Sleep and Pain

There is a very direct connection between sleep and pain, making anesthesia for surgery possible. The link is the hypothalamus, which sits just below the thalamus.

Brain imaging is done with either positron emission tomography or more commonly functional MRI. When a scientist wants to know about specific tracts or fiber bundles, it is often helpful to add tensor MRI.

The hypothalamus is a very tiny brain part which sits just below the thalamus. The anterior hypothalamus controls sleep facilitation, while the posterior hypothalamic grey matter controls SLEEP and PAIN control. How interesting that these two areas, so central to general anesthesia, both relate to the posterior hypothalamus. Montagna found in Neurol Sci. 2006 May;27 Suppl 2:S138-43, that hypothalamic pain is related to the autonomic nervous system. We have suggested before that the autonomics may represent a way for pain to bypass the cord and reach the brain.

Montagna showed connections between pain centers in the hypothalamus with other known pain areas:
“[the] brainstem periaqueductal grey, the locus coeruleus and the median raphe nuclei, all involved in sleep mechanisms and also in the descending control of pain perception.” Bear in mind that most of what you read at painonline is about the conscious ska somatic nervous system aka voluntary nervous system. Autonomic pain is more heavily related to the above named structures.

So called “migraine” is linked to the dorsal pons (part of the brainstem) according to Montagna, while things like cluster headache and trigeminal autonomic cephalagia (autonomic headache) is related to activation of the posterior hypothalamic grey.

By comparison, using PET scans, May recently confirmed the neuroimaging of head pain, but also showed in J Headache Pain. 2006 Aug 11 that the “(hypothalamic grey) is involved in the pain process in either a permissive or triggering manner rather than simply as a response to first-division nociception per se.” Translated, this means activation of the posterior hypothalamus is involved in the severe pain state known as “cluster headache” which is similar in most ways to the dural headache found in some with central pain in the head, or the dura supplied by the trigeminal nerve. Cluster headache seems to have more of an episodic quality, and is somewhat associated with smoking, just as some with central pain have reported that it also can be triggered by smoking. May relates this phenomenon to a socalled “trigeminovascular reflex”. Of course, he is using a functional term rather than an anatomical one, as no one really truly knows what is going on.

We welcome this valuable research and hope the imaging doctors will continue to do this work. If we can find a way to correlate the imaging studies with molecular pain chemistry, we are going to know a great deal more about nerve injury pain, which types link to the autonomic system, and which link to the somatic nervous system.

It has been a long time coming in interesting the radiologists in pain, but the day has arrived. PET involves expensive radioactive tracers, whereas functional MRI can do essentially the same thing by studying oxygen consumption, which is the equivalent of activation. Tensor analysis is still so new that few can manage it, and fewer still direct their expertise to pain. Still, we are on the right path, and it has become impossible to say those with pain are imagining it.