Volker Zschorlich

High-frequency magnetic stimulation induces long-term potentiation in rat hippocampal slices

Recent reports indicate that the exposure of brain tissues to transcranial magnetic stimulation induces persistent changes in neuronal activity and influences hippocampal synaptic plasticity. However, the modulation of synaptic efficiency by magnetic stimulation in vitro is still unclear. In the present study, we investigated whether high-frequency magnetic stimulation (HFMS) can induce long-term potentiation (LTP) in rat hippocampal slices in vitro. During baseline recording and after HFMS, field excitatory postsynaptic potentials (fEPSPs) were recorded within the CA1 stratum radiatum in response to electrical stimulation of the Schaffer collateral inputs. For LTP induction, HFMS was delivered through a circular coil positioned closely above the slices using two different paradigms (A: 10 trains of 20 pulses at 100 Hz with 1s intervals, 5 repetitions with 10s intervals; B: 3 trains of 100 pulses at 100 Hz with 20s intervals). The intensity of the magnetic stimulus was adjusted to 60-75 A/micros. After application of HFMS, electrically evoked CA1 fEPSPs were enhanced showing significant levels of LTP by both paradigms (A: 142+/-9% of baseline, n=6; B: 129+/-7%, n=8). Furthermore, HFMS-induced LTP induced by paradigm A was prevented by the presence of the selective N-methyl-D-aspartate receptor (NMDAR) blocker D-AP5 (50 microM) in the bath solution (95+/-6% of the baseline, n=6; p<0.01 compared to control condition without D-AP5). Further, the lack of changes in paired-pulse ratio and the afferent fiber volleys exclude presynaptic involvement in HFMS-induced LTP. In summary, we have demonstrated that HFMS can induce NMDAR-dependent LTP in the CA1 region in vitro.

Investigation of first ray mobility during gait by kinematic fluoroscopic imaging-a novel method

BackgroundIt is often suggested that sagittal instability at the first tarso-metatarsal joint level is a primary factor for hallux valgus and that sagittal instability increases with the progression of the deformity. The assessment of the degree of vertical instability is usually made by clinical evaluation while any measurements mostly refer to a static assessment of medial ray mobility (i.e. the plantar/dorsal flexion in the sagittal plane). Testing methods currently available cannot attribute the degree of mobility to the corresponding anatomical joints making up the medial column of the foot. The aim of this study was to develop a technique which allows for a quantification of the in-vivo sagittal mobility of the joints of the medial foot column during the roll-over process under full weight bearing.MethodsMobility of first ray bones was investigated by dynamic distortion-free fluoroscopy (25 frames/s) of 14 healthy volunteers and 8 patients with manifested clinical instability of the first ray. A CAD-based evaluation method allowed the determination of mobility and relative displacements and rotations of the first ray bones within the sagittal plane during the stance phase of gait.ResultsTotal flexion of the first ray was found to be 13.63 (SD 6.14) mm with the healthy volunteers and 13.06 (SD 8.01) mm with the patients (resolution: 0.245 mm/pixel). The dorsiflexion angle was 5.27 (SD 2.34) degrees in the healthy volunteers and increased to 5.56 (SD 3.37) degrees in the patients. Maximum rotations were found at the naviculo-cuneiform joints and least at the first tarso-metatarsal joint level in both groups.ConclusionsDynamic fluoroscopic assessment has been shown to be a valuable tool for characterisation of the kinematics of the joints of the medial foot column during gait.A significant difference in first ray flexion and angular rotation between the patients and healthy volunteers however could not be found.

Effects of muscular activation patterns on the ankle joint stabilization: An investigation under different Degrees of Freedom

Altered biomechanical demands of a stabilization task lead to specific changes in coordination patterns among the involved muscles. The objective of this work was to investigate the effects of increased Degrees of Freedom (DoF) of an external object on the stabilization process of the ankle joint in a voluntary force production task. Four muscles (vastus medialis, VM; tibialis anterior, TA; peroneus longus, PL; gastrocnemius medialis, GM) were recorded using surface electromyography and synchronized to dynamometric data. The subjects task was to exert force against the external object by performing a knee extension under 0, 1 or 3 DoF. Forces were measured using three dimensional force transducers and temporal coordination was assessed using the cross-correlation function (CCF). While the force decreased with increasing DoF the muscles showed a selective gain scaling in order to stabilize the ankle joint. Muscles fulfilling mainly stabilizing functions (TA and PL) tended to increase their activities, while the muscles with motor functions either decreased (GM) or increased (VM). The CCF revealed different intermuscular coordination strategies depending on the environmental condition, showing an advanced phasing in the ankle stabilizing PL in unstable environmental conditions (3 DoF). Nevertheless, the overall sequence of muscle activation was preserved. It is concluded that the process of joint stabilization is controlled in dependency of the status of the external system. The associated neuromuscular system adjustments underline the role of movement coordination in the stabilization process.

