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.     

Hindlimb unweighting for 2 weeks alters physiological properties of rat hindlimb motoneurones

We sought to determine whether decreased neuromuscular use in the form of hindlimb unweighting (HU) would affect the properties of innervating motoneurones. Hindlimb weight-bearing was removed in rats for a period of 2 weeks via hindlimb suspension by the tail. Following this the electrophysiological properties of tibial motoneurones were recorded under anaesthesia in situ . After HU, motoneurones had significantly ( P < 0.05) elevated rheobase currents, lower antidromic spike amplitudes, lower afterhyperpolarization (AHP) amplitudes, faster membrane time constants, lower cell capacitances, and depolarized spike thresholds. Frequency–current ( f – I ) relationships were shifted significantly to the right (i.e. more current required to obtain a given firing frequency), although there was no change in f – I slopes. 'Slow' motoneurones (AHP half-decay times, > 20 ms) were unchanged in proportions in HU compared to weight-bearing rats. Slow motoneurones had significantly lower minimum firing frequencies and minimum currents necessary for rhythmic firing than 'fast' motoneurones in weight-bearing rats; these differences were lost in HU rats, where slow motoneurones resembled fast motoneurones in these properties. In a five-compartment motoneurone model with ion conductances incorporated to resemble firing behaviour in vivo , most of the changes in passive and rhythmic firing properties could be reproduced by reducing sodium conductance by 25% and 15% in the initial segment and soma, respectively, or by increasing potassium conductance by 55% and 42%, respectively. This supports previous conclusions that changes in chronic neuromuscular activity, either an increase or decrease, may result in physiological adaptations in motoneurones due to chronic changes in ion conductances.     

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.     

Disynaptic brainstem—propriospinal projections to mammalian motoneurones

In experiments carried out on cats simultaneous recording from hindlimb motoneurones and propriospinal interneurones receiving monosynaptic inputs from the brain stem was accomplished. The recording of the unitary postsynaptic potentials produced in motoneurones by direct stimulation of individual propriospinal cells has shown that supraspinal projections can govern spinal motor centres via propriospinal cells that establish direct monosynaptic contacts with alpha-motoneurones.     

Double stimulation modulates afterhyperpolarization phase following action potentials evoked in rat motoneurones

The influence of a pair of stimuli running in time sequence between 5-10 ms (a doublet) on the basic parameters of antidromic action potentials was studied in rat motoneurones. Electrophysiological experiments were based on stimulation of axons in the sciatic nerve and intracellular recording of antidromic action potentials from individual motoneurones located in L4-L5 segments of the spinal cord. The following parameters were analyzed after application of a single stimulus and a doublet: amplitude and duration of the antidromic spike, amplitude, total duration, time to minimum, half-decay time of the afterhyperpolarization (AHP). It was demonstrated that application of a pair of stimuli resulted in: (1) a prolongation of action potentials, (2) a prolongation of the total duration and half-decay time of the AHP, (3) a decline of the time to minimum of the AHP, (4) an increase of the AHP amplitude of the spike evoked by the second stimulus. Significant differences in AHP parameters were found either in fast or slow motoneurones. We suppose that doublet-evoked changes in the AHP amplitude and duration are linked to intrinsic properties of individual motoneurones and may lead to the prolongation of the time interval to subsequent motoneuronal discharges during voluntary activity.     

Lack of Specificity of Motoneurone Responses to Microiontophoretically Applied Phenolic Amines

Intracellular recordings from spinal motoneurones have revealed that iontophoresed noradrenaline (NA) causes a membrane hyperpolarization, sometimes leading to a block of spike generation (Curtis 1968, Phillis, Tebëcis and York 1968). Similar alterations of membrane activity were produced by serotonine and histamine. The hyperpolarization is possibly due to a reduction of the resting sodium permeability (P_Na) as the membrane conductance is decreased during NA application (Engberg and Marshall 1971, 1973)—a similar mechanism as in the slow synaptic inhibition in bullfrog sympathetic ganglion cells (Weight and Padjen 1973)—or it may be due to an activation of electrogenic ion transport as has been suggested for cerebellar Purkinje cells (Siggins et al. 1971).     

