A quantitative assessment of presynaptic inhibition of la afferents in spastics

Abstract

Soleus H-reflex facilitation evoked by a supramaximal conditioning stimulation to the femoral nerve was investigated in 28 healthy control subjects and 35 spastic patients of whom 17 were paraplegics with bilateral spinal cord lesion and 18 were hemiplegics with unilateral cerebral lesion. Heteronymous facilitation from quadriceps to soleus was measured 0.4 ms after onset, while the monosynaptic Ia excitation is still uncontaminated by any non-monosynaptic effect and can be used to assess ongoing presynaptic inhibition on Ia terminals to soleus motor neurons. In paralegics, this heteronymous Ia facilitation was significantly larger than in control subjects (all individual results in these patients being above the mean observed in controls). This must reflect a decrease in presynaptic inhibition of Ia terminals in the paraplegics explored here. There was no correlation between this decreased presynaptic inhibition of Ia terminals and the degree of spasticity measured by Ashworth's scale. Surprisingly, the amount of heteronymous Ia facilitation in hemiplegics was the same as in normal subjects. This indicates that presynaptic inhibition of Ia terminals is unchanged in these patients and disagrees with the usual interpretation of reduced vibratory inhibition of the soleus H-reflex in hemiplegics. It is argued that this disagreement is due to the fact that vibratory inhibition of the reflex also depends on post-activation depression following repetitive synaptic transmission.

Restoration of frequency-dependent depression of the H-reflex by passive exercise in spinal rats

“…9,[12][13][14][15] Decreased presynaptic inhibition is frequently linked with decreased low-frequency depression of the H-reflex. 14,21,38 It is thought that the loss of inputs from descending pathways leads to a reorganization of spinal circuitry that promotes hyper-reflexia.…”

“…The physiological changes that have been postulated to contribute to hyper-reflexia include alpha motoneuron hyperexcitability, [1][2][3] changes in the intrinsic properties of alpha motoneurons, [4][5][6][7] reduced post-activation depression of transmission from Ia fibers, 8,9 synapse growth; 10 alterations in morphology of alpha motoneurons, 11 and decreased presynaptic inhibition of Ia terminals. 9,[12][13][14][15] The time course of spinal changes after injury has been proposed to include an early postsynaptic mechanism, possibly involving an increase in excitability and/or receptor upregulation, and a late change involving presynaptic mechanisms possibly involving synaptic growth in spared descending pathways and in reflex pathways. 10 One measure used by numerous investigators to quantify hyper-reflexia is the electrical analogue of the classic tendon jerk reflex, referred to as the Hoffman or H-reflex.…”

“…10 One measure used by numerous investigators to quantify hyper-reflexia is the electrical analogue of the classic tendon jerk reflex, referred to as the Hoffman or H-reflex. 2,[15][16][17][18][19] The H-reflex is a compound electromyographic (EMG) response elicited by the synaptic activation of motoneurons by muscle afferents following stimulation of muscle nerves. Thompson et al 20 investigated four measures of H-reflex excitability in a contusion model of SCI in the rat.…”

Spastic movement disorder

“…In addition, reduction of pre-synaptic inhibition of group Ia a erents is stronger in patients with paraplegia compared to those with hemiplegia. 10 However, no correlation is seen between decreased presynaptic inhibition of Ia terminals and the degree of spastic muscle tone measured by Ashworth's scale. 10 Studies on muscle tone and re¯ex activity in patients with spastic hemiparesis following stroke have usually been performed under passive motor conditions.…”

“…10 However, no correlation is seen between decreased presynaptic inhibition of Ia terminals and the degree of spastic muscle tone measured by Ashworth's scale. 10 Studies on muscle tone and re¯ex activity in patients with spastic hemiparesis following stroke have usually been performed under passive motor conditions. In such a condition a displacement of the elbow joint is associated with an increased EMG activity in the stretched muscles of the spastic side compared to the una ected side in patients with spastic hemiparesis.…”