Intracellular tetraethylammonium ions enhance group Ia excitatory post-synaptic potentials evoked in cat motoneurones

Efficient and robust coding in heterogeneous recurrent networks

Cortical networks show a large heterogeneity of neuronal properties. However, traditional coding models have focused on homogeneous populations of excitatory and inhibitory neurons. Here, we analytically derive a class of recurrent networks of spiking neurons that close to optimally track a continuously varying input online, based on two assumptions: 1) every spike is decoded linearly and 2) the network aims to reduce the mean-squared error between the input and the estimate. From this we derive a class of predictive coding networks, that unifies encoding and decoding and in which we can investigate the difference between homogeneous networks and heterogeneous networks, in which each neurons represents different features and has different spike-generating properties. We find that in this framework, 'type 1' and 'type 2' neurons arise naturally and networks consisting of a heterogeneous population of different neuron types are both more efficient and more robust against correlated noise. We make two experimental predictions: 1) we predict that integrators show strong correlations with other integrators and resonators are correlated with resonators, whereas the correlations are much weaker between neurons with different coding properties and 2) that 'type 2' neurons are more coherent with the overall network activity than 'type 1' neurons.

Effects of anti-epileptic drugs on spreading depolarization-induced epileptiform activity in mouse hippocampal slices

Epilepsy and spreading depolarization (SD) are both episodic brain disorders and often exist together in the same individual. In CA1 pyramidal neurons of mouse hippocampal slices, induction of SD evoked epileptiform activities, including the ictal-like bursts, which occurred during the repolarizing phase of SD, and the subsequent generation of paroxysmal depolarization shifts (PDSs), which are characterized by mild depolarization plateau with overriding spikes. The duration of the ictal-like activity was correlated with both the recovery time and the depolarization potential of SD, whereas the parameters of PDSs were not significantly correlated with the parameters of SD. Moreover, we systematically evaluated the effects of multiple anti-epileptic drugs (AEDs) on SD-induced epileptiform activity. Among the drugs that are known to inhibit voltage-gated sodium channels, carbamazepine, phenytoin, valproate, lamotrigine, and zonisamide reduced the frequency of PDSs and the overriding firing bursts in 20–25 min after the induction of SD. The GABA uptake inhibitor tiagabine exhibited moderate effects and partially limited the incidence of PDSs after SD. AEDs including gabapentin, levetiracetam, ethosuximide, felbamate, and vigabatrin, had no significant effect on SD-induced epileptic activity. Taken together, these results demonstrate the effects of AEDs on SD and the related epileptiform activity at the cellular level.

The bulbospinal network controlling the phrenic motor system: Laterality and course of descending projections

The respiratory rhythm is generated by the parafacial respiratory group, Bötzinger complex, and pre-Bötzinger complex and relayed to pre-motor neurons, which in turn project to and control respiratory motor outputs in the brainstem and spinal cord. The phrenic nucleus is one such target, containing phrenic motoneurons (PhMNs), which supply the diaphragm, the primary inspiratory muscle in mammals. While some investigators have demonstrated both ipsi- and contralateral bulbophrenic projections, there exists controversy regarding the relative physiological contribution of each to phasic and tonic drive to PhMNs and at which levels decussations occur. Following C1- or C2 spinal cord hemisection-induced silencing of the ipsilateral phrenic/diaphragm activity, respiratory stressor-induced, as well as spontaneous, recovery of crossed phrenic activity is observed, suggesting an important contribution of pathways crossing below the level of injury in driving phrenic motor output. The precise mechanisms underlying this recovery are debated. In this review, we seek to present a comprehensive discussion of the organization of the bulbospinal network controlling PhMNs, a thorough appreciation of which is necessary for understanding neural respiratory control, accurate interpretation of studies investigating respiratory recovery following spinal cord injury, and targeted development of therapies for respiratory neurorehabilitation in patients sustaining high cervical cord injury.

Motor unit

Movement is accomplished by the controlled activation of motor unit populations. Our understanding of motor unit physiology has been derived from experimental work on the properties of single motor units and from computational studies that have integrated the experimental observations into the function of motor unit populations. The article provides brief descriptions of motor unit anatomy and muscle unit properties, with more substantial reviews of motoneuron properties, motor unit recruitment and rate modulation when humans perform voluntary contractions, and the function of an entire motor unit pool. The article emphasizes the advances in knowledge on the cellular and molecular mechanisms underlying the neuromodulation of motoneuron activity and attempts to explain the discharge characteristics of human motor units in terms of these principles. A major finding from this work has been the critical role of descending pathways from the brainstem in modulating the properties and activity of spinal motoneurons. Progress has been substantial, but significant gaps in knowledge remain.