Absence of recurrent axon collaterals in motoneurones to the extrinsic digit extensor muscles of the cat forelimb.

Forelimb alpha-motoneurones were intracellularly recorded in anaesthetized cats and iontophoretically filled with horseradish peroxidase (HRP). All motoneurones to the elbow flexors, elbow extensor and to the extensor carpi radialis muscles displayed in parallel homonymous recurrent inhibitory postsynaptic potentials (RIPSPs) and axon collaterals. Homonymous RIPSPs and axon collaterals were missing in the nuclei to the long digit extensor muscles. Two populations of motoneurones, with and without recurrent axon collaterals, seem to be present in the extensor carpi ulnaris motor nucleus. These results are consistent with the hypothesis that the motoneurones to the extrinsic digit extensors lack a recurrent axonal system. This indicates that the contribution of the recurrent Renshaw systems to motor control may be more complex than hitherto assumed.     

Fourier analysis of the relation between the discharge of quadriceps motor units and periodic mechanical stimulation of cat knee joint receptors

It has been widely assumed that joint receptors contribute to the reflex regulation of movement and posture, although there have been few clear demonstrations of joint-mediated reflex actions on alpha-motoneurones other than those attributable to flexor reflex afferents. The present study extends our recent work on joint-mediated reflexes by using Fourier analysis of spike train interactions to demonstrate that restricted mechanical activation of a small number (one to five) of knee joint receptors by localized indentation of the joint capsule can modify the firing pattern of quadriceps motoneurones in decerebrated cats. The modulation of this discharge pattern can be reversibly abolished by application of droplets of lignocaine directly to the joint nerve and consequently can be attributed unambiguously to articular afferents. Activation of single joint afferents could on occasion produce changes in motor unit firing patterns, but usually activation of two or more was required before secure effects were observed. Increasing the intensity of indentation (resulting in activation of greater numbers of joint receptors) increases the strength of coupling between joint afferent input and motor unit responses, although the relationship is not linear. The relation between the discharge patterns of pairs of motor units was also examined, and it was found that significant coupling occurred at the stimulus frequency, superimposed on a 'background' coupling from unidentified sources. The phase relationship between pairs of motor units was not affected by the periodic stimulus. However, it was observed that if two motor units were firing independently of one another in the absence of capsule probing, maintained capsular indentation produced a striking synchronization between the discharges of the two motor units. These experiments show strong joint-mediated reflex effects on motor unit firing indicating that joint receptors may have an important role to play in motor control.     

Does elimination of afferent input modify the changes in rat motoneurone properties that occur following chronic spinal cord transection?

The purpose of this study was to determine the effects of 6-8 weeks of chronic spinal cord isolation (SI, removal of descending, ascending and afferent inputs), compared with the same duration of spinal cord transection (ST, removal of descending input only) on hindlimb motoneurone biophysical properties. Adult female Sprague-Dawley rats were placed into three groups: (1) control (no removal of inputs), (2) ST and (3) SI. The electrophysiological properties from sciatic nerve motoneurones were recorded from deeply anaesthetized rats. Motoneurones in SI rats had significantly (P < 0.01) lower rheobase currents and higher spike afterhyperpolarization amplitudes and input resistances compared with motoneurones in control rats. A higher percentage (chi2, P = 0.01) of motoneurones in SI than control rats demonstrated frequency-current (f-I) relationships consistent with activation of persistent inward currents. Motoneurone steady state f-I slopes determined by increasing steps of 500 ms current pulses were significantly lower (P < 0.02) in SI than control rats. Motoneurone spike frequency adaptation measured using 30 s square-wave current injections (1.5-3.0 nA above the estimated rhythmic firing threshold), was similar for control and SI motoneurones. Changes in motoneurone properties following SI did not differ from ST. These findings indicate that the removal of afferent and ascending inputs along with descending inputs has little additional affect on motoneurone properties than removal of descending inputs alone. This study is the first to demonstrate that intact ascending and afferent input does not modify the effects of spinal transection on basic and rhythmic firing properties of rat hindlimb motoneurones.     

