An Estimate of the Ratio of Propriospinal to Long Tract Neurons in the Sacral Spinal Cord of the Rat

There are, to our knowledge, no estimates of the number of propriospinal and long tract neurons to mammalian spinal cord. The present study uses retrograde transport of horseradish peroxidase from axons transected by cutting the cervical spinal cord as a method for marking long tract neurons in the S2 segment of rat spinal cord. The findings are that long tract neurons make up approximately 1% of the total neurons of the spinal cord. Since motor neurons make up 1-2% of the total number of neurons in this area of cord, the remaining 97% are almost certainly propriospinal cells and thus propriospinal neurons are numerically predominant.