This includes both upward and downward nerve fibre masses. The nerve cell bodies are located in the gray spinal cord substance and emit their axons into the white matter close to the gray matter. Some of the axons are short and extend only one spinal cord segment up or down before diving into the gray matter and contacting - forming synapses with - nerve cells located there. Some other axons are very long and extend along almost the entire length of the spinal cord and connect e.g. neck segments with thoracic and lumbar segments.
The function of the fasciculi propriis is to coordinate the activities of the different spinal cord segments with each other.