Resting potential and electrical properties of frog slow muscle fibres. Effect of different external solutions

1. The electrical properties of frog slow muscle fibres were investigated with intracellular micropipettes to determine their characteristic length (lambda), specific membrane resistance (R(m)) and specific membrane capacitance.2. The value of lambda was about 1 cm in fibres of 1.2 cm length. The ;short cable model' was used to calculate R(m). Its mean value was 1.12 x 10(5) ohm cm(2), about 10-20 times larger than the value for twitch fibres. The mean value for C(m) was 3.24 x 10(-6) F/cm(2).3. Resting potentials measured immediately after penetration with a single micropipette were about - 80 mV. Lower values can be attributed to the effects of damage or leakage produced by micropipette insertion.4. Changes in external K concentration produced changes in the initially recorded resting potentials which follow the constant field theory using a ratio of Na: K permeabilities P(Na)/P(K) = 0.02. Changes in external Cl concentration produced little or no change in the resting potential or membrane resistance, indicating a low Cl permeability.5. In agreement with previous work, slow fibres showed a time-dependent decrease in resistance (;delayed rectification') for membrane potentials more positive than - 60 mV. ;Anomalous rectification' observed in twitch fibres was not seen in slow fibres. In high external K concentrations the resistance of slow fibres is almost unaffected by changes in membrane potential.6. Increasing the concentration of external Ca (up to isotonic) has two distinct effects on slow fibres. It increases R(m) up to ten times, and it improves the stability of trans-membrane recordings, probably by reducing the leakage due to micropipette penetrations. Magnesium does not appear to have either of these effects.     

Influence of dendritic location and membrane properties on the effectiveness of synapses on cat motoneurones

1. Measurements of the specific membrane properties and neuronal geometry of cat motoneurones were used to calculate the excitatory postsynaptic potentials (e.p.s.p.s) produced by unit (quantal) conductance changes occurring at various locations on the dendritic tree.2. Calculations demonstrate that conductance changes of 80-190 x 10(-10) mho are required to produce e.p.s.p.s having the same rise time and peak amplitude as the quantal e.p.s.p.s recorded in motoneurones by Kuno & Miyahara (1969b). Because quantal conductance changes are so large, synaptic activity can significantly reduce the effective specific resistance of the motoneuronal membrane.3. A quantal conductance change occurring at a high-impedance distal dendritic site is calculated to produce an e.p.s.p. of 15-20 mV peak amplitude at that site. Significant non-linear summation will occur between the e.p.s.p.s produced by conductance changes occurring simultaneously on the same dendritic branch.4. Calculations which take into account both non-linear summation and the loss of synaptic charge through dendritic membranes predict that for these motoneurones the time integral of soma-recorded quantal e.p.s.p.s originating on distal dendrites should be at least 20% as great as the time integral of a quantal e.p.s.p. originating directly on the soma. Quantal conductance changes occurring on 76% of the dendritic tree should produce soma e.p.s.p. time integrals at least 50% as great as those produced by somatic synapses.     

Electrotonic parameters of rat dentate granule cells measured using short current pulses and HRP staining

The passive electrotonic parameters of nerve cells in the dentate gyrus of the rat were studied in vitro. Intracellular recordings from 30 granule cells and 3 pyramidal basket cells followed by intracellular injection of horseradish peroxidase (HRP), allowed calculations of input resistance (RN), membrane time constant (tau m), electrotonic length (L), ratio of dendritic to somatic conductance (rho), membrane specific capacitance and resistance (Rm, Cm), and specific axoplasmic resistance (Ri). The analysis of the voltage decays from long saturating (100 ms) and short (0.5 ms) current pulses showed that the short-pulse method gave better resolution for the measurement of the time constants and avoided some of the time-dependent nonlinearities but required larger currents than the long pulse. Morphological analysis of 49 branching points taken from the dendritic trees of granule cells showed that the branching power, n, is equal to 1.56 +/- 0.186 and was fairly constant throughout the tree. Given the fact that all dendrites have approximately the same length and number of branch points, the granule cell dendritic tree can be meaningfully collapsed into an equivalent cable. Moreover, electrophysiological data suggested that the cable had a "sealed" end or at least a high-impedance termination. Based on an equivalent cable model with a sealed end and a lumped soma impedance, a method was implemented to analyze the multiexponential decays from hyperpolarizing current pulses and to solve the equations of the model. This was done successfully in only 40% of the cells and yielded the following mean values for L = 1.13 and rho = 7.58. From the measurements of the soma surface area (S) and the equivalent cable diameter (D), the average specific membrane parameters were calculated: Rm = 2,726 alpha x cm2, Cm = 5.24 microF/cm2, Ri = 101 alpha x cm. The input resistance and time constant of the granule cells as measured from the short-pulse technique averaged to RN 58.57 M alpha and tau m = 16.21 ms. The failure of the model to fit 60% of the cells was interpreted to be due to the presence of a somatic shunt resulting from electrode injury, tonic synaptic activity, a lower somatic membrane specific resistance, or electronic coupling.(ABSTRACT TRUNCATED AT 400 WORDS)     

