Physical Factors Involved in Ultrasonically Induced Changes in Living Systems: I. Identification of Non-Temperature Effects

Abstract

The changes in physical variables which accompany a high intensity ultrasonic disturbance in liquid media and which appear important in understanding the effects of such disturbances on tissue are discussed with special reference to temp. Ultrasound (35 w./sq. cm., frequency 1 mc.) is without effect on excitability, wave form of the spike potential, or propagation velocity of excised peripheral nerve, even after prolonged exposures. The excised crayfish ventral nerve cord exposed to ultrasound exhibits a reduction of spontaneous activity after several seconds exposure and recovers its original activity about one minute after the ultrasound is turned off. Frogs positioned so that ultrasound is incident on the dorsal surface over the lumbar enlargement of the spinal cord exhibit paralysis of the hind legs after 4.3-sec. exposure at room temp. and after 7.3-sec. exposure at 1[degree]-2[degree]C. Histological examination of the sciatic nerves shows extensive degeneration of nerves, and the spinal cord shows marked pathology of the lower motor neurones. Temp. measurements indicate that peripheral nerve and crayfish ventral nerve cord exhibit a maximal rise of 1-2[degree]C. whereas the spinal cord of intact frogs exhibit temp. increases in temp. of the order of 25[degree]C. The effect of ultrasound on the system studied is produced by physical factors other than temp. Of these factors, cavitation is the one most easily controlled and will be investigated in the future. A method is presented for obtaining acoustic absorption coefficients by measuring the initial rate of change of temp. in various test objects.