Age-related differences in corticomotor facilitation indicate dedifferentiation in motor planning

Efficient motor control requires motor planning. Age-related changes in motor control are well described, e.g. increased movement variability and greater antagonistic muscle co-activation, as well as less functional and less regional specific brain activation. However, less is known about age-related changes in motor planning. By use of transcranial magnetic stimulation we investigated differences in corticomotor facilitation during motor planning in 17 young (25±3years) and 17 older healthy adults (70±13years) in a delayed movement paradigm for wrist movements. Motor evoked potentials (MEPs) were recorded for the flexor and extensor carpi radialis during movement preparation of wrist flexion and extension as well as during rest. We found that MEPs were less specifically facilitated during planning in older as compared to younger adults, as indicated by an Age×Condition×Muscle interaction. Young participants showed significantly facilitated MEPs in the respective muscle needed for wrist flexion or extension. By contrast MEPs in older participants were less specifically modulated. We conclude that age relates to dedifferentiated activation of the primary motor cortex already during preparation of distinct movements which might contribute to less efficient motor control in older adults.

How Thoughts Give Rise to Action - Conscious Motor Intention Increases the Excitability of Target-Specific Motor Circuits

The present study shows evidence for conscious motor intention in motor preparation prior to movement execution. We demonstrate that conscious motor intention of directed movement, combined with minimally supra-threshold transcranial magnetic stimulation (TMS) of the motor cortex, determines the direction and the force of resulting movements, whilst a lack of intention results in weak and omni-directed muscle activation. We investigated changes of consciously intended goal directed movements by analyzing amplitudes of motor-evoked potentials of the forearm muscle, flexor carpi radialis (FCR), and extensor carpi radialis (ECR), induced by transcranial magnetic stimulation over the right motor cortex and their motor outcome. Right-handed subjects were asked to develop a strong intention to move their left wrist (flexion or extension), without any overt motor output at the wrist, prior to brain stimulation. Our analyses of hand acceleration and electromyography showed that during the strong motor intention of wrist flexion movement, it evoked motor potential responses that were significantly larger in the FCR muscle than in the ECR, whilst the opposite was true for an extension movement. The acceleration data on flexion/extension corresponded to this finding. Under no-intention conditions again, which served as a reference for motor evoked potentials, brain stimulation resulted in undirected and minimally simultaneous extension/flexion innervation and virtually no movement. These results indicate that conscious intentions govern motor function, which in turn shows that a neuronal activation representing an “intention network” in the human brain pre-exists, and that it functionally represents target specific motor circuits. Until today, it was unclear whether conscious motor intention exists prior to movement, or whether the brain constructs such an intention after movement initiation. Our study gives evidence that motor intentions become aware before any motor execution.

The effect of increasing external degrees of freedom on force production and neuromuscular stabilisation

Abstract Evidence suggests that during interaction with different environmental dynamics the necessity to stabilise the involved joints leads to altered efficiency in force transmission to the surroundings and a specific orchestration of motor control strategies. However, little is known about the modalities of the changes associated with altered environmental dynamics. In 29 healthy participants, electromyographic (EMG) signals from four muscles of the right leg (M. peroneus longus, M. tibialis anterior, M. vastus medialis, M. gastrocnemius medialis) and three dimensions of force (Fx, Fy, Fz) were recorded. The participants were to exert force against an external object by performing a unilateral leg extension task with the task being influenced by either 0, 1 or 3 mechanical degrees of freedom. We hypothesised that the ankle stabilising muscles would increase their activities with increasing degrees of freedom (DoF), and that increasing external degrees of freedom results in decreased muscle force exerted during the movement task. The progressive change in the type of mechanical interaction from stable to unstable caused a loss of the ability to apply force in movement direction (Fz) which was accompanied by a reduction of Fy and Fx force dimensions. These reductions corresponded to maximum losses of 23% for Fz, 33% for Fx and 41% for Fy in the three degrees of freedom condition (all P < 0.001). Next, the individual muscles showed specific tuning effects, depending on the type of mechanical interaction. Our results suggest that the loss of the ability to exert force effectively against the external object is due to the neuromuscular stabilisation process of the involved joints. The change of the degrees of freedom conditions allowed for assessment of movement- or stabilisation-related adjustments of the motor system.

The effect of age on coordination of stabilization during changing environmental dynamics.

