The rodent corticospinal tract (CST) is widely used in studies of regeneration in the spinal cord but its functional connectivity is poorly understood. Although the CST is generally considered an important descending motor pathway, there is also evidence that it may be involved in the regulation of sensory processing. In this study we have investigated the actions of the CST within the cervical spinal cord of the rat.

Experiments were conducted on adult Fischer rats anaesthetised with sodium pentobarbitone (50 mg kg^-1, I.V.) and neuromuscular blockade was maintained with pancuronium (see Riddell & Hadian (1998) for full details of anaesthetic monitoring methodology). Subsequent protocol followed local guidelines and the animals were humanely killed at the end of the experiment. Electrical stimuli were applied to the pyramidal tract in the brainstem using a ventral approach and to mixed or cutaneous nerves in the forelimb (up to 5 T). Recordings were made from the surface of the cervical segments exposed by laminectomy from C1-C8 and from within the grey matter using glass microelectrodes.

Single shocks to the pyramids (< 150 µA, 0.2 ms) evoked descending volleys that could be recorded from the surface of the dorsal columns. The fastest conducting fibres contributing to these volleys conducted at about 15 m s^-1, indicating the involvement of small diameter myelinated fibres. Descending volleys were followed by negative cord dorsum potentials that could be detected from C1 to C6 segments (mean maximal amplitude 200 µV, n = 4). Single corticospinal volleys also evoked negative field potentials within the grey matter of the cervical cord. In the C4 segment, where the potentials were most extensively mapped, they could be recorded from the most superficial laminae to the base of the dorsal horn. The largest potentials occurred within the medial aspects of lamina IV and V. No fields were detectable in the ventral horn. Conditioning stimuli applied to the pyramidal tract depressed field potentials within the dorsal horn, evoked by stimulation of peripheral nerves. Single shocks were sufficient, but trains of three or four stimuli were most effective. Such stimuli reduced the amplitude of the cutaneous potentials by up to 45 % (mean 30 %, 7 tests, 3 animals). The depression was most marked at relatively short conditioning-test intervals (10-40 ms) but usually persisted at intervals longer than 70 ms.

We conclude that the CST has relatively powerful synaptic actions in the dorsal horn of cervical segments and exerts a modulatory effect on synaptic transmission from cutaneous afferents. These results suggest that the CST tract in rodents may play a significant role in the control of segmental sensory processing.

This work was supported by the ISRT.

Riddell, J.S. & Hadian, M. (1998). J. Comparative Neurol. 394, 357-373.