaches a peak about 0.3-0.4 msec after unitary e.p.p.s of the shortest latency and returns to zero about 1.0 msec after the peak. At m.g. end-plates the distribution of latencies shows less uniformity. 5. At m.s. end-plates m is approximately 5 in normal Ringer solution (2 mM-Ca, 1 mM-Mg). The relation between log m and log [Ca]o is linear, with a slope of 1.0-1.5. The K dependence of m.e.p.p. frequency appears reduced at m.s. end-plates. 6. Assuming a co-operative mechanism for transmitter release at normal human motor nerve terminals, the dissociation constant for the Ca complex is about 1.6X10(-3) M and the dissociation constant for the Mg complex is about 1.0X10(-3) M. 7. It is concluded that the presynaptic changes, at m.g. end-plates, are not the primary cause of the defect in nerve muscle transmission. At m.s. end-plates the presynaptic changes are sufficient to account for failure in transmission. Possible mechanisms for the abnormalities in transmitter release are considered.

The role of muscle mass in the cardiovascular response to static contractions.

Abstract: 1. Eleven men performed static contractions with the fingers (digits II and III), forearm (handgrip) and the knee extensors (knee angle 90°) at 40% maximal voluntary contraction (MVC) for 2 min. In seven of the subjects handgrip and knee extension were combined, both contractions held at 40% MVC. At the end of the contraction, cuffs were inflated to 250 mmHg for 3 min around the extremity which had been contracting. Continuous measurements were performed of force, heart rate and intra-arterial blood pressure (a. brachialis; 20 cm proximally), before and during contraction as well as during the recovery with and without cuffs inflated. 2. Heart rate and blood pressure increased momentarily with the onset of a contraction whereafter a gradual further increase took place. This pattern of response was similar for all muscle groups studied. The increases during the contractions were in the order: fingers, forearm, knee extensors and combined forearm—knee extensors, with the difference between each muscle group contraction being significant. 3. In the recovery period from a contraction with the cuff(s) inflated, heart rate returned to control level. Blood pressure also dropped, but remained elevated above pre-contraction level until the cuff(s) was released. 4. The present data during contraction are at variance with earlier observations showing that the cardiovascular response to a static contraction is proportional to the% MVC regardless of the muscle mass involved in the contraction. Our findings are in line with the traditional concept of central and peripheral nervous inputs playing a role in the cardiovascular adjustments to exercise, with both the central and the peripheral factors being related to the mass of the muscles engaged in the exercise.

The adequacy of stretch receptors in the cat knee joint for signalling joint angle throughout a full range of movement.

Abstract: 1. The present experiments were performed to resolve the discrepancy between the experiments of Boyd & Roberts (1953) in which receptors active at intermediate angles (mid range units) were frequently encountered, and the experiments of Burgess & Clark (1969) where mid range units were rarely encountered. 2. The discharge of knee joint afferents in the cat posterior articular nerve was recorded from both the dorsal roots and from the intact joint nerve. 3. No receptors were isolated which exhibited intermediate adaptation rates (similar to the "phasic" receptors described by Burgess & Clark, 1969). 4. The proportion of mid range units isolated in the present experiments (17.8%) was substantially larger than in the experiments of Burgess & Clark (4.8%). 5. Direct recordings from the posterior articular nerves of a series of cats revealed that there were always receptors tonically active at all intermediate positions. 6. Afferents from the popliteus muscle did not make a major contribution to the population of mid range units in the posterior articular nerve as removal of the popliteus muscle did not significantly alter the mean number of mid range units in the joint nerves of a series of nine cats. 7. Succinylcholine (SCh) in low doses (0.05-0.1 mg/kg) was found not to be specific for spindle afferents as these doses of SCh could elevate the resting discharge of mid range units of true articular origin. 8. It is concluded that slowly adapting joint receptors can adequately signal joint angle throughout the full normal range of movement and hence could make an important contribution to position sense.

Interactions between the regulation of the intracellular pH and sodium activity of sheep cardiac Purkinje fibres.

