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Corticomotoneuronal synaptic connections in normal man: an electrophysiological study
A M de Noordhout1, G Rapisarda, D Bogacz
1University Department of Neurology, Hôpital de la Citadelle, Liège, Belgium.
Brain : a Journal of Neurology
|July 2, 1999
Summary
Direct cortical stimulation reveals that monosynaptic connections predominantly influence low-threshold motor units in both upper and lower limbs. This suggests a dominant direct pathway from the cortex to spinal motor neurons for precise motor control.
Area of Science:
- Neuroscience
- Motor Control
- Human Physiology
Background:
- Understanding the directness of cortical pathways to spinal motor neurons is crucial for deciphering motor control mechanisms.
- Transcranial electrical stimulation offers a method to probe these pathways non-invasively.
Purpose of the Study:
- To determine if corticomotor connections to spinal motor neurons are primarily mono- or oligosynaptic.
- To compare the synaptic properties of corticomotor pathways with known monosynaptic reflexes.
Main Methods:
- Constructed peri-stimulus time histograms (PSTHs) of single motor unit firing probability.
- Used slightly suprathreshold anodal transcranial electrical stimulation of the motor cortex.
- Compared PSTH peak durations with those evoked by 1A afferent stimulation (known monosynaptic reflex).
Main Results:
- Anodal cortical stimuli elicited synchronized firing in upper and lower limb motor units.
- PSTH peaks in flexor carpi radialis and soleus motor units were narrower than 1A reflex peaks, indicating monosynaptic transmission.
- Non-monosynaptic pathways were not evident even with near-threshold stimuli.
- Inhibition was observed in some muscles, consistent with 1A inhibitory interneuron activation.
Conclusions:
- Monosynaptic corticomotor neuronal transmission is dominant for low-threshold motor units across limb muscles.
- The direct cortical pathway plays a significant role in fine motor control, particularly for hand and finger muscles.
- Evidence supports the existence of inhibitory corticomotor pathways acting via interneurons.