Efferent fibers in the cervical sympathetic nerve influenced by light

Abstract

Spontaneous electrical activity of single-fiber preparations isolated from the central stump of the cervical sympathetic nerve has been recorded, the discharge frequency of which was greatly modified by changing the intensity of light reaching the cat's eyes. These fibers showed the highest firing frequency during darkness and the lowest in light (sometimes complete inhibition) and a gradation of the response intermediate illumination values. Light stimulations ranged from 0 to 1500 lux. The close correlation between the discharge frequency of these fibers and at the light stimulus intensity suggested that the fibers originated from the Budge's ciliospinal center and were directed to the dilatator pupillae muscle. Analysis of these units, besides confirming the intervention of the sympathetic system in the development of the pupillary light reflex, corroborates the hypothesis of a larger participation of sympathetic nerves in the reflex response to darkness than in the response to light. The role of sympathetic nerves in the direct light reflex appeared larger than in the consensual one. The conduction velocities of the light-responsive components were in a range of 6 to 25 m/sec (usually between 10 and 15 m/sec) as determined by (i) the action potential evoked in the single-fiber preparations, (ii) the functional analysis of compound action potentials, and (iii) the use of the antidromic occlusion method on the whole cervical sympathetic nerve. Very small doses of Nembutal (5 mg/kg) greatly decreased the resting discharge of the sympathetic light-responsive fibers and completely abolished the response to darkness.