Measurement of passive membrane parameters with whole-cell recording from neurons in the intact amphibian retina

1. Whole-cell recordings have been obtained from intact, photoactive retinal neurons using patch-clamp electrodes in the amphibian superfused retina eyecup preparation. 2. After removal of the vitreous humor from the surface of the retina, using a collagenase with low tryptic activity, high-resistance seals (1-10 G omega) could be formed between the patch pipette and the cell membrane by applying mild suction to the pipette. Additional suction broke the membrane patch and provided continuity between the low-resistance pipette and the interior of the neuron. 3. Measurements of input resistance and time constant were obtained from bipolar, amacrine, and ganglion cells. Assuming the membrane capacitance was 1 microF/cm2, time constant data were used to derive the specific membrane resistance. The average specific membrane resistance for the inner retinal neurons in our sample was 68,000 omega.cm2. 4. Analysis of the charging curve induced by a brief current pulse applied to the soma was used to analyze the average electrotonic length of dendrites. The charging curves of some ganglion cells were well represented by a single exponential, suggesting that they were essentially isopotential. 5. The voltage decay along an equivalent cylinder model of a ganglion cell was calculated, using the experimentally obtained values of membrane resistance to compute decay of steady-state voltages along the dendritic tree. The calculations indicate that with the high membrane resistance values implied by this study, the electrotonic length of dendritic cables were short, and there may be relatively little attenuation of the synaptic potentials irrespective of their location along the dendritic tree.     

Electrical constants of trabecular muscle from mammalian heart

1. The passive electrical properties of muscle bundles obtained from the right ventricle of sheep or calf hearts were determined. Preparations were kept in silicon oil; through extracellular electrodes constant current pulses were made to flow between the ends of the bundles.2. Using micro-electrodes for potential recording, the following data were obtained: (i) a space constant of 880 mu; (ii) a membrane time constant of 4.4 msec; (iii) a ratio of intra-to-extracellular longitudinal resistance of 3.5: 1; (iv) a conduction velocity of 0.75 m/sec.3. The intracellular specific resistance (R(i)) in the longitudinal direction was 470Omega cm, corresponding to 3 times R(i) of Purkinje fibres or 9 times the specific resistance of Tyrode solution.4. A calculation of specific membrane resistance (R(m)) and capacity (C(m)) was up against uncertainties in estimating the surface area. Taking morphological data as obtained by light microscopy, R(m) works out at 9100Omega cm(2), C(m) 0.81 muF/cm(2). Electron micrographs suggest that the true surface membrane might be either larger (T-tubules) or smaller (tight junctions between parallel fibres) than the surface area as seen by the light microscope.5. The apparently small value of C(m) seems to indicate that the flow of current between ;outside' and ;inside' is restricted to only a fraction of the fibre surface, while a considerable part of the contact area between parallel fibres is of the low-resistance type. This would provide for functional connexions not only at the level of intercalated disks, but also along parallel-running fibres.     

