Neuronal Communication: The Need for Persistence
16 May, 2007 03:43 pm
Scientists at the Feinberg School of Medicine at Northwestern University in Chicago, Illinois, USA, studying mammalian spinal cord cells, recently published new research that sheds light on how information is processed in the central nervous system (i.e. the brain and the spinal cord) to control movement. The findings are relevant to understanding mechanisms underlying movement and disorders such as spinal cord injury and motor neuron disease (ALS).
The laboratory of CJ Heckman, Ph.D. has a strong record of neuroscience research including the study of cellular properties of “motor neurons” (or “motoneurons” – the spinal cord cells that directly control muscles) and intermediate processing/relay cells called “interneurons.” Interneurons 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 motor neurons to control movement.
Spinal interneurons exhibit a remarkable variety of response patterns (output) to incoming signals (input). For example, some cells produce sustained, “repetitive” output in response to sustained input, some cells produce a brief “burst” output and then stop responding even if the input continues, and other cells produce only one “single” response irregardless of the input duration. (I often compare this “single-spike” behavior to having a conversation with me in the morning—I hear the entire input, but am not awake enough to respond with more than single grunts or one word responses.) Different response patterns of spinal interneurons and the resulting input/output relationship have important implications for information processing.
Building on an idea first theorized by Robert Lee (now at Emory University) and CJ Heckman, my colleagues Jason J. Kuo and CJ Heckman and I took advantage of the diverse response patterns to uncover the intrinsic mechanisms responsible for producing a sustained, “repetitive” response as opposed to a “single-spike” response. We discovered that a small, yet important component of a particular type of cellular chemical current, a persistent sodium current, could account for the different output responses. First, we demonstrated that the size of the persistent sodium current was significantly different among the cells with different responses. For example, “single-spike” cells had the least amount of the current, while “repetitive” cells had the largest amount of current. The next step was then to alter the size of the current and measure the effect on the response. Using the drug riluzole, which targets and reduces the persistent sodium current, we were able to show that eliminating this current converted both “repetitive” and “burst” responding cell patterns to the “single-spike” pattern. Thus we supported Lee and Heckman’s hypothesis that persistent sodium currents are essential for sustained responses to sustained inputs.
To us, it was a delightful result that the persistent sodium current, though only a small component of the entire sodium current, could so powerfully shape how a cell responds to incoming signals. This small component could have profound effects on signal processing— different response patterns can change how cells integrate and relay incoming information. Interneuron outputs that are too long or too short would send inappropriate signals to the muscle-controlling motor neurons, and normal movements could be completely disrupted.
These factors highlight the significance of this study. We were able to show that a small sodium current component can powerfully shape the responses of cells in the spinal cord. Since these cells receive information from both the senses (such as touch or muscle stretch) and the brain (such as movement commands), appropriate signal processing and relay are very important. When these currents are not appropriately regulated, the processing and relay capacity of these spinal cord cells could be detrimentally impaired and have a serious effect on normal movement patterns, such as is seen in disease states like spinal cord injury and ALS.
Renée D. Theiss, et al, Persistent inward currents in rat ventral horn neurones, The Journal of Physiology, April issue