Introduction
The tooth is a vital organ that accomplishes various integral bodily functions. To efficiently function, they need to be protected and supported appropriately. Periodontal ligament is one such fibrous joint that anchors the teeth to the underlying bone. It is a highly specialized connective tissue that is supportive, formative, and protective in nature and is essential for withstanding the masticatory forces or oral motor behavior functions like chewing, grinding, biting, etc.
Mechanoreceptors are certain specialized neural receptors that help transmit any form of external stimulus to the organ. The periodontal ligament comprises several such receptors. These periodontal mechanoreceptors are significant for conveying the receptivity of tooth load, stress, and force to the central nervous system. They are also considered crucial for the control of oral motor behavior.
What Are Periodontal Mechanoreceptors?
Periodontal mechanoreceptors are also known as Ruffini-type endings. They are located within the periodontal ligament of the tooth. These Ruffini endings may be in fact rather large and unencapsulated with finger-like extensions into the ligaments. There is a considerable variation or complex morphology discovered within these mechanoreceptors within the periodontal ligament, however, it is only these mechanoreceptors that the periodontal ligament possesses. These mechanoreceptors are supplied by approximately 1 to 15-micrometer large nerve fibers that are myelinated.
What Is the Functional Importance of Periodontal Mechanoreceptors?
The periodontal mechanoreceptors play a crucial role in transmitting the sensory information in the form of electrophysiological responses from the tooth to the brain. There are two types of mechanoreceptors found:
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RA Receptors.
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SA Receptors.
RA stands for rapidly acting mechanoreceptors while SA stands for slow-acting mechanoreceptors. While the RA receptors are present towards the tooth fulcrum or axis, the SA receptors are closer to the apex of the root, that is, towards the end part of the tooth that is embedded within the jaw bone. Experiments also suggest that even if the same kind of force is applied to the tooth, the receptors towards the RA receptors are much less stimulated in comparison to those receptors that are located towards the root apex (SA receptors). These mechanoreceptors have a specialized role of being very directionally sensitive which could be attributed to both the direction of the impact or stress-bearing forces and the magnitude of force applied to the tooth structure.
What Is the Neurophysiology of Periodontal Mechanoreceptors?
The neurophysiologic response of the periodontal ligament is not only diverse but also complex as there is a convergence of multiple afferent nerve fibers at the level of the thalamus in the brain. Be it the stimulation of a single tooth or multiple teeth from different quadrants of our dentition, the directional sensitivity of the periodontal mechanoreceptors has been hypothesized to be more with the stimulation of the SA receptors as in comparison with the RA receptors. Research also indicates that SA receptors have an increasing potential or associated frequency with the magnitude of the force exerted on the tooth. Even in the absence of any applied loads, the afferent fibers in relation to the periodontal mechanoreceptors always remain active and have the ability to generate action potentials. The afferent fibers of these receptors eventually connect with the second-order neurons located within the trigeminal sensory and mesencephalic nucleus regions.
What Is the Periodontal Neural Response Mechanism?
The afferent fibers from the cell bodies present within the trigeminal ganglion (TG) or the trigeminal mesencephalic nucleus (MS), are mainly responsible for innervating the mechanoreceptors of the periodontal ligament. Neurophysiologic experiments of the periodontal ligament show the presence of MS mechanoreceptors that tend to be more concentrated around the root apex of the tooth. In contrast, the TG mechanoreceptors tend to be found near the mid-root of the tooth. Hence the mechanoreceptor's afferent fibers supply would be distributed diversely through the periodontal ligament.
The periodontal ligament acts as a cushion and a shock absorber for the forces that are dissipated or imposed upon the natural tooth surface. Any application of mechanical force, impact, or stimulus tends to create a region of secondary tension that builds up within the periodontal ligament that activates these mechanoreceptors. For example, any stressful force that would impact the tooth or cause the tooth to be displaced or to move from its original position within the socket will cause the tooth to experience a fulcrum force at the junction of the crown and root.
Also, the activation of the receptors depends upon the amount of force that is needed to move the tooth or displace it from its position. Hence, there is a minimum threshold of force that needs to be exerted in order to activate the mechanoreceptors of the periodontal ligament. These findings have been confirmed by oral clinicians and pathologists through oral physiology research that demonstrates why the periodontal ligament is so crucial for the functioning, protection, and absorption of forces as well as the experience of discomfort upon tooth trauma especially when the stress is towards the root apex of the tooth or at the junction of the crown and root.
At the level of the thalamus, the periodontal neurons can also be responsive toward the tooth being impacted from any direction. The TG periodontal mechanoreceptors have links with various primary functional areas of the brain. These multiple links of the periodontal neurons suggest the fact that every single tooth has an inherent receptivity that activates the mechanoreceptors in response to a stimulus or force or trauma upon the tooth structure.
Conclusion
To conclude, it is essential to understand that the human periodontal ligament plays a pivotal role in tooth protection, tactility, and jaw motor control responses. The mechanoreceptors are sensory receptors that carry external sensory stimuli from the tooth to the brain. They also play an important role in the reflexes that aid in mastication and salivation. Thus, periodontal mechanoreceptors are essential for the efficient tactile function of the teeth.
