Impulse coding of ramp currents intracellularly injected into pyramidal tract neurones

Recovery of neuronal and network excitability after spinal cord injury and implications for spasticity

The state of areflexia and muscle weakness that immediately follows a spinal cord injury (SCI) is gradually replaced by the recovery of neuronal and network excitability, leading to both improvements in residual motor function and the development of spasticity. In this review we summarize recent animal and human studies that describe how motoneurons and their activation by sensory pathways become hyperexcitable to compensate for the reduction of functional activation of the spinal cord and the eventual impact on the muscle. Specifically, decreases in the inhibitory control of sensory transmission and increases in intrinsic motoneuron excitability are described. We present the idea that replacing lost patterned activation of the spinal cord by activating synaptic inputs via assisted movements, pharmacology or electrical stimulation may help to recover lost spinal inhibition. This may lead to a reduction of uncontrolled activation of the spinal cord and thus, improve its controlled activation by synaptic inputs to ultimately normalize circuit function. Increasing the excitation of the spinal cord with spared descending and/or peripheral inputs by facilitating movement, instead of suppressing it pharmacologically, may provide the best avenue to improve residual motor function and manage spasticity after SCI.

Human stretch reflex pathways reexamined

Reflex responses of tibialis anterior motor units to stretch stimuli were investigated in human subjects. Three types of stretch stimuli were applied (tap-like, ramp-and-hold, and half-sine stretch). Stimulus-induced responses in single motor units were analyzed using the classical technique, which involved building average surface electromyogram (SEMG) and peristimulus time histograms (PSTH) from the discharge times of motor units and peristimulus frequencygrams (PSF) from the instantaneous discharge rates of single motor units. With the use of SEMG and PSTH, the tap-like stretch stimulus induced five separate reflex responses, on average. With the same single motor unit data, the PSF technique indicated that the tap stimulus induced only three reflex responses. Similar to the finding using the tap-like stretch stimuli, ramp-and-hold stimuli induced several peaks and troughs in the SEMG and PSTH. The PSF analyses displayed genuine increases in discharge rates underlying the peaks but not underlying the troughs. Half-sine stretch stimuli induced a long-lasting excitation followed by a long-lasting silent period in SEMG and PSTH. The increase in the discharge rate, however, lasted for the entire duration of the stimulus and continued during the silent period. The results are discussed in the light of the fact that the discharge rate of a motoneuron has a strong positive linear association with the effective synaptic current it receives and hence represents changes in the membrane potential more directly and accurately than the other indirect measures. This study suggests that the neuronal pathway of the human stretch reflex does not include inhibitory pathways.

Doublet of action potentials evoked by intracellular injection of rectangular depolarization current into rat motoneurones

A doublet of action potentials is frequently observed at the beginning of motoneuronal discharge patterns and its appearance leads to a considerable increase in the motor unit force. The aims of this study were (1) to determine the relationship between the intensity of rectangular depolarization currents injected into motoneurones and their ability to generate doublets and (2) to evaluate the influence of the initial doublets on changes in motoneuronal firing frequency. Experiments were performed on anesthetized rats, and recordings were taken from motoneurones located in the L4-L5 segments of the spinal cord. The depolarization current necessary to evoke the initial doublet of action potentials was measured and expressed in multiples of the rheobase. A gradual increase in the intensity of current injected into motoneurones resulted in initial doublets in 80% of the cases studied, at doublet threshold ranges between 1.25 and 4.0 times the rheobase. This suggests that doublets are an effect of strong synaptic excitation of motoneurones that may precede a sudden change in force during a movement. Moreover, in the great majority of the studied motoneurones, this initial doublet caused changes in the subsequent firing rate by the prolongation of the first interspike interval.

Effects of large excitatory and inhibitory inputs on motoneuron discharge rate and probability

We elicited repetitive discharge in hypoglossal motoneurons recorded in slices of rat brain stem using a combination of a suprathreshold injected current step with superimposed noise to mimic the synaptic drive likely to occur during physiological activation. The effects of repetitive en mass stimulation of afferent nerves were simulated by the further addition of trains of injected current transients of varying shapes and sizes. The effects of a given current transient on motoneuron discharge timing and discharge rate were measured by calculating a peristimulus time histogram (PSTH) and a peristimulus frequencygram (PSF). The amplitude and time course of the simulated postsynaptic potentials (PSPs) produced by the current transients were calculated by convolving the current transient with an estimate of the passive impulse response of the motoneuron. We then compared the shape of the injected current transient and the simulated PSP to the profiles of the PSTH and the PSF records. The PSTHs produced by excitatory PSPs (EPSPs) were characterized by a large, short-latency increase in firing probability that lasted slightly longer than the rising phase of the EPSP, followed by a reduced discharge probability during the falling phase of the EPSP. In contrast, the PSF analysis revealed a proportionate increase in discharge rate over the entire profile of the EPSP, even though relatively few spikes occurred during the falling phase. The PSTHs associated with inhibitory PSPs (IPSPs) indicated a reduction in discharge probability during the initial, hyperpolarizing phase of the IPSP, followed by an increase in the discharge probability during its subsequent repolarizing phase. Using the PSF analysis, the initial phase of the IPSP appeared as a large hole in the record where a very small number or no discharges occurred. The subsequent phase of the IPSP was associated with frequency values that were lower than the background values. The primary features of both PSTHs and PSFs can be used to estimate the relative amplitudes of the underlying EPSPs and IPSPs. However, PSTHs contain secondary peaks and troughs that are not directly related to the underlying PSP but instead reflect the regular recurrence of spikes following those affected by the PSP. The PSF analysis is more useful for indicating the total duration and the profile of the underlying PSP. The shape of the underlying PSP can be obtained directly from the PSF records because the discharge frequency of the spikes follow the PSPs very closely, especially for EPSPs.

The dynamic response of cat alpha-motoneurones investigated by intracellular injection of sinusoidal currents

Sine-wave currents intracellularly injected into spinal alpha-motoneurones were found to modulate sinusoidally the regular rhythmic firing (carrier frequency) evoked by a current step. Cycle histograms of the instantaneous frequency could be accurately fitted by sinusoidal functions. Those functions were treated as the cell output. For a given modulation frequency between 2 and 14-18 Hz, the amplitude of the cell output was linearly related to the amplitude of the sine-wave current, all over a wide range of current intensities. The sensitivity (gain) and the phase relationships were estimated by varying the modulation frequency of a given sine-wave. When modulation frequency varied from 1-2 Hz to 14-18 Hz, there was a progressive increase of the gain and a phase advance. The experimental gain curve closely conformed to the response of an ideal linear transducer sensitive to both the intensity and the velocity of the input. The phase advance was instead less than that predicted by the model. No "carrier dependent" variations of gain and phase were detected. Differences among motoneurones regarded both the static gain and the "corner frequency" (a measure of the dynamic sensitivity). In 10 motoneurones, the corner frequency ranged between 5 and 10 Hz.

Properties of the spike afterhyperpolarization in pyramidal tract neurons

Features of the spike afterhyperpolarization (AHP) recorded intracellularly have been analyzed in fast pyramidal tract neurons of cats. Cell input conductance increases during the AHP, possibly because of a change in potassium conductance, as suggested by an AHP equilibrium potential 10--15 mV negative to the resting membrane potential. When more spikes are evoked in succession, AHPs following the first one are strongly reduced in amplitude. The effect is virtually maximal (30--50% of the control) after a single spike and fades out by 200-400 ms after the last spike. At short interspike intervals the initial time course of the depression is hidden by summation occurring between consecutive AHPs.