The Sympathetic Component of the Sciatic Nerve

Abstract

Our present knowledge of the course of the preganglionic nerve fibers is based largely on the early experiments of Langley 1 , 2 and Bayliss and Bradford 3 who used erection of hairs, sweating and vascular changes for recording sympathetic activity. These types of effector activity do not lend themselves readily to quantitative estimation. Moreover sweating and vasomotor changes offer additional difficulties in that they are complex responses. Sweating is influenced by accompanying vascular changes while vasomotor effects in turn are subject to rapid modification by compensatory reflexes.

In the present experiments a direct insight into the kind and magnitude of the sympathetic nerve impulses themselves was obtained by recording the action potentials in the efferent nerves. A direct current amplifier, which Marrazzi 4 had found, in recording action potentials from other parts of the sympathetic system, to be especially suitable, was utilized to actuate a Matthews oscillograph. This technique provides not only a means of tracing the pathway of preganglionic fibers to an organ or a limb but enables the analysis of an exact point-to-point representation of the sympathetic component of each ventral root in any peripheral nerve.

Method. In a series of experiments on 19 cats, lightly anesthetized with nembutal or sodium amytal the spinal nerve roots from T7 to L7, inclusive, were severed from the cord, the dorsal root and ganglion excised; and the distal cut end of the ventral root stimulated, after insulation from surrounding tissues, by accurately controlled shocks from a thyratron stimulator. The activity resulting in the sciatic nerve was detected by electrodes placed on the main branches of the ipsilateral nerve and connected to the amplifier. The B and C waves thus recorded enabled us to map the exact roots through which sympathetic fibers passed to the lower limb via the sciatic nerve.