Food Oral Processing E-Book

Jianshe Chen

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 885

Veröffentlichungsjahr: 2012

Beschreibung

This volume provides an overview of the latest research findings on the physics, physiology, and psychology of food oral consumption, as well as the experimental techniques available for food oral studies. Coverage includes the main physical and physiological functionalities of the mouth; the location and functionalities of various oral receptors; the main sequences of eating and drinking, and the concomitant food disintegration and destabilisation. Chapters also explain oral processing and its relation to flavour release and texture perception, and there is an introduction to the principles of food rheology as they relate to eating.

Food Oral Processing is directed at food scientists and technologists in industry and academia, especially those involved in sensory science and new product development. It will also be of interest to oral physiologists, oral biologists and dentists. The book will be a useful reference for undergraduate and postgraduate students of these disciplines.

Leseprobe

Table of Contents

Cover

Title page

Copyright page

Preface

Contributors

Part One: Oral Anatomy and Physiology

1 Oral Cavity

1.1 INTRODUCTION

1.2 THE ORAL CAVITY

1.3 SALIVARY GLANDS AND SALIVA SECRETION

1.4 OROFACIAL MUSCLES

1.5 THE TONGUE

1.6 CONCLUDING REMARKS

ACKNOWLEDGEMENTS

2 Oral Receptors

2.1 INTRODUCTION TO ORAL RECEPTORS

2.2 TASTE

2.3 MECHANORECEPTION

2.4 NOCICEPTION

2.5 THERMAL PERCEPTION

2.6 OLFACTION

2.7 CONCLUDING REMARKS

3 Role of Saliva in the Oral Processing of Food

3.1 INTRODUCTION

3.2 CONTROL OF SALIVARY SECRETION

3.3 FUNCTIONALITIES OF SALIVA

3.4 SALIVA IN BOLUS FORMATION, SWALLOWING AND ORAL CLEARANCE

3.5 CONCLUDING REMARKS

ACKNOWLEDGEMENTS

Part Two: Food Oral Management

4 Oral Management of Food

4.1 INTRODUCTION

4.2 FACTORS INFLUENCING ORAL FUNCTION

4.3 INFLUENCE OF FOOD CHARACTERISTICS ON CHEWING

4.4 NEUROMUSCULAR CONTROL OF CHEWING AND SWALLOWING

4.5 CONCLUDING REMARKS

5 Breaking and Mastication of Solid Foods

5.1 INTRODUCTION

5.2 MECHANICAL PROPERTIES AND FOOD TEXTURE

5.3 CHARACTERISATION OF MECHANICAL PROPERTIES

5.4 ORAL SELECTION OF FOOD PARTICLES

5.5 BREAKAGE FUNCTION

5.6 CONCLUDING REMARKS

6 Oral Behaviour of Food Emulsions

6.1 INTRODUCTION

6.2 FOOD EMULSIONS IN GENERAL

6.3 INTERFACIAL LAYERS

6.4 EMULSION STABILITY

6.5 BEHAVIOUR OF EMULSIONS UNDER ORAL CONDITIONS

6.6 CONCLUDING REMARKS

7 Bolus Formation and Swallowing

7.1 INTRODUCTION

7.2 MECHANISMS OF SWALLOWING

7.3 THE FORMATION OF A FOOD BOLUS AND THE TRIGGERING CRITERIA OF BOLUS SWALLOWING

7.4 CONCLUDING REMARKS

Part Three: Food Oral Processing and Sensory Perception

8 Oral Processing and Texture Perception

8.1 INTRODUCTION

8.2 WHERE IS TEXTURE SENSED IN THE MOUTH?

8.3 TEXTURE VERSUS FOOD STRUCTURE

8.4 THE MEASUREMENT OF ORAL PROCESSES

8.5 TEXTURE VERSUS ORAL PROCESSING

8.6 TEXTURE ATTRIBUTES ARE SYSTEMATICALLY RELATED

8.7 THE ROLE OF SALIVA IN TEXTURE PERCEPTION

