The use of local anaesthetic microinjections to identify central pathways: a quantitative evaluation of the time course and extent of the neuronal block

Summary The time course and extent of local anaesthetic blocks within the spinal cord of cats were evaluated. A monopolar stimulation electrode with the tip lowered into the dorsal columns (DC) 1000 m below cord surface was used to activate antidromically DC fibers at the T13 level and evoke cord dorsum potentials at the level of the lumbar spinal cord. The amplitude of the negative deflection, the N-wave, was determined for various stimulation intensities (stimulation-response-function, SRF). Lidocaine (1%) was microinjected in volumes of 0.5 or 1.0 l into the DC from a glass micropipette 1 mm caudal to the stimulation site. Conduction block was characterized by a reversible shift of the SRFs to higher stimulation intensities. The diameter of the blocked area in the transverse plane was evaluated from threshold intensities and was found to be 0.9±0.1 mm 4 to 30 min after the injection of 0.5 l lidocaine and 1.6±0.36 mm 10 to 45 min after the injection of 1.0 l lidocaine. In the sagittal plane, the diameter of the blocked area following 1.0 l lidocaine was found to be up to 2.8 mm. The DC-block was reversible within 92 min following injection of 1.0 l and 69 min after the injection of 0.5 l lidocaine. The application of the present findings for blocks in other CNS structures is discussed.