An analysis of the cable properties of spinal motoneurones using a brief intracellular current pulse

1. A brief intracellular current pulse, with duration less than 500 musec, has been applied to lumbosacral motoneurones in anaesthetized cats. The resulting voltage transients have been analysed by the procedure suggested in Jack & Redman (1971b) to obtain the cable parameters for each motoneurone.2. Forty-three motoneurone responses were analysed. In all cases the voltage response indicated that the dendrites could be represented as uniform, finite length cables, with either a sealed distal end, or at least a high resistance distal termination. The electrical length of the equivalent uniform dendritic cable ranged from 1.0 to 2.1 space constants, with a mean value of 1.5.3. The initial decay of the membrane potential following the removal of the current pulse was more rapid than was predicted by the Rall model for the motoneurone, in approximately two thirds of the responses. Consequently a value of dendritic to soma conductance ratio could not be obtained for these motoneurones.4. The explanation given for the departure from the theoretical response to a brief current pulse is that the specific resistivity of the soma membrane is lower than the specific resistivity of the dendritic membrane. This explanation is complicated by the possibility of the electrode tip not lodging in the isopotential soma region. The contribution that each of these effects has on the early decay phase of the current pulse response has been assessed.5. It is concluded that the specific resistivity of the soma membrane could be as low as one third of the dendritic membrane resistivity. Tonic inhibitory activity restricted to the soma is suggested as an explanation.     

The amplitude, time course and charge of unitary excitatory post-synaptic potentials evoked in spinal motoneurone dendrites

1. Group Ia e.p.s.p.s were recorded from lumbosacral motoneurones in anaesthetized cats after almost complete section of the appropriate dorsal roots. The cable parameters of these same motoneurones were obtained from the voltage response to a brief intracellular current pulse, as described in Iansek & Redman (1973).2. A total of thirty-three e.p.s.p.s, recorded in thirty different motoneurones, were analysed. E.p.s.p.s which were recorded in motoneurones which were not studied using an intracellular current pulse, or in which the resting membrane potential fell below 50 mV, were not considered. Also, e.p.s.p.s whose time course indicated more than one synaptic site of origin were not analysed. The selected e.p.s.p.s were plotted on a semilogarithmic amplitude scale, and their 10-90% rise time, half-width and peak amplitudes were measured.3. Using the previously determined values of the cable parameters L, rho(infinity) and tau(m), the rise time and half-width of each e.p.s.p. were used to determine the synaptic location (X), and the synaptic current time course (alpha). Twenty-seven e.p.s.p.s had time courses which allowed a value of X and alpha to be determined within the constraints of the measured cable parameters. The remaining six e.p.s.p.s either required an extension of the dendritic cable to be localized, or their time course was not compatible with a brief synaptic current.4. The synaptic locations lie in the range 0 (soma) to 1.25 space constants. When expressed as a fraction of the length of the dendritic cable, all but four of the twenty-seven e.p.s.p.s were located on the proximal half of the dendrites.5. The time to peak of synaptic current for each e.p.s.p. ranged from 30 to 390 musec, although a majority (70%) lay in the range 50 to 200 musec. There was no significant correlation between time to peak of synaptic current and synaptic location.6. The peak amplitude of e.p.s.p.s at the soma showed no significant correlation with synaptic location.7. The peak amplitude, and the cable parameters for each e.p.s.p. were used to compute the time course and amplitude of each e.p.s.p. at its point of generation on various fractions of the total dendritic cable, using the results derived in Redman (1973). These calculations showed the greatly increased rate of decay of e.p.s.p.s at their point of generation. Assuming that the synaptic input was restricted to one tenth of the total dendritic tree, the range of peak amplitudes at the synaptic site was from less than 100 muV (soma) to 20 mV.8. The net inward positive charge crossing the synaptic junction was calculated from the voltage-time integral of the e.p.s.p., as was the net outward positive charge crossing the soma membrane. These calculations showed that dendritic synapses caused up to ten times more net charge to be displaced across the synaptic junction than did synapses on or near to the soma, for similar durations of synaptic current. Similarly, dendritic synapses were generally more effective than somatic synapses in displacing charge across the soma membrane. It was concluded that the average quantal content in the conductance change at dendritic synapses is significantly greater than for somatic synapses.9. Some implications of the results for general integrative mechanisms in dendrites are discussed.     

Electrotonic characteristics of alpha motoneurones of varying size

1. The neuronal membrane responses to long constant current pulses (essentially current steps) have been studied in cat triceps surae motoneurones identified as to the type of muscle fibres, fast twitch (type F) or slow twitch (type S), innervated by the cell being studied. For each motoneurone the membrane time constant, τ_M, and input resistance, R_N, were determined from the response to a current step. In addition, shorter time constants (`equalizing time constants') resulting from current spread into the dendrites were estimated by graphical analysis.