8.8 ORAL TEMPERATURE AND TEXTURE PERCEPTION

8.9 CONCLUDING REMARKS

9 Oral Processing and Flavour Sensing Mechanisms

9.1 INTRODUCTION

9.2 MECHANISMS FOR SENSING AND MEASURING TASTE

9.3 MECHANISMS FOR SENSING AND MEASURING AROMA

9.4 MECHANISMS FOR SENSING AND MEASURING TEXTURE

9.5 MULTI-SENSORY INTERACTIONS

9.6 MEASURING FOOD BREAKDOWN AND DEPOSITION IN VIVO

9.7 BIOCHEMICAL FLAVOUR CHANGES DURING ORAL PROCESSING

9.8 APPLICATIONS OF KNOWLEDGE TO REAL FOOD PRODUCTS

9.9 CONCLUDING REMARKS

ACKNOWLEDGEMENTS

10 Multi-sensory Integration and the Psychophysics of Flavour Perception

10.1 INTRODUCTION

10.2 TASTE/GUSTATION

10.3 OLFACTORY–GUSTATORY INTERACTIONS IN MULTI-SENSORY FLAVOUR PERCEPTION

10.4 ORAL–SOMATOSENSORY CONTRIBUTIONS TO MULTI-SENSORY FLAVOUR PERCEPTION

10.5 AUDITORY CONTRIBUTIONS TO MULTI-SENSORY FLAVOUR PERCEPTION

10.6 ‘VISUAL FLAVOUR’: VISUAL CONTRIBUTIONS TO MULTI-SENSORY FLAVOUR PERCEPTION

10.7 THE COGNITIVE NEUROSCIENCE OF MULTI-SENSORY FLAVOUR PERCEPTION

10.8 CONCLUDING REMARKS

Part Four: Principles and Practices of Instrumental Characterisation for Eating and Sensory Perception Studies

11 ‘Oral’ Rheology

11.1 INTRODUCTION TO FOOD RHEOLOGY AND ORAL PROCESSING

11.2 LIQUID FOOD RHEOLOGY AND STRUCTURE

11.3 SOFT FOOD RHEOLOGY AND MICROSTRUCTURE

11.4 SOLID FOOD BREAKDOWN AND RHEOLOGY

11.5 SALIVA AND RHEOLOGY

11.6 SENSORY PERCEPTION AND THE FLUID DYNAMICS BETWEEN TONGUE AND PALATE

11.7 CONCLUDING REMARKS

12 ‘Oral’ Tribology

12.1 INTRODUCTION

12.2 PRINCIPLES OF TRIBOLOGY

12.3 FOOD LUBRICATION

12.4 CONCLUDING REMARKS

ACKNOWLEDGEMENTS

13 Applications of Electromyography (EMG) Technique for Eating Studies

13.1 INTRODUCTION

13.2 PRINCIPLES OF ELECTROMYOGRAPHY TECHNIQUE

13.3 EMG EXPERIMENTAL DESIGN AND SET-UP

13.4 DATA ANALYSIS

13.5 CASE STUDIES

13.6 CONCLUDING REMARKS

14 Soft Machine Mechanics and Oral Texture Perception

14.1 INTRODUCTION

14.2 SENSORY TERMS AND VOCABULARY

14.3 SOFT MACHINE MECHANICS

14.4 THE ‘AMPLIFIER’ AND SENSORY SENSITIVITY

14.5 ADAPTATION AND FATIGUE

14.6 CONCLUDING REMARKS

Part Five: Applications and New Product Developments

15 Appreciation of Food Crispness and New Product Development

15.1 INTRODUCTION

15.2 APPRECIATION OF CRISPY AND CRUNCHY TEXTURE

15.3 MECHANICAL AND STRUCTURAL FEATURES OF CRISPY/CRUNCHY FOOD

15.4 CHARACTERISATION OF CRISPY/CRUNCHY TEXTURES

15.5 INFLUENCE OF THE PRODUCT DESIGN AND FORMULATION, PROCESS AND STORAGE CONDITIONS IN THE ATTAINMENT, ENHANCEMENT AND MAINTENANCE OF THE CRISPY/CRUNCHY CHARACTER IN WET, DRY AND CRUSTED FOOD PRODUCTS

15.6 CONCLUDING REMARKS

16 Design of Food Structure for Enhanced Oral Experience

16.1 INTRODUCTION

16.2 BIOPHYSICS OF ORAL PERCEPTION

16.3 STRUCTURAL STIMULI OF MECHANORECEPTORS

16.4 ENGINEERING OF MICROSTRUCTURES IN FOOD

16.5 ACKNOWLEDGEMENTS

Index

Color Plates

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Library of Congress Cataloging-in-Publication Data

Food oral processing : fundamentals of eating and sensory perception / edited by Jianshe Chen, Lina Engelen.

p. cm.

 Includes bibliographical references and index.