Frequency-dependent amplification of stretch-evoked excitatory input in spinal motoneurons

Voltage-dependent calcium and sodium channels mediating persistent inward currents (PICs) amplify the effects of synaptic inputs on the membrane potential and firing rate of motoneurons. CaPIC channels are thought to be relatively slow, whereas the NaPIC channels have fast kinetics. These different characteristics influence how synaptic inputs with different frequency content are amplified; the slow kinetics of Ca channels suggest that they can only contribute to amplification of low frequency inputs (<5 Hz). To characterize frequency-dependent amplification of excitatory postsynaptic potentials (EPSPs), we measured the averaged stretch-evoked EPSPs in cat medial gastrocnemius motoneurons in decerebrate cats at different subthreshold levels of membrane potential. EPSPs were produced by muscle spindle afferents activated by stretching the homonymous and synergist muscles at frequencies of 5-50 Hz. We adjusted the stretch amplitudes at different frequencies to produce approximately the same peak-to-peak EPSP amplitude and quantified the amount of amplification by expressing the EPSP integral at different levels of depolarization as a percentage of that measured with the membrane hyperpolarized. Amplification was observed at all stretch frequencies but generally decreased with increasing stretch frequency. However, in many cells the amount of amplification was greater at 10 Hz than at 5 Hz. Fast amplification was generally reduced or absent when the lidocaine derivative QX-314 was included in the electrode solution, supporting a strong contribution from Na channels. These results suggest that NaPICs can combine with CaPICs to enhance motoneuron responses to modulations of synaptic drive over a physiologically significant range of frequencies.

Contribution of intrinsic properties and synaptic inputs to motoneuron discharge patterns: a simulation study

Motoneuron discharge patterns reflect the interaction of synaptic inputs with intrinsic conductances. Recent work has focused on the contribution of conductances mediating persistent inward currents (PICs), which amplify and prolong the effects of synaptic inputs on motoneuron discharge. Certain features of human motor unit discharge are thought to reflect a relatively stereotyped activation of PICs by excitatory synaptic inputs; these features include rate saturation and de-recruitment at a lower level of net excitation than that required for recruitment. However, PIC activation is also influenced by the pattern and spatial distribution of inhibitory inputs that are activated concurrently with excitatory inputs. To estimate the potential contributions of PIC activation and synaptic input patterns to motor unit discharge patterns, we examined the responses of a set of cable motoneuron models to different patterns of excitatory and inhibitory inputs. The models were first tuned to approximate the current- and voltage-clamp responses of low- and medium-threshold spinal motoneurons studied in decerebrate cats and then driven with different patterns of excitatory and inhibitory inputs. The responses of the models to excitatory inputs reproduced a number of features of human motor unit discharge. However, the pattern of rate modulation was strongly influenced by the temporal and spatial pattern of concurrent inhibitory inputs. Thus, even though PIC activation is likely to exert a strong influence on firing rate modulation, PIC activation in combination with different patterns of excitatory and inhibitory synaptic inputs can produce a wide variety of motor unit discharge patterns.

Persistent inward currents in spinal motoneurons and their influence on human motoneuron firing patterns

Persistent inward currents (PICs) are present in many types of neurons and likely have diverse functions. In spinal motoneurons, PICs are especially strong, primarily located in dendritic regions, and subject to particularly strong neuromodulation by the monoamines serotonin and norepinephrine. Because motoneurons drive muscle fibers, it has been possible to study the functional role of their PICs in motor output and to identify PIC-mediated effects on motoneuron firing patterns in human subjects. The PIC markedly amplifies synaptic input, up to fivefold or more, depending on the level of monoaminergic input. PICs also tend to greatly prolong input time course, allowing brief inputs to initiate long-lasting self-sustained firing (i.e., bistable behavior). PIC deactivation usually requires inhibitory input and PIC amplitude can increase to repeated activation. All of these behaviors markedly increase motoneuron excitability. Thus, in the absence of monoaminergic input, motoneuron excitability is very low. Yet PICs have another effect: once active, they tend to sharply limit efficacy of additional synaptic input. All of these PIC effects have been detected in motoneuron firing patterns in human subjects and, hence, PICs are likely a fundamental component of normal motor output.

Differential modulation by tetraethylammonium of the processes underlying network bursting in the neonatal rat spinal cord in vitro

In the rat spinal cord in vitro, block of synaptic inhibition evokes persistent, regular disinhibited bursting which is a manifestation of the intrinsic network rhythmicity and is readily recorded from ventral roots. This model is advantageous to explore the network mechanisms controlling burst periodicity, and duration. We questioned the relative contribution of K+ conductances to spontaneous rhythmicity by investigating the effects of the broad K+ channel blocker tetraethylammonium (TEA). In TEA (10 mM) solution, bursts occurred at the same rate but became substantially longer, thus showing an unusual dissociation between mechanisms of burst periodicity and duration. In the presence of TEA, electrical stimulation of a single dorsal root or N-methyl-D-aspartate application (5 microM) could, however, fasten bursting associated with immediate decrease in burst length, thus demonstrating maintenance of short-term plasticity. Either riluzole (1 microM) or surgical sectioning that isolated a single spinal segment strongly depressed bursting which could, however, be revived by TEA. In the presence of TEA, the L-type channel blocker nifedipine (20 microM) made bursting faster and shorter. Our data are best explained by assuming that TEA increased network excitability to generate rhythmic bursting, an effect that was counteracted by intrinsic mechanisms, partly dependent on L-type channel activity, to retain standard periodicity. TEA-sensitive mechanisms were, nevertheless, an important process to regulate burst duration. Our results are consistent with the proposal of a hierarchical structural of the central pattern generator in which the circuits responsible for rhythmicity (the clock) drive the discharges of those creating the motor commands (pattern).

