Motor threshold as indicator of premotor and motor cortex excitability

The premotor cortex is a second generator of motorics, involved in mass inborn movements performance, and in pathology--in genesis of spasticity and other motor disorders. As far as the repetitive transcranial magnetic stimulation (TMS) is expected to be a therapeutic tool in some movement disorders, the investigation of premotor cortex response to TMS seems to be an important first step. The goal of our work was to picture the difference in motor responses of premotor and primary motor areas to TMS, by means of motor threshold (MT), and to give a simple and easy testing method, which may be of use before trying therapeutic TMS in some motor disorders. It is based on the motor threshold values for arms and legs motor responses, as a primary motor and premotor cortex excitabity indicator. Only a transcranial magnetic stimulator is necessary for the investigation. A MagPro stimulator (Medtronic, Denmark) with an original C125 coil have been used. The hand and finger contraction motor threshold by TMS at C(z) was measured. After that the stimulation intensity (combined with facilitation), necessary for gaining muscle contraction in every arm and leg, contra- and ipsilaterally, by TMS at C(z), C3, C4, CF1 and CF2 (left and right premotor zones) was tried. The responses have been assessed visually. The results showed a bilateral arm and leg motor response to unilateral TMS of premotor area. The stimulation intensity necessary to evoke contraction in leg musculature was significant lower at premotor area than this at C(z). On the contrary the TMS at C3 and C4 resulted always only in a contra-lateral arm and sometimes leg contraction. The visual assessment of contra- and ipsilateraly leg and arm muscles participation allowed this to be scanned all over the body.

The two motor generators (premotor and primary motor) show different behavior by TMS. The proposed method of motor threshold comparison in TMS of both motor and premotor cortex, necessary to gain motor responses in four limbs may be useful as an easy, fast and noninvasive functional test.