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

W Michael Panneton,Bingbing Pan,Qi Gan

doi:10.3389/fneur.2017.00522

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.

Highlights

Much behavior is reflex in nature and serves basic vegetative functions that are usually less com plex and more uniform across species
“dermatomes” by definition do not exist in the MDH, our data support the hypothesis in humans [8] of sequential bands of innervation continuing from rostral cervical dermato mes over the facial skin and going intraorally (Figure 5), and transitioning to visceral structures in the throat via the paratrigeminal nucleus
Our tract-tracing studies show the somatotopic map of the trigeminocervical complex is reinforced by topographic projections to the facial motor nucleus

Summary

INTRODUCTION

Much behavior is reflex in nature and serves basic vegetative functions that are usually less com plex and more uniform across species. An injection into the ventrolateral part of the rostral MDH (Figure 4E; blue arrow), where sensory fibers from the cornea and anterior ethmoidal nerve project, labeled dorsolateral areas of the facial nucleus (Figure 4D; green arrows), which includes motoneurons innervating the orbicularis oculi muscle. After injections of FG into dorso lateral facial areas, which contain motoneurons projecting to the orbicularis oculi muscle, most retrogradely labeled neurons were found in ventrolateral portions of the rostral trigeminocervical complex (Figures 3I–K) and others were noted more caudally (Figures 3M,N) These areas overlap the central projections of the cornea and conjunctiva, respectively [7]. Dorsomedial neurons in the subnucleus oralis are considered important as premotor to the trigeminal motor nucleus for jaw reflexes; we consider them as closely associated with lamina V neurons found more caudally and as interneurons in reflex behavior

SUMMARY AND PERSPECTIVES

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