Intracellular recording from spinal motoneurones in cats with post-asphyxial rigidity

1. Intracellular recordings were obtained from lumbar spinal motoneurones in cats with post-asphyxial rigidity of the hind limbs.2. Membrane potentials, latencies and (or) appearance of excitatory post-synaptic potentials, initial segment responses and soma-dendritic spikes were not materially different from those observed in cells of normal cords.3. Dorsal root stimulation activated all the motoneurones examined through monosynaptic pathways in contrast to cells in normal cords in which such a stimulus sometimes elicits only a post-synaptic potential. In a number of cells subsequent polysynaptic activation caused a spike about 10 msec after the monosynaptic response, notwithstanding the serious interneuronal destruction which characterized these preparations.4. In a few preparations the effects of acute asphyxiation could be examined. The soma depolarized at a rate of 3-4 mV/min. Synaptic activation was more resistant to O(2) lack than antidromic and direct excitation, in contrast to the experience with normal cells. Survival times of 8.5, 11 and 16 min were found. At certain levels of depolarization ;spontaneous' spikes were observed, which, since they were preceded by post-synaptic potentials, could be considered as the result of synaptic activation.5. To account for the enhanced reflex activity of rigid preparations, it was postulated that the substantial loss of interneurones in the cord had caused denervation supersensitivity of the motoneurones to the transmitter compound without materially changing their electrical excitability.6. It was postulated that the early presynaptic failure during asphyxiation in normal preparations was dependent on a mechanism resembling presynaptic inhibition. The prolonged asphyxial survival of reflex activity in rigid preparations may be due to the destruction of interneurones involved in this form of inhibition.     

Relationships between the spike components and the delayed depolarization in cat spinal neurones.

1. Changes in the delayed depolarization (DD) following composite (IS-SD) intracellular spikes in motoneurones and neurones of the ventral spinocerebellar tract were recorded in a variety of experimental conditions. Cell activation was either antidromic or by direct intracellular stimulation. 2. It was observed that under all conditions in which IS-SD coupling changes took place (as a consequence of spontaneous fluctuations, membrane conductance variations, variations of direct-stimulation parameters, changes in steady membrane polarization), SD spike delays were always accompanied by a progressive concomitant reduction of the DD depolarizing hump amplitude. 3. Under the same conditions the latency of the DD peak from the stimulus artifact remained constant. Accordingly, any increase of the SD delay was accompanied by a reciprocal reduction of the time interval between the SD spike and the DD peak. This variability of temporal relationships between SD spike and DD would appear to contradict the hypothesis that the DD might represent the image of the excitation spreading from the soma to the dendrites (Kernell, 1964; Nelson & Burke, 1967). 4. as the gradual reduction of the DD hump progressed, the time course of the decay phase of the afterhyperpotential more and more closely approximated the decay phase of the IS spike. As an alternative hypothesis it is suggested that the DD might originate from the current which generates the IS spike.     

Firing of spinal motoneurones due to electrical interaction in the rat: An in vitro study

