Information processing in the central nervous system, the signalling system controlling movement

Interneurones in the spinal cord integrate command signals from the brain, with information from the senses, and their own internal pattern generating activities to send appropriate instructions to motorneurones controlling movement. Spinal interneurones exhibit a remarkable variety of firing patterns in response to a pulse of injected current, with important implications for information processing. These patterns range from repetitive to delayed, to bursting and to single spiking.

In the ventral spinal cord, interneurones process both motor commands and sensory inputs. Steady firing interneurones integrate these inputs, while bursting neurons may emphasize input variations and single spiking neurons probably serve as coincidence detectors. Although these different processing modes suggest a diversity in ion channels, Robert Lee (now at Emory University) and C J Heckman hypothesized that a small component of the total current mediated by sodium channels plays a critical role in determining firing patterns. This component is persistent instead of transient and is essential for action potential initiation during prolonged input.

The research by Theiss et al. on slices of spinal cord taken from rats indicates that reducing persistent sodium current in ventral interneurones converted both steady firing and bursting patterns into a single spike pattern, and thus its modulation may provide the CNS with the capacity to mediate dramatic changes in neural computations. This result is an important step forward in our understanding of neuronal processing and should lead to more research on how persistent sodium currents interact with other currents to generate the full array of firing patterns of neurons throughout the CNS.

Dr. Theiss noted that “Abnormal regulation of persistent sodium currents in disease states like spinal injury and ALS could seriously impair the integration of motor commands with sensory inputs, which is essential for normal movement patterns”.