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Last updated: Tue, Nov 19, 2024
As we've seen, there are several paths by which signals can ascend the spinal column: 1) Sensory fibers make their way up and down the spine, in some cases for many segments; 2) Interneurons carry signals up or down for several or many segments. These are called propriospinal interneurons; 3) Ascending projection neurons extend from their point of origin in the cord all the way to specific destinations in the brain stem, the thalamus, and even the cortex.
The ascending projection neurons are believed to carry most of the important sensory signals to the brain. These fibers originate with neurons in several laminae of the dorsal and ventral horns, and work their way out of the gray matter and into the white, where they come together into groups called tracts. There are a large number of these tracts (depending partly on how many you define).
Some important basic distinctions can be made between nociceptive signals and innocuous sensory signals: 1) Innocuous sensations ascend in the “dorsal columns,” which are in the area between the two dorsal horns, while nociceptive signals ascend in the “anterolateral pathways,” which are at each side near the ends of the two ventral horns. 2) Innocuous signals travel two to three times as fast as nociceptive signals. While innocuous signals might travel the length of the cord in just a few thousandths of a second, nociceptive signals might take ten to sixty thousandths. 3) Nociceptive signals are less accurately localized spatially. 4) The nociceptive pathways are less able to transmit rapidly changing or rapidly repeating signals.
The functions of the ascending signals will be described in the later section, The Ascending Tracts.