Cortical Modulation of Spinal Reflexes in Healthy Humans

Considerable evidence suggests that monosynaptic and polysynaptic spinal reflexes are prone to corticospinal inputs. The goal of this study was to establish the cortical modulation of monosynaptic and polysynaptic spinal reflexes in healthy humans. Cutaneomuscular responses were evoked following stimulation of the medial edge of the right foot with a 30 ms pulse train every 10 s via a bipolar electrode and recorded from the ipsilateral tibialis anterior (TA) muscle. Across subjects, medial edge foot stimulation was delivered at 1.2 to 1.5 times the response threshold. The soleus H-reflex was evoked and recorded via conventional methods. The effects of transcranial magnetic stimulation (TMS) applied over left primary motor cortex at 0.7 and 1.2 times the motor evoked potentials (MEPs) resting threshold on the cutaneomuscular TA responses were established at the conditioning-test (C-T) intervals of 0, 25, 50, 70, 90, and 110 ms and on the soleus H-reflex at C-T intervals that ranged from negative 4 to positive 20 ms. We found that the soleus H-reflex was modulated in a similar pattern regardless the intensity of TMS. Subthreshold and suprathreshold TMS induced soleus H-reflex depression at negative C-T intervals and soleus H-reflex facilitation at positive C-T intervals. In contrast, the middle-latency cutaneous TA responses were depressed in all C-T intervals tested. Our findings strongly support that direct corticospinal and cortico-cortical neuronal pathways affect the excitability of spinal neuronal circuits in healthy humans