Abstract: 1. We have investigated the influence of the H+ and Na+ gradients across the cell membrane on the regulation of the intracellular pH (pHi) and of the intracellular Na activity (aNai) in sheep heart Purkinje fibres, using Na+- and pH-sensitive microelectrodes. 2. In oxygenated, nominally bicarbonate-free solutions (buffered with HEPES) the steady-state pHi changed linearly with the extracellular pH (pHo) by 0.23 pH units/pHo unit change over the pHo range of 5.4-8.4. The H+ equilibrium potential changed by about 47 mV/pHo unit change. 3. Both the steady-state pHi and the pHi recovery from acidification induced by lowering the pHo to 6.4 were affected only to a small extent by reducing the extracellular Na concentration [Na]o, to one half or to one tenth normal. 4. The steady-state aNai decreased by 5 to 20% when the pHo was reduced to 6.4 and increased by 3 to 8% when the pHo was raised to 8.4. These changes in aNai were still present when the Na-K pump had been inhibited by the cardioactive steroid strophanthidin (10(-5) M). 5. Exposure to K-free solutions caused an increase in aNai. Following addition of 6 mM-K (to re-activate the Na-K pump) the rate of decrease of aNai was not affected by pHo changes from 6.4 to 8.4. 6. Inhibition of the Na+-K+ pump by strophanthidin (10(-5) M) caused aNai to rise rapidly within 2-3 min. The pHi remained unchanged for the first 1-30 min after the pump inhibition, but then decreased by several tenths of a pH unit. 7. Amiloride (10(-3) M) caused a small decrease in a Nai and an intracellular acidification of up to 0.2 pH units. 8. Under conditions, where aNai was high due to inhibition of the Na-K pump by strophanthidin, lowering the [Na]o to one tenth normal produced a very large intracellular acidification, while aNai decreased. Amiloride increased this intracellular acidification even more, while the decrease in aNai remained unaffected. 9. Application of NH4Cl (20 mM) produced a decrease of aNai and a rapid intracellular alkalinization, followed by a slower acidification. Upon removal of NH4Cl the pHi dropped by several tenths of a pH unit but rapidly recovered. During this pH recovery there was a small transient increase in aNai above the control level before returning to normal. 10. The pHi recovery after the removal of NH4Cl was slowed by lowering the [Na]o to one tenth normal, and it was greatly inhibited in the presence of amiloride. The transient overshoot of aNai after NH4Cl removal was suppressed by amiloride. 11. We conclude that under some conditions there appears to be an exchange of intracellular H+ for extracellular Na+ across the cell membrane and that this exchange can help regulate the intracellular pH.

Motoneurone projection patterns in the chick hind limb following early partial reversals of the spinal cord.

Abstract: 1. The development of motoneurone projection patterns in the chick hind limb from reversed spinal cord segments was studied from the onset of axonal outgrowth (St. 24) to the establishment of mature connectivity patterns (St. 36). Approximately the first three lumbosacral cord segments were reversed along the anterior-posterior axis at St. 15-16. 2. Projection patterns from reversed cord segments were assessed electrophysiologically by direct spinal cord and spinal nerve stimulation and anatomically by retrograde horseradish peroxidase (HRP) labelling of motoneurones in St. 30-36 embryos. In younger embryos, paths taken by reversed axons were characterized by orthograde HRP labelling of motoneurones in specific reversed cord segments. 3. Lumbosacral motoneurones formed appropriate functional connexions with individual limb muscles in spite of anterior-posterior shifts in their spinal cord position aned consequent shifts in their spinal nerve entry point into the limb bud. Reversed motoneurones supplying individual hind limb muscles formed discrete nuclei in the transverse plane of the cord. Each nucleus and the lateral motor column as a whole showed reversed topographical characteristics when compared to control embryos. These observations were made before (St. 30) and after (St. 35-36) the major period of motoneurone cell death. 4. Correct connectivity resulted from specific alterations in axonal pathways within the plexus or major nerve trunks proximal to the branching of individual muscle nerves. Further such directed outgrowth was present from the earliest times that axons could be traced into the limb which is before the onset of motoneurone cell death and muscle cleavage. 5. It is concluded that motoneurones are specified to project to individual muscles or to follow particular pathways prior to motoneurone birthdays and limb bud formation. The establishment of specific motoneurone connectivity can not be accounted for by passive or mechanical guidance models alone. Rather, motoneurones must also actively respond to cues within the limb or interact among themselves on the basis of an early central specification.

The action of serotonin in the rat hippocampal slice preparation.

