Ascending collaterals of cutaneous neurons in the fasciculus gracilis of the cat

Primary sensory neurons with myelinated axons in the sural nerve of the cat were found to be divisible into 3 systems on the basis of the length of their central collaterals in the dorsal columns. The short system consists of neurons that ascend only a segment or two in the fasciculus gracilis above their level of entry into the spinal cord. It is composed of all neurons with peripheral conduction velocities in the Aδ range and thus includes both D hair and nociceptive neurons. Approximately 35% of the Aα neurons join the intermediate system and ascend 4–12 segments before leaving the dorsal columns. This system is composed of all sural type I neurons, as well as about 40% of the G 2 hair, 40% of the intermediate field, and 50% of the F 2 field neurons in the nerve. Those nociceptive neurons conducting at Aα velocities also contribute to the intermediate system. The remaining G 2 hair, intermediate field, and F 2 field neurons, together with almost all the sural type II, G 1 hair, intermediate hair, and F 1 field neurons, join the long system and ascend to the nucleus gracilis. Fibers in the intermediate system showed a relatively abrupt decrease in conduction velocity usually of 50% or more (median 71%) a few millimeters rostral to their entry into the spinal cord. Members of the long system also decreased in conduction velocity at this point, but the magnitude of the change was typically less than 50% of the peripheral velocity (median 36%). In addition, the ascending collaterals of the long system underwent a second reduction in conduction velocity near the cervical enlargement.