Fictive locomotor patterns generated by tetraethylammonium application to the neonatal rat spinal cord in vitro

Intrinsic spinal networks generate a locomotor rhythm characterized by alternating electrical discharges from flexor and extensor motor pools. Because this process is preserved in the rat isolated spinal cord, this preparation in vitro may be a useful model to explore methods to reactivate locomotor networks damaged by spinal injury. The present electrophysiological investigation examined whether the broad spectrum potassium channel blocker tetraethylammonium could generate locomotor-like patterns. Low (50-500 microM) concentrations of tetraethylammonium induced irregular, synchronous discharges incompatible with locomotion. Higher concentrations (1-10 mM) evoked alternating discharges between flexor and extensor motor pools, plus large depolarization of motoneurons with spike broadening. The alternating discharges were superimposed on slow, shallow waves of synchronous depolarization. Rhythmic alternating patterns were suppressed by blockers of glutamate, GABA(A) and glycine receptors, disclosing a background of depolarizing bursts inhibited by antagonism of group I metabotropic glutamate receptors. Furthermore, tetraethylammonium also evoked irregular discharges on dorsal roots. Rhythmic alternating patterns elicited by tetraethylammonium on ventral roots were relatively stereotypic, had limited synergy with fictive locomotion induced by dorsal root stimuli, and were not accelerated by 4-aminopyridine. Horizontal section of the spinal cord preserved irregular ventral root discharges and dorsal root discharges, demonstrating that the action of tetraethylammonium on spinal networks was fundamentally different from that of 4-aminopyridine. These results show that a potassium channel blocker such as tetraethylammonium could activate fictive locomotion in the rat isolated spinal cord, although the pattern quality lacked certain features like frequency modulation and strong synergy with other inputs to locomotor networks.

Effect of nonlinear summation of synaptic currents on the input-output properties of spinal motoneurons

A single spinal motoneuron receives tens of thousands of synapses. The neurotransmitters released by many of these synapses act on iontotropic receptors and alter the driving potential of neighboring synapses. This interaction introduces an intrinsic nonlinearity in motoneuron input-output properties where the response to two simultaneous inputs is less than the linear sum of the responses to each input alone. Our goal was to determine the impact of this nonlinearity on the current delivered to the soma during activation of predetermined numbers and distributions of excitatory and inhibitory synapses. To accomplish this goal we constructed compartmental models constrained by detailed measurements of the geometry of the dendritic trees of three feline motoneurons. The current "lost" as a result of local changes in driving potential was substantial and resulted in a highly nonlinear relationship between the number of active synapses and the current reaching the soma. Background synaptic activity consisting of a balanced activation of excitatory and inhibitory synapses further decreased the current delivered to the soma, but reduced the nonlinearity with respect to the total number of active excitatory synapses. Unexpectedly, simulations that mimicked experimental measures of nonlinear summation, activation of two sets of excitatory synapses, resulted in nearly linear summation. This result suggests that nonlinear summation can be difficult to detect, despite the substantial "loss" of current arising from nonlinear summation. The magnitude of this "loss" appears to limit motoneuron activity, based solely on activation of iontotropic receptors, to levels that are inadequate to generate functionally meaningful muscle forces.

Estimation of the Electrical Parameters of Spinal Motoneurons Using Impedance Measurements

Electrical parameters of spinal motoneurons were estimated by optimizing the parameters of motoneuron models to match experimentally determined impedance functions with those of the models. The model was described by soma area, somatic and dendritic membrane resistivities, and the diameter of an equivalent dendritic cable having a standard profile. The impedance functions of motoneurons and optimized models usually differed (rms error) by <2% of input resistance. Consistent estimates for most parameters were obtained from repeated impedance determinations in individual motoneurons; estimates of dendritic resistivity were most variable. The few cells that could not be fit well had reduced impedance phase lag consistent with dendritic penetrations. Most fits were improved by inclusion of a voltage-dependent conductance GV with time constant τV. A uniformly distributed GV with τV >5 ms provided a better fit for most cells. The magnitude of this conductance decreased with depolarization. Impedance functions of other cells were adequately fit by a passive model or by a model with a somatic GV and τV <5 ms. Most of these neurons (7/8) had resting potentials positive to −60 mV. The electrotonic parameters ρ, τ, and L, estimated from model parameters, were consistent with published distributions. Most motoneuron parameters obtained in somatic shunt and sigmoidal models were well correlated, and parameters were moderately affected by changes in dendritic profile. These results demonstrate the utility and limitations of impedance measurements for estimating motoneuron parameters and suggest that voltage-dependent conductances are a substantial component of resting electrical properties.

Hyperexcitable dendrites in motoneurons and their neuromodulatory control during motor behavior

Dendrites contain voltage-sensitive conductances that, in vivo, can be influenced by neuromodulatory inputs. In spinal motoneurons, dendrites have voltage-activated persistent inward currents that are facilitated by neuromodulatory input from monoaminergic axons originating in the brainstem. The highest levels of monoamine input are likely to occur during 'fight or flight' behavioral situations. At these high levels, the persistent currents are so large that the dendrites of motoneurons become hyperexcitable, enhancing ionotropic inputs by fivefold or more and allowing the firing rates required for maximal activation of muscle fibers to be generated by surprisingly small inputs. Moderate dendritic excitability (twofold to threefold enhancement) is likely to be a standard component of many normal motor behaviors, such as locomotion. Thus, during normal motor behavior, synaptic integration might be dominated by active currents intrinsic to the dendritic tree rather than by the synaptic current entering via ionotropic channels.

