Research
Fatigue
During prolonged activation a decline in force generating capacity can be observed (fatigue). The underlying cause can lie anywhere in the complex chain of events necessary to produce force, including e.g. changes in the behavioral 'will', changes in the input/output relations of the motoneurons and a decline in actine-myosine interactions at the level of the peripheral muscle. During a sustained sub-maximal voluntary contraction, a constant muscle force can be obtained at the cost of an increase in central drive to the motoneurons. However, little is known about the interaction between underlying central mechanisms driving motor behavior and cognitive processes. Together with Hiske van Duinen and Monique Lorist experiments were performed to study the nature of this behavior in more detail.
In addition, data obtained in patients suffering from chronic fatigue syndrome is analyzed and summarized by Reina Timmer.
Contralateral activation
During high effort force productions activity is not restricted to the target muscles but also other muscles become active. The general existence of this associated activity has long been known and the phenomena are of considerable theoretical and practical interest. The contralateral activity that is seen at high levels of activation may reveal inborn patterns of motor organization that are successfully suppressed under normal motor behavior. Our experiments (together with Ilse Broeders, Anneke Steens and Daniel Kernell) have shown a decrease in the amount of associated activity in fatigued muscles. These fatigue effects could not be explained by changes in muscle properties, but electrical activation of contralateral muscles did enhance the amount of associated activity. These results show that fatigue has a depressing effect on the excitability of pathways used for associated activity.
During bilateral contractions of two homologeous muscle (groups) less force is produced per muscle than during a unilateral contraction. This phenomenon is called 'bilateral deficit'. A bilateral deficit is found for different muscle (groups), it thought that a decline of the neuronal dive during the bilateral contaction is the underlying cause of this deficit. In collaboration with Marijn Post, Hiske van Duinen, Anneke Steens and Bart Kuiper we have shown that the cortical activity in the primary motor cortex is lower during a bilateral contraction than during a unilateral contraction. This observation provide important evidence for the acceptance of the neuronal hypothesis for the bilateral deficit.
Imagery and observation training
Motor imagery is an active process during which the representation of a motor act is internally reproduced within the working memory without any motor output. Imaging studies have provided evidence that during motor imagery similar anatomical areas become activated as during motor performance.
With the use of transcranial magnetic stimulation we have shown (together with Meyke Roosink) that during movement observation also motor areas become activated. Moreover, the activity was larger during observation of a finger sequence than during imagination of the same finger sequence. The data suggest that motor imagery as well as motor observation could be an interesting potential too for rehabilitation.
Motor unit activity in spinal cord injured subjects
The activity of muscle fibres is the resultant of activation of their motoneuron by the central nervous system. Force gradation during voluntary contractions occurs by recruitment and rate gradation of motor units. Reduced number of motoneurons often controls muscles that are left under some voluntary control after spinal cord injury, both because the descending drive has been compromised and because some motoneurons die. These changes may alter both the strength and the distribution of the synaptic input within the motoneuron pool. Hence, it is quite possible that the manner in which motoenurons are managed during voluntary contractions has changed after spinal cord injury. Together with Christine Thomas (The Miami Project to Cure Paralysis, Miami, USA) we have documented the relative contributions of motor unit recruitment and rate modulation during voluntary contractions of thenar muscles that have been paralysed in part by chronic spinal cord injury.
We have also shown that after spinal cord injury subjects were unable to stop activity in motor units that were activated voluntarily (Figure 1). This self-sustained activity might result from the activation of persistent inward currents in motoneurons. These inward currents are the resultant of the persistent opening of ion channels in the motoneurons, as is found in rat and turtle spinal cord preparations. Since persistent inward currents are activated below the spike initiating-threshold, any consequent depolarisation may bring a motoneuron nearer to or above the threshold for spike-initiation. In control-subjects, persistent inward currents in motoneurons may be switched off after a voluntary contraction by voluntary inhibition. After spinal cord injury, both the amounts of supraspinal excitation and inhibition are probably reduced. Lack of inhibition could therefore result in prolonged inward currents that keep some motoneurons above their threshold for spike initiation, thereby resulting in ongoing motor unit activity. Thus, the prolonged motor unit firing in spinal cord injured subjects may not be an expression of deviant firing behavior in these motoneurons.
The existence of the persistent inward currents could potentially be very important after spinal cord injury. For example, voluntary tasks that involve low absolute forces in control subjects require relatively strong forces in muscles of spinal cord injured subjects because of the reduction in maximal voluntary force generating capacity. This makes the muscles of spinal cord injured subjects more vulnerable to central fatigue. Thus, persistent inward current may be beneficial in keeping motoneurons active despite any decline in central drive. Therefore, it is important to evaluate the influences that various medications have on these currents and thus on muscle force.
Grants
- NWO-cognition (2003-2007)
- Junior Scientific Masterclass (to Reina Timmer) and BCN (2003-2004)
- Junior Scientific Masterclass (to Anniek Visser) and BCN (2005).
Future plans
Fatigue
The grant of NWO-cognition enables us to accomplish future plans involving the investigation of the nature of the interaction between cognitive and motor tasks and the involved anatomical structures, combining experimental setups using EMG, EEG, and fMRI- techniques (together with Hiske van Duinen, Koen Vaartjes, Hans Hoogduin en natasha Marits). Together with Marijn Post en Gretha Boersma central fatigue will be studied with twitch interpolation in combination with fMRI.
Contralateral activation
The nature of the contralateral activation will be studied in more detail. In collaboration with J. Butler, J. Taylor and S. Gandevia, we will focus attention on differentiating between contralateral effects on spinal and supraspinal levels. Motor unit analysis during contralateral contractions will performed in collaboration with Marijn Post.
Motor unit activity in spinal cord injured subjects
The underlying mechanism of the spontaneous activity of motor unit in spinal cord injured subjects will be further analysed. Furthermore, the importance of this activity for voluntary force gradation will be investigated.
Collaborations
- CK Thomas, The Miami Project to Cure Paralysis, Departments of Neurological Surgery, Physiology and Biophysics, Miami, USA
- JA Butler, J. Taylor and SC Gandevia, Prince of Wales Medical Research Institute, Randwick, Australia
- JM Kuks, Department of Neurology, UMCG, Groningen
- H. Hoogduin, BCN-Neuroimaging Center, UMCG, Groningen
- N. Maurits, Department of Neurology, UMCG, Groningen
- D. Stegeman, Radboud Medical Center, Nijmegen
Teaching
- Lecturer: Force gradation (medical students 1.1; Human movement sciences)
- Kinesiology (medical students 1.1)
- Motor system (Human movements science; tandheelkunde 1.1)
- Physiological basis of fMRI (BCN)
- Organiser: Bouwen aan Gezondheid (Blok 1.3-G2010)
- Practical course: EMG (Human movements sciences)
Supervisor undergraduate thesis:
* Gretha Boersma (Biology; 2005)
* Kristel Brown (Human movement sciences, Maastricht 2005)
* Jasper Dijkstra (Biology; 2005)
* Laurens Hoogeweg (Medical student; 2005)
* Bart Kuiper (Medical student; 2004)
* Christian Nusselder (Human movement sciences; 2005)
* Meyke Roosink (Biology; 2004)
* Anneke Steens (Biology; 2004)
* Willemijn van der Veen (Medical student; 2005)
