Study the Concomitant Effect of Topical Anesthesia and Cold on the Recruitment Curve Parameters of H-Reflex

There is a substantial controversy about the role of skin receptors in the excitation of spinal motor neurons. The purpose of this study was to determine the effect of topical anesthesia and cold temperature on the excitation of spinal motor neurons in healthy non-athletic men.

10 volunteers were tested in four separated sessions. Control group #1 (placebo ointment with ambient temperature), test group #1 (anesthetic ointment with ambient temperature), control group #2 (cold placebo ointment), test group # 2 (cold anesthetic ointment). Soleus H-reflex and Mh- wave recruitment curve parameters were used to evaluate the changes in the excitability of motor neuron. The anesthetic ointment used was benzocaine 5% which was rubbed around T11. Vaseline was used as the placebo ointment. H-reflex was recorded five times for nine different intensities, prior to, immediately after and 30 minutes after using the ointment.

Fit L3 slope, fit L5 slope and peak to peak amplitude of H-reflex decreased 30 minutes after the application of anesthetic ointment at ambient temperature (p<0.05).Fit L3 slope, fit L5 slope and peak to peak amplitude of H-reflex decreased immediately and 30 minutes after the application of cold placebo ointment (p<0.05).Peak to peak amplitude of H-reflex decreased 30 minutes after the application of cold anesthetic ointment (p<0.05).

Based on the above results, it was concluded that topical anesthesia causes a decrease in the stimulation of motor neurons by desensitizing the low threshold skin receptors and reducing their basic activity. Additionally, cold temperature could cause a decrease in the stimulation of motor neurons by the stimulation of thermo-receptors and reducing the firing rate of surface mechanoreceptors. Different skin receptors depend on the intensity, duration and place of the stimulus, have different effects on the excitability of the spinal motor neurons. Cutaneous afferents in addition with muscle afferents, have convergence on common spinal interneurons and determines motor neurons excitability.