INTRACELLULAR-RECORDING FROM SYMPATHETIC PREGANGLIONIC NEURONS IN CATLUMBAR SPINAL-CORD

Sympathetic preganglionic neurons (SPN) are responsible for the contro l of many autonomic targets including the heart and blood vessels. Pre vious intracellular studies have examined the morphology of SPN in the thoracic spinal cord, but there are no intracellular studies of SPN i n the lumbar spinal cord. In this study we identified lumbar SPN using intracellular recording and dye-filling so that we could study their entire soma-dendritic tree, as well as their axons. At the same time, axonal conduction velocity was measured, and any evidence of an input in phase with phrenic nerve discharge was noted. Intracellular recordi ngs were made from SPN in the L(3) (n = 125) and T-3 (n = 17) segments of the cat spinal cord. Axonal conduction velocities ranged from 0.6- 8.4 m/s. In 85 lumbar SPN, the recordings lasted long enough to assess respiratory-related modulation. A respiratory-related modulation of t he membrane potential was seen in 7 of these 85 neurons. All 7 respira tory-related neurons had a conduction velocity of 2.0 m/s or less, whi le none of the SPN with conduction velocities of more than 2.0 m/s had a respiratory rhythmicity. Histological analysis of 50 biocytin-fille d SPN, including 3 with a respiratory-related modulation of their memb rane potential, revealed that they occurred mostly in the principal pa rt of the intermediolateral cell column and tended to be elongated in the rostro-caudal direction. Dendrites ramified in the intermediolater al cell column, the dorsolateral white matter and the ventral and medi al gray matter. Axons arose either from cell bodies or from primary de ndrites and did not bifurcate or have varicose intraspinal collaterals . This is the first report of the morphology of intracellularly filled SPN in the lumbar spinal cord.