 ISBN 978-1-4443-3012-0 (hard cover : alk. paper)

 ISBN 978-1-4443-6091-2 (epdf)

 ISBN 978-1-4443-6092-9 (epub)

 ISBN 978-1-4443-6093-6 (mobi)

 1. Ingestion. 2. Drinking (Physiology) 3. Food habits. 4. Taste. I. Chen, Jianshe, 1961– II. Engelen, Lina.

QP147.F665 2012

612.31–dc23

2011035807

A catalogue record for this book is available from the British Library.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Preface

‘It is critically important not only what we eat but also how we eat!’

Eating, or food oral consumption, is an essential part of our daily life. It is a routine process of obtaining the energy and nutrients essentially required for living and well-being and also the appreciation of sensory pleasure and enjoyment. The eating process can be seen as the ultimate stage of the food supply chain and is the starting point of food disintegration and the digestion process. Therefore, the eating quality and sensory experience of a food always remains a top concern to food researchers, food manufacturers and retailers, as well as consumers. How a food is broken down inside the mouth could also have important implications for our well-being and health, as indicated by Horace Fletcher (1849–1919) almost a century ago. Even though the practice of eating is well-known to most, the fundamental principles involved in eating and sensory perception of food are not as obvious as they are normally perceived. This book endeavours to review the latest research findings on food oral processing and sensory perception. The main objective of the book is to provide readers with up-to-date knowledge and understanding of the underpinning principles of food physics, oral physiology and sensory psychology of an eating process.

Studies of food texture, taste, flavour, aroma and colour as independent scientific disciplines began only around the middle of the last century, shortly after food science and technology became the subject of degree courses. Knowledge of food sensory properties was in urgent demand due to largely industrialised food manufacturing and supply, which led to huge expansions of research activities in these areas during the second half of last century. Approaches during the early stages of eating and food sensory studies were mostly either through an objective instrumental characterisation or a human subject sensory description method. For example, for food texture studies, rheology and mechanical investigations were most commonly used, where food was essentially treated as a material, that is mechanical and rheological properties (e.g. hardness, springiness, viscosity, cohesiveness, etc.) were characterised using instrumental devices, and results were interpreted in relation to sensory perception. On the other hand, food taste and aroma studies focused mainly on small molecules, their release, characterisation and detection. It is only during the last one or two decades that cross-disciplinary approaches were introduced into eating and food sensory studies. During the last decade, increased use of physiological methodologies and techniques has been reported by food scientists. Food texture studies have been conducted in combination with the observation of orofacial muscle activities and the analysis of saliva interactions. Very recently, fNMI (functional Nuclear Magnetic Imaging) observation by neuroscientists revealed positive correlations between increased brain activities and the eating and sensory pleasure perception. Eating is no longer seen as a simple process of food break down, but is recognised as a highly sophisticated process of human responses (physiological, psychological and neurological) to the changing physicochemical properties of the food.

Based on this background, we feel that there is a need for a book that elucidates the multi-disciplinary nature of eating and sensory perception and that reviews the latest progress in related areas, from fundamental studies to industrial applications. This book endeavours to be a multi-disciplinary source of stimulation and reference, and we hope it will encourage further researches in these areas. The book is divided into five sections: 1 Oral anatomy and physiology; 2 Food oral management; 3 Oral processing and sensory perception; 4 Principles and practices of instrumental characterisation for eating and sensory perception studies; and 5 Applications and new product development.

The first section covers the oral cavity, where Luciano Pereira describes the anatomy and function of the different parts of the oral cavity; oral receptors, where Lina Engelen reviews the oral tactile and chemosensory receptors; and saliva, where Guy Carpenter discusses the origins and composition of saliva as well as its role in the oral processing of food. In Section 2 Andries van der Bilt starts by discussing the strategies of food oral management, from ingestion to swallow; followed by a chapter on the oral break down and mastication of solid foods and the determining physical principles (Carolyn Ross and Clifford Hoye Jr.). Anwesha Sarkar and Harjinder Singh introduce food emulsions and their behaviour in the mouth. This chapter explains the possible mechanisms of oral destabilisation of food emulsions and their implications on sensation. The section ends with a review by Jianshe Chen on the mechanisms of food bolus formation and the critical criteria in triggering a swallowing action.

The third section of the book covers the interactions between oral processing and sensory perception, regarding texture by Lina Engelen and Rene de Wijk, and flavour by Sara Adams and Andrew Taylor, followed by an account of sensory integration and psycho-physics by Charles Spence.