Summary The excitatory interaction between spinal motoneurones was investigated by means of electromyogram (EMG) recordings from hindlimb muscles as well as intracellular ones from their innervating motoneurones in the isolated preparation of immature rats.Stimulation of the muscle nerve to biceps femoris or medial gastrocnemius or of the L5 ventral root evoked early and late EMG responses in the muscle of the preparations with the dorsal roots cut. The early response was produced directly by volleys in the motor nerve. The late response was of spinal origin, since it disappeared after the severance of the ventral root. The thresholds and the conduction velocities of nerve fibres, which conducted the centripetal impulse causing the late response, were compatible with those of motor nerve fibres. The amplitude of the late response was 5–10% of that of the maximum early EMG response.Intracellular recordings from spinal motoneurones revealed that stimulation of the ventral root elicited the double discharge composed of antidromic and delayed spike potentials. The delayed spike was never evoked after the spike potential elicited directly by a short depolarizing pulse. The double discharge was observed in about 6% of the motoneurones examined. The threshold of the stimulus intensity evoking the double discharge was in the range of those of motor nerve fibres. The latencies of the delayed excitation were 7.0–9.0 ms, comparable to the intraspinal delays of the late EMG response.Stimulation of the ventral root at intensities subthreshold for antidromic activation was found to produce a small depolarizing potential in about 60% of the motoneurones examined. The amplitudes were 0.5–5.0 mV, and the onset and the peak latencies 2.0–7.0 ms and 5.0–8.0 ms, respectively. The potential was unaffected by the deficiency of calcium ions in the perfusing medium and persisted after the degeneration of the afferent fibres in the ventral root. It was thus concluded that the depolarizing potential was generated by electrical synapses between motoneurones.In a few motoneurones the electrical synaptic potential was found to elicit spike potentials. Latencies of these spikes were similar to those of the delayed excitation in motoneurones with the double discharge. The time course of changes in the excitability in these motoneurones showed that the delayed excitation, hence the late EMG response, was also caused by the electrical synaptic potential.     

On the control of myotomal motoneurones during “fictive swimming” in the lamprey spinal cord in vitro

Intracellular recordings have been made from myotomal motoneurones during “fictive swimming” in the in vitro preparation of the lamprey spinal cord, while monitoring the efferent burst activity in the ventral roots. The pattern of rhythmic activity in the motoneurones is described, as well as how synaptic inputs from the premotoneuronal level exert their control of motoneurone activity. (1) All motoneurones investigated displayed rhythmic, symmetric oscillations of their membrane potential during “fictive swimming”. The period of depolarization occurred in phase with the burst discharge in the ventral root containing the motoneurone axon. (2) About one-third of the cells fired bursts of action potentials during the depolarized phase, while the remaining motoneurones exhibited subthreshold oscillations. (3) Intracellular injection of chloride ions reversed the sign of the hyperpolarized phase, demonstrating phasic active inhibition of the motoneurones during rhythmicity. (4) The depolarized phase was unaffected after chloride injection, showing that the motoneurones also received phasic active excitation. (5) “Pre-triggered” averaging of the motoneurone recording (using the ventral root spikes from other motoneurones for triggering), revealed that some degree of synchronous excitation of several motoneurones occurred, suggesting common excitation from the same premotor-interneurones. It is concluded that the rhythmic oscillations of membrane potential in lamprey myotomal motoneurones during “fictive locomotion” depend on phasic excitation alternating with phasic active inhibition. The premotoneuronal mechanism responsible for this control may consist of reciprocally organized groups of excitatory and inhibitory interneurones.     

Intracellular recording from cat spinal motoneurones during acute asphyxia

1. Changes in membrane and action potentials of cat spinal motoneurones during acute asphyxiation and re-oxygenation were recorded with an intracellular technique.2. The asphyxial potential of the grey matter, which develops in the first 2-2.5 min of asphyxiation, can be expected to interfere with the membrane potential record. After correcting for this effect a gradual depolarization of the soma at a rate of 3-4 mV/min was found, commencing within a fraction of a minute after the start of asphyxiation.3. The orthodromic responses of the motoneurones were the most vulnerable to O(2) lack. They failed earlier than the responses to antidromic and to direct excitation of the cell through the micro-electrode. After failure of the orthodromic spike an excitatory post-synaptic potential remained for a short time. Failure of antidromic excitation began by the dropping out of the some dendritic potential, followed by the arrest of the initial segment response.4. It was concluded that the early arrest of orthodromic excitation is caused by presynaptic failure.5. All changes in membrane and action potentials were completely reversible by re-oxygenation after periods of asphyxia lasting from 4 to 6 min. The orthodromic response recovered markedly slower than the antidromic and direct ones.     

