2.1 Signal pathways. PDMR (afferent neurons) are triggered (sensory and motor) and the impulse is detected in the trigeminal ganglion and the trigeminal mesencephalic nucleus. The information is then relayed at the brainstem and finally projected to the primary somatosensory cortex. The output from the cortex seems to be important for initiating and co‐ordinating masticatory movement and adapting to the hardness of the bolus.
Source: Modified from Morquette et al. (2012).
Incisors – designed to grab and cut through food (to incise).
Canines – designed to grab and tear through food (cornerstone of the arch).
Premolars – designed to crush food and seen as transitional due to having anatomical features of both canines and molars (the equilibrium point of the arch).
Molars – designed for grinding food.
The muscles involved in mastication are responsible for moving the jaws in a manner that brings the teeth into contact rhythmically. When the muscles are inflamed this process can be painful and uncomfortable. Certain activities can cause this such as:
hypernormal function – habits such as nail biting, chewing gum
parafunction – clenching (static) and bruxism (dynamic).
Muscles involved in jaw opening (smaller muscle mass group).
Lateral pterygoid.
Suprahyoid muscles – anterior digastric, mylohyoid, geniohyoid.
Muscles involved in jaw closing (larger muscle mass group underlying where the greater activity is).
Temporalis.
Masseter.
Medial pterygoid
The innervation for these muscles is via the trigeminal nerve (V) but other cranial nerves such as the facial (VII), glossopharyngeal (IX) and hypoglossal (XII) are also involved in the whole process of mastication and swallowing, which comprises more than 30 nerves and muscles (Matsuo and Palmer 2008). Some of these muscles are also involved in respiration and are considered accessory respiratory muscles as discussed by Van Lunteren and Dick (1997).
The pattern of mastication is made up of three successive cycles as described by Lund (1991).
1 The preparatory phase – also called the gathering stage, when the incisors bring the food into the mouth and shift it deeper onto the posterior teeth.
2 The reduction phase – breakdown of food in a rhythm called the chewing cycle; as the food gets smaller, the teeth start to contact, letting us know that the food is ready for swallowing
3 The preswallowing phase – the bolus is prepared for swallowing, the tongue places the food posteriorly and the swallowing reflex is initiated.
The evidence also supports sensory feedback controls for a large part of the masticatory process. Soft foods mean a short masticatory sequence and tough foods provoke a longer sequence, as discussed by Plesh (1986).
Let's look at the sensory feedback system in more detail (Figure 2.2).
Figure 2.2 Chewing cycle data collected using MODJAW (for further details on MODJAW see Chapter 12).
Sensory and Motor Feedbacks
PDMRs
Situated close to the collagen fibres and in between like a sandwich within the periodontal ligament and described as Ruffini‐like by Byers (1985) and Lambrichts (1992) in humans. The main periodontal Ruffini nerve endings have been classified as type 1 and 2. Type 1 shows lamellar terminal Schwann cells and expanded axon terminals with axonal spines which penetrate surrounding tissue; type 2 is characterised by lesser branched Ruffini endings with fewer axonal spines, less basal lamina and fewer Schwann cells. Both of these receptor types are present in the periodontal ligament (Maeda et al. 1999).These are crucial for force interpretation and control of mastication and therefore it would be assumed that the different teeth (incisors–molars) would have differing sensitivity thresholds and this has been shown to be correct by Johnsen et al. (2007). This is called the interocclusal tactility threshold (ITT) as discussed by Herren (1988) and this study uses foils of varying thickness between healthy teeth. The smallest ITT recorded is between 8 and 60 μm but during chewing the threshold increases by a factor of 5 (range 2.5–8×) and the occlusal perception is reduced due to descending inhibition (sensory gating), meaning there is a filtering out of irrelevant information which allows for enhanced detection of unexpected stimuli. There are also interindividual variations which are ascribed to differing attentional degrees in the higher brain centres and this will be linked to adaptability and neuroplasticity (capability to change and adapt to new demands).
The direction of force also shows that the PDMRs respond more when the forces are axial, i.e. in the direction in which they function best – ‘Directional bias may reflect functional adaptation’ (Sato 1988). When force is applied, the collagen fibrils are compressed which fires the mechanoreceptors. The use of the split and hold tests by Trulsson and Johansson (1996) also determined the amount of force required through a positive feedback loop, meaning the harder the food, the more force is applied through the initial feedback from the mechanoreceptors. A recent systematic review by Piancino et al. (2017) summarises studies that looked at this in greater detail.
What happens, then, when we lose teeth and the PDMRs are lost? Several studies such as Svensson and Trulsson (2011) and Svensson et al. (2013) have shown higher biting and food holding forces, indicating that optimal restoration design is fundamental.
Pulp
The pulp also provides proprioception. Randow and Glanz (1986) showed that non‐vital teeth had mean pain threshold levels that, on cantilever loading, were more than twice as high as those of neighbouring or contralateral vital teeth. This partly explains the vulnerability to fracture of root‐filled teeth.
Muscles
The control of position and movement of the muscle/tendon complex is achieved by a combined Golgi tendon organ (GTO) and muscle spindle (MS) feedback. The jaw closing muscles are richly supplied with MSs which provide information related to length and contraction velocity of muscle fibres and generate muscle activation patterns in a predictable manner. The GTO provides information related to tension of the tendons (Kistemaker et al. 2013). This is a feedback loop system which plays a role in CPGs and given the superior tactile discriminative abilities of dentate subjects in comparison with those with implant‐supported or removable prostheses, PDMRs provide a more sensitive indicator of jaw position and movement.
What happens when we increase the occlusal vertical dimension (OVD)? An increase in OVD results in stretching or increased MS length. Muscles show a high degree of functional
