ABSTRACT
Voluntary movements arise from activity in the cerebral cortex, and the instructions are then carried to the spinal cord in order to activate motoneurones and produce movement. Voluntary inputs from the cortex do not have private access to spinal motoneurones, separated from any other systems. In the vast majority of cases, cortical inputs first contact interneurones which then relay the commands to motoneurones. Since the same interneurones also receive continuous input from sensory receptors (and hence might be thought to participate in spinal reflexes) as well as from interneurones from other parts of the spinal cord, this means that by the time cortical input reaches motoneurones it has been filtered by multiple lower level systems. In higher primates and in man, cortical input can access some motoneurones via a special direct pathway (the corticomotoneuronal pathway), which is often supposed to play a critical role in volitional movement. However, even if this input is strong (and there is little comparative evidence on this) the excitability of motoneurones will have been biased by the multiple other inputs that each one receives. Thus, even this connection does not guarantee the brain a straightforward control of muscle.
