CORTICAL MOTOR THRESHOLD AND THE THRESHOLDS AND DISTRIBUTION OF EXCITED BETZ CELLS IN THE CAT

¹ The University Laboratory of Physiology, Oxford

1. In "monopolar" stimulation of the motor cortex of lightly anæsthetized cats with single rectangular pulses of the 10·0 msec. duration, the least current needed to evoke a flick movement of the forelimb is 0·5 to 1·5 mA. This motor threshold has been compared with the thresholds for single and for iterative firing of single Betz cells. Some lay as close as possible to the point of stimulation, but at different depths in the wall of the cruciate sulcus. Others lay at different horizontal distances and in different directions from the stimulated point.

2. Sixteen Betz cells (proved to be such by intracellular recording of their responses to antidromic stimulation of the pyramidal tract) were stimulated from the nearest surface point: eight with surface positive and surface negative pulses, eight with surface negative only.

3. Cells on the convexity of a gyrus have thresholds of 25 to 50 µA (surface positive) for the discharge of a single impulse. Thresholds of cells in the depths of the sulcus do not usually exceed 250 µA. For the discharge of a high frequency burst the lowest threshold is 100 µA (surface positive).

4. Eight Betz cells were also stimulated from one or more distant points. High-frequency bursts were obtained in response to shocks at points 4 to 6 mm. away. These shocks were usually below the threshold for movement.

5. It is concluded that in punctate stimulation of the cat's motor cortex with single 10 msec. shocks just strong enough to cause movement, Betz cells as far as 4 mm. distant in all directions (including cells lying on the far side and in the walls of the cruciate sulcus) are stirred into high-frequency iterative firing. It is probable that there is also an outer zone of Betz cells firing single impulses.

Note:

These experiments owe much to the assistance of Mr. F. Elvidge.