2. The electrotonic length of the combined motoneurone soma and dendritic tree was estimated from the current step data using the neuronal equivalent cylinder model formulated by Rall (Rall, 1969). The mean electrotonic length of the motoneurone equivalent cylinder was approximately 1·5 in both type F and type S motoneurones. The mean membrane time constant of type F cells was 5·6 msec and that of type S motoneurones was 6·7 msec. This difference in mean τ_M values was of border line statistical significance.

3. The results indicate that the electrotonic length of the combined dendritic trees of both large type F and small type S motoneurones is essentially the same. The implication of this conclusion for interpretation of previous analyses of the monosynaptic EPSP is discussed.

     

Electrotonic architecture of type-identified alpha-motoneurons in the cat spinal cord

1. Measurements of input resistance (RN), time constant (tau 0), and electrotonic length (Lpeel) were derived from intracellular voltage changes produced by injection of current pulses in six type-identified triceps surae alpha-motoneurons. The motoneurons were labeled with horseradish peroxidase and subsequently reconstructed and measured from serial sections. These quantitative morphological and physiological data were incorporated into detailed computer models of the motoneurons. 2. Steady-state and dynamic models were used to determine values for specific membrane resistivity (Rm) that matched the experimental estimates of RN, tau 0, and Lpeel for each motoneurons. The models were based on the following assumptions 1) the membrane was electrically passive, 2) cytoplasmic resistivity (Ri) was 70 omega-cm, and 3) "sealed-end" boundary conditions were present at dendritic terminations. We also considered the nature and magnitude of possible errors introduced by using linear (passive) computer models to match responses from motoneurons with nonlinear (i.e., voltage-dependent) conductances. 3. If we assume that the experimental measurements of RN and tau 0 were correct, uniform Rm values that reproduced the experimentally measured RN required widely varying values of Cm (1.4-8.6 microF/cm2) to match the experimental tau 0. Furthermore, the electrotonic distance to dendritic terminals was generally much greater than expected from physiological estimates of Lpeel. However, if we assumed that the RN measurements could have been underestimated by as much as 30% and that Cm = 1.0 microF/cm2, it was possible to choose spatially uniform Rm that matched the observed tau 0 in three of six cases. 4. Relaxing the assumption of spatially uniform membrane resistivity permitted us to reconcile the anatomical and physiological characteristics of all six motoneurons. Two qualitatively different models of Rm nonuniformity gave equally good fits to the experimental results 1) a step-wise increase in Rm from a low value at the soma to a much higher but uniform value over the entire dendritic tree, and 2) a monotonic increase in Rm from soma to distal dendrites as a sigmoidal function of path distance along the dendrites. The step and sigmoidal models of the spatial distribution of Rm generated different electrotonic architectures in motoneuron dendritic trees, but both gave essentially identical electrical responses at the soma.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cable properties of external intercostal muscle fibres from myotonic and nonmyotonic goats

1. In preparations of about 200 fibres each from thirty-nine biopsies of external intercostal muscle taken from nine myotonic and six nonmyotonic goats, cable properties were determined at 38 degrees C for individual fibres with a pair of intracellular micro-electrodes.2. In each preparation the mean fibre dimensions, determined histologically and corrected for shrinkage, were used to calculate the mean membrane resistance, R(m), fibre capacitance, C(t), and myoplasmic resistivity, R(i). In the 124 nonmyotonic fibres the mean values were: R(m), 1897 Omega.cm(2), C(t), 4.1 muF/cm(2), and R(i), 112 Omega.cm. In 151 myotonic fibres R(m) was 5589 Omega.cm(2), C(t), 4.4 muF/cm(2), and R(i), 103 Omega.cm.3. Conductance of the fibre core times unit length increased with cross-sectional area, and fibre capacitance per unit length increased with perimeter. There was little correlation of membrane resistance per unit length of fibre with either fibre perimeter or resting potential.4. The principal abnormality of cable properties in the myotonic fibre is its threefold higher membrane resistance, which accounts for its decreased electrical current rheobase.     

