Effect of Lissauer tract stimulation on activity in dorsal roots and in ventral roots

Abstract

In decerebrate spinal cats, microelectrode stimulation of the lumbar lateral Lissauer tract (LLT), which does not spread to the dorsal roots, produces a volley in the tract that is conducted at 0.5 to 2.5 m/s. In 38 cats, such a volley in the LLT produced a negative dorsal root potential (DRP) beginning at 15 to 20 ms and lasting 30 to 40 ms, followed by a positive DRP lasting 90 to 140 ms. In seven cats, LLT stimulation produced a positive DRP alone beginning at 30 to 40 ms and lasting 80 to 110 ms. The LLT-evoked DRP was limited to the dorsal rootlets located within a few millimeters on either side of the stimulus point, unlike the dorsal root-evoked DRP. This local distribution parallels the limited spread of the LLT-evoked compound action potential. The LLT-evoked DRP occluded the dorsal root-evoked DRP. Conditioning stimuli applied to the LLT produced early facilitation and late inhibition of the monosynaptic ventral root reflex. The polysynaptic reflexes were inhibited for as long as 150 ms. With higher stimulus intensities, the polysynaptic reflexes were first inhibited and then facilitated for 150 to 1000 ms. Barbiturates first prolonged and then attenuated the LLT-evoked DRP. Picrotoxin reduced the LLT-evoked DRP but failed to have any effect on the facilitation or inhibition of the ventral root responses. Naloxone produced an enhancement of dorsal root-evoked polysynaptic reflexes. Although naloxone had no effect on the LLT facilitation of the monosynaptic reflex, it antagonized the LLT inhibition of the polysynaptic reflex.