Excitatory Connections Between CA1 Pyramidal Cells Revealed by Spike Triggered Averaging in Slices of Rat Hippocampus are Partially NMDA Receptor Mediated

Spike triggered averaging was used to record local circuit connections between pairs of CA1 pyramidal neurons in isolated slices of rat hippocampus. Of 795 pairs of neurons tested, six were connected. These epsps were only partially blocked by 2-amino-5-phosphonovalerate (AP-5), which decreased the amplitude and half width of the epsp, but did not affect the early rising phase. In contrast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked all phases of the epsp and combinations of AP-5 and CNQX blocked the epsp almost entirely. These results indicate that these epsps were mediated by both N-methyl-d-aspartate (NMDA) and non-NMDA excitatory amino acid receptors. Moreover, they exhibited a voltage relation typical of neuronal responses to NMDA, increasing in amplitude and duration as the postsynaptic cell was depolarized. These epsps were brief (10 - 90% rise time < 5 ms, width at half amplitude < 20 ms), indicating a proximal location. Increasing presynaptic firing rate (1 - 4 spikes/s) reduced average epsp amplitude by almost 50%. When epsps were evoked by pairs of spikes (interval 3 - 25 ms), a large response to the first spike precluded a large response to the second. No evidence for selective enhancement of the NMDA receptor component by paired spike activation was found. It is concluded that a significant NMDA receptor mediated input to CA1 is provided by local circuit CA1 - CA1 connections and that these synapses can be demonstrated under control conditions.

Input-output functions of mammalian motoneurons

Our intent in this review was to consider the relationship between the biophysical properties of motoneurons and the mechanisms by which they transduce the synaptic inputs they receive into changes in their firing rates. Our emphasis has been on experimental results obtained over the past twenty years, which have shown that motoneurons are just as complex and interesting as other central neurons. This work has shown that motoneurons are endowed with a rich complement of active dendritic conductances, and flexible control of both somatic and dendritic channels by endogenous neuromodulators. Although this new information requires some revision of the simple view of motoneuron input-output properties that was prevalent in the early 1980's (see sections 2.3 and 2.10), the basic aspects of synaptic transduction by motoneurons can still be captured by a relatively simple input-output model (see section 2.3, equations 1-3). It remains valid to describe motoneuron recruitment as a product of the total synaptic current delivered to the soma, the effective input resistance of the motoneuron and the somatic voltage threshold for spike initiation (equations 1 and 2). However, because of the presence of active channels activated in the subthreshold range, both the delivery of synaptic current and the effective input resistance depend upon membrane potential. In addition, activation of metabotropic receptors by achetylcholine, glutamate, noradrenaline, serotonin, substance P and thyrotropin releasing factor (TRH) can alter the properties of various voltage- and calcium-sensitive channels and thereby affect synaptic current delivery and input resistance. Once motoneurons are activated, their steady-state rate of repetitive discharge is linearly related to the amount of injected or synaptic current reaching the soma (equation 3). However, the slope of this relation, the minimum discharge rate and the threshold current for repetitive discharge are all subject to neuromodulatory control. There are still a number of unresolved issues concerning the control of motoneuron discharge by synaptic inputs. Under dynamic conditions, when synaptic input is rapidly changing, time- and activity-dependent changes in the state of ionic channels will alter both synaptic current delivery to the spike-generating conductances and the relation between synaptic current and discharge rate. There is at present no general quantitative expression for motoneuron input-output properties under dynamic conditions. Even under steady-state conditions, the biophysical mechanisms underlying the transfer of synaptic current from the dendrites to the soma are not well understood, due to the paucity of direct recordings from motoneuron dendrites. It seems likely that resolving these important issues will keep motoneuron afficiandoes well occupied during the next twenty years.

Laminar differences in recurrent excitatory transmission in the rat entorhinal cortex in vitro

Paired intracellular recordings were used to investigate recurrent excitatory transmission in layers II, III and V of the rat entorhinal cortex in vitro. There was a relatively high probability of finding a recurrent connection between pairs of pyramidal neurons in both layer V (around 12%) and layer III (around 9%). In complete contrast, we have failed to find any recurrent synaptic connections between principal neurons in layer II, and this may be an important factor in the relative resistance of this layer in generating synchronized epileptiform activity. In general, recurrent excitatory postsynaptic potentials in layers III and V of the entorhinal cortex had similar properties to those recorded in other cortical areas, although the probabilities of connection are among the highest reported. Recurrent excitatory postsynaptic potentials recorded in layer V were smaller with faster rise times than those recorded in layer III. In both layers, the recurrent potentials were mediated by glutamate primarily acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazole receptors, although there appeared to be a slow component mediated by N-methyl-D-aspartate receptors. In layer III, recurrent transmission failed on about 30% of presynaptic action potentials evoked at 0.2Hz. This failure rate increased markedly with increasing (2, 3Hz) frequency of activation. In layer V the failure rate at low frequency was less (19%), and although it increased at higher frequencies this effect was less pronounced than in layer III. Finally, in layer III, there was evidence for a relatively high probability of electrical coupling between pyramidal neurons. We have previously suggested that layers IV/V of the entorhinal cortex readily generate synchronized epileptiform discharges, whereas layer II is relatively resistant to seizure generation. The present demonstration that recurrent excitatory connections are widespread in layer V but not layer II could support this proposal. The relatively high degree of recurrent connections and electrical coupling between layer III cells may be a factor in it's susceptibility to neurodegeneration during chronic epileptic conditions.