Section 4 begins with two chapters by Jason Stokes on ‘oral’ rheology and ‘oral’ tribology, in which he discusses the underlying physical principles of food oral break down and food oral movement and their roles in sensory perception. This is followed by a chapter on the EMG (electromyography) technique (by Yadira Gonzalez and Jianshe Chen), covering the theories and practices of the technique and its application to eating studies. Micha Peleg and Maria Corradini conclude Section 4 with a chapter on food–body interactions, where, by treating the human mouth as a soft machine, soft machine mechanics are discussed in relation to instrumental characterisation of textural properties of a food.

The final section is dedicated to possible applications of recent research findings for new product developments. Paula Varela and Susana Fiszman focus mainly on crispy and crunchy foods and the principles and practices applied in industry in designing and providing such products. Adam Burbidge finishes off the book by reviewing the biomechanics of oral stress and strain, which (micro-)structures elicit these effects, and considers potential routes for creating these structures in a food context.

Integrated studies of eating and sensory perception have been adopted only fairly recently and this book is probably the first of its kind. We anticipate that this book will be of interest to scientists, technologists and engineers in food-related areas, as well as to those from other disciplines such as oral physiology, oral biology, dentistry and sensory science. This book could also be used as a useful reference for undergraduate and postgraduate students studying in above disciplines and for R&D researchers in food manufacturing and food service industries.

We would like to take this opportunity to thank all the contributors; their expert knowledge, enthusiasm and hard work have enabled us to put a book together of high scientific quality; the editorial staff at Wiley-Blackwell for their support and advices; and our families and friends for bearing with us through the long nights and weekend hours.

Jianshe Chen (Leeds, UK)

Lina Engelen (Sydney, Australia)

Contributors

Sarah Adams

Unilever Discover

Port Sunlight

Wirral, Meseyside, CH63 3JW

UK

Adam Burbidge

Nestlé Research Centre

vers-chez-les Blanc

CH-1000 Lausanne 26

Switzerland

Guy Carpenter

Salivary Research Unit

Dental Institute

King’s College London

Guy’s Hospital

London, SE1 9RT

UK

Jianshe Chen

School of Food Science and Nutrition

University of Leeds

Leeds LS2 9JT

UK

Maria G. Corradini

Instituto de Tecnología

Facultad de Ingeniería y Ciencias Exactas

Universidad Argentina de la Empresa

Cdad. de Buenos Aires

Argentina

René A. de Wijk

WUR/Food & Biobased Research.

P.O. Box 17,

6700 AA Wageningen

The Netherlands

Lina Engelen

University of Sydney

Faculty of Health Sciences

75 East Street, J013

Lidcombe NSW 2141

Australia

Susana Fiszman

Instituto de Agroquímica y Tecnología de Alimentos (CSIC)

Agustín Escardino 7

46980 Paterna – Valencia

Spain

Yadira Gonzalez Espinosa

School of Food Science and Nutrition

University of Leeds

Leeds LS2 9JT

UK

Clifford L. Hoye, Jr.

School of Food Science

Washington State University

Pullman, WA 99164-6376

USA

Micha Peleg

Department of Food Science

University of Massachusetts

Amherst, MA 01003

USA

Luciano José Pereira

DMV – Physiology and Pharmacology

Federal University of Lavras – UFLA

Minas Gerais, Caixa Postal 3037

CEP 37200-000

Brazil

Carolyn F. Ross

School of Food Science

Washington State University

Pullman, WA 99164-6376

USA

Harjinder Singh

Riddet Institute

Massey University

Palmerston North

New Zealand

Anwesha Sarkar

Riddet Institute

Palmerston North

New Zealand

Charles Spence

Crossmodel Reseaerch Lab

Department of Experimental Psychology

Somerville College

Oxford

UK

Jason R. Stokes

School of Chemical Engineering

University of Queensland, Brisbane

Australia

Andrew J. Taylor

Division of Food Sciences

University of Nottingham

Sutton Bonington Campus

Loughborough LE12 5RD

UK

Andries van der Bilt

Department of Oral-Maxillofacial Surgery,

Prosthodontics and Special Dental Care

University Medical Center Utrecht

P.O. Box 85500, 3508 GA Utrecht

The Netherlands

Paula Varela

Instituto de Agroquímica y Tecnología de Alimentos (CSIC)

Agustín Escardino 7

46980 Paterna – Valencia

Spain

Part One: Oral Anatomy and Physiology

1

Oral Cavity

Luciano José Pereira

1.1 INTRODUCTION

The oral cavity is the first part of the digestive tract. However, the mouth is not only responsible for digestive functions. It also plays a role in breathing, behavioural and social activities (talking, smiling, yawning, sucking) and taste perception. The oral cavity consists of two parts: the vestibule, which is limited externally by the lips and cheeks and internally by the gums and teeth; and the oral cavity itself (1.1), which is limited laterally and ventrally by the alveolar process and teeth and dorsally communicates with the pharynx through the isthmus faucium (Gray, 2000).

