Influences of neck receptors on soleus motoneuron excitability in man.

We studied the influence of the asymmetric tonic neck reflexes on the excitability of the human soleus motoneuronal pool by mapping the H amplitude as a function of rotation of the body relative to the fixed head. Eight normal adult volunteers were tested. On each subject 15 consecutive H reflexes were recorded from the right soleus muscle, for each of the following test position, 4 degrees, 8 degrees, 12 degrees, 16 degrees, as well as at the control position (0 degrees), both before and after each change in body position. Our results showed that the H reflex amplitude was progressively facilitated for contralateral rotation in respect to the recording side and conversely inhibited for ipsilateral rotation. The results indicate that neck receptors of one side enhance the excitability of the contralateral soleus motoneurons and depress the ipsilateral ones.