Changes in motoneuron excitability in the lower extremity induced by afferents of the upper extremity

A study was conducted to investigate the neural connection between the afferents of the upper extremity and the motoneurons innervating the extensors of the lower extremity. Healthy young males and females were recruited, The median nerve was cutaneously stimulated, and the soleus H reflex, reflecting the excitability of motoneurons, was evoked. The C-T interval and intensity of the conditioning stimulus were modulated. A conditioning stimulus with an intensity of 1.4 xMT and a C-T interval of 69 ms inhibited the soleus H reflex pathway (I-1.4) Additionally, a conditioning stimulus with an intensity of 2.1 xMT and a C-T interval of 13 ms inhibited the soleus H reflex pathway (I-2.1). The conduction velocity of the afferents responsible for I-1.4 was 19 m/s and that for I-2.1 was 39 m/s. The conduction velocity of the afferent fiber type responsible for I-1.4 was Similar to that of the group III afferents, and the conduction velocity for I-2.1 was similar to that of the group II afferents. However, further investigation is needed to identify the afferents responsible, because the results conflict with those of previous studies in terms of inhibition thresholds.