Peter Jacob Slot

Simulations of the alpha motoneuron pool electromyogram reflex at different preactivation levels in man

The alpha motoneuron pool and the surface electromyogram (EMG) of the human soleus muscle are modelled, respectively, by an alpha motoneuron pool model generating the firing patterns in the motor units of the muscle and by a muscle model using these discharge patterns to simulate the surface EMG. In the alpha motoneuron pool model, we use a population of motoneurons in which cellular properties like cell size and membrane conductance are distributed according to experimentally observed data. By calculating the contribution from each motor unit, the muscle model predicts the EMG. Wave forms of the motor unit action potentials in the surface EMG are obtained from experimental data. Using the model, we are able to give a quantitative prediction of the motoneuron pool activity and the reflex EMG output at different preactivation levels. The simulated data are consistent with experimentally obtained results in healthy humans. During static isometric muscle preactivations, the simulations show that the reflex strength is highly dependent on the intrinsic thresh-old properties of the alpha motoneuron pool.

Modeling of the H-reflex facilitation during ramp and hold contractions

Healthy subjects were asked to make a voluntary ramp and hold contraction. The duration of the ramp stage was 500 ms, and the torque increment in this period was set to 15 Nm. The contraction was made from a relaxed and from a 5 Nm background torque situation. Hoffmann (H-) reflexes were elicited during the voluntary contraction, mostly with 100 ms intervals. These experiments showed an increase (facilitation) in the H-reflex before the torque or the EMG started to increase. This facilitation of the H-reflex remained during all the stages of the voluntary movement and declined to normal levels again only at the very end of the hold phase, which lasted for one second. This specific pattern of facilitation during a voluntary contraction was modeled using a modeling language, that is specifically designed to calculate neuronal systems with a high degree of reality (Ekeberg et al., 1991). Our model consisted of a motoneuron pool with 200 neurons connected to an EMG-model of the human soleus muscle and an extra group of higher-level neurons for controlling the amount of decrease of presynaptic inhibition. The model was used to simulate the observed modulation of the H-reflex with both a presynaptic and a postsynaptic mechanism. Simulations showed that a continuous change in the descending control signals is needed to make the model based on postsynaptic mechanism fit with the experimental data, whereas no extra control from the CNS over the excitatory drive to the motoneuron pool is needed when the decrease of presynaptic inhibition mechanism is applied.

Time-frequency analysis of mechanically evoked ENG

This paper deals with analysis of the evoked response of a sensory nerve signal in the time-frequency domain due to mechanical perturbations. The objective of the project is to determine whether other than amplitude information can be extracted from the sensory neural activity (ENG) to classify the mechanical events. Wavelet analysis has been applied because of its inherent capability to analyse non-stationary signals such as ENG.

An alpha motoneuron pool model to analyse EMG measured reflex output

The intrinsic properties of the human soleus alpha motoneuron pool and the surface EMG are modelled. The model consists of an alpha motoneuron pool model generating the firing patterns of motor units in a human soleus muscle and a muscle model simulating the soleus surface EMG. The model is able to give a quantitative prediction of the motoneuron pool activity and the reflex EMG output at different excitation levels. The simulated data are consistent with experimentally obtained results. The simulations show that the reflex strength is highly dependent on the intrinsic threshold properties of the alpha motoneuron pool during static isometric muscle contractions.