Mastication is the most important function of the mouth. Teeth, muscles of mastication and salivary glands all work together to shred and break down food for swallowing. The teeth are the hardest tissues in the jaw and are involved in different activities, such as food ingestion and pronunciation of words, and also play an important role in facial aesthetics (Honda et al., 2008; Koussoulakou et al., 2009). The muscles of mastication promote the force needed to elevate the jaw so that food can be shredded between the teeth as the upper and lower arches come into contact (Fontijn-Tekamp et al., 2000). Simultaneously, saliva is produced by major and minor salivary glands. The water in saliva moistens food particles and salivary mucins bind masticated food into a coherent, moist bolus that can be easily swallowed (Pedersen et al., 2002).

This chapter reviews the main anatomical and physiological aspects of the oral cavity – teeth, tongue, salivary glands and major orofacial muscles. The review focuses on the physiological behaviour of the mouth and fundamental knowledge of oral operations covered in four main sections: the oral cavity (including teeth and periodontal tissue); saliva (saliva glands, saliva secretion, composition, physical and chemical properties); orofacial muscles (location, function, activity) and tongue (tongue muscles, function).

1.2 THE ORAL CAVITY

The oral cavity is delimited anteriorly by the upper and lower lips (vermilion surface, mucosal lip, labial mucosa), laterally by the cheeks, superiorly by the hard palate and inferiorly by the tongue and muscles attached to the internal side of the mandible, including the geniohyoid, mylohyoid and digastric muscles. The upper and lower dentition, salivary glands, mucosal glands, tongue and the mucosal tissue covering the hard palate are found in this cavity (German and Palmer, 2006) (Figure 1.1).

Figure 1.1 The oral cavity.

The oral cavity is continuous with the pharyngeal cavity. The region where the pharynx connects to the oral cavity is called the oropharynx, and it embraces the base of the tongue, vallecula, soft palate, uvula, lateral pharyngeal walls (including the palatine tonsils and tonsillar pillars) and the posterior pharyngeal wall extending from the plane of the soft palate/hard palate junction to the level of the pharyngoepiglottic folds at the hyoid bone. The base of the tongue is the part posterior to the circumvallate papillae (Yousem and Chalian, 1998).

The mucous membrane that covers the mouth connects to the integument at the free margin of the lips and with the mucous covering the pharynx. It has a rose-pink colour and it becomes thicker on hard parts limiting the cavity. The mucous membrane is covered by stratified squamous epithelium (Gray, 2000).

The bones adjacent to the oral cavity are the maxilla and mandible. These bones support the dentition and form the hard palate, which is made up of the palatine process of the maxilla and the maxillary process of the palatine bones. The final portion of the oral cavity is formed by muscle, with the hyoid bone and cartilages of the larynx functioning as the pharyngeal arch structures (German and Palmer, 2006).

The dentition is placed in the maxilla and mandible and consists of 32 teeth. Children are born edentulous; the first deciduous (primary) teeth erupt approximately six months after birth. There are five types of deciduous teeth: medial incisor, lateral incisor, canine, first molar and second molar. These teeth are replaced by permanent teeth. However, the permanent dentition is composed of two additional premolars and a third molar. The permanent dentition is usually complete (except for the third molar) at 12 years of age. The third molar erupts at around 16 to 20 years of age and frequently fails to erupt at all (German and Palmer, 2006). Some individuals do not even present those teeth (agenesia).

The main component of a tooth is dentine, which is calcified tissue produced by odontoblasts (Koussoulakou et al., 2009). The dentine surrounds the pulp, which is rich in fibroblast-like cells, blood vessels and nerves. The dentine that forms the tooth crown (the visible part of the tooth in the oral cavity) is covered by a layer of enamel, which is produced by ameloblasts. The enamel is the hardest tissue in the human body and is collagen free. Its main proteins are amelogenin (90%), ameloblastin, enamelin and tuftelin. The teeth are firmly attached to the jaw by their roots, which support the teeth within an alveolar socket by means of the periodontal ligament. The periosteum is connected to the fibrous structure of the gums (Gray, 2000).