A diverse pattern of the spike threshold changes in feline gastrocnemius–soleus motoneurons during stretch reflex activation

The aim of the study was to define the spike threshold changes in motoneurons during naturally evoked activation. Spike activity was evoked in the gastrocnemius–soleus motoneurons of decerebrate cats by controlled stretches of the homonymous muscles. The spike thresholds were defined by using the first derivative of the membrane potential and statistical boundaries of its change. The evoked firing was analyzed in terms of the spike thresholds and the membrane potential trajectories between spikes, which were dependent on the intensity of the firing. The main inference of the study is the absence of a single-valued dependence between the spike rates and thresholds; the last ones can both increase and decrease with a rise in the firing rate; moreover, opposite directions in the threshold changes are often observed at various phases of the stretch-evoked firing. The dependency of the spike thresholds on firing rates was checked in 42 of the total set of 57 motoneurons. In a group of cells (n = 27) showing statistically significant correlation (P < 0.05), this parameter was negative in 18 and positive in 9 motoneurons. The obtained results indicate that the threshold-crossing models with a single-valued dependency of the spike thresholds on the firing rates are not completely suitable to analyze the spike generation processes in the motor unit records.     

Relative contribution from different nerves to recurrent depression of Ia IPSPs in motoneurones

1. The pattern of depression of Ia IPSPs by volleys in recurrent motor axon collaterals was investigated in motoneurones supplying hind-limb muscles in the cat. The test IPSPs were evoked by stimulation of dorsal roots and the conditioning antidromic volleys by stimulation of motor fibres in different peripheral muscle nerves.2. In all motor nuclei investigated the strongest depression of Ia IPSPs is evoked from motor fibres to muscles whose Ia afferents produce the IPSPs. For example, the Ia IPSP from the knee extensor recorded in motoneurones to a knee flexor is most effectively depressed by antidromic stimulation of motor fibres to the knee extensor.3. The origin of recurrent inhibition of alpha-motoneurones and of Ia inhibitory interneurones with the same Ia input display a striking similarity. This suggests that the same population of Renshaw cells mediates effects to motoneurones and to Ia inhibitory interneurones.4. The functional significance of impulses in motor axon collaterals was discussed and it was suggested that they have an important role in the control of the excitatory as well as inhibitory Ia actions to motoneurones. The recurrent inhibition may limit the Ia effects to excitation of homonymous motoneurones, which would provide optimal conditions for control of individual muscles via the gamma-loop.     

Synaptic actions of single interneurones mediating reciprocal Ia inhibition of motoneurones

1. The investigation was aimed at defining the function of the interneurones which, according to indirect evidence, mediate the reciprocal Ia inhibition of motoneurones (Hultborn, Jankowska & Lindstrom, 1971 b) by studying their direct synaptic actions. These actions were tested by recording post-synaptic potentials in motoneurones following spike activity of single interneurones activated by iontophoretic application of glutamate. The interneurones were found to produce unitary monosynaptic IPSPs in those motoneurones in which disynaptic IPSPs are evoked by the group Ia afferents which monosynaptically excite the interneurones.2. Unitary IPSPs were found in more than 80% of the motoneurones impaled in the immediate vicinity of the axonal branches of the investigated Q interneurones in the PBSt motor nucleus. It is estimated that each interneurone might inhibit about every fifth PBSt motoneurone. The amplitudes of the unitary IPSPs ranged between 8 and 220 muV and were 10-200 times smaller than the maximal Ia IPSPs evoked in the same motoneurones.3. The synaptic delay in the generation of unitary IPSPs was measured in relation to the spike potentials recorded from the terminal branches of interneurones in the immediate vicinity of the impaled motoneurones. The synaptic delays ranged between 0.28 and 0.42 msec.4. From chloride reversal tests and an analysis of the time course of the unitary IPSPs it was concluded that the terminals of the investigated interneurones make synaptic contact predominantly on the soma and/or on the proximal parts of the dendrites of the motoneurones, their distribution being, however, not quite uniform.     

