Afterhyperpolarization Half-decay Time Research Articles

Expression of postsynaptic Ca2+-activated K+ (SK) channels at C-bouton synapses in mammalian lumbar -motoneurons.

Small-conductance calcium-activated potassium (SK) channels mediate medium after-hyperpolarization (AHP) conductances in neurons throughout the central nervous system. However, the expression profile and subcellular localization of different SK channel isoforms in lumbar spinal α-motoneurons (α-MNs) is unknown. Using immunohistochemical labelling of rat, mouse and cat spinal cord, we reveal a differential and overlapping expression of SK2 and SK3 isoforms across specific types of α-MNs. In rodents, SK2 is expressed in all α-MNs, whereas SK3 is expressed preferentially in small-diameter α-MNs; in cats, SK3 is expressed in all α-MNs. Function-specific expression of SK3 was explored using post hoc immunostaining of electrophysiologically characterized rat α-MNs in vivo. These studies revealed strong relationships between SK3 expression and medium AHP properties. Motoneurons with SK3-immunoreactivity exhibit significantly longer AHP half-decay times (24.67 vs. 11.02 ms) and greater AHP amplitudes (3.27 vs. 1.56 mV) than MNs lacking SK3-immunoreactivity. We conclude that the differential expression of SK isoforms in rat and mouse spinal cord may contribute to the range of medium AHP durations across specific MN functional types and may be a molecular factor distinguishing between slow- and fast-type α-MNs in rodents. Furthermore, our results show that SK2- and SK3-immunoreactivity is enriched in distinct postsynaptic domains that contain Kv2.1 channel clusters associated with cholinergic C-boutons on the soma and proximal dendrites of α-MNs. We suggest that this remarkably specific subcellular membrane localization of SK channels is likely to represent the basis for a cholinergic mechanism for effective regulation of channel function and cell excitability.

Diversification of Intrinsic Motoneuron Electrical Properties During Normal Development and Botulinum Toxin–Induced Muscle Paralysis in Early Postnatal Mice

During early postnatal development, between birth and postnatal days 8-11, mice start to achieve weight-bearing locomotion. In association with the progression of weight-bearing locomotion there are presumed developmental changes in the intrinsic electrical properties of spinal -motoneurons. However, these developmental changes in the properties of -motoneuron properties have not been systematically explored in mice. Here, data are presented documenting the developmental changes of selected intrinsic motoneuron electrical properties, including statistically significant changes in action potential half-width, intrinsic excitability and diversity (quantified as coefficient of variation) of rheobase current, afterhyperpolarization half-decay time, and input resistance. In various adult mammalian preparations, the maintenance of intrinsic motoneuron electrical properties is dependent on activity and/or transmission-sensitive motoneuron-muscle interactions. In this study, we show that botulinum toxin-induced muscle paralysis led to statistically significant changes in the normal development of intrinsic motoneuron electrical properties in the postnatal mouse. This suggests that muscle activity during early neonatal life contributes to the development of normal motoneuron electrical properties.

Summation of afterhyperpolarization after a doublet in fast motoneurones of the rat spinal cord.

Motoneurones located in lumbar segments (L4-L5) of the rat spinal cord were activated antidromically by stimulation of the sciatic nerve. Records following a single pulse and double stimuli were collected from fast motoneurones and in each case voltage and time parameters of the action potential and afterhyperpolarization (AHP) were compared to respective values calculated for a simulated doublet - obtained by mathematical summation of two individual single potentials. The most prominent differences between experimentally recorded and simulated potentials concerned spike duration and the AHP half-decay times, with significantly lower values obtained for the simulated doublets. Tendencies for a higher AHP amplitude, shorter AHP peak time and higher AHP area were observed in mathematically modeled doublets, though no statistical differences were present in comparison to experimental data. Results indicated a non-linear summation of the AHP after double stimuli running in a short interval. These observations suggest that alterations in the generation of the AHP, mainly based on the sustained activity of K(Na+) channels, influence time-intervals in motoneuronal discharge pattern and this way force development of individual motor units during muscle activity.

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.

Endurance training alters the biophysical properties of hindlimb motoneurons in rats.