Abstract: 1. Intracellular activity was recorded from neurones in the CA1 pyramidal layer of slices of rat hippocampus maintained in vitro. 2. Application of 5-HT in a droplet or via ionophoresis produced a 3-5 mV hyperpolarization associated with a 30% decrease in input resistance. 3. The response to 5-HT was minimal with a drop concentration of 1 microM and maximal with 100 microM. The responses appeared to be blocked by methysergide applied in the superfusion medium. 4. The responses to 5-HT were minimal when the drug was applied in the apical dendritic region and maximal when it was applied near the soma. 5. 5-HT produced no substantial changes in e.p.s.p.s evoked by stimulation of the Schaffer collateral-commissural system or in i.p.s.p.s which were occasionally encountered following stimuli to the stratum radiatum. 6. The responses to 5-HT are true post-synaptic responses and are not indirect effects since they are present in a Ca2+-deficient Mg2+-enriched medium which blocks synaptic transmission. 7. The responses to 5-HT were not dependent on extracellular Cl- concentration. 8. These experiments indicate that 5-HT produces its effects in the rat hippocampus by activating K+ channels.

Potential clamp analysis of membrane currents in rat myelinated nerve fibres.

Abstract: 1. Large myelinated nerve fibres were isolated from rats and the membrane action potential was recorded in single nodes of Ranvier. Potential clamp experiments were performed at 24 degrees C and in one fibre also at 33 degrees C. 2. Positive potential steps were associated with an initial, mainly Na carried, current with an equilibrium potential (Ue) of 40 +/- 13 mV. The INa vs. U curve showed rectification at large positive potentials. 3. The Na permeability (PNa) curve was characteristically S-shaped with a half value at -48 +/- 6 mV and a ceiling value of 3.7 +/- 0.7 cm.sec-1.10(-3). PNa as defined by the constant field equation accounted for the rectification of the INa/U curve. 4. The potential and time dependence of the rapid inactivation (h) process was described quantitatively in terms of its steady state (h infinity) vs. U curve and its rate constants (alpha h and beta h) vs. U curves. h infinity (U = -80 mV) was 0.68 +/- 0.09. 5. The delayed currents were small and the ionic specificity of the delayed permeability changes was not identified. Tentatively calculated PK at large potentials was 0.21 +/- 0.06 cm.sec-1.10(-3). The leak conductance (gL) was 130 +/- 33 mS.cm-2. 6. A temperature rise in one experiment from 24 to 33 degrees C increased the rate constants (alpha h and beta h), but did not significantly change the size of the delayed currents. 7. INa was completely blocked by 25 nM-TTX in the outside solution. External application of 5 mM-TEA only slowly and incompletely blocked the small delayed currents. This effect was not fully reversible. 8. A comparison with frog fibres showed that the node in rat fibres had (a) a lower Ue of the initial current (probably because of a lower Na selectivity of the channel for the initial current), (b) approximately equal max. peak PNa, (c) about 10 mV negatively shifted PNa vs. U and h infinity vs. U curves, (d) quantitatively similar relations between rate constants of inactivation (alpha h and beta h) and U at 20--24 degrees C, (e) PK that was 1/5 of PK in frog fibres, (f) 4--5 times larger leak conductance, (g) membrane action potentials of smiliar amplitude and duration.

The control of sets of motoneurones by local interneurones in the locust.

Abstract: 1. A motoneurone innervating a muscle in a hind leg of a locust is controlled in a graded manner by many non-spiking, local interneurones. There is overlap and fractionation of control between these interneurones. Some interneurones depolarize the motoneurone over part of its range, others hyperpolarize it, whilst some do both. 2. The interneurones organize the small number of motoneurones that innervate one muscle into overlapping sets of various sizes. A motoneurone can therefore be activated individually or in particular combinations with its fellow motoneurones. 3. The motoneurones innervating two muscles of a joint are also organized into overlapping sets by many local interneurones. This permits the motoneurones to the two muscles to be activated reciprocally, together, or independently. 4. One interneurone can elicit a co-ordinated movement of one, two or even three joints in a hind leg that are components of the normal behaviour of the locust. 5. A single interneurone acting alone does not usually elicit the maximum output from one motoneurone, nor a complete piece of behaviour. A stronger contraction of a muscle and a more complete movement results from the action of groups of interneurones. 6. It is suggested that local interneurones, exerting graded control over motoneurones are a major element in the organization of motor patterns in the locust.

