Spinal neurons integrate information from descending systems and peripheral afferents to create meaningful outputs for proper movement execution. Sensory processing in the central nervous system occurs first in the spinal cord; afferents associated with skin mechanical and thermal properties, muscle length change and tension and joint position all synapse on to spinal motoneurons and interneurons as their signals make their way to higher centers. The spinal cord is not a passive conduit for these signals. Chapter 1 shows that an accurate representation of limb kinematics is present in the firing behavior of dorsal root sensory afferents and intermediate zone spinal interneurons. The output of populations of these cells was shown to strongly correlate to parameters associated with limb configuration, position and velocity in space. Further this coding was found to occur in a joint and spatial specific manner and differed between afferents and interneurons. Synaptic inputs to spinal motonerons interact with the cells intrinsic properties. One strong intrinsic property is the persistent inward current (PIC). This depolarizing current is the result of voltage sensitive ion channels located primarily along the cells dendrites. The PIC’s presence and strength depends on the amount of monoaminergic drive from the brainstem which is related to state of arousal and can greatly amplify synaptic inputs. The nature of the interaction between synaptic inputs and this intrinsic property has been an important focus of our lab. Chapter 2 shows how the temporal interplay between excitation and inhibition serves along with PICs to modify the input-output gain of spinal motoneurons. We show that when these synaptic inputs are modulated in a push-pull fashion from a mixed background of both excitation and inhibition gain is increased. With a baseline of tonic activity of both types of input this push-pull control system achieves maximum depolarization by coupling excitation with disinhibition and maximum hyperpolarization by coupling inhibition with disfacilitation. We show that spinal motoneurons rely on these tonic inputs and that in order to achieve full excitatory gain inhibition needs to be paradoxically increased. We demonstrate this phenomenon at the cellular level by directly measuring the effects of synaptic currents at the motoneuron soma and at the system level by measuring force production, the ultimate function of the motor system. This push-pull control system works along with PIC amplification to optimize motor control and offers a surprising revelation: for optimum force generation inhibition needs to be increased.
|Advisor:||Miller, Lee E.|
|Commitee:||Heckman, Charles J., Singer, Joshua H., Tresch, Matthew C.|
|Department:||Neuroscience Institute Graduate Program|
|School Location:||United States -- Illinois|
|Source:||DAI-B 72/12, Dissertation Abstracts International|
|Keywords:||Interneurons, Motoneurons, Spinal neurons|
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