Investigations on the nervous mechanisms underlying the somatosensory cervical response in man.

The main features (amplitude, latency and shape) of the cervical activity evoked by stimulation of the median nerve, recorded throughout the cervical spine, have been concurrently investigated by monopolar, bipolar longitudinal and bipolar transverse recordings. In some subjects the derivation C7-Sn (suprasternal notch) has been employed as well. A comparative evaluation of the refractory period of each component of the cervical responses under investigation has been performed to differentiate presynaptic from postsynaptic events. Additional information has been obtained by cervical activity recorded by longitudinal and transverse bipolar derivations upon stimulation of the lower limb. It was thus demonstrated that both presynaptic and postsynaptic events were responsible for the cervical sensory evoked potential, as appearing when recorded against a cranial reference (that is the upper midfrontal region). The structures involved were the brachial plexus (N9), the cervical roots (P10 and a minor part of N11a), the dorsal columns both at caudal (N11a) and rostral (N11b) cervical levels, and the dorsal column nuclei (N13). However a contribution of the spinal segmental activity to the postsynaptic portion of the cervical response, more specifically to N13, should be considered as well, though direct evidence is still inadequate.