Читать книгу Practical Procedures in Dental Occlusion - Ziad Al-Ani - Страница 17

The goal of mastication is to increase the surface area of food to enhance enzymatic action. Therefore, our teeth (incisors, canines, premolars and molars) are designed to crush and shear the food.

Figure 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).