Early alterations in the electrophysiological properties of rat spinal motoneurones following neonatal axotomy

Early in development, motoneurones are critically dependent on their target muscles for survival and differentiation. Previous studies have shown that neonatal axotomy causes massive motoneurone death and abnormal function in the surviving motoneurones. We have investigated the electrophysiological and morphological properties of motoneurones innervating the flexor tibialis anterior (TA) muscle during the first week after a neonatal axotomy, at a time when the motoneurones would be either in the process of degeneration or attempting to reinnervate their target muscles. We found that a large number (∼75%) of TA motoneurones died within 3 weeks after neonatal axotomy. Intracellular recordings revealed a marked increase in motoneurone excitability, as indicated by changes in passive and active membrane electrical properties. These changes were associated with a shift in the motoneurone firing pattern from a predominantly phasic pattern to a tonic pattern. Morphologically, the dendritic tree of the physiologically characterized axotomized cells was significantly reduced compared with age-matched normal motoneurones. These data demonstrate that motoneurone electrical properties are profoundly altered shortly after neonatal axotomy. In a subpopulation of the axotomized cells, abnormally high motoneurone excitability (input resistance significantly higher compared with control cells) was associated with a severe truncation of the dendritic arbor, suggesting that this excitability may represent an early electrophysiological correlate of motoneurone degeneration.     

Influence of activity on the passive electrical properties of denervated soleus muscle fibres in the rat.

The technique of direct electrical stimulation of denervated muscle was used to study the role of muscle activity per se in controlling the passive electrical properties of muscle fibres. 2. Specific membrane resistance and capacitance of the denervated and the denervated-stimulated muscle fibres were measured by a sinewave technique at frequencies between 5 and 240 Hz. The parameter values were constant at low frequencies up to a variable transition frequency and declined rapidly at higher frequencies. 3. Following denervation the low-frequency value of specific membrane resistance increased (2291 omega cm2 for 19-day denervated fibres vs. 766 omega cm2 for innervated fibres), the specific membrane capacitance declined (2-7 muF/cm2 vs. 3-6 muF/cm2) and the transition frequency shifted towards lower frequencies. The specific internal resistance was higher in denervated fibres (301 omega cm for 19-day denervated fibres vs. 240 omega cm in innervated fibres) apart from a transient decline after 5 days of denervation (164 omega cm). 4. Direct electrical stimulation for 2 weeks beginning on the 5th day after denervation restored all parameters listed above to their original values before denervation. 5. Stimulation arrested in most cases further atrophy from the time of stimulation but did not restore normal fibre size.     

Enhancement of synaptic transmission by dendritic potentials in chromatolysed motoneurones of the cat

1. Monosynaptic transmission in cat lumbosacral motoneurones undergoing chromatolysis was studied by intracellular recording from 7 to 20 days after section of the appropriate ventral roots.2. The average input resistance measured by passing polarizing currents across the cell membrane showed no significant difference between normal and chromatolysed motoneurones. Average rheobasic current for chromatolysed motoneurones was significantly lower (by about 30%) than that for normal motoneurones.3. Spike-like partial responses were commonly superimposed on monosynaptic EPSPs in chromatolysed motoneurones. These responses could be eliminated by stimulation of the bulbar inhibitory reticular formation, but could not be blocked by hyperpolarization applied to the motoneurone soma.4. The spike-like partial response in chromatolysed motoneurones showed a refractory period following (i) the antidromic invasion of the neurone generated by ventral root stimulation, and (ii) in response to two successive afferent stimuli. The refractory period ranged from 5 to 13 msec.5. Initiation of the partial response had no direct relation with the amplitude of the underlying EPSP. The partial response could be evoked by small EPSPs of about 0.5 mV.6. The action potential of a chromatolysed motoneurone arose from the partial response at different levels of depolarization, showing multiple trigger zones for spike initiation. Occasionally, chromatolysed motoneurones discharged in response to stimulation of a single afferent fibre.7. In neurones where more than one spike-like response was obtained, interaction between dendritic responses showed no refractoriness.8. It is concluded that the partial response is an all-or-none event originating at some discrete site on dendrites, and that its presence increases the efficacy of synaptic excitation in chromatolysed motoneurones.     

