Persistent inward currents in motoneuron dendrites: Implications for motor output

C. J. Heckman*, Monica A. Gorassini, David J. Bennett

*Corresponding author for this work

Research output: Contribution to journalReview articlepeer-review

318 Scopus citations


The dendrites of motoneurons are not, as once thought, passive conduits for synaptic inputs. Instead they have voltage-dependent channels that provide the capacity to generate a very strong persistent inward current (PIC). The amplitude of the PIC is proportional to the level of neuromodulatory input from the brainstem, which is mediated primarily by the monoamines serotonin and norepinephrine. During normal motor behavior, monoaminergic drive is likely to be moderately strong and the dendritic PIC generates many of the characteristic features of motor unit firing patterns. Most of the PIC activates at or below recruitment threshold and thus motor unit firing patterns exhibit a linear increase just above recruitment. The dendritic PIC allows motor unit derecruitment to occur at a lower input level than recruitment, thus providing sustained tonic firing with little or no synaptic input, especially in low-threshold units. However the dendritic PIC can be readily deactivated by synaptic inhibition. The overall amplification due to the dendritic PIC and other effects of monoamines on motoneurons greatly increases the input-output gain of the motor pool. Thus the brainstem neuromodulatory input provides a mechanism by which the excitability of motoneurons can be varied for different motor behaviors. This control system is lost in spinal cord injury but PICs nonetheless recover near-normal amplitudes in the months following the initial injury. The relationship of these findings to the cause of the spasticity syndrome developing after spinal cord injury is discussed.

Original languageEnglish (US)
Pages (from-to)135-156
Number of pages22
JournalMuscle and Nerve
Issue number2
StatePublished - Feb 2005


  • Dendrites
  • Motoneurons
  • Norepinephrine
  • Persistent currents
  • Serotonin
  • Spinal cord injury

ASJC Scopus subject areas

  • Clinical Neurology
  • Physiology (medical)
  • Cellular and Molecular Neuroscience
  • Physiology


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