The teeth are important to the masticatory system, as they break down food particles during occlusal contact (Pereira et al., 2006). A significant reduction in masticatory function occurs following the loss of post-canine teeth. Moreover, individuals with natural dentition present better masticatory function than those who wear removable dentures or have an implant-supported prosthesis (van der Bilt, 1994; Wilding, 1993; Julien et al., 1996; Fontijn-Tekamp et al., 2000; Hatch et al., 2001; van Kampen et al., 2004). A linear relationship has been found between masticatory performance and the number of occluding teeth (van der Bilt et al., 1993). However, individuals who have lost posterior teeth do not necessarily chew longer before swallowing than individuals with all teeth. This indicates that, on average, people with a bad masticatory performance swallow larger food particles (Fontijn-Tekamp et al., 2004).

Tooth loss is related not only to a reduced occlusal area, but also to the disappearance of the periodontal ligament. Mechanoreceptors located in the periodontal ligament obtain detailed information on the spatial relationship and load modulation in the process of food fragmentation (Johanson et al., 2006). Thus, chronic periodontal disease can cause the destruction of the support tissue, with consequent loss of periodontal mechanoreceptors, resulting in tooth mobility and masticatory impairment (Alkan et al., 2006). The subjective perception of the impact of oral health on mastication diminished after periodontal treatment (Pereira et al., 2011).

1.3 SALIVARY GLANDS AND SALIVA SECRETION

The major salivary glands are characterized by three pairs of organs: parotid, submandibular (Figure 1.2) and sublingual glands that work simultaneously to produce saliva for the oral cavity (Denny et al. 1997). The major salivary glands secrete more than 90% of the total volume of saliva and the remaining amount is secreted by the minor glands. These glands are located all over the mouth except the gums and anterior portion of the hard palate (Tenovuo, 1997). Salivary glands are made up of acinar and ductal cells. The formation of saliva inside the salivary glands occurs in a similar manner to the action of the tubular filtration in the kidneys. A plasma-like filtrate is formed by the acinar cells. Initially, this fluid is isotonic with respect to blood plasma. During its way through the gland ducts the filtrate becomes hypotonic due to resorption and secretion of ions and other components. (Turner et al., 2002; Dodds et al., 2005). Secretion is controlled by the autonomic nervous system. Parasympathetic stimulation induces the output of a large volume of saliva with a low protein concentration, whereas sympathetic stimulation has the opposite effect, causing the release of a relatively small volume of saliva, with a high protein concentration (Anderson et al., 1984). Even though both parasympathetic and sympathetic stimulation can evoke salivary flow, stress situations can cause dry mouth symptoms due to vasoconstriction.

Figure 1.2 Parotid and submandibular salivary glands.

The parotid gland (Figure 1.2) is located in the retromandibular fossa anterior to the ear and sternocleidomastoid muscle. Parts of the superficial lobe cover the ramus of the mandible and the posterior part of the masseter muscle (Bialek et al., 2006). The acinar cells of the parotid gland produce a largely serous secretion and synthesise most of the α-amylase (Llena-Puy, 2006).

The submandibular gland (Figure 1.2) is located in the posterior portion of the submandibular triangle. The submandibular triangle is limited by the anterior and posterior bellies of the digastric muscle as well as the body of the mandible. (Bialek et al., 2006). The sublingual gland lies between the muscles of the oral cavity floor – geniohyoid muscle, hyoglossal muscle (medially), mylohyoid muscle and intrinsic muscles of the tongue. Its lateral side is adjacent to the mandible (Bialek et al., 2006). Mucins are glycosylated proteins, mainly produced by the submandibular and sublingual glands, whereas proline-rich and histatin-rich proteins are produced by the parotid and submandibular glands. The minor salivary glands are basically mucus (Llena-Puy, 2006) and they play an important role in lubricating the mucosa, thereby accounting for a large fraction of the total secretion of salivary proteins. The minor glands, which are distributed throughout the oral mucosa (labial, buccal, lingual, palatinal mucosa), are mixed glands largely comprising mucous acinar cells (Pedersen et al., 2002).

During non-stimulated salivary flow, about 20% of the volume is secreted by the parotid glands; about 65 to 70% by the submandibular glands, around 7 to 8% by the sublingual glands and less than 10% by the minor salivary glands. When salivary flow is stimulated, the parotids contribute more than 50% of total salivary secretion (Edgar et al., 1992).

Saliva is basically composed of water. However, it also contains several diluted electrolytes (sodium, potassium, calcium, chloride, magnesium, bicarbonate, phosphate); proteins (albumin) and enzymes; immunoglobulins and mucosal glycoproteins, among other peptides. There is also glucose, urea and ammonia (Edgar, 1992; Humphrey and Williamson, 2001).