Electrotonic parameters of neurons following chronic ethanol consumption

The electronic parameters of nerve cells in the dentate gyrus following long-term ingestion of ethanol were studied in vitro. The ethanol was administered in a liquid diet for a period of 20 wk followed by a 3-wk withdrawal period. A control group received a similar diet with the ethanol replaced by maltose-dextrins. Intracellular recordings were obtained from 44 neurons, and the voltage decays following current injections were analyzed with a recent electrical model of granule cells to take into account a somatic shunt already detected in previous studies. The new model accurately accounted for the fast voltage transients and showed that the membrane time constant in the dendrites is, on average, five times larger than the somatic time constant. Injection of horseradish peroxidase into the neurons for the morphological analysis showed that neurons in the ethanol group have a longer dendritic tree than neurons in the control group. Estimation of the membrane surface area showed that the membrane area in the dendrites is at least 60% greater (in both control and ethanol groups) when the membrane foldings and irregularities are taken into account. The results of the modeling analysis showed that the membrane time constant and the input resistance are not affected by ethanol. However, the membrane resistance is significantly increased in the ethanol group (6,632 versus 18,460 omega X cm2), and the capacitance is significantly decreased (4.48 versus 1.71 microF/cm2). The electrotonic length is also increased by chronic ethanol treatment (0.85 versus 0.94). Higher values of membrane specific resistance (Rm) mean larger transmission coefficients. However, since the neurons from the ethanol group are on average longer than neurons in the control group, it is suggested that the change in Rm compensates for the increase in the length of the dendrites, thereby maintaining a value of the electrotonic length under 1.0. The observed changes in the passive parameters are in opposite direction from the recently measured effect of acute doses of ethanol on hippocampal neurons. These results support a model of chronic alcohol intake where homeostatic adaptive changes lead to the development of long-term changes in cellular physiology.     

Differential reaction of fast and slow α-motoneurones to axotomy

1. The properties of medial gastrocnemius (m.g., fast alpha) and soleus (sol., slow alpha) motoneurones of the cat were examined with intracellular electrodes 8-119 days after section of the muscle nerves.2. The axonal conduction velocity was significantly decreased in both m.g. and sol. motoneurones after chronic section of the muscle nerves.3. The amplitude of overshoot of action potentials was significantly increased in both m.g. and sol. motoneurones following section of the muscle nerves.4. No significant changes in the resting membrane potential or the input resistance were observed for sol. motoneurones, whereas m.g. motoneurones showed a slight decrease in the resting potential and a slight increase in the input resistance.5. The duration of after-hyperpolarization was significantly decreased in sol. motoneurones, whereas that in m.g. motoneurones remained virtually unchanged or increased slightly following section of the muscle nerves.6. The changes described above were not seen in the preparations examined 29-46 days after section of the lumbosacral dorsal roots, suggesting that alterations in the motoneurone properties observed after section of the muscle nerves resulted from axotomy of the motoneurones rather than from sensory deprivation.7. The differences in electrophysiological properties between m.g. and sol. motoneurones were less prominent in axotomized animals than in control, unoperated cats.8. It is concluded that fast (m.g.) and slow (sol.) alpha-motoneurones have qualitatively different properties. A possible ;dedifferentiation' of fast and slow alpha-motoneurones by axotomy is discussed.     

Effects of daily spontaneous running on the electrophysiological properties of hindlimb motoneurones in rats

No evidence currently exists that motoneurone adaptations in electrophysiological properties can result from changes in the chronic level of neuromuscular activity. We examined, in anaesthetized (ketamine/xylazine) rats, the properties of motoneurones with axons in the tibial nerve, from rats performing daily spontaneous running exercise for 12 weeks in exercise wheels ('runners') and from rats confined to plastic cages ('controls'). Motoneurones innervating the hindlimb via the tibial nerve were impaled with sharp glass microelectrodes, and the properties of resting membrane potential, spike threshold, rheobase, input resistance, and the amplitude and time-course of the afterhyperpolarization (AHP) were measured. AHP half-decay time was used to separate motoneurones into 'fast' (AHP half-decay time < 20 ms) and 'slow' (AHP half-decay time ≥ 20 ms), the proportions of which were not significantly different between controls (58 % fast) and runners (65 % fast). Two-way ANOVA and ANCOVA revealed differences between motoneurones of runners and controls which were confined to the 'slow' motoneurones. Specifically, runners had slow motoneurones with more negative resting membrane potentials and spike thresholds, larger rheobasic spike amplitudes, and larger amplitude AHPs compared to slow motoneurones of controls. These adaptations were not evident in comparing fast motoneurones from runners and controls. This is the first demonstration that physiological modifications in neuromuscular activity can influence basic motoneurone biophysical properties. The results suggest that adaptations occur in the density, localization, and/or modulation of ionic membrane channels that control these properties. These changes might help offset the depolarization of spike threshold that occurs during rhythmic firing.     

