562 MAKOTO SATO, GEORGE AUSTIN, AND WALTER STAHL 



injected intravenously, 0.2 cc per hour. Dorsal and ventral roots of 6th and 

 5th lumbar segments were cut intradurally and used for orthodromic and 

 antidromic stimulations, respectively. A tiny hole was made in the pia mater 

 approximately 2 mm lateral from the entry of the stimulated dorsal root, 

 using jeweler's forceps under a binocular microscope. A glass pipette micro- 

 electrode, filled with 2.75 MKCl solution, was used for both extracellular 

 and intracellular recordings from the neural elements in the spinal cord, 

 being inserted through the hole made on the surface of the cord. Gross 

 electrodes of silver wire, 1 mm in diameter, were used for recordings of 

 cord potential and monosynaptic reflex, and a large platinum plate, 3X3 

 cm, was placed between the skin and subcutaneous tissues of the back as 

 an indifferent electrode. Two different microelectrode tip sizes, 5 /x and less 

 than 0.5 ix in diameter, were used for extracellular and intracellular 

 recordings, respectively. 



The maximum rate of x-irradiation used was 1,270 r per minute at 10 

 cm from the x-ray tube. The x-ray tube, made by General Electric Co., 

 was set 10 to 15 cm above the surface of the spinal cord at the lumbar 

 segment, and in most cases a thin aluminum filter 0.25 mm thick was used 

 for homogeneous irradiation. As the main flux of the x-radiation from the 

 tube had a stereoangle of 35 degrees, the irradiated area of the cat 1,270 

 covered some of the thoracic segments as well as the sacral and caudal seg- 

 ments. 



A cathode follower input preamplifier was used with a positive feedback 

 system for improvement of frequency characteristics. In addition to this, a 

 rectangular pulse of 1 msec (Lettvin et al., 1958) was used as a monitor to 

 see the resistance of the microelectrode and the adjustment of the positive 

 feedback during recordings, being synchronized with the sweep of the 

 oscilloscope. 



Results 



Cord Potential and the Monosynaptic Reflex 



The stimulus strength and amplitude relationship was investigated for 

 each element of the cord potential and monosynaptic reflex. As shown in 

 Fig. 1, the cord potential did not show any significant change in the shape 

 of the curves after irradiation, while the monosynaptic reflex showed a 

 remarkable increase in maximum amplitude after irradiation. There was a 

 slight decrease in threshold strength of the stimulating current for all re- 

 sponses after successive irradiations. Figure 2 shows the responses of each 

 element during a series of irradiation experiments. Each point represents 

 a mean value of 7 measurements of amplitude. The control values of each 



