Teeth are sensory organs. Teeth are specialized organs that function to nourish and sustain life. While people eat, the brain rapidly compares food’s texture and hardness in the mouth to previous encounters and determines the best chewing strategy. Optimal chewing forces and rhythms are developed based on tactile (touch) sensory feedback from the food particle’s contact with the teeth and soft tissues as the particle progressively becomes smaller. The ability of a tooth to endure the rigors of mastication depends on having a durable stone-like structure and a complex neural control system to maintain the tooth’s integrity. The cornerstone of this neural control system is an exquisitely sensitive network of mechanoreceptors within the tooth and its neurovascular bundle (periodontal ligament). Dental mechanoreceptors play a crucial role in providing tactile sensory feedback that minimizes the stresses that the teeth endure while they pulverize vast quantities of food in a lifetime. Under the influence of pathologic conditions such as malocclusion or central nervous system disease, the teeth’s mechanosensory system can play a key role in promoting destructive or motor (oral cavity) behaviors, such as bruxism and clenching.
Mastication triggers a unique and complex neural control system that is designed to protect teeth’s structure. As part of this process, mechanoreception is the unconscious sensing or conscious perception of touch or mechanical displacement caused by stimuli such as tension, pressure, and vibration. Endodontically (root canal) treated teeth and dental implant-retained prostheses provide less mechanosensory information than vital teeth. Consequently, tooth wear and catastrophic failures of nonvital teeth may occur.
Mechanoreception is the unconscious sensing or conscious perception of touch or mechanical displacement arising from stimuli outside the body. Mechanoreceptors are sensory end organs that respond to mechanical stimuli such as tension, pressure, or vibration. The encoding of mechanoceptors, which include slowly adapting (SA)
Perception and recognition of a finely textured object that is handle or bitten relies on the ability to encode tactile cues arising from its size, shade, and roughness. The encoding of these cues occurs primarily as a result of two types of mechanoreceptors, which include slowly adapting (SA) and rapidly adapting (RA) mechanorecptors. SA mechanoreceptors fire continuous streams of action potentials as long as the stimulus (eg, touch) remains active. Because they fire continuously during contact, SA mechanoreceptors are best suited for providing awareness that an object is between the teeth.
Vibrations are produced when textured objects rub against the surfaces of the skin or teeth. RA mechanoreceptors fire briefly upon initiation of vibrating or rapidly accelerating stimulation, stop quickly, and are able to re-fire rapidly in response to a new stimulus. The rapid on/off firing characteristics of RA mechanoreceptors make them well suited for sensing the vibrations associated with textural assessment. Vibration perception through human skin is essential for accurate perception of textured objects that are grasped. Similarly, vibration perception through the teeth enables accurate assessment of textured objects placed in the mouth.
Research confirms the presence of intra-dental mechanoreceptors and suggests endodontic procedures may limits patients’ abilities to perceive vibrations associated with textured assessment of objects with their teeth. Having lost intra-dental mechanoreceptors, missing and non-vital teeth may allow the use of stronger-than normal biting forces. Eventually, elevated occlusal forces may lead to tooth wear, over stimulations of the muscles of mastication and catastrophic fractures in non-vital teeth.
Sensory Motor Integration
Sensory motor integration is a feedback process during mastication which sensory inputs from peripheral parts of the body modify actions initiated by the central nervous system. This process occurs primarily in the brainstem, thalamus, and cortex (Figure 2). In the masticatory system, sensory motor integration coordinates fundamental activities such as breathing, eating, and swallowing with sensations that arise during their performance.
Occlusion of the jaws and teeth, as defined is “the act or process of closure.”
Occlusion is a dynamic process during which willful and rhythmic jaw movements are integrated with sensations experienced during movement and memories of prior movements. Efferent motor commands from the cortex, cerebellum, and brainstem are integrated with peripheral sensory feedback from the teeth, muscles, temporomandiibular joints, bones, and soft tissues. Occlusion relies on sensory motor integration to coordinates the activities of the muscles of mastication.
Movement coordination in most of the body’s joint systems (eg, arm and leg) is facilitated by proprioceptors (eg, muscles spindles) in antagonist muscle groups (ie, abductor and adductor muscles) and sensory receptors in the skin and joints. The masticatory system is unique in that only its adductor muscles (ie, jaw-closing muscles) are innervated by muscle spindles. As a result of this unique neural architecture, control of the jaw-opening may reliant on tactile sensory feedback from mechanosensory receptors (ie, intradental and periodontal mechanorecptors) than in other joint systems.
Therefore, mechanical tooth contacts produce very rapid jaw reflex behaviors. Jaw reflexs are thought to protect the teeth from excessively strong biting forces. Whether tooth contacts induce inhibition or excitation of the jaw-closing muscles depends on several variables, including rate of force application and background clenching level. Reflex inhibition of the jaw-closing muscles after mechanical tooth stimulation may be referred to as the jaw-opening reflex or silent period. In humans, the jaw-opening reflex is characterized by rapid inhibition of the jaw-closing muscles (ie, masseter, temporalis, and medial pterygoid) and bite-force reduction, following tooth contact. When an unexpectedly high biting force occurs the jaw-opening reflex may prevent tooth fracture by rapidly shutting down the –closing muscles.
In conclusion the results demonstrated that bending forces applied to vital teeth evoke the jaw-opening reflex and subsequent endodontic procedures abolish this reflex. In speculation a specialized sensory transducer mechanism exists in dentin that is activated by deformation or bending of the crown of a tooth.
There are striking disturbances in the control of certain jaw motor behaviors in people lacking dental mechanoreceptors. Participants with dentures, missing teeth, and implants could not position their jaws as precisely as participants with nature teeth and used four times the biting force to hold a peanut between their teeth. These findings imply implant and denture prostheses are likely to undergo mechanical damage as a consequence of poor biting control.
Occlusal tooth contact competition can induce muscle hyperactivity in the orafacial region as jaw muscles become overworked. This may cause pain in these muscles. It becomes clear that bite force magnitude is affected by mechanosensory feedback that can restrain muscle activity and limit structural damage to the teeth, temporomandibular joints, and periodontal apparatus. The goal of health teeth is to foster changes in oromotor behavior that reduce functional occlusal forces and positively affect the health and longevity of the masticatory system.
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Washington Women's Journal Article
Published 12-01-2009
