2
Neuroanatomy - Why is It Important?

Introduction

What is the role of the teeth? An important question which underpins our clinical dentistry because we are routinely involved in possibly changing it if we don't follow a careful process.

The roles of the teeth can be thought of as follows.

• Mastication.
• Swallowing.
• Speech/phonetics.

This is a simplified view because the impact of teeth is far greater for both the individual but also when interacting with the wider community.

• Aesthetics - emotional and psychosocial perspective; this is specific to the individual and also has an impact on their self-esteem.
• Psychophysical - the ability to appreciate the food via texture, volume and taste.
• Occlusal stability and jaw support - maintain the elements that maximise function.
• Cognition - decreased mastication is a risk factor for dementia.
• Mortality - Osterberg et al. 2008 in numerous studies demonstrate the statistically significant correlation between the number of teeth remaining and mortality, with the data suggesting a 4% decrease in mortality for each remaining tooth above 20 occluding pairs. I am not suggesting that we tell our patients they will live longer if we provide more teeth but the link between quality of life and having fixed teeth is certain.

So how do we avoid altering this system in an uncontrollable manner? We use protocols and processes. The acronym for the process is STOP! STOP picking up that drill before you assess the occlusion. A preassessment of the occlusion is crucial to ensure we have not potentially affected the role or performance of the teeth. Therefore, we use our senses to preassess the occlusion. This is essential in both conformative and reorganised occlusion.

• S - Survey - visual assessment using coloured paper to analyse contacts.
• T - Touch - fremitus.
• O - Observe and listen.
• P - Patient feedback.

The goals of occlusion are as follow.

• To provide equal contacts on as many teeth as possible when the patient swallows - centric occlusion position. This will aid muscle health.
• To provide incisional guidance (protrusive guidance) on the anterior teeth. This will aid temporomandibular joint health.
• To provide group function when chewing using cuspal inclines.
• To avoid introducing new contacts (unless in a controlled manner) which may strain the adaptive capacity of the patient.
• To biomechanically distribute the forces so as not to cause failure of the restoration or other teeth.

Neuroanatomy

The aim of this chapter is to?provide a clear understanding of the complex neural framework involved in mastication, swallowing and speech. The key objective is the information the brain requires to understand the position of the jaw in space and it acquires this information from the teeth, temporomandibular joint (TMJ), muscles and soft tissues.

An understanding of the neural framework involved in dental occlusion is essential in determining the protocols within clinical dentistry. The neural framework comprises the central nervous system (CNS) (spinal cord and brain) and the peripheral nervous system (connects the rest of the body to the spinal cord and brain). This is a feedback and feedforward system made up of sensory fibres (registering pain, pressure and temperature) and motor fibres (providing a function such as muscle contraction).

Anatomically, another structure which is important in our understanding of the masticatory system is the brainstem, which is the posterior part of the brain continuous with the spinal cord which is composed of three regions:

• medulla oblongata
• pons
• mesencephalic area.

Why do I need to know this, I hear you ask? Well, within this area are the central pattern generators (CPGs) generally defined as a network of neurons (nerve cells) capable of enabling the production of central commands, specifically controlling stereotyped rhythmic motor behaviours such as mastication, deglutition, respiration and locomotion, among others. There is increasing evidence suggesting that some of these CPGs are interconnected for co-ordinated control.

In this chapter we will only be dealing with mastication and deglutition. For further reading, the article by Steuer and Guertin (2019) goes into greater detail. Kandel (2012) stated that the brainstem is an important element of motor and sensory function and plays a key role in the control of mastication and deglutition.

Mastication

This process needs sensory input for the CPGs and this comes from the periodontal mechanoreceptors (PDMRs), muscles, bones, TMJ and soft tissues. Other inputs from the higher centres of the brain can also affect the basic output from the CPG. The output through the motor fibres is relayed in the descending pathway to the muscles to apply forces to break food down. This process is constant and if we bite on something hard which we are not expecting then a reflex arc is created, i.e. jaw opening reflex (Figure 2.1).

Most foods that we are used to eating do not require attention, but when we try a new food the higher order brain centres are involved as we investigate (attention is required) this new substance in regard to texture and taste and a decision is made whether we will eat this again. This is feedforward learning. Age and types of food can also modulate mastication activity as stated by Peyron et al. (2004).

What is the Goal of Mastication?

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).

Sensory and Motor Feedbacks

PDMRs

Situated close to the collagen fibres and in between like a sandwich within the periodontal ligament and described as...