The trigemino-vascular system, its role in migraine
In a few simple words:
The trigeminovascular system is an essential component in the pathophysiology of migraine . The trigeminal nerves are the fifth pair of cranial nerves that innervate the sensitivity of the face, the meninges (the envelope of the brain) and its blood vessels .
During a migraine, it is this system that is activated . Indeed, the brain does not hurt in itself, it is not innervated. The pain felt during a migraine or headache attack comes mainly from the meninges. This system is activated during a headache and transmits information through its lower nucleus then to the thalamus and finally the cortex . When the information reaches the cortex, the patient is able to describe and characterize his pain. If it is a question of non-painful stimuli, like touching the face, the information travels in the same way, but transits at the level of higher nuclei. The set of nuclei is called the trigeminal sensory complex.
In the case of migraine, the sensory complex of the trigeminal is gradually sensitized . That is to say that its activation threshold decreases . It therefore triggers more easily. This sensitization is called central , because this complex is located in the brainstem, therefore in the central nervous system. This central sensitization increases as migraine attacks progress and is the main factor in a transition to chronic migraine . In other words, the more the patient has migraines, the more easily and intensely the sensory complex of the trigeminal activates and the more the frequency of attacks will tend to increase. This article reviews the anatomy of the trigemino-vascular system to better understand the pathophysiology of migraine and headaches.
However, we advise you to read " the basics of migraine " before going any further.
From the trigemino-vascular system to the sensory complex of the trigeminal
The headaches felt during a migraine attack are distributed over the entire hemicrane, from the frontal to the occiput and seem to be the consequence of the activation of the trigemino-vascular system .
The term trigemino-vascular encompasses the entire pathway from the innervation of the meninges , their blood vessels and sinuses , to the sensory complex of the trigeminal and first cervical segments of the spinal cord . Activation of these structures by mechanical, electrical, or chemical stimuli results in headaches similar to those experienced during a migraine attack .
The anatomy of the trigemino-vascular system has been extensively studied over the past 70 years and has led to a better understanding of the physiology of migraine and the distribution of its pain. Peripheral messages from the meninges originate at the level of polymodal nociceptors which constitute the free endings of weakly myelenized Aδ fibers and unmyelinated C fibers (nociceptive fibers). They mainly convey within the ophthalmic branch of the trigeminal nerve (V1) and project into the trigeminal sensory complex (also called caudalis or Sp5C) (Goadsby et al. 2017).
To better understand, you can read the following article: The basics: mechanism of migraine
Relay of sensitivity and nociception
The sensory complex of the trigeminal (CST) is the first relay of somatosensory information (mechanical, thermal and proprioceptive) coming from the orofacial sphere and the meninges . It extends in the brainstem from the first cervical segments to the caudal limit of the midbrain.
This complex is divided into 2 nuclei : rostrally, the main nucleus and caudally, the spinal nucleus. The latter is itself subdivided into 3 sub-nuclei which are in the rostro-caudal direction: the oral sub-nucleus (Sp5O), the interpolar sub-nucleus and the caudal sub-nucleus (Sp5C) (Marieb et al. 2014 ).
The main subnucleus along with the interpolar subnucleus are the relays for tactile, proprioceptive, and discriminative sensitivity . Sp5C and Sp5O are the relays of nociception . Only Sp5C receives afferents from C-fibers, which project into outer layers I and II of Rexed for cutaneous primary nociceptive afferents and exclusively to layer I for meningeal afferents (Melin et al. 2017).
Myelinated Aδ fibers project to the three subnuclei of the spinal nucleus of the trigeminal sensory complex. Those originating from deep tissue only project to the Sp5C level. (Nishimori et al. 1986). Nonspecific nociceptive neurons (WDR) are also present in the deep layers. The Sp5C also receives afferents from the C1 to C3 roots, responsible among other things for the innervation of the neck muscles. This anatomical interaction of Sp5C and the cervical cord would explain the projection of cephalic or dental pain at the cervical level and vice versa.
The rest of this article is here: ALTERATION OF THE SENSORY COMPLEX OF THE TRIGEMINAL AND THE ASCENDING TRACT
Marieb, Elaine, and Katja Hoehn. 2014. Human Anatomy and Physiology . Pearson Education France.
Melin, Céline, Florian Jacquot, Nicolas Vitello, Radhouane Dallel, and Alain Artola. 2017. “Different Processing of Meningeal and Cutaneous Pain Information in the Spinal Trigeminal Nucleus Caudalis.” Cephalalgia 37 (12): 1189-1201. https://doi.org/10.1177/0333102416673204.
Nishimori, T., M. Sera, S. Suemune, A. Yoshida, K. Tsuru, Y. Tsuiki, T. Akisaka, T. Okamoto, Y. Dateoka, and Y. Shigenaga. 1986. “The Distribution of Muscle Primary Afferents from the Masseter Nerve to the Trigeminal Sensory Nuclei”. Brain Research 372 (2): 375-81. https://doi.org/10.1016/0006-8993(86)91148-0.
