Kristian Hennings

The effects of training time, sensory loss and pain on human motor learning

This study determined, in humans, the effects of (i) the number of within-session task repetitions (72 or 144 over a period of 15 or 30 min, respectively) on the time course of motor learning in a long-term (seven consecutive daily motor-training sessions and a 1-week post-follow-up) novel tongue-task training regime and (ii) somatosensory manipulations (capsaicin-induced intra-oral pain or lidocaine-induced sensory loss of the tongue tip) on motor learning in a short-term (single motor-training session consisting of 72 within-session task repetitions over a period of 15 min) novel tongue-task training regime. Novel tongue-task training consisted of tracking a moving target box by generating a pre-set amount of tongue-protrusion force onto a force lever. Analysis of motor behaviour revealed (i) a higher within-session gain for the 30-min tongue-task training regime, but this difference did not differentially affect the time course of the overall motor performance or additional motor performance variables between the 15- and 30-min tongue-task training regimes in subsequent training sessions. (ii) somatosensory manipulations of the tongue tip reduced the gains in overall motor performance, and this reduced motor performance was mainly characterized by exaggerated undershoot errors and delayed reaction times for the lidocaine tongue-task training regime and exaggerated overshoot and undershoot errors as well as delayed reaction times for the capsaicin tongue-task training regime. It is concluded that extended within-session task repetitions do not facilitate additional long-term gains in overall motor performance and intra-oral sensory loss or pain hinders motor learning.

Experimental validation of the nerve conduction velocity selective recording technique using a multi-contact cuff electrode

The earthworm (Lumbricus terrestris) is presented as an in vitro model of a peripheral nerve containing only two fibers each with distinctly different conduction velocities, the median and lateral giant fibers (MGF and LGF). The worm model is used with a multi-contact cuff electrode to validate the spatial-temporal filtering effect of different electrode contact configurations and the effect of applying a delay adder and matched filter tuned to either the MGF or LGF action potential (AP) to extract conduction direction and velocity from the recording. The results confirmed the known effect of inter-electrode spacing and bipolar and tripolar recording configuration on the AP amplitude. It also demonstrates a crossover point where the amplitude of the tripolar recording is larger than the monopolar recording, an effect of the slower action potential conduction velocities in the worm. The delay adder was found to be an effective velocity sensitive filter, able to discriminate units based on conduction velocity. The matched filter to be an effective means to eliminate artifact and increase signal to noise ratios, however was not found to improve selectivity.

Acute Performance of the Thin-Film Longitudinal Intra-Fascicular Electrode

A thin-film microfabricated multisite longitudinal intra-fascicular electrode (LIFE) array was realized as a patterned thin film on polyimide structure. These electrodes were tested in the acute rabbit animal preparation to determine their recording characteristics for comparison to standard metal wire LIFEs and the cuff electrode. It was found that the tfLIFE provided nearly as high signal amplitudes as the metal electrodes, with greater recording selectivity, making them a promising peripheral nerve interface for advanced FNS and neuroprosthetic applications

Selective activation of small-diameter motor fibres using exponentially rising waveforms: a theoretical study

The present study investigated the possibility of using exponentially rising waveforms for selectively activating small motor fibres in a nerve bundle enclosed by a cuff electrode. Exponentially rising waveforms were studied using models of motor fibres and a volume conductor model. With an exponentially rising waveform (duration: 2 ms, time constant: 1 ms) large (15.5 μm) and small (8 μm) nerve fibres located at the edge of the nerve bundle had a current threshold of 125 μA and 53 μA, respectively. These reversals in the recruitment order of large and small nerve fibres located at the edge of the nerve bundle were observed for exponentially rising waveforms of 2, 4, and 6 ms in duration with time constants of 0.9, 0.6 and 0.6 ms, respectively. Reversals of the same nerve fibres located at the centre of the nerve bundle were observed for exponentially rising waveforms of 4 and 6 ms in duration, with a time constant of 0.6 ms for both waveforms. The underlying mechanism for selective activation of small nerve fibres with exponentially rising waveforms was found to be a combination of a decrease in the size of the local excitations in the centre node due to sodium channel inactivation and blocking of action potentials in large nerve fibres due to their larger difference in the membrane potential of adjacent nodes. The exponentially rising waveforms were compared with both rectangular prepulses and ramp prepulses. The rectangular prepulses were found to be unable selectively to activate small nerve fibres with the volume conductor model and criteria used in the present study, whereas the ramp prepulses performed as well as the exponentially rising waveforms. In conclusion, a novel stimulation paradigm has been proposed that may provide smooth, gradual control of muscle force with minimum fatigue.

Orderly activation of human motor neurons using electrical ramp prepulses.

