When neurons from a motoneuron pool are subjected to increasing excitatory synaptic input, the resulting patterns of motoneuronal recruitment and rate modulation are usually highly ordered, and bear a near-optimal relation to the contraction speed and fatigability of the innervated muscle fibers. In acute spinal cord injury, these relations appear to break down, so that there is a compression of recruitment range, a substantial reduction in minimum firing rate, and an increase in motoneuron synchrony. The sources of these changes in motoneuron response are not understood. They could be mediated by alterations in the patterns of excitatory and inhibitory interneuronal input to spinal motoneurons (so that smaller motoneurons are relatively inhibited and larger motoneurons are excited), or they could be induced by changes in the intrinsic properties of spinal motoneurons. These changes in intrinsic properties could be mediated by reductions in monoamines released at segmental levels by pathways traversing the lesioned dorsolateral funiculus. To investigate these issues, we will pursue four objectives. First, we will refine the design of a cold-block system, which will reversibly block fiber conduction in dorsal pathways of the cord. We will then determine whether this cold block replicates the changes in motoneuron responses observed in the surgically lesioned cord. Second, using extracellular recordings of motoneuron discharge, we will determine whether monoamine agonists or antagonists modify the response patterns of motoneurons in normal or cold-blocked cords. Third, we will evaluate the intrinsic characteristics and synaptic inputs to spinal motoneurons using intracellular recordings in normal and cold-blocked spinal cord. Fourth, we will examine the effects of intrathecally administered monoamines on cellular responses using intracellular recordings in motoneurons. These studies should materially improve our understanding of the disturbances in segmental function which follow spinal injury, and help rationalize pharmacological intervention in this important disorder. In addition, they should clarify the factors regulating normal neuronal discharge rate, and help specify which properties are legitimately identified as """"""""intrinsic"""""""" to the motoneuron.

Agency
National Institute of Health (NIH)
Institute
National Institute of Neurological Disorders and Stroke (NINDS)
Type
Research Project (R01)
Project #
5R01NS028076-02
Application #
3414550
Study Section
Neurology B Subcommittee 2 (NEUB)
Project Start
1990-09-20
Project End
1995-08-31
Budget Start
1991-09-01
Budget End
1992-08-31
Support Year
2
Fiscal Year
1991
Total Cost
Indirect Cost
Name
Northwestern University at Chicago
Department
Type
Schools of Dentistry
DUNS #
005436803
City
Chicago
State
IL
Country
United States
Zip Code
60611
Chen, D; Theiss, R D; Ebersole, K et al. (2001) Spinal interneurons that receive input from muscle afferents are differentially modulated by dorsolateral descending systems. J Neurophysiol 85:1005-8
Lin, D C; Rymer, W Z (2001) Damping actions of the neuromuscular system with inertial loads: human flexor pollicis longus muscle. J Neurophysiol 85:1059-66
Lee, R H; Heckman, C J (2000) Adjustable amplification of synaptic input in the dendrites of spinal motoneurons in vivo. J Neurosci 20:6734-40
Lin, D C; Rymer, W Z (2000) Damping actions of the neuromuscular system with inertial loads: soleus muscle of the decerebrate cat. J Neurophysiol 83:652-8
Lee, R H; Heckman, C J (1999) Paradoxical effect of QX-314 on persistent inward currents and bistable behavior in spinal motoneurons in vivo. J Neurophysiol 82:2518-27
Heckman, C J; Lee, R H (1999) Synaptic integration in bistable motoneurons. Prog Brain Res 123:49-56
Heckman, C J; Lee, R H (1999) The role of voltage-sensitive dendritic conductances in generating bistable firing patterns in motoneurons. J Physiol Paris 93:97-100
Lee, R H; Heckman, C J (1999) Enhancement of bistability in spinal motoneurons in vivo by the noradrenergic alpha1 agonist methoxamine. J Neurophysiol 81:2164-74
Maltenfort, M G; Heckman, C J; Rymer, W Z (1998) Decorrelating actions of Renshaw interneurons on the firing of spinal motoneurons within a motor nucleus: a simulation study. J Neurophysiol 80:309-23
Lin, D C; Rymer, W Z (1998) Damping in reflexively active and areflexive lengthening muscle evaluated with inertial loads. J Neurophysiol 80:3369-72

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