Active Integration of Movement Related Synaptic Inputs by Spinal Motoneurons

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During movement, the dendrites of spinal motoneurons receive steady excitatory and inhibitory synaptic input from supraspinal sources, interneurons, and sensory afferents. Motoneurons also have dendritic voltage sensitive ion channels. Most notable is a persistent inward current (PIC), which can enhance the amplitude of synaptic input by several fold. PICs are subject to the state dependent level of brainstem monoaminergic drive. Previously, studies of the PIC had focused on cellular mechanisms and the basics of integration of excitatory input. My goal was to determine how the PIC interacted with the mixture of excitation and inhibition required for generation of movement. Data from Chapter 1 showed that when synaptic inputs were applied separately, the PIC resulted in a dendritic amplification of excitatory and inhibitory inputs that occurred in different voltage ranges. The predicted algebraic summation of excitatory and inhibitory inputs would thus result in a large variability in effective synaptic current (In) amplitude as a function of voltage. However, non-linear summation during simultaneous activation of both inputs tended to compensate for this dual-range amplification, producing better balancing between excitation and inhibition as function of membrane potential. In addition to regulation by levels of monoaminergic drive, we found that the amplitude of the PICs of ankle extensor motoneurons could be modulated by as much as 50% as a function of joint angle through reciprocal inhibitory pathways (Chapter 2). This evidence indicates how the modulation of PICs within a given motor pool can be shaped by the biomechanics of the limb. In Chapter 3, we demonstrated that PICs can amplify the movement related IN of ankle extensor, knee extensor, and hip flexor MNs generated by passive flexion and extension rotations of the ankle, knee, and hip joints without altering the overall pattern of each cell's movement related receptive field. The presence and regulation of PICs optimize motor control by providing an intrinsic source of excitation that can be modulated by several mechanisms in order to match the motor requirement.

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  • 08/30/2018
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