From periodontal mechanoreceptors to chewing motor control: A systematic review
Introduction
Functional movements of the stomatognathic system and their relative forces depend on signals arising from various sensory organs in the orofacial structures.
A special role is played by periodontal mechanoreceptors and their sensory innervation, located in the periodontal ligament, that is the optimal location for detecting the functional forces on the teeth. They are involved in mechanotransduction and chewing motor control, but there are important limitations of knowledge in the field. For exemple (Piancino & Kyrkanides, 2016), even though mastication is a dynamic process, studies regarding periodontal mechanoreceptors are usually conducted in static conditions and mostly in animals, that are characterized by different teeth and occlusion with respect to the humans, often disregarding the functional differences of teeth.
There are, mainly, two type of studies, hystological and electrophysiological, that are not easy to correlate and without agreement as regards the results.
This article aims to review the progress in the field, especially during the last three years, with a special attention to the functional significance of experimental results.
There have been a number of molecular reports; however, to understand the impact of these reports on the mechanisms of motor control we need to go back to the earliest physiological studies and these are briefly described, commented and integrated with recent molecular data.
The main results of basic research will be summarized in the first part of this review, dividing the animal from the human studies, the second part being dedicated to the signal pathways arising from mechanotransduction.
Section snippets
Search strategy
A systematic review of the literature was conducted. Original articles were searched through Pubmed, Cochrane Central database and Embase until january 2016.
The research has been done with the following free words: periodontal AND (‘mechanoreceptor’/exp OR ‘mechanoreceptor’), periodontal mechanorecept* NOT dental implant*, periodontal mechanorecept* AND brain NOT dental implant*; and with the following MESH Terms (“Periodontium/innervation”[MeSH Terms]) AND “Neurons/physiology” [MAJR],
Search results
1466 articles were identified through database searching and screened by reviewing the abstracts. 160 full-text were assessed for eligibility, and after 109 exclusion, 51 articles were included in the review process as reported in Fig. 1.
Type of selected studies
Studies selected by the review process were mainly divided in studies on animal (Chen and Wong, 2013, Higuchi et al., 2008, Hitomi et al., 2009; Honma, Kato, Shi, Yatani, & Wakisaka, 2012; Honma, Taki, Lei, Niwa, & Wakisaka, 2010; Iizuka et al., 2009; Jabbar et
Cytological features and cytochemistry
Periodontal mechanoreceptors are known to be receptors in the periodontal ligament that respond to surprisingly low contact force levels (<1 N) (Newton) applied to the teeth. The functions of nerve fibers in the periodontal ligament, junctional epithelium and gingiva are coordinated with the dental pulp and dentin innervation to form an integrated set of sensory systems needed for normal somatosensory reflexes and perception of the external forces on teeth. Their ultrastructural specialization
Histological studies
The studies in humans regarding the macroscopic anatomy are few and limited, due to the difficult isolation of the integral human PDL (Table 1). Huang (Fig. 3) studied the human PDL in the lower canine of a cadaver and he found dense innervations by myelinated nerve fibres in close proximity to collagen fibres and alveolar bone and a more densely innervation in the apical as well as mesial and buccal sites of the human canine (Huang et al., 2011).
Some recent studies have been conducted on
Central connections of the trigeminal primary afferent neurons: is there a bias?
Understanding the signal pathways arising from periodontal mechanoreceptors is important for the comprehension of the mechanisms that control the masticatory forces. For this reason, special attention has been dedicated to review the literature regarding the central connections of the afferent neurons of periodontal mechanoreceptors. In particular, studies regarding the location of the cell body of the primary afferent that conveys information from the mechanoreceptor to the central nervous
Sensory feed-back to the rhythmic pattern generator
Recent research has underlined the significance of feed-back from periodontal input to the central pattern generator in the brainstem, during chewing (Morquette et al., 2012). The afferent neurons in the trigeminal ganglion are active during mastication, when using the molar teeth (Fig. 6, Fig. 7), especially during the power phase of closure, and are sensitive to the force (or to changes of force) applied to the tooth. A model has recently been proposed in which sensory feedback might be
Future directions
The actual knowledge of the periodontal mechanoreceptors let us conclude that they are very refined neural receptors deeply involved in the activation and coordination of the masticatory muscles. Strictly linked to the rigid structure of the teeth, they determine all the functional physiological and pathological processes of the stomatognathic system, playing an important role in the sensorimotor cortical processes both adaptive and compensatory. The knowledge of their complex features is
Funding
Authors reported no study funding.
Conflict of interests
No conflict of interest.
Acknowledgment
Thank you to Dr. Luca Cortina for his technical assistance with the images.
The authors declare that they have no proprietary, financial, professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in this manuscript.
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