Somatic & Visceral Referred Pain Explained | Pain Neurophysiology
Before you start watching this video, we highly recommend you read our post on somatic and visceral referred pain in which we explain the basic underlying mechanism of referred pain. You already know the basics? Okay, let’s jump right in:
In the case of referred pain, pain is perceived in a region other than the site of the painful stimulus. Thus, pressure or load on the place where the pain is felt usually does not lead to an increase of pain intensity. However, pressure or load on the place of the sensitized nociceptors results in an increase in pain intensity in the referred area. In the case of cervicogenic headache, nociception in the structures of the neck leads to referred pain to the head. But how does this happen?
If we follow the convergence-projection theory outlined in our other post, we first need a structure responsible for nociception in the high cervical area that has a low density of nociceptive afferent innervation. These are usually structures that are lying deep, such as the facet joints including their joint capsules at C2/C3 or the alar ligaments reaching from the dens of C2 to the occiput for example. The affererent innervation of those structures converges on the second-order neuron in the dorsal horn at the height of C1/C2.
At the same time, our face has a very high density of afferent nociceptive innervation and receives sensory innervation from cranial nerve number V – the trigeminal nerve. The trigeminal nerve in turn converges onto the second-order neuron in the trigeminal nerve nucleus, which is the largest cranial nerve nucleus. It extends all the way from the midbrain, to the pons and medulla into the spinal cord until C1/C2. So the trigeminal nerve’s innervation and the innervation of deep structures of the upper cervical spine converge onto the same spinal segment.
So when the afferent nociceptive stimulus from the neck, travels to the second-order neuron in the dorsal horn at segment C1/C2 and finally reaches the somatosensory cortex, this part of the brain then has to figure out the origin of the stimulus. In this case, the brain makes a projection error and decides that the nociceptive stimulus must be coming from the area with the higher nociceptive afferent innervation, which is the face rather than the poorly innervated upper cervical area. In other words, the brain projects pain into the fronto-orbital area of the head.
If the whole face is innervated by the trigeminal nerve, why do we only feel headaches in the fronto-orbital area and not the cheek and the jaw? The trigeminal nerve splits into 3 different branches, which are:– The ophthalmic nerve supplying the scalp, forehead, and orbital area amongst others. The maxillary nerve supplies the cheek, upper lip, and upper teeth amongst others. And the mandibular nerve supplies the lower lip, chin, and jaw all the way up to the temporal area
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When those 3 nerve branches reach the trigeminal nerve nucleus, they are basically inverted. Remember that the trigeminal nerve nucleus is large and it consists of three different subparts. The mandibular and maxillary nerves converge onto the pars oralis and pars interpolaris of the trigeminal nerve nucleus respectively, which both don’t reach as far caudal as the spinal cord. Only the ophthalmic nerve converges onto the pars caudalis of the trigeminal nerve nucleus which is located in the spinal cord at the height of C1/C2, exactly where the afferents from structures in the upper cervical spine converge.
It’s important to mention that we are talking about unilaterally innervated structures of the neck and face. So referred pain from the structures of the neck on one the right for example will always lead to headache on the right side and the left side will refer to the left side.
Johnston, M. M., Jordan, S. E., & Charles, A. C. (2013). Pain referral patterns of the C1 to C3 nerves: implications for headache disorders. Annals of neurology, 74(1), 145-148.
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