THE RELATIONSHIP BETWEEN THE SOLEUS H-REFLEX AMPLITUDE AND VIBRATORY INHIBITION IN CONTROLS AND SPASTIC SUBJECTS .2. COMPUTER-MODEL

A computer model is presented-that describes soleus H-reflex recruitme nt as a function of electric stimulus intensity. The model consists of two coupled non-linear transfer functions. The first transfer functio n describes the activation of muscle spindle (Ia) afferent terminals a s a function of the electric stimulus intensity; whereas the second de scribes the activation of a number of motoneurons as a function of the ; number of active Ia afferent terminals. The effect of change in thes e transfer functions on the H-reflex recruitment curve is simulated. I n spastic patients, a higher average maximal H-response amplitude is o bserved in combination with a decreased H-reflex threshold. Vibration of the Achilles tendon reduces the H-reflex amplitude, presumably by r educing the excitatory afferent input. Vibratory inhibition is diminis hed in spasticity. In the model, the afferent-motoneuron transfer func tion was modified to represent the possible alterations Occurring in s pasticity. The simulations show that vibratory suppression of the H-re flex is determined only in part by the inhibition level of the afferen t input. With a constant level of presynaptic inhibition, the suppress ion of reflexes of different sizes may vary. A lowering of the motoneu ron activation thresholds in spastic patients will directly contribute to a decrease of vibratory inhibition in spasticity. Copyright (C) 19 96 Published by Elsevier Science Ltd.