Persistent inward currents in motoneuron dendrites: Implications for motor output

• Published in: Muscle & nerve Vol. 31; no. 2; pp. 135 - 156
• Main Authors: Heckman, C. J., Gorassini, Monica A., Bennett, David J.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01-02-2005; Wiley
• Subjects: Action Potentials - physiology; Animals; Biogenic Monoamines - physiology; Biological and medical sciences; Central nervous system; Central neurotransmission. Neuromudulation. Pathways and receptors; dendrites; Dendrites - physiology; Fundamental and applied biological sciences. Psychology; Humans; Medical sciences; motoneurons; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor Neurons - physiology; Muscle Spasticity - physiopathology; Nervous system (semeiology, syndromes); Nervous system as a whole; Neurology; norepinephrine; persistent currents; serotonin; Spinal Cord Injuries - physiopathology; spinal cord injury; Vertebrates: nervous system and sense organs; Motor pathway; Central nervous system; Motor neuron; Ionic current; Synaptic inhibition; Neuromodulation; Motor unit; Recruitment; Neurological disorder; Inward current; spinal cord injury; Nervous system diseases; Serotonin; Discharge pattern; Excitability; Spasticity; dendrites; Muscle tonus alteration; motoneurons; Catecholamine; Dendrite; Amplification; Striated muscle disease; Neurotransmitter; Norepinephrine; persistent currents
• ISSN: 0148-639X; 1097-4598
• DOI: 10.1002/mus.20261

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. Muscle Nerve, 2005