Adjustable amplification of synaptic input in the dendrites of spinal motoneurons in vivo

The impact of neuromodulators on active dendritic conductances was investigated by the use of intracellular recording techniques in spinal motoneurons in the adult cat. The well known lack of voltage control of dendritic regions during voltage clamp applied at the soma was used to estimate dendritic amplification of a steady monosynaptic input generated by muscle spindle Ia afferents. In preparations deeply anesthetized with pentobarbital, Ia current either decreased with depolarization or underwent a modest increase at membrane potentials above -40 mV. In unanesthetized decerebrate preparations (which have tonic activity in axons originating in the brainstem and releasing serotonin or norepinephrine), active dendritic currents caused strong amplification of Ia input. In the range of -50 to -40 mV, peak Ia current was over four times as large as that in the pentobarbital-anesthetized preparations. Exogenous administration of a noradrenergic agonist in addition to the tonic activity further enhanced amplification (sixfold increase). Amplification was not seen in preparations with spinal transections. Overall, the dendritic amplification with moderate or strong neuromodulatory drive was estimated to be large enough to allow the motoneurons innervating slow muscle fibers to be driven to their maximum force levels by remarkably small synaptic inputs. In these cells, the main role of synaptic input may be to control the activation of a highly excitable dendritic tree. The neuromodulatory control of synaptic amplification provides motor commands with the potential to adjust the level of amplification to suit the demands of different motor tasks.

Adjustable amplification of synaptic input in the dendrites of spinal motoneurons in vivo

The impact of neuromodulators on active dendritic conductances was investigated by the use of intracellular recording techniques in spinal motoneurons in the adult cat. The well known lack of voltage control of dendritic regions during voltage clamp applied at the soma was used to estimate dendritic amplification of a steady monosynaptic input generated by muscle spindle Ia afferents. In preparations deeply anesthetized with pentobarbital, Ia current either decreased with depolarization or underwent a modest increase at membrane potentials above -40 mV. In unanesthetized decerebrate preparations (which have tonic activity in axons originating in the brainstem and releasing serotonin or norepinephrine), active dendritic currents caused strong amplification of Ia input. In the range of -50 to -40 mV, peak Ia current was over four times as large as that in the pentobarbital-anesthetized preparations. Exogenous administration of a noradrenergic agonist in addition to the tonic activity further enhanced amplification (sixfold increase). Amplification was not seen in preparations with spinal transections. Overall, the dendritic amplification with moderate or strong neuromodulatory drive was estimated to be large enough to allow the motoneurons innervating slow muscle fibers to be driven to their maximum force levels by remarkably small synaptic inputs. In these cells, the main role of synaptic input may be to control the activation of a highly excitable dendritic tree. The neuromodulatory control of synaptic amplification provides motor commands with the potential to adjust the level of amplification to suit the demands of different motor tasks.

Summation of effective synaptic currents and firing rate modulation in cat spinal motoneurons

The aim of this study was to examine how cat spinal motoneurons integrate the synaptic currents generated by the concurrent activation of large groups of presynaptic neurons. We obtained intracellular recordings from cat triceps surae motoneurons and measured the effects of repetitive activity in different sets of presynaptic neurons produced by electrical stimulation of descending fibers or peripheral nerves and by longitudinal vibration of the triceps surae muscles (to activate primary muscle spindle Ia afferent fibers). We combined synaptic activation with subthreshold injected currents to obtain estimates of effective synaptic currents at the resting potential (I(Nrest)) and at the threshold for repetitive discharge (I(Nthresh)). We then superimposed synaptic activation on suprathreshold injected current steps to measure the synaptically evoked change in firing rate. We studied eight different pairs of synaptic inputs. When any two synaptic inputs were activated concurrently, both the effective synaptic currents (I(Nrest)) and the synaptically evoked changes in firing rate generally were equal to or slightly less than the linear sum of the effects produced by activating each input alone. However, there were several instances in which the summation was substantially less than linear. In some motoneurons, we induced a partial blockade of potassium channels by adding tetraethylammonium (TEA) or cesium to the electrolyte solution in the intracellular pipette. In these cells, persistent inward currents were evoked by depolarization that led to instances of substantially greater-than linear summation of injected and synaptic currents. Overall our results indicate that the spatial distribution of synaptic boutons on motoneurons acts to minimize electrical interactions between synaptic sites permitting near linear summation of synaptic currents. However, modulation of voltage-gated conductances on the soma and dendrites of the motoneuron can lead to marked nonlinearities in synaptic integration.

Synaptic integration in spinal motoneurones

Spinal motoneurones receive thousands of presynaptic excitatory and inhibitory synaptic contacts distributed throughout their dendritic trees. Despite this extensive convergence, there have been very few studies of how synaptic inputs interact in mammalian motoneurones when they are activated concurrently. In the experiments reported here, we measured the effective synaptic currents and the changes in firing rate evoked in cat spinal motoneurones by concurrent repetitive activation of two separate sets of presynaptic neurons. We compared these effects to those predicted by a linear sum of the effects produced by activating each set of presynaptic neurons separately. We generally found that when two inputs were activated concurrently, both the effective synaptic currents and the synaptically-evoked changes in firing rate they produced in motoneurones were generally linear, or slightly less than the linear sum of the effects produced by activating each input alone. The results suggest that the spatial distribution synaptic terminals on the dendritic trees of motoneurones may help isolate synapses from one another, minimizing non-linear interactions.

