Last updated: Wed, Jun 21, 2017
The ascending projection neurons are concentrated in lamina I. Some ascending projection neurons also originate in lamina III through VI, as well as from the ventral horn. Detailed studies on the spinal cord of the rat show that there are only about four hundred ascending lamina I projection neurons on each side of the body at level L4 (lumbar level 4), although they are much more numerous at the lumbar and cervical enlargements of the spine in cats and monkeys. About 95% of these four hundred neurons project to an area of the brain stem called the lateral parabrachial area; one third project to another brain stem area called the periaqueductal gray, and about one quarter project to another brain stem area called the solitary tract nucleus. Only five percent project to the thalamus at the center of the brain.
Ascending projection neurons send out side branches as they ascend, and so they are able to participate in the processing of information within the spinal cord.