Saliva is involved in taste perception, as its high water content provides the capacity to dissolve substances and allows the gustatory buds to perceive different flavours (de Almeida et al., 2008). Additionally, saliva mucins lubricate the food bolus and protect oral tissues from irritating agents (Nagler et al., 2004). The water in the saliva moistens food particles, allowing salivary amylase to access available starch. The salivary mucins bind masticated food into a coherent, moist bolus that can easily be swallowed (Pedersen et al., 2002). The dilution effect seems to be the most important factor related to digestive properties, since the act of adding fluids to the food significantly reduces the number of chewing cycles and total muscle effort. The type of fluid (water, artificial saliva containing mucins or a solution of α-amylase) has been found to have no significant effect on the chewing process (van der Bilt et al., 2007) and salivary flow does not seem to have a significant influence on masticatory performance (de Matos et al., 2010). In addition to diluting substances, saliva provides the mechanical removal of residues, non-adherent bacteria and food debris (Almeida et al., 2008).

The most known enzyme of saliva is α-amylase, which breaks carbohydrates down to maltoses by cleaving the α-1-4 glycosidic bindings. Salivary α-amylase is considered to be of small significance in digestion because of its rapid inactivation in stomach (Pedersen et al., 2002). Salivary α-amylase is secreted mainly from the serous acinar cells of the parotid and submandibular gland. An additional salivary digestive enzyme is lingual lipase, which is secreted from acinar cells of the serous von Ebner’s glands located on the posterior region of the tongue and beneath the circumvallate papillae. Lingual lipase is, however, considered to be of limited significance (Pedersen et al., 2002).

1.4 OROFACIAL MUSCLES

The anterior limit of the oral cavity is formed by the orbicularis oris muscle, which surrounds the opening of the mouth. The labial muscles also control the lips and therefore the movements of the mouth: levator labii superioris, depressor anguli oris and risorius. The buccinator is the cheek muscle. These are superficial facial muscles and receive motor supply from branches of the facial nerve (VII) (German and Palmer, 2006).

Although they do not form the boundaries of the oral cavity or pharynx, the muscles of mastication are critical to moving the jaws and therefore oral function. The muscles of mastication are the masseter, temporalis, internal pterygoid (raisers of the mandible) and external pterygoid muscle (mandible protruder) (Figure 1.3). These muscles act in a group more than individually. They move the mandible in different directions, with the temporomandibular joint acting as a fulcrum. They are innervated by the motor root of the trigeminal nerve (Madeira, 2003).

Figure 1.3 Muscles of mastication.

The masseter consists of two portions, superficial and deep. The superficial portion, which is larger, arises from a thick, tendinous aponeurosis of the zygomatic process of the maxilla and from the anterior two thirds of the lower border of the zygomatic arch (zygomatic-temporal suture); its fibres pass downward and backward (Gray, 2000). The smaller deep portion arises from the posterior third of the lower border and from the whole of the medial surface of the zygomatic arch; its fibres are more vertical and pass downward and forward. Both portions are inserted into the angle and lower half of the lateral surface of the ramus of the mandible (Gray, 2000). The masseter is the most powerful jaw elevator muscle.

The temporal muscle arises from the whole of the temporal fossa and from the deep surface of the temporal fascia. Its fibres converge as they descend and end in a tendon, which passes into the zygomatic arch and is inserted into the medial surface, apex and anterior border of the coronoid process as well as the anterior border of the ramus of the mandible (Gray, 2000). It is divided into three portions based on fibre position: anterior, mid and posterior. The fibres are more vertical in the anterior portion and gradually become horizontal in the posterior region. Thus, the fibres of the anterior portion are more active during mouth closing and the posterior fibres are basically jaw retruders.

The external pterygoid muscle extends almost horizontally between the infratemporal fossa and the condyle of the mandible. It arises from two heads: an upper head from the lower part of the lateral surface of the great wing of the sphenoid and from the infratemporal crest; and a lower head from the lateral surface of the lateral pterygoid plate. Its fibres pass horizontally backward and laterally and are inserted into a depression in front of the neck of the condyle of the mandible as well as into the front margin of the articular disk of the temporomandibular articulation (Gray, 2000). The simultaneous contraction of both right and left external pterygoid muscles causes the jaw to move forward. When associated to contraction of the suprahyoid muscles (especially the digastric muscle), the mandible rotates and the mouth opens. If only one external pterygoid acts at a time, it moves the jaw to the opposite side (lateral movement) (Madeira, 2003).