Contribution of voltage-dependent potassium channels to the somatic shunt in neck motoneurons of the cat

The specific membrane resistivity of motoneurons at or near the soma (Rms) is much lower than the specific membrane resistivity of the dendritic tree (Rmd). The goal of the present experiments was to investigate the contribution of tonically active voltage-dependent potassium channels at or near the soma to the low Rms. These channels were blocked with the use of intracellular injections of cesium. Input resistance (RN), Rms/Rmd, a conductance representing voltage-dependent potassium channels on the soma, GK, and a conductance attributed to damage caused by electrode impalement, GDa, were estimated from voltage responses to a step of current. The effect of intracellular injections of cesium on electrotonic structure was determined with the use of two strategies: 1) a population analysis that compared data from two groups of motoneurons, those recorded with potassium acetate electrodes (control group) and those recorded with cesium acetate electrodes after the motoneurons were loaded with cesium; and 2) an analysis of changes in electrotonic structure that occurred over the course of multiple injections of cesium or during similar periods of time in control cells. RN of control and cesium-loaded motoneurons was similar. Over the course of the recordings, RN of control cells usually increased and this increase was associated with a hyperpolarization. In contrast, increases in RN after successive injections of cesium were closely linked to a depolarization. At corresponding membrane potentials, Rms/Rmd of cesium-loaded motoneurons was greater than Rms/Rmd of control motoneurons. Over the course of cesium injections, Rms/Rmd increased and the membrane potential depolarized. In contrast, increases in Rms/Rmd observed during the course of recordings from control cells were associated with a hyperpolarization. Compared with control cells at corresponding membrane potentials, GK was less in cesium-loaded cells. Increases in GK that occurred over the course of recordings in control cells were closely coupled to a depolarization. In cesium-loaded cells, GK usually decreased as the cell depolarized during the injections. In control cells, increases in GDa that occurred during the recording period were closely coupled to a depolarization. In contrast, changes in GDa that occurred during cesium injections were not related to the change in membrane potential and did not explain the corresponding changes in Rms/Rmd and membrane potential. The results of this study indicate that voltage-dependent potassium channels contribute to the somatic shunt (low Rms) in neck motoneurons and provide a voltage-dependent mechanism for the dynamic regulation of motoneuron electrotonic properties.

Spatial integration of local transmitter responses in motoneurones of the turtle spinal cord in vitro

1. Integration of responses to local activation of transmitter receptors in the dendrites of motoneurones was investigated in a slice preparation of the turtle spinal cord. Membrane-active substances were applied from up to three independent iontophoresis electrodes during intracellular recording from the cell body. 2. Responses to glutamate could be evoked from dendrites closer than 20 microns from the tip of the glutamate electrode. The effects of other substances were more widespread. 3. In normal medium the configuration of a glutamate response was affected by time-dependent anomalous rectification. In the presence of muscarine the sum of glutamate responses from two different dendrites recruited a voltage-sensitive plateau potential. 4. The response to glutamate from one dendrite could be attenuated by local application of gamma-aminobutyric acid (GABA) without effects on soma conductance or glutamate responses from other dendrites. 5. The response to glutamate from one dendrite could be selectively enhanced by local application of tetraethylammonium (TEA) or N-methyl-D-aspartate (NMDA) without effects on soma conductance or glutamate responses from other dendrites. 6. NMDA could convert a tonic glutamate response from one dendrite into a phasic response without affecting the configuration of glutamate responses from other dendrites. 7. The effects of TEA and NMDA were facilitated by depolarization and reduced by hyperpolarization. 8. We conclude that the cable structure of motoneurones and the distribution of synapses and voltage-sensitive ion channels provide relative autonomy to non-linear synaptic processing and modulation in confined dendritic regions.

Synaptically triggered action potentials begin as a depolarizing ramp in rat hippocampal neurones in vitro

1. During just-suprathreshold synaptic activation of CA1 pyramidal cells in rat hippocampal slices in vitro the action potential begins as a slow depolarizing ramp, superimposed on the underlying EPSP and forming an integral part of the action potential. We call this ramp a synaptic prepotential (SyPP). 2. In order to examine the SyPP, a procedure for subtraction of the underlying EPSP was necessary. Because action potentials were only elicited by a subset of EPSPs with larger than average amplitude, a subtraction of the mean subthreshold EPSP would not give valid results. Instead, an EPSP to be subtracted was selected from an assemblage of subthreshold EPSPs, so that its amplitude matched the initial part of the spike-generating EPSP. 3. Virtually all action potentials started with a SyPP. Using an amplitude criterion of 1 S.D. of the mean of the matching subthreshold EPSPs, just-suprathreshold EPSPs gave prepotentials in 72-100% of all action potentials from fifteen randomly selected cells. With a criterion of 2 S.D.S, the frequency of occurrence ranged from 36 to 100%. 4. With a constant stimulus strength, there was a certain variability of the spike latencies. Shorter latency spikes had steeper, but smaller SyPPs than later spikes, suggesting that the slope of SyPP influenced the timing of the cell discharge. 5. The SyPP was best fitted by a single, exponentially rising curve, and was both smaller and slower than the large amplitude action potential. Its amplitude was 1-6 mV and the time constant 1-5 ms, which was 10-50 times slower than that of the upstroke of the action potential. 6. A properly timed hyperpolarizing current pulse could block the large amplitude action potential, thereby unmasking the SyPP as an initial depolarizing ramp. 7. The SyPP was more sensitive than the large amplitude action potential to intracellular injection of QX-314, a lidocaine derivative. At the concentrations used (10 or 30 mM) no detectable changes were seen in the large amplitude action potential. 8. Droplet application of a specific N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovaleric acid (1 mM), reduced both the EPSP and the firing probability, but did not change the SyPP. 9. The SyPP amplitude and time course depended upon the membrane potential at which the cell was activated. Depolarization enhanced and prolonged the SyPP, while hyperpolarization gave opposite effects. In part, the depolarization-induced amplitude increase could be attributed to membrane accommodation. 10. Antidromically evoked action potentials never started with a prepotential.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction by general anaesthetics of group Ia excitatory postsynaptic potentials and currents in the cat spinal cord

