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J Physiol Volume 586, Number 5, 1225-1231, March 1, 2008 DOI: 10.1113/jphysiol.2007.145078
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SYMPOSIUM REPORT

Active properties of motoneurone dendrites: diffuse descending neuromodulation, focused local inhibition

 C. J. Heckman1, Allison S. Hyngstrom1 and Michael D. Johnson1

1 Department of Physiology and Department of Physical Medicine and Rehabilitation, Northwestern University Feinberg School of Medicine, Chicago IL 60126, USA

The dendrites of spinal motoneurones are highly active, generating a strong persistent inward current (PIC) that has an enormous impact on processing of synaptic input. The PIC is subject to regulation by descending neuromodulatory systems releasing the monoamines serotonin and noradrenaline. At high monoaminergic drive levels, the PIC dominates synaptic integration, generating an intrinsic dendritic current that is as much as 5-fold larger than the current entering via synapses. Without the PIC, motoneurone excitability is very low. Presumably, this descending control of the synaptic integration via the PIC is used to adjust the excitability (gain) of motoneurones for different motor tasks. A problem with this gain control is that monoaminergic input to the cord is very diffuse, affecting many motor pools simultaneously, probably including both agonists and antagonists. The PIC is, however, exquisitely sensitive to the reciprocal inhibition mediated by length sensitive muscle spindle Ia afferents and Ia interneurones. Reciprocal inhibition is tightly focused, shared only between strict mechanical antagonists, and thus can act to ‘sculpt’ specific movement patterns out of a background of diffuse neuromodulation. Thus it is likely that motoneurone gain is set by the interaction between diffuse descending neuromodulation and specific and focused local synaptic inhibitory circuits.

(Received 15 September 2007; accepted after revision 12 October 2007; first published online 18 October 2007)
Corresponding author C. J. Heckman: Departments of Physiology and Physical Medicine and Rehabilitation, Northwestern University Feinberg School of Medicine, Chicago, IL 60126, USA. Email: c-heckman{at}northwestern.edu

This report was presented at The Journal of Physiology Symposium on The cortex, interneurones and motoneurones in the control of movement, IBRO World Congress of Neuroscience, Darwin, Australia, 19 July 2007. It was commissioned by the Editorial Board and reflects the views of the authors.




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