Corticomotoneuronal synaptic connections in normal man - An electrophysiological study

In order to determine the mono- or oligosynaptic character of connections b etween pyramidal axons and individual spinal motor neurons, we constructed peristimulus time histograms (PSTHs) of the firing probability of voluntari ly activated single motor units (SMUs) of various upper and lower limb musc les upon slightly suprathreshold transcranial anodal electrical stimulation s of the motor cortex in normal subjects, Weak anodal cortical stimuli are known to activate preferentially fast-conducting pyramidal axons directly, bypassing cell bodies and cortical interneurons. A narrow bin width (0.1 ms ) was chosen to measure precisely the duration of the PS TH excitatory peak , which corresponds to the rise time of the underlying compound excitatory post-synaptic potentials (EPSP), A short duration PSTH peak indicates sharp -rising EPSPs, most commonly encountered in the case of monosynaptic connec tions, In flexor carpi radialis and soleus SMUs, the PSTHs of built-in resp onses to anodal cortical stimuli were compared with those produced by 1A af ferent stimulation able to elicit a Hoffmann reflex, which is known to be l argely monosynaptic. In all upper and lower limb muscles, excitable SMUs re sponded to anodal cortical stimuli with a highly synchronized peak of incre ased firing probability In flexor carpi radialis and soleus SMUs, the mean duration of this peak was significantly narrower than that evoked by 1A aff erent stimulation, indicating that monosynaptic corticomotor neuronal trans mission dominates low-threshold motor units, even in proximal arm and leg m uscles. In the various muscles studied, and particularly in forearm SMUs, w e did not observe broad PSTH peaks against the activation of non-monosynapt ic corticomotor neuronal pathways, even with near-threshold stimuli. In som e triceps and forearm flexor SMUs, subthreshold anodal pulses caused signif icant inhibition of their voluntary firing, with a latency consistent with activation of 1A inhibitory interneurons by the descending volleys. Measure ments of the maximal number of counts in the excitatory PSTH peak upon anod al cortical stimuli provide comparisons of the strength of monosynaptic inp uts to various muscles which seems to be maximal for hand and finger extens or muscles, and also for deltoid.