The anesthesia effect of vertebral column skin receptors on the motoneuron excitability and H-reflex recovery in Soleus and Gastrocnemius muscles

There are many disputes concerning the role of skin receptors on excitability of spinal motoneurons. The purpose of the present study is to evaluate the motoneuron excitability and H-reflex recovery curve of Soleus and Gastrocnemius muscles after application of local anesthesia on the skin of vertebral column.

Thirty healthy non-athlete female volunteers with a mean age of 27.20 ± 3.11 years old, were tested in two separated sessions. The H-reflex recovery curve of Soleus and Gastrocnemius were recorded before intervention and 15 minutes after the application of anesthesia (lidocaine 10%) or placebo anesthesia (water), with two excitation intensity of 30% and 100% of Hmax and interval interpulses of 1-202 ms. After extraction the amplitude of H1 and H2 response and fitting the recovery curve; their amplitude and interval of the peak, area under the curve and, thresholds of the early and secondary facilitation were calculated and analyzed.

After application of lidocaine, the sensory threshold of the vertebral column skin increased significantly. Due to a significant increase in H1 amplitude, peak, and area under the curve of the early facilitation showed a significant decrease (p<0.05). Threshold of the secondary facilitation showed a significant decrease in the Gastrocnemius and Soleus muscles (p<0.05).

The results suggest that decrease of skin receptors activity after anesthesia can facilitate motoneurons excitability and H-reflex recovery. It seems that using the method of paired stimulation and H-reflex recovery curve may be useful to investigate the effect of physical therapy modalities on the fast and slow motoneuron excitability.