Motoneuron excitability: The importance of neuromodulatory inputs

• Published in: Clinical neurophysiology 2009-12, Vol.120 (12), p.2040-2054
• Main Authors: Heckman, C.J, Mottram, Carol, Quinlan, Kathy, Theiss, Renee, Schuster, Jenna
• Format: Article
• Language: English
• Subjects: Action Potentials - physiology; Animals; Biological and medical sciences; Electrodiagnosis. Electric activity recording; Excitatory Postsynaptic Potentials - physiology; Fundamental and applied biological sciences. Psychology; Humans; Investigative techniques, diagnostic techniques (general aspects); Medical sciences; Motoneuron; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor neuron; Motor Neurons - physiology; Nervous system; Nervous System Diseases - diagnosis; Nervous System Diseases - physiopathology; Neurology; Neuromodulation; Neurotransmitter Agents - physiology; Norepinephrine; Reflex; Reflex excitability; Serotonin; Vertebrates: nervous system and sense organs
• ISSN: 1388-2457; 1872-8952
• DOI: 10.1016/j.clinph.2009.08.009

Abstract The excitability of spinal motoneurons is both fundamental for motor behavior and essential in diagnosis of neural disorders. There are two mechanisms for altering this excitability. The classic mechanism is mediated by synaptic inputs that depolarize or hyperpolarize motoneurons by generating postsynaptic potentials. This “ionotropic” mechanism works via neurotransmitters that open ion channels in the cell membrane. In the second mechanism, neurotransmitters bind to receptors that activate intracellular signaling pathways. These pathways modulate the properties of the voltage-sensitive channels that determine the intrinsic input–output properties of motoneurons. This “neuromodulatory” mechanism usually does not directly activate motoneurons but instead dramatically alters the neuron’s response to ionotropic inputs. We present extensive evidence that neuromodulatory inputs exert a much more powerful effect on motoneuron excitability than ionotropic inputs. The most potent neuromodulators are probably serotonin and norepinephrine, which are released by axons originating in the brainstem and can increase motoneuron excitability fivefold or more. Thus, the standard tests of motoneuron excitability (H-reflexes, tendon taps, tendon vibration and stretch reflexes) are strongly influenced by the level of neuromodulatory input to motoneurons. This insight is likely to be profoundly important for clinical diagnosis and treatment.