Movement of Substance Proximo-Distally in Nerve Axons as Studied With Spinal Cord Injections of Radioactive Phosphorus

Abstract

Small volumes of radioactive phosphorus (P³²) were injected into the lumbar 7th (L7) spinal cord of cats by means of fine glass capillaries to study the movement of substance down the motor axons. A gradient of flow over a period of days was indicated when the ventral roots were divided into portions proximal and distal from the cord and each measured for its content of P³² activity. The ratio of P³² activity of the lower to upper portions increased exponentially when plotted as a function of the number of days after injection. The possibilities of a) intra-axonal distal flow following an uptake of P³² by the cell body and dendrite, and b) an adventitious interaxonal (endoneural) distal flow was examined. Activity in the ventral roots was always greater than that in the dorsal roots. The asymmetry supports the concept of an intra-axonic movement since interaxonic flow should be equally possible in the ventral and dorsal roots. A group of animals had the L7 segments of their cords asphyxiated by means of local pressure which destroyed the motoneurons. In these animals, P³² was injected 1 week later. There was a greatly diminished P³² content in the ventral roots of animals so treated when sampled 1 week after injection. This evidence also supports the concept of an intra-axonic movement of substance distally in the axons.