The purpose of the study was to determine the effect of daily endurance treadmill training (2 h/day, 30 m/min) on motoneuron biophysical properties. Electrophysiological properties of tibial motoneurons were measured in situ in anesthetized (ketamine/xylazine) control and trained rats using sharp glass microelectrodes. Motoneurons from trained rats had significantly hyperpolarized resting membrane potentials and spike trigger levels, and faster antidromic spike rise-times. "Fast" motoneurons (after-hyperpolarization half-decay time <20 ms) in trained rats also had a significantly larger mean cell capacitance than those in control rats, suggesting that they were larger, although this had no effect on indices of excitability (rheobase, cell input resistance). Motoneurons are thus targets for activity-induced adaptations, which may have clinical significance for the role of physical activity as a therapeutic modality in cases of neurological deficit. The specific adaptations noted, which reflect alterations in ionic conductances, may serve to offset decreases in membrane excitability that occur during sustained excitation.

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.

Partial peripheral motor nerve lesions induce changes in the conduction properties of remaining intact motoneurons.

A partial injury or loss of peripheral motor axons is followed by compensatory sprouting of remaining intact motor axons in order to reinnervate muscle. Little is known, however, about the electrophysiologic properties proximally of these intact motoneurons and their axons following injury of neighboring motor axons. We studied the conduction properties of intact cat motor axons and motoneurons proximal to the site of a partial peripheral nerve section. Twelve weeks after the partial transection of the cat medial gastrocnemius motor nerve, there was a significant (7%) reduction in conduction velocity and a 13% prolongation in afterhyperpolarization half-decay time in the remaining intact motoneurons, compared with controls. Partial injury to motor nerves thus induces reactive electrophysiologic changes in the remaining intact motoneurons and their axons, perhaps associated with compensatory sprouting within partially denervated muscle.

Electrophysiological and morphological properties of neurons in the ventral horn of the turtle spinal cord

In this report, we present recent findings on the electrophysiological and morphological properties of spinal motoneurons (MNs) and interneurons (INs) of the adult turtle which were studied in slices of the spinal cord. The range of values for the measured electrophysiological parameters in 96 tested cells included: resting potential, −57 to −83 mV; input resistance, 2.5–344 MΩ; time constant, 2.5–63 ms; rheobase current, 0.04–5.3 nA; after-hyperpolarization (AHP) duration, 72–426 ms; AHP half-decay time; 11–212 ms; and, slope of the stimulus current-spike frequency relationship, 3.4–235 Hz/nA. For another 20 cells, we made both morphological and electrophysiological measurements (the latter values within the above ranges). Their ranges in morphological properties included: soma diameter, 20–54 μm; soma surface area, 299–2045 μm 2; soma volume, 2.3–45 μm 3 × 10 4; rostro-caudal dendritic projection distance, 150–1200 μm; and, sum of dendritic lengths, 1.5–16 μm × 10 3. The emphasized findings include: 1) the quality and robustness of the intracellular recordings, which enabled accurate measurement of the action potential's shape parameters (spike, afterhyperpolarization [AHP]); 2) the substantial AHP of the INs'AP; 3) no single action-potential shape parameter (nor combination of parameters) being cardinal for its (or their combined) changes matching the profile of the initial and later phases of spike-frequency adaptation; 4) the utility and flexibility of a cluster analysis (using varying combinations of passive, transitional and active cell properties) for providing a provisional classification of low (like cat S) and high (like cat F) threshold MNs, and groups of INs with non-spontaneous versus spontaneous discharge; 5) the clear-cut morphological confirmation of the provisional classification strategy; 6) the basis for testing the possibility that one of the provisionally classified MN types innervates non-twitch muscle fibers; and 7) the heuristic value of comparing the properties of MNs versus INs across vertebrate species, with an emphasis on the lamprey, turtle, and cat.

The effects of tetrodotoxin-induced muscle paralysis on the physiological properties of muscle units and their innervating motoneurons in rat.

