SOLEUS H-REFLEX EXTINCTION IN CONTROLS AND SPASTIC PATIENTS - ORDEREDOCCLUSION OR DIFFUSE INHIBITION

Extinction of the soleus H reflex at higher stimulus intensities is co mmonly attributed to retrograde conduction of action potentials in mot or axons. This study was designed to gain further insight into the mec hanisms underlying the extinction. The decrease of the H reflex was qu antified in a group of controls and spastic patients, with and without depression of the H response by continuous tendon vibration. Response amplitudes were normalized as a percentage of the maximal M wave ampl itude. Stimuli were normalized as a multiple of the M wave threshold. After normalization, the mean M recruitment curves, and similarly the fractions of motor axons activated, were equal in each group. In contr ast, the mean H reflex amplitudes at the M threshold were different. T he mean H reflex decrease, between 1.0 and 1.5 times the M threshold, was found to be the same fraction of the maximal H reflex amplitude in each group. The largest motor fibres, belonging to the largest motone urones, are traditionally thought to have the lowest threshold for ele ctrical excitation. Collision or retrograde inactivation should theref ore preferentially affect the largest motoneurones, employed in only t he largest H reflexes, at the lowest stimulus intensities. Our results are contrary to this hypothesis. Renshaw and/or Ib inhibition is like ly to play a role in the initial decrease of the H reflex at higher st imulus intensities.