Somatotopy in the Medullary Dorsal Horn As a Basis for Orofacial Reflex Behavior

Abstract

The somatotopy of the trigeminocervical complex of the rat was defined as a basis for describing circuitry for reflex behaviors directed through the facial motor nucleus. Thus, transganglionic transport of horseradish peroxidase conjugates applied to individual nerves/peripheral receptive fields showed that nerves innervating oropharyngeal structures projected most rostrally, followed by nerves innervating snout, periocular, and then periauricular receptive fields most caudally. Nerves innervating mucosae or glabrous receptive fields terminated densely in laminae I, II, and V of the trigeminocervical complex, while those innervating hairy skin terminated in laminae I–V. Projections to lamina II exhibited the most focused somatotopy when individual cases were compared. Retrograde transport of FluoroGold (FG) deposited into the facial motor nucleus resulted in labeled neurons almost solely in lamina V of the trigeminocervical complex. The distribution of these labeled neurons paralleled the somatotopy of primary afferent fibers, e.g., those labeled after FG injections into a functional group of motoneurons innervating lip musculature were found most rostrally while those labeled after injections into motoneurons innervating snout, periocular and preauricular muscles, respectively, were found at progressively more caudal levels. Anterograde transport of injections of biotinylated dextran amine into lamina V at different rostrocaudal levels of the trigeminocervical complex confirmed the notion that the somatotopy of orofacial sensory fields parallels the musculotopy of facial motor neurons. These data suggest that neurons in lamina V are important interneurons in a simple orofacial reflex circuit consisting of a sensory neuron, interneuron and motor neuron. Moreover, the somatotopy of primary afferent fibers from the head and neck confirms the “onion skin hypothesis” and suggests rostral cervical dermatomes blend seamlessly with “cranial dermatomes.” The transition area between subnucleus interpolaris and subnucleus caudalis is addressed while the paratrigeminal nucleus is discussed as an interface between the somatic and visceral nervous systems.

Multiple Morphometric Assessment of Microglial Cells in Deafferented Spinal Trigeminal Nucleus

“…The "activation" of MG was most prominent in laminae I-II, decreased in laminae III-IV, and was essentially absent from the deeper laminae. Medio-laterally, it covered the lateral 4/5 of the nucleus, which somatotopically barely exceeds the target region for IoN afferents toward adjacent terminal territories of the ophthalmic (ventrolaterally) and mandibular (dorsomedial) trigeminal branches (Hayashi, 1982;Jacquin et al, 1986;Panneton et al, 2017;Fernández-Montoya et al, 2018).…”

Transcranial direct‐current stimulation reduces nociceptive behaviour in an orofacial pain model

“…The trigeminal primary afferents pass into the brainstem and terminate in the trigeminal brainstem sensory nuclear complex (subnuclei interpolaris, oralis and caudalis) . The subnucleus caudalis has been specifically implicated in orofacial nociceptive mechanisms, and it is compared to the spinal dorsal horn due to its many analogous morphological and physiological features and now it has been termed the medullary dorsal horn . In these regions, the afferent nociceptive fibres release neuropeptides (substance P and ATP) and excitatory amino acids such as glutamate, that increasingly excite nociceptive neurons (nociceptive‐specific, NS; and wide dynamic range, WDR), a process known as central sensitisation.…”

“…Previous study has shown that, in rats, CFA administration into the TMJ induces an increase in the calcitonin gene‐related peptide (CGRP), nerve growth factor (NGF), interleukin‐1β (IL‐1β) and tumour necrosis factor‐α (TNF‐α) levels, until 6 weeks after administration . Inflammation in the TMJ region results in increased excitability of the trigeminal subnucleus caudalis, which is a sensory nucleus with a laminated structure similar to the spinal dorsal horn and, due to this close similarity has been termed the medullary dorsal horn …”

Antagonism of Transient Receptor Potential Ankyrin Type-1 Channels as a Potential Target for the Treatment of Trigeminal Neuropathic Pain: Study in an Animal Model

“…According to Terayama et al [ 116 ] and Panneton et al [ 117 ], the central afferent innervations of the IoN are mostly distributed in (but not restricted to) the dorsal and lateral part of the Sp5C, projecting to all the laminae. The pattern of CGRP and SP protein related to the painful component of the IoN was investigated in the superficial laminae of the Sp5C.…”