1. Although the inactivity of a slow muscle (cat soleus) induced via nerve impulse blockade has been demonstrated to have some axotomy-like effects (decreased after-hyperpolarization (AHP) duration) on its innervating motoneurons, the reported effects of inactivity on motoneurons which innervate fast muscles containing mixtures of motor unit types are equivocal. This study was designed to determine the effect of a period (2 weeks) of complete hindlimb muscle paralysis, via tetrodotoxin (TTX) blockade of sciatic nerve impulses, on the contractile (muscle units) and electrophysiological (motoneurons) properties of motor units in the rat gastrocnemius. Motoneuron properties were also compared with those of rats subjected to sciatic nerve axotomy 2 weeks earlier. 2. At the time of the terminal experiment (24 h after the removal of the TTX delivery system) in anaesthetized animals, properties of tibial motoneurons (i.e. rheobase current, input resistance, time course of after-potentials) were determined using conventional microelectrode techniques. For those tibial motoneurons innervating the gastrocnemius, muscle unit responses (i.e. twitch force and time course, maximum tetanic tension, fatigability) were also recorded in response to current injection. 3. Consistent with previously reported whole-muscle responses to TTX-induced disuse, the TTX-treated gastrocnemius muscle units showed weaker tetanic forces, prolonged twitches and elevated twitch/tetanic ratios. These effects were similar for motor units classified as small, medium and large according to their tetanic tension-generating capacities. Muscle unit fatigue resistances appeared to be unchanged. 4. The mean values, distributions and ranges of tibial motoneuron properties were similar between control and TTX-treated groups for rheobase, input resistance and AHP half-decay time. In the case of the latter, the proportion of motoneurons possessing "slow' AHP half-decay times (> 20 ms) was not significantly different in control (17%) and TTX-treated groups (11%). 5. Motoneurons axotomized 2 weeks earlier had a significantly higher (42-69%) mean input resistance and a longer (34-42%) mean AHP half-decay time when compared with the control and TTX-treated groups. 6. It appears that, for fast muscles containing several different motor unit types, TTX-induced axon blockade does not produce similar effects on motoneuron intrinsic properties to those evoked by axotomy. This lack of effect on the distribution and range of these properties of tibial motoneurons indicates that none of the motoneurons which innervate muscles of mixed fibre type are particularly susceptible to the decreased activity and the atrophy-associated muscle changes produced by this condition. Thus, the apparent 'retrograde signalling' of muscle on motoneuron properties reported previously for the cat soleus may be specific to this particular muscle or species.

Physiological properties of motoneurons innervating different muscle unit types in rat gastrocnemius

1. The contractile properties of gastrocnemius muscle units and the electrophysiological properties of their innervating motoneurons were examined in anesthetized adult male Sprague-Dawley rats in situ, using conventional microelectrode techniques. 2. Muscle units (n = 70) were classified as fast (F) or slow (S) on the basis of the degree of force summation during stimulation at 25 Hz, as well as fatigue resistance, in response to current injection into innervating motoneurons. S units categorized using these criteria were also found to invariably demonstrate twitch half-relaxation times (RT1/2) > 28 ms, with F units demonstrating RT1/2 < 27 ms. Some overlap was present between F and S units in twitch time-to-peck tension (TPT). 3. S muscle units were innervated by motoneurons with significantly higher afterhyperpolarization (AHP) times-to-peak and half-decay times, AHP amplitudes, and input resistances than F units. Motoneurons innervating S units also demonstrated slower mean axon conduction velocity than F units. 4. F units were further classified as fast fatiguing (FF), fast fatigue-resistant (FR), or fast intermediate (FI) on the basis of their fatigue resistance. Muscle unit forces were different among the unit types such that FF > FI > FR > S. Twitch TPT and RT1/2 differed such that FF < FI, FR < S. 5. No differences among FF, FI, and FR units were found for measures of AHP time course, AHP amplitude, rheobase, or input resistance. The only motoneuron property that differed among F unit types was axon conduction velocity, which was lower for FF than for FI and FR units. 6. The best relationships between muscle unit and motoneuron physiological properties were between expression of twitch time course, which included RT1/2 and AHP half-decay time (r = 0.73 to 0.74). S units always had AHP half-decay times > 20 ms, whereas for F units, values never exceeded 19 ms. Correlations between these variables increased when only S units were considered (r = 0.86-0.97), and were weak and not significant among F units only. 7. The only other significant correlations between muscle unit and motoneuron properties > 0.5 were between input resistance and expressions of twitch time course (r = 0.56-0.66). These relationships were nonexistent when type S units were not included in the analysis. 8. A sample of tibial motoneurons (n = 98) that innervated hindlimb muscles other than gastrocnemius was combined with the gastrocnemius motoneurons to examine interrelationships among motoneuron properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Motor-unit properties following cross-reinnervation of cat lateral gastrocnemius and soleus muscles with medial gastrocnemius nerve. II. Influence of muscle on motoneurons.