1. The effects of thiopentone and halothane on excitatory synaptic transmission at group Ia afferent synapses on lumbosacral motoneurones were studied in the anaesthetized or decerebrate cat. 2. Thiopentone (10 mg kg-1) infused on a background of light pentobarbitone anaesthesia caused a decrease in single-fibre monosynaptic group Ia excitatory postsynaptic potentials (EPSPs) of between 0 and 24%. A step increase in inspired halothane concentration in the range 0.7-0.9% produced a decrease in EPSP amplitude of between 0 and 31%. These effects were reversible when the anaesthetic level was reduced. 3. Fluctuation analysis of selected single-fibre group Ia EPSPs revealed that these effects could be accounted for by a decrease in the probability of occurrence of EPSPs of larger amplitude, and an increase in the probability of occurrence of EPSPs of smaller amplitude. The mean separation between discrete amplitudes was not altered by either anaesthetic agent. 4. EPSPs whose time course indicated a somatic site of origin were voltage clamped to study the effect of the anaesthetics on the time course of the synaptic currents. Neither thiopentone nor halothane produced a consistent effect on the time constant of decay of the current, although they both depressed its peak amplitude. 5. The results are interpreted as indicating a presynaptic site of action of both anaesthetics at the concentrations studied: the probability of release of neurotransmitter is reduced, without any detectable change in the mean duration of the postsynaptic conductance increase. These findings are discussed in relation to the mechanisms of action of anaesthetics on exocytosis and presynaptic inhibition.

Monosynaptic EPSPs in cat lumbosacral motoneurones from group Ia afferents and fibres descending in the spinal cord

1. Excitatory postsynaptic potentials (EPSPs) were elicited in lumbosacral motoneurones of pentobarbitone-anaesthetized cats by stimulating the ventral quadrants (VQ) of the thoracic spinal cord. These EPSPs were compared with monosynaptic EPSPs from small numbers of group Ia afferents, obtained by stimulating hindlimb muscle nerves with most of the dorsal roots severed. 2. EPSPs with average peak amplitude less than 1 mV were selected for fluctuation analysis. Three out of fourteen (21%) VQ EPSPs with peak voltage less than 150 mu V fluctuated in amplitude from trial to trial no more than could be accounted for by the background intracellular noise. Similarly, nine out of thirty-nine (23%) Ia EPSPs smaller than 150 mu V fluctuated to a comparable extent as the noise. These results are consistent with the view that there is little variation in the postsynaptic signal produced by an individual transmitter release event. 3. Of the EPSPs which did fluctuate more than the background noise, maximum likelihood estimates were obtained for the fluctuation patterns of ten VQ and fourteen Ia EPSPs. This was achieved by assuming that synaptic signals sum linearly with noise, but without constraining the results to conform to a statistical description of transmitter release. The fluctuation of both VQ and Ia EPSPs was made up of discrete amplitudes separated by roughly equal increments, in accordance with the quantal hypothesis of synaptic transmission. 4. Fluctuation patterns were obtained simultaneously for VQ and Ia EPSPs in seven motoneurones. The amplitudes of the quanta, defined as the mean increments between discrete amplitudes, were correlated (r = 0.90), suggesting common postsynaptic mechanisms. 5. For most EPSPs the time course of the voltage transient could be used to estimate the electrical distance from the soma at which the synaptic current was injected. There was a comparable distribution for VQ and Ia EPSPs. For those in which a quantal analysis was performed (nine VQ and eleven Ia), quantal size measured at the soma appeared to be independent of the deduced site of origin. 6. The results indicate no qualitative or quantitative differences in the behaviour of VQ and Ia EPSPs.