The internal pterygoid muscle arises from the medial surface of the lateral pterygoid plate and the grooved surface of the pyramidal process of the palatine bone; it has a second slip of origin from the lateral surfaces of the pyramidal process of the palatine and tuberosity of the maxilla. Its fibres pass downward, laterally and backward and are inserted by a strong tendinous lamina into the lower and back part of the medial surface of the ramus and angle of the mandible at the height of the mandibular foramen (Gray, 2000).

The supra-hyoid muscles comprise the muscles of the oral floor. These are sheets of parallel fibrous tissue running from the hyoid bone to the mandible and include the digastric (V3 and VII), mylohyoid (V3) and geniohyoid (XII and C1) muscles (Figure 1.4). The digastric muscle is believed to be the principal muscle of jaw opening, whereas the geniohyoid is the most important muscle for elevation of the hyoid bone. The supra-hyoid muscles are in a group designated jaw retruders and mouth-opening muscles (Gray, 2000).

Figure 1.4 The supra-hyoid muscles.

Masticatory muscle activation and coordination determine the direction of jaw movement and control occlusal force (Herring, 2007). The thickness of the muscles of mastication affects facial dimensions and bite force (Pereira et al., 2007; Castelo et al., 2010). The functioning of the jaw muscles is highly dependent on the physiological properties of their motor units. These properties (force output, fatigability and contraction speed) vary considerably (Van Eijden and Turkawski, 2001). The jaw-closing muscles seem more adapted to performing slow, tonic movements and producing a smooth, gradable force. In contrast, the jaw-opening muscles seem more adapted to producing faster, phasic movements (Korfage et al., 2005).

The soft palate is the upper limit of the oropharynx and consists of several muscles joining in an aponeurosis: tensor veli palatini, levator veli palatini, palatopharyngeus, uvulus and palatoglossus. The principal elevator of the soft palate is the levator veli palatini, but all of these muscles play an important role in opening or closing the airway during swallowing (German and Palmer, 2006).

1.5 THE TONGUE

The tongue plays a major role in food ingestion. When the tongue moves during the mastication process the food progresses distally through the oral cavity, from the anterior region to the pharynx, for bolus formation and swallowing. Chemoreceptors and mechanoreceptors on the tongue surface sense the nature and mechanical properties of food (Hiimae and Palmer, 2003). In addition, tongue position is also important for breathing and talking.

The dorsum of the tongue is convex and marked by a median sulcus, which divides it into two symmetrical halves. This sulcus ends in a depression called foramen cecum, from which a shallow groove denominated the sulcus terminalis runs laterally and forward on both sides of the tongue. The anterior surface of the tongue is covered with papillae; the posterior region is smoother and contains numerous muciparous glands and lymph follicles (lingual tonsil) (Gray, 2000). There are different kinds of papillae. Circumvallate papillae are located on the dorsum of the tongue right in front of the foramen cecum and sulcus terminalis, forming a row on both sides; these papillae run backward and medially and meet at the mid line, forming an inverted V shape. Foliate papillae are clustered into two groups positioned on each side of the tongue just in front of the ‘V’ of the vallate papillae; these papillae are involved in taste sensation and have taste buds on their surfaces. Fungiform papillae are found both at the sides and apex, but are also scattered irregularly and sparingly over the dorsum; these papillae are differentiated by their larger size, rounded eminences and deep red colour. Filiform papillae are very small, hair-like papillae that cover the anterior two-thirds of the tongue (Figure 1.5). Additionally, taste buds are placed all over the mucous membrane of the mouth and tongue at irregular intervals and are the end organs of the gustatory sense (Gray, 2000).

Figure 1.5 The tongue: taste areas and papillae disposition.

Taste is an important part of feeding behaviour. The gustatory cells detect nutritionally important and harmful components in food and triggers innate behaviour leading to either the acceptance or rejection of potential food sources (Yarmolinsky et al., 2009). Taste receptor cells are located on the surface of the tongue and palate. These receptors are organised into taste buds, which are round structures with 50 to 100 cells (Lindemann, 2001). Taste signals from the fungiform taste buds and palate are transmitted to neurons in the geniculate ganglion via the chorda tympani and greater superficial petrosal nerve, respectively, whereas the circumvallate and foliate papillae are innervated primarily by the glossopharyngeal nerve, composed of fibres initiating from the petrosal ganglion. Taste information from sensory ganglia converges on the rostral portion of the nucleus of the solitary tract in the brainstem, from where it is sent to the ventral posteromedial nucleus of the thalamus. From the thalamus, projections connect to the primary