We tested whether the muscle innervated may influence the expression of motoneuron electrical properties. Properties of individual motor units were examined following cross-reinnervation (X-reinnervation) of cat lateral gastrocnemius (LG) and soleus muscles by the medial gastrocnemius (MG) nerve. We examined animals at two postoperative times: 9-10 wk (medX) and 9-11 mo (longX). For comparison, normal LG and soleus motoneuron properties were also studied. Motor units were classified on the basis of their contractile responses as fast contracting fatigable, fast intermediate fast contracting fatigue resistant, and slow types FF, FI, FR, or S, respectively) (9, 21). Motoneuron electrical properties (rheobase, input resistance, axonal conduction velocity, afterhyperpolarization) were measured. After 9-11 mo, MG motoneurons that innervated LG muscle showed recovery of electrical properties similar to self-regenerated MG motoneurons. The relationships between motoneuron electrical properties were largely similar to self-regenerated MG. For MG motoneurons that innervated LG, motoneuron type (65) predicted motor-unit type in 74% of cases. LongX-soleus motoneurons differed from longX-LG motoneurons or self-regenerated MG motoneurons in mean values for motoneuron electrical properties. The differences in overall means reflected the predominance of type S motor units. The relationships between motoneuron electrical properties were also different than in self-regenerated MG motoneurons. In all cases, the alterations were in the direction of properties of type S units, and the relationship between normal soleus motoneurons and their muscle units. Within motor-unit types, the mean values were typical for that type in self-regenerated MG. Motoneuron type (65) was a fairly strong predictor of motor-unit type in longX soleus. MG motoneurons that innervated soleus displayed altered values for axonal conduction velocity, rheobase, and input resistance, which could indicate incomplete recovery from the axotomized state. However, although mean afterhyperpolarization (AHP) half-decay time was unaltered by axotomy (25), this parameter was significantly lengthened in MG motoneurons that innervated soleus muscle. There were, however, individual motoneuron-muscle-unit mismatches, which suggested that longer mean AHP half-decay time may also be due to incomplete recovery of a subpopulation of motoneurons. Those MG motoneurons able to specify soleus muscle-fiber type exhibited motoneuron electrical properties typical of that same motoneuron type in self-regenerated MG.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane electrical properties and prediction of motor-unit type of medial gastrocnemius motoneurons in the cat.

Membrane electrical properties [time constant, action potential afterhyperpolarization (AHP), rheobase, input resistance, and axonal conduction velocity] were measured in motoneurons of cat medial gastrocnemius (MG) motor units. Motor units were classified on the basis of their mechanical responses as fast twitch, fast fatiguing (FF); fast twitch with intermediate fatigue resistance (FI); fast twitch, fatigue resistant (FR); or slow twitch, fatigue resistant (S; 11, 22). Motoneuron membrane time constant, estimated from the voltage response at the onset or termination of long (50-100 ms) current pulses and corrected for voltage-response nonlinearities (32), was found to differ significantly among the major motor-unit types, increasing in the order FF less than FR less than S. Afterhyperpolarization magnitude, half-decay time, and duration were all significantly greater for the fast (FF + FI + FR) versus the slow (S) motor units. The AHP half-decay time was correlated with muscle unit twitch time over the entire motoneuron population and within the type S motor-unit population. There was no significant correlation between twitch time and AHP half-decay time among the types FF and FR motor-unit populations. In agreement with previous studies, we found a significant difference in both rheobase and input resistance among the major motor-unit types, with rheobase increasing in the order S less than FR less than FF and input resistance decreasing in that order (S greater than FR greater than FF). The differences in input resistance were present both before and after correcting for voltage-response nonlinearities (32). Also in agreement with previous studies, the mean axonal conduction velocity was significantly faster among the fast (FF + FI + and FR) compared with the slow (S) motor units. These data were used to examine the properties alone to determine motor-unit type, which has traditionally been defined on the basis of the muscle unit's mechanical properties (11, 22). We used a discriminant analysis program to classify 73 mechanically typed motor units for which we had measures of rheobase, input resistance, membrane time constant, and AHP half-decay time. This model was able to properly classify 71 of the 73 motor units of this data set, indicating that the motor units of this data set could be grouped into three categories representing the three major motor-unit types (FF, FR, and S) on the basis of their rheobase, input resistance, membrane time constant, and AHP half-decay time.(ABSTRACT TRUNCATED AT 400 WORDS)