And importantly: for these phasic responses it is the superficial dorsal horn (orange) that seems activated!

We found promising results in our first dataset, then pre-registered this analysis for the second dataset. And if we look into these phasic response results, we see a more focused activation on the ipsilateral dorsal horn:

We wondered whether at least for a certain amount of time the superficial dorsal horn would be involved and analyzed only the first 3 s of each heat block. There is evidence from recordings of action potentials of A delta fibers that some of them may fire in the beginning of a stimulus only!

Result: it is especially the deep dorsal horn that shows the strongest activation (dark red)! The superficial dorsal horn is not much involved in our activation. This was a surprise as the superficial dorsal horn is supposed to be the major termination zone of nociceptive primary afferents!

We wanted to come closer to these laminae by separating the dorsal horn into superficial (laminae I/II), middle (laminae III/IV) and deep layers (laminae V/VI) and created a high-resolution layer-specific grey matter atlas in our template space based on an annotated spinal cord cross section.

Based on cell structure the spinal cord has traditionally been divided into smaller functional units – the laminae. The dorsal horn consists of 6 laminae I-VI with the superficial (laminae I/II) and deep (lamina V) being the main entry points for afferent fibers conducting heat pain information.

This is not new: compare for example the glucose utilization that Abram & Kostreva report in 1986(!) using noxious heat pain in cats. It also fits with all the 3T spinal cord fMRI literature about heat pain that report similar activation patterns with particular focus on the dorsal horn.

And when we specifically test for activation in the ipsilateral dorsal horn, we can report significant results. 🥳🥳

If we look into this without strict thresholding, indeed we see activations on the ipsilateral side – stronger than on the contralateral side! Overall lots of activation in dorsal and ventral horns!

To investigate this, we use a simple design with blocks of 30s each using heat pain on the forearm – we have to establish the basics, right? What would we expect? Hopefully activation on the ipsilateral side and in the dorsal horn, as this is where the textbooks say heat pain is processed.

Unfortunately, these inhomogeneity artifacts increase with higher field strength, causing significant signal loss and distortions in typical EPI acquisitions! Here, you see EPI scans of participants with varying data quality. The green outline indicates the anatomical location of the spinal cord.

Within that very small spinal cord, we are interested in how the gray matter processes inputs, e.g. noxious inputs like heat pain. The gray matter is the small butterfly shape and consists of the ventral horns (associated with motor function) and the dorsal horns (associated with sensory function).