Excitation and inhibition of motoneurones in the tortoise

1. Intracellularly recorded responses of lumbosacral motoneurones in the tortoise are described. The preparation was decerebrate and unanaesthetized. Cells were tested by stimulation of the main branches of the ipsilateral sciatic nerve.2. The distribution of cells according to resting membrane potential is given; highest recorded values were around 70 mV.3. The action potential has a step on the rising phase and has a total duration in the region of 4 msec.4. The motoneurone input resistance, measured by the spike height method, was 5-8 MOmega.5. Orthodromic excitation of motoneurones occurred after a central latency of 2 msec.6. Inhibition, and inhibitory (hyperpolarizing) potentials were demonstrated. Peak amplitude of maximum response was frequently greater than 10 mV. The potentials were influenced by injected currents in a manner resembling IPSPs in mammalian motoneurones. Minimum central latency for inhibitory potentials was about 3 msec.7. Membrane potentials and dimensions of action potentials for motoneurones in the cat are given for purposes of comparison with the tortoise.     

A longitudinal gradient of synaptic drive in the spinal cord of Xenopus embryos and its role in co-ordination of swimming.

1. Intersegmental co-ordination in Xenopus embryos could be influenced by longitudinal gradients in neuronal properties or synaptic drive. To determine if such gradients exist intracellular recordings were made from putative motoneurones at different spinal levels. 2. No evidence was found of a longitudinal gradient in neuronal resting potentials. In a rostrocaudal direction the duration of current-evoked spikes increased and the amplitude of the spike after-hyperpolarization (AHP) decreased. 3. During fictive swimming the amplitude of the tonic excitatory synaptic input and the mid-cycle IPSPs declined in a rostrocaudal direction. The rise-time and fall-time of mid-cycle IPSPs increased in a rostrocaudal direction. 4. Rostral to the eighth post-otic segment mid-cycle IPSPs occurred on all cycles of fictive swimming episodes. More caudally IPSPs became irregular in occurrence and caudal to the twelfth post-otic segment no mid-cycle IPSPs could be detected, even during the injection of depolarizing current or when recording with KCl-filled electrodes. 5. The duration of spikes occurring during fictive swimming increased and the amplitude of spike AHP decreased in a rostrocaudal direction. A spike AHP was absent during fictive swimming activity in neurones caudal to the ninth post-otic segment even though it was present in current-evoked spikes in the same neurones. 6. On-cycle IPSPs (occurring shortly after the spike at phase values less than 0.4) were observed predominantly at the beginning of swimming episodes in neurones recorded rostral to the eighth segment, but were not detected at all in more caudal neurones. 7. If the rostrocaudal gradients in synaptic excitatory and inhibitory drive to putative motoneurones during fictive swimming are also present in premotor spinal interneurones they would be expected to have a strong influence on rostrocaudal delays. Such gradients could therefore be important components of the mechanism underlying intersegmental co-ordination.     

Cholinergic control of excitability of spinal motoneurones in the salamander

The cholinergic modulation of the electrical properties of spinal motoneurones was investigated in vitro , with the use of the whole-cell patch-clamp recording technique in lumbar spinal cord slices from juvenile urodeles ( Pleurodeles waltlii ). Bath application of acetylcholine (20 μ m ) with eserine (20 μ m ) induced an increase in the resting membrane potential, a decrease of the input resistance, a decrease of the action potential amplitude, and a reduction of the medium afterhyperpolarization (mAHP) that followed each action potential. Moreover, the firing rate of motoneurones during a depolarizing current pulse and the slope of their stimulus current–spike frequency relation were increased. All of these effects were mimicked by extracellular application of muscarine (20 μ m ), and blocked by application of the muscarinic receptor antagonist atropine (0.1–1 μ m ). They were not observed during bath application of nicotine (10 μ m ). These results suggest that the cholinergic modulation of spinal motoneurone excitability was mediated by activation of muscarinic receptors. Our results further show that the muscarinic action primarily resulted from a reduction of the Ca²⁺-activated K⁺ current responsible for the mAHP, an inhibition of the hyperpolarization-activated cation current, I _h, and an enhancement of the inward rectifying K⁺ current, I _Kir. We conclude that cholinergic modulation can contribute significantly to the production of motor behaviour by altering several ionic conductances responsible for the repetitive discharge of motoneurones.