The cortical control of motor neurones: some principles of operation

There are several ways in which the motor control areas of the brain can elicit action potentials in motor neurones. In this article, it is argued that the basis for the cortical control of motor neurones is that each corticospinal cell gives excitatory inputs to all or most motor neurons innervating a given muscle. When a muscle contracts under voluntary control with a steadily-increasing force, the asynchronous activity in many corticospinal cells will effectively result in a steadily-increasing injection of current into most or all motor neurones. When this current exceeds the level necessary to bring individual cells to threshold, they will begin to fire. The threshold current for each cell is determined by the cell's size and the biophysical properties of its membrane. As the level of this "common drive" from the cortex increases, all active cells will tend to increase their firing frequency proportionately: decreases in the common drive will cause their individual frequencies to fall proportionately. Damage to corticospinal pathways (as in strokes) will decrease the total excitatory input to all motor neurones. This will mean that a larger effort is necessary to produce more activity in cortical cells, resulting in an increased sense of exertion and an increase in the subjective sense of muscle fatigue.