OBJECTIVE Conventional electrical stimulation (rectangular pulses) recruits large before small diameter motor neurons and motor neurons close to the electrode before more distant motor neurons. The present study investigated the possibility for changing the recruitment order of electrical stimuli with sub-threshold ramp prepulses. METHODS The median nerve was stimulated using surface electrodes at the wrist and elbow. Compound motor action potentials were recorded from abductor pollicis brevis and flexor carpi radialis. Stimulus-response curves, nerve conduction velocity and excitation thresholds of abductor pollicis brevis and flexor carpi radials, with and without ramp prepulses, were recorded in order to study the effect of ramp prepulses on axonal excitability. RESULTS The conduction velocity of the initial response (10% of the maximal response) was decreased by 4.3+/-0.83m/s with ramp prepulses (500ms, 80% of the excitation threshold). The ramp prepulses also had a differential effect on the excitation thresholds of abductor pollicis brevis and flexor carpi radialis. In addition, ramp prepulses increased the threshold of 10% of the maximal response more than the threshold of 90% of the maximal response. CONCLUSIONS These results demonstrate that large diameter and motor neurons close to the electrode accommodate more to ramp prepulses than small diameter and distant motor neurons, which suggests that ramp prepulses may be used to change the recruitment order of rectangular pulses. SIGNIFICANCE This technique of ramp prepulses allows stimulation of alternate subsets of motor nerves.

Velocity recovery function of the compound muscle action potential assessed with doublet and triplet stimulation

Normative values of muscle fiber conduction velocity depend on the conditions in which conduction velocity is measured due to the velocity recovery function (VRF) of muscle fibers. In this study the VRF of the compound muscle action potential (CMAP) was assessed following doublet and triplet stimulation in order to investigate the effect of repetitive muscle activation on muscle fiber conduction velocity. The VRF from doublet and triplet activation showed a peak of 4.6%–15.0% and 6.4%–25.9%, respectively, which is not significantly different. The VRF of the CMAP with doublet stimulation had a plateau between 25–75 ms, similar to that reported for single muscle fibers, and changed as a consequence of previous activation. The VRFs with doublet and triplet stimulation were different for interstimulus intervals in the range of 12–250 ms, where the triplet resulted in a plateau of supernormal conduction velocity. The VRF of the triplet could be explained by linear summation of the effects from doublet stimulations only for small distances between the two conditioning stimuli. These results provide new information on the adaptation of membrane properties of muscle fibers to repetitive activation. Changes in CMAP properties due to repeated activation may influence the accuracy of techniques based on CMAP recordings, such as collision methods. Muscle Nerve, 2007

The recruitment order of electrically activated motor neurons investigated with a novel collision technique

OBJECTIVE The development of a novel collision technique for assessment of the activation order of electrically activated nerve fibers, which is an important question in functional electrical therapy or for interpretation of results of motor unit number estimates. METHODS Compound muscle action potentials were recorded with the belly-tendon configuration from the abductor digiti minimi. A novel modified Hopfs collision technique was applied on ten healthy male subjects to determine the distributions of conduction velocities (DCV) of all ulnar nerve fibers and of the fibers activated by electrical stimuli eliciting 20%, 50%, and 80% of the maximal muscle response. RESULTS The maximum nerve conduction velocity was (means+/-SE) 64.1+/-0.85m/s. The median conduction velocity of estimated DCV was 58.9+/-0.97m/s (stimulus at 20%), 58.0+/-0.98m/s (50%), 57.2+/-0.91m/s (80%), and 56.5+/-0.84m/s (whole nerve) (all different between each other, P<0.001). CONCLUSIONS The proposed collision technique allows the assessment of nerve conduction velocity distributions at maximal and sub-maximal stimulation levels and provided evidence for an inverse activation order of nerve fibers with electrical stimulation. SIGNIFICANCE The excessive fatigue seen with nerve electrical stimulation can be explained by a preferential activation of large diameter nerve fibers. The motor units first activated with electrical stimulation are likely not representative of the motor unit pool in the muscle, which poses limitations in the reliability of some of the proposed methods for motor unit counting.

Nerve Fiber Activation During Peripheral Nerve Field Stimulation: Importance of Electrode Orientation and Estimation of Area of Paresthesia.

Low back pain is one of the indications for using peripheral nerve field stimulation (PNFS). However, the effect of PNFS varies between patients; several stimulation parameters have not been investigated in depth, such as orientation of the nerve fiber in relation to the electrode. While placing the electrode parallel to the nerve fiber may give lower activation thresholds, anodal blocking may occur when the propagating action potential passes an anode.

Breakdown of accommodation in nerve: a possible role for persistent sodium current

BackgroundAccommodation and breakdown of accommodation are important elements of information processing in nerve fibers, as they determine how nerve fibers react to natural slowly changing stimuli or electrical stimulation. The aim of the present study was to elucidate the biophysical mechanism of breakdown of accommodation, which at present is unknown.ResultsA model of a space-clamped motor nerve fiber was developed. It was found that this new model could reproduce breakdown of accommodation when it included a low-threshold, rapidly activating, persistent sodium current. However, the phenomenon was not reproduced when the persistent sodium current did not have fast activation kinetics or a low activation threshold.ConclusionThe present modeling study suggests that persistent, low-threshold, rapidly activating sodium currents have a key role in breakdown of accommodation, and that breakdown of accommodation can be used as a tool for studying persistent sodium current under normal and pathological conditions.

Differences in perception and brain activation following stimulation by large versus small area cutaneous surface electrodes: Differences in perception and brain activation

Application of electrical stimulation through conventional surface electrodes activates both non‐nociceptive and nociceptive fibres. To encompass this problem, electrical stimulation through small area pin electrode was introduced where subjective description of stimulation quality indicated preferential activation of nociceptors. The present study aimed to show that brain areas involved in nociceptive processing are activated by stimulation through cutaneous pin electrode (CPE) to a larger extent than conventional surface electrodes.