Voltage‐sensitive outward currents in cat motoneurones.

Abstract: 1. The soma membrane of cat motoneurones was voltage-clamped in vivo using intracellular current and voltage electrodes whose tips were separated by at least 5 micrometer. 2. Depolarization activates two separate, non-interacting K conductance systems whose rates of activation and decay differ by a factor of about 10. These conductances have a similar reversal potential, in the range of -6 to -21 mV (these and all subsequent voltages are expressed relative to the resting potential). Both conductances show linear 'instantaneous' current-voltage relationships. The steady-state magnitudes of both conductances increase with increasing depolarization. Neither conductance inactivates substantially during prolonged depolarizations. 3. The faster K conductance is similar to that described for squid axons and frog node. Activation begins at about +30 mV and is more than 90% complete within 5 msec of a depolarizing voltage step to +50 mV. Activation kinetics appear to be nonlinear. This fast K conductance contributes to the fast falling phase of the action potential. Following repolarization, this conductance decays with a time constant of 2-4 msec. 4. The slower K conductance activates during depolarizations of 10 mV or greater. The activation and decay of this conductance can be described by first-order exponential functions with time constants ranging from 20 to 50 msec. The slow K conductance underlies the prolonged hyperpolarization that follows motoneurone action potentials. Evidence from other studies suggests that this slow K conductance is regulated by intracellular Ca ions. 5. In addition to the two K conductance systems activated by depolarization, motoneurones exhibit another distinct conductance system that is activated by hyperpolarization. This third system has a reversal potential near the resting potential. Activation of this conductance during a hyperpolarizing voltage step can be fitted by a single exponential function with a time constant of 50-60 msec over the range -20 to -50 mV. This hyperpolarization-activated conductance accounts for some aspects of the anomalous rectification reported in cat motoneurones. 6. When the clamp circuit was turned off and the motoneurones were stimulated to discharge repetitively by depolarizing current steps, the apparent soma threshold voltage increased as the applied current (and discharge frequency) increased. 7. The basic features of the motoneurone action potential were reconstructed by simulations based on voltage clamp measurements of the voltage dependent conductance systems and previous measurements of passive membrane properties. These simulations assumed that the kinetics of the fast Na and K conductance systems in motoneurones can be described by equations of the same form as the Hodgkin-Huxley equations. These action potential reconstructions indicated that a major portion of the delayed depolarization following the action potential is attributable to capacitative currents from the dendrites...

Intracellular pH in early Xenopus embryos: its effect on current flow between blastomeres.

Abstract: 1. Electrophysiological techniques were used to monitor the flow of electric current from one cell to the next in Xenopus laevis embryos between the 4-cell and early blastula stages of development. Intracellular pH and blastocoel pH were determined using pH-sensitive micro-electrodes. 2. The resting intracellular pH was 7.74+/-0.02 (S.E. of mean, n = 29); there were no systematic differences between developmental stages. Blastocoel cavity pH was 8.4+/-0.06 (S.E. of mean, n = 10). The intracellular buffer value was 18 m-equiv. H+/pH unit per litre. 3. In embryos treated with bicarbonate buffered Holtfreter solution equilibrated with 100% CO2 the intracellular pH fell to 6.3+/-0.17 (S.D., n = 8). The membrane potential fell and the input resistance increased. The size of the effect on membrane potential and input resistance varied. 4. From the 32-cell stage onwards current flow from one cell to the next was abolished when the intracellular pH fell to below 6.5; the effect was rapid in onset and completely reversible. At cleavage stages of development lowering intracellular pH with CO2 had no effect on current flow from cell to cell. 5. The relationship between intracellular pH and current flow from cell to cell was sigmoid and covered between 0.2 and 0.4 pH units. The pH at which current flow was completely abolished ranged from 6.85 to 6.4. 6. Alterations in extraembryonic pH over the range 5.8-7.5 had no effect on any parameter measured. 7. We conclude that lowering the intracellular pH increases the resistance of both non-junctional junctional membranes. The data do not allow us to extract the pH junctional conductance relationship. 8. Variations in intracellular pH may provide a useful tool for the study of the functional role of direct cell to cell communication in both adult organs and early embryos.