CORTICAL INFLUENCES ON CERVICAL MOTONEURONS IN THE RAT - RECORDINGS OF SYNAPTIC RESPONSES FROM MOTONEURONS AND COMPOUND ACTION-POTENTIAL FROM CORTICOSPINAL AXONS

The synaptic responses of cervical motoneurons to intracortical stimul ation (ICS) of the motor cortex were studied in the rat by means of in tracellular recordings. Motoneurons (n = 80) were identified either by their antidromic response to peripheral nerve electrical stimulation and/or by intracellular staining with biocytin. As a result of ICS (0. 6-1.5 mA) of the contralateral motor cortex, the vast majority of moto neurons responded with EPSPs (77 out of 80), while only three motoneur ons exhibited IPSPs. For increasing ICS intensities, the amplitude of the EPSPs in a given motoneuron increased, whereas their latency was n ot substantially affected. For the whole population of motoneurons, id entified mainly by their antidromic response, the latency of the EPSPs was on average 8.45 ms (SD 1.6 ms), ranging from 4.7 to 12.6 ms. A ve ry comparable latency distribution was obtained from the subpopulation of biocytin stained motoneurons (n = 23). In 7 of 19 tested motoneuro ns EPSPs could follow high frequencies (50-100 Hz) of stimulation with out change of latency. The compound action potential (descending volle y) travelling along corticospinal fibers reached the level of intracel lular recording with a minimal latency estimated to be about 3 ms afte r ICS. The conduction velocity of corticospinal axons contributing to the descending volley was calculated to range from 9 to 19.7 m/s, base d on morphometric measurements of conduction distance from the motor c ortex and duration of the compound action potential. The time delay be tween the latency of descending volley and the latency of early EPSPs on the one hand, and frequency following properties of EPSPs on the ot her hand, suggest that some cervical motoneurons receive secure, most likely, indirect (presumably disynaptic) inputs from fast conducting c orticospinal axons or direct contacts from slower conducting corticosp inal fibers. The biocytin labeled cervical motoneurons exhibited extra ordinary long dendritic trees, extending both laterally in the white m atter near the edge of the spinal cord and medially in the gray matter as far as the midline of the spinal cord. The motoneurons were also c haracterized by the presence of one or several recurrent axon collater als, ramifying profusely in the neuropil, with numerous boutons en pas sant and terminaux contacting most likely neighboring cervical neurons .