Electrophysiological and morphological properties of rat abducens motoneurones

Summary The electrical and morphological properties of abducens motoneurones were investigated in the rat with intracellular recordings and intracellular HRP-staining. Motoneurones were identified by their antidromic response to electrical stimulation of the lateral rectus muscle. The antidromic action potential was followed by a delayed depolarization and an after hyperpolarization lasting 20 ms to 45 ms. The whole neurone input resistance (R_N) calculated from I/V curves, was found to lie between 2 M and 15 M with a bimodal distribution (mean values 4.9 M and 12 M). In some cases, anomalous rectification was observed with low current intensities. Prolonged hyperpolarizing current pulses revealed the presence of a time dependant inward rectification and slow rebound depolarization. The intensity/frequency curves suggest the existence of three ranges of discharge. The average intensity frequency slope during the steady state was 43 imp/s/ nA. Eight abducens motoneurones were intracellularly labelled with HRP and fully reconstructed. The soma (23 m to 40 m in diameter) gave off 5 to 7 primary dendrites. The general organization and extension of the dendritic trees depended on the location of the soma within the abducens nucleus. The mean diameter of primary dendrites was 4.17 m with similar average values in all motoneurones. The soma size of abducens motoneurones was not correlated with either the size of the proximal tree or the whole neurone input resistance.     

Mechanisms regulating the activity of facial nucleus motoneurones--2. Synaptic activation from the caudal trigeminal nucleus

Field and postsynaptic potentials of facial motoneurones evoked by stimulation of the caudal trigeminal nucleus were studied in cats by means of extra- and intracellular recording. Mono- and polysynaptic input onto facial motoneurones from the caudal trigeminal nucleus were shown. Four types of responses were distinguished: excitatory postsynaptic potentials generating a single action potential; a gradual shift of depolarization inducing multiple discharges; a rhythmic discharge of action potentials appearing at a low level of depolarization; excitatory postsynaptic potentials or a sequence of excitatory and inhibitory postsynaptic potentials. Multiple discharge was shown to appear as a result of effective summation of high frequency excitatory influences from efferent neurones of the caudal trigeminal nucleus projecting into the facial nucleus. Factors facilitating the development of gradual depolarization are: dendritic localization of synaptic terminals, dendritic origin of after-depolarizing processes and the high input resistance of the facial motoneurone membrane. It is thought that specific features of facial motoneurones and properties of afferent inputs are supposed to provide high sensitivity of neuronal organization of the facial nucleus to afferent signals as well as wide diversity in controlling its activity.     

Membrane capacity of the cardiac Purkinje fibre

1. The basis for the relatively high membrane capacitance of the cardiac Purkinje fibre has been investigated.2. The capacitance measured by analysis of the cable response to a current step (square wave) was compared in the same fibres to the capacitance calculated from the foot of the propagated action potential. The square wave value for capacitance was 12.8 +/- 1.3 muF/cm(2) and that from the foot of the action potential, 2.4 +/- 0.5 muF/cm(2).3. The capacitative filling at the beginning of a voltage clamp in short Purkinje fibres was measured. The current-time course deviated from that predicted by a model membrane containing resistance and capacitance in parallel.4. The results obtained by both methods are consistent with two components to the membrane capacitance, with part in parallel with the membrane resistance (2.4 muF/cm(2)) and part (7 muF/cm(2)) in series with a resistor (300 Omega cm(2)).5. The value of the series resistor could be increased by decreasing the conductivity of the extracellular fluid.6. The possible anatomical basis for these findings is discussed.7. Implications of this model on the shape of the Purkinje fibre action potential and on the electrical triggering of contraction are considered.