Presynaptic inhibition and homosynaptic depression - A comparison between lower and upper limbs in normal human subjects and patients with hemiplegia

Presynaptic inhibition of Ia terminals and postactivation depression at the Ia fibre-motor neuron (MN) synapses were compared in the upper and lower l imbs of both sides in subjects from different populations: 49 spastic patie nts with hemiplegia [mainly with a lesion in the middle cerebral artery (MC A) area], two tetraplegics and 35 healthy subjects. Presynaptic inhibition was assessed using D1 inhibition of the soleus and the flexor carpi radiali s (FCR) H reflexes elicited by electrical stimuli applied to the nerve supp lying antagonistic muscles, and postactivation depression was explored by v arying the time interval between two consecutive H reflexes. In normal subj ects no right-left asymmetry was found in the amount of presynaptic Ia inhi bition, homosynaptic depression or the H-max/M-max ratio. In the hemiplegic side of patients with MCA area lesions, the H-max/M-max ratio was signific antly increased in the soleus but not in the FCR, Presynaptic inhibition of Ia terminals, which was significantly reduced at the cervical level on the hemiplegic side land also, hut to a lesser extent, on the unaffected side) , was unchanged at the lumbar level. Homosynaptic depression was similarly reduced at the cervical and lumbar levels on the hemiplegic side but not mo dified on the unaffected side, It is argued that the decrease in presynapti c inhibition of Ia terminals is more a correlate of spasticity than a mecha nism underlying it. The decrease in postactivation depression, which very p robably contributes to the exaggeration of the stretch reflex characterizin g spasticity, might be a consequence of the changes in the pattern of activ ation of Ia afferents and MNs following the motor impairment.