Cable properties of cat spinal motoneurones measured by combining voltage clamp, current clamp and intracellular staining

1. Spinal alpha-motoneurones were injected with horseradish peroxidase after measuring their voltage response to a brief current pulse and their current response to a small voltage step. 2. The morphology of each motoneurone was reconstructed from serial sections. The diameters and lengths of dendritic segments were used to build a compartmental model of each neurone's electrotonic structure. The specific resistivity of the membrane (Rm) was assumed to be constant throughout the dendrites, but it was lowered for the somatic membrane by the introduction of a somatic shunt resistance. 3. The specific resistances of the somatic and dendritic membrane were adjusted in the compartmental model until the responses of the model to the same current and voltage steps as those used in the experiment gave the best fits to the recorded transients. Satisfactory fits were obtained for six out of seven motoneurones. Dendritic Rm varied from 7 to 35 k omega cm2 and somatic Rm varied from 100 to 420 omega cm2. The dendritic Rm was 100-300 times the somatic Rm for different neurones. 4. The calculated dendritic Rm was used to determine the geometric profile of the equivalent dendritic cable. This was found to be an approximately uniform cylinder for about 0.5 lambda and thereafter to taper rapidly to a final termination at 2-3 lambda from the soma. 5. The results indicate that motoneurone dendrites are more electrically compact than was hitherto believed. The different Rm values for somatic and dendritic membrane, and the tapering of the dendritic cable, means that the cable model developed by Rall (1959, 1964) must be revised to take account of these spatial and electrical non-uniformities.

Electrical properties of motoneurons in the spinal cord of rat embryos

Electrical properties of immature motoneurons were studied in vitro using isolated segments of spinal cords of rat embryos aged 14-21 days of gestation. Stable resting potentials and evoked synaptic potentials were recorded for more than 9 hr, indicating that motoneurons remain viable for many hours. Motoneurons are electrically excitable at 14 days of gestation and from the onset of excitability the action potentials are Na+-dependent but slow rising long-duration Ca2+-dependent action potentials can be evoked if K+ conductance is reduced. Thus, during embryonic development the regenerative potential inward current is Na+-and Ca2+-dependent. During motoneurons' differentiation there are some changes in their electrical properties: resting membrane potential increases, input resistance decreases, input capacitance increases, threshold for action potential decreases, and maximum rate of rise of action potential increases. Afferent motoneuron contacts are formed at 16-18 days of gestation when excitatory synaptic potentials can first be evoked in response to dorsal root stimulation. The changes in input capacitance and threshold for action potential occur at the onset of functional afferent motoneuron contacts, but it is not known whether these changes are autonomous or are influenced by the newly formed sensory inputs.

Monosynaptic transmission of respiratory drive to phrenic motoneurons from brainstem bulbospinal neurons in rats

The termination patterns in the rat phrenic nucleus of neurons within two respiratory cell groups of the ventrolateral medulla (Bötzinger Complex and the rostral ventral respiratory group) were determined. The plant lectin, Phaseolus vulgaris leuco-agglutinin, was used as an anterograde tracer to label presynaptic processes of bulbospinal neurons, and horseradish peroxidase was used simultaneously to label phrenic motoneurons. Labeled bulbospinal axons ended with dense terminal arborizations within the phrenic cell column and on radial phrenic motoneuron dendrite bundles, which represented the exclusive site of termination of Bötzinger Complex and rostral ventral respiratory group neurons in the lower cervical spinal cord. Terminals of these descending axons formed presumptive synaptic contacts within longitudinal and radial dendrite bundles, and on the cell somata of phrenic motoneurons.

Waveform and amplitude characteristics of evoked responses to dendritic stimulation of CA1 guinea-pig pyramidal cells

1. Dendritic synaptic responses were evoked in CA1 hippocampal pyramidal cells using a microstimulation protocol which included focal excitation of proximal and distal apical afferents. Ensembles of excitatory postsynaptic potentials (EPSPs) were analysed for magnitude, waveform parameters and fluctuation characteristics between responses. 2. The peak amplitude of the minimal detectable responses to dendritic stimulation ranged from 0.12 to 0.89 mV. Control experiments showed separation of the minimal EPSPs from extracellular field potentials and somatic inhibition. The EPSPs demonstrated no significant amplitude trends over ensembles of 150-300 responses, at a 2 Hz stimulation rate. 3. Both minimal (less than 1 mV) and large (1-5 mV) proximal and distal evoked EPSPs were clearly different in terms of all waveform parameters analysed. However, the large EPSPs exhibited substantially less proximal-distal separation than the minimal responses. 4. The separation of minimal proximal and distal responses was similar to that predicted by earlier dendritic model simulations, after revision of the model parameter determining shape of the synaptic input. The proximal and distal synapses are separated by approximately 0.6 dendritic length constants (lambda), within an average apical dendritic tree of 0.9 lambda. 5. The decrease in proximal-distal separation with increasing EPSP size may stem from recruitment of non-laminar dendritic afferents and the addition of feed-forward inhibitory potentials. These circuitry features of the CA1 region lead to an effective electrical shortening of the apical dendritic tree for large EPSPs and in particular to an enhanced efficacy of distal synapses.

The effect of intracellular tetraethylammonium ions on the reversal potential of monosynaptic EPSPs and excitatory amino acids in cultured spinal cord neurones

Using the whole cell patch clamp recording technique, the reversal potential of currents generated by the application of excitatory amino acids and by stimulation of excitatory synapses has been examined in cultured spinal cord neurones, with the partial substitution of intracellular K+ by TEA+. A previous report (Clements et al. 1986) showed that intracellular injection of TEA into the soma of spinal motoneurones in the cat shifted the reversal of the 1a EPSC to more positive potentials, but the reason for this shift was not clear. The results reported here show that the observed change in the reversal potential for both excitatory amino acid agonists and the excitatory synaptic current, was not due to a change in the relative permeability of the channels to potassium or sodium, but could be explained by a change in driving force for the permeant intracellular ions (K+ and/or Cs+) on their substitution with the impermeant TEA+ ion.