Coordination as part of the stabilization process of joints is compromised in older adults. We addressed changes in neuromuscular control and force output during a ballistic force production task influenced by different environmental dynamics. Aged participants (AP) and young participants (YP) were asked to perform a unilateral maximal leg extension against a movable sled in a reaction-time task. The task was performed in a sitting posture and involved a stable (1 degree of freedom; DoF) or an unstable (3 DoF) condition of the sled. Electromyographic and dynamometric recordings were made and analyzed using the cross-correlation-function, assessment of peak EMG-activity and peak force. Initial motor strategies (i.e. motor system adjustments in order to meet the demands of the particular task while respecting individual constraints) were assessed by analyzing total reaction times (TRT), premotor time (PMT) and electromechanical delay (EMD). The AP group showed motor control strategies governed by prolonged TRT in both conditions. However, the change of mechanical interactions (i.e. the interaction between the participant and the sled in its particular mechanical state) caused group specific motor system adjustments in PMT and EMD. Force measures showed reduced peak forces in AP accompanied by less loss of force between conditions compared to YP. Inter- and intramuscular coordination strategies differed between YP and AP reflected in changes in CCF and peak EMG values. We conclude that change in environmental dynamics is associated with specific adjustments of control properties of the motor system. These adjustments were sensitive to age and mechanical condition (1 or 3 DoF) and might contribute to declines in motor output seen in AP. However, due to the nature of the task, our results do not allow a direct transfer to situations involving whole body balance.

NMDA Receptor-Dependent Metaplasticity by High-Frequency Magnetic Stimulation

High-frequency magnetic stimulation (HFMS) can elicit N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) at Schaffer collateral-CA1 pyramidal cell synapses. Here, we investigated the priming effect of HFMS on the subsequent magnitude of electrically induced LTP in the CA1 region of rat hippocampal slices using field excitatory postsynaptic potential (fEPSP) recordings. In control slices, electrical high-frequency conditioning stimulation (CS) could reliably induce LTP. In contrast, the same CS protocol resulted in long-term depression when HFMS was delivered to the slice 30 min prior to the electrical stimulation. HFMS-priming was diminished when applied in the presence of the metabotropic glutamate receptor antagonists (RS)-α-methylserine-O-phosphate (MSOP) and (RS)-α-methyl-4-carboxyphenylglycine (MCPG). Moreover, when HFMS was delivered in the presence of the NMDA receptor-antagonist D-2-amino-5-phosphonovalerate (50 µM), CS-induced electrical LTP was again as high as under control conditions in slices without priming. These results demonstrate that HFMS significantly reduced the propensity of subsequent electrical LTP and show that both metabotropic glutamate and NMDA receptor activation were involved in this form of HFMS-induced metaplasticity.

Relationship between muscle volume and contractile properties of the human knee extensors.

The present study was designed to investigate the relationship between volume and electrically evoked twitch properties of the quadriceps muscle. Supramaximal single and doublet stimulation of the femoral nerve was used to assess contractile properties at 45° and 80° knee flexion. Muscle volume was measured using a 1.5-Tesla magnetic resonance imaging scanner. Quadriceps muscle volume was only significantly correlated (r = 0.629) with peak twitch torque induced by doublet stimulation at 80° but not at 45° knee flexion.

Mental Fatigue Increases Gait Variability During Dual-task Walking in Old Adults

Background Mental fatigue is a psychobiological state induced by sustained periods of demanding cognitive activity and is characterized by feelings of tiredness which are common in everyday life. Recently, it has been hypothesized that mental fatigue might have an impact on gait performance in old adults. Therefore, the effect of mental fatigue on gait performance under single- and dual-task conditions was investigated in young and old participants. Methods Spatio-temporal gait parameters of 16 young and 16 old healthy participants were measured using a photoelectric system during single- and dual-task walking before and after a randomly assigned mental fatigue (performing a stop-signal task for 90 minutes) and control intervention (watching a video for 90 minutes), respectively. Changes in subjective fatigue, wakefulness, mood, arousal, and psychophysiological workload (heart rate variability indices) were assessed. Results Psychometric measures indicated increased subjective fatigue and arousal as well as decreased mood and wakefulness after the mental fatigue task. Heart rate variability indices revealed a higher psychophysiological workload during the mental fatigue intervention in old compared to young participants. Gait measures (coefficient of variation of speed, stride length, and stance time) revealed impaired dual-task walking performance following the mental fatigue intervention only in old participants. Conclusion Data indicate that mental fatigue, induced by sustained cognitive activity, can impair gait performance during dual-task walking in old adults. The susceptibility to mental fatigue could be a new intrinsic risk factor for falls in older people and should be taken into account when dual-task gait analyses are performed.