04 - Spinal Input

Cross references:     

04 - Spinal Input

I want to say more about the spinal cord.  Since I am advocating for a "bottom-up" view of human behavior, it was important to me that I introduce the spinal locomotor generator (SLG) and make the point that, at least to some extent, behavior begins in the spinal cord.  However, there is much more to the spinal cord than just the SLG, and I wish to now specify the spinal inputs to the reticular formation.  

Most importantly, there is the Spinoreticular Tract.  In case you're not familiar with the notation, tracts are named by their begining and ending points; thus the "spinoreticular tract" begins in the spinal cord and ends in the reticular formation.  Carpenter says "Spinoreticular fibers originate from all spinal levels, presumably from cells located in the posterior horn."  [C:280]  Note the ambiguous term "presumably".  The reticular formation is so totally crossconnected with itself that, as of 1983, specific details still had not been worked out.  

The neurons in the spinoreticular tract lead directly from the spinal cord to the reticular formation.  However, Carpenter lists 11 other tracts from the spinal cord which end primarily in structures other than the reticular formation but which have collaterals which do end in the reticular formation.  Collaterals are branches of an axon that are distinguished from the main axon by ending in a different part of the brain than the main axon and by being smaller and having fewer branches at their end.  Since they are smaller, their output is a similar but somewhat abbreviated version of the output of the main axon.  Nerve impulses travel in trains of varying frequency.  If several closely spaced impulses arrive at a large collateral or axon branch, most, perhaps even all, of them will be transmitted by the large collateral or branch.  However, if the collateral or branch is small, only a few of them will be transmitted. 

K&W briefly mentions the dorsal column nuclei on [K&W:385].  These receive tactile and kinesthetic input for all of the body below the face.  If I understand correctly, sensory nerves are considered to end in sensory nuclei specific to that sense modality.  [K&W:385] shows that temperature and pain neurons end in the spinal cord.  This suggests that there is a temperature-pain sensory nucleus in the spinal cord.  However, I was not able to find any reference to it inspite of a diligent search. Carpenter actually lists two tracts which carry temperature and pain information; the Lateral Spinothalamic Tract [C:273] and the Spinotectal Tract [C:276].  The Anterior Spinothalamic Tract conveys the sensation of light touch [C:270-271].  The Anterior [C:279] and Posterior [C:276-278] Spinocerebellar Tracts carry information from muscle spindles, pressure receptors, and stretch receptors from individual muscles.  Carpenter also briefly mentions the Spino-olivary Tract [C:280], the Spinopontine Tract [C:282], the Spinovestibular Tract [C:282] and the Spinocortical Tract [C:282].  

As far as I know, all of the above tracts are stimulatory so that input along them causes the reticular formation to relay stimulatory input both to structures farther forward, such as the cerebral cortex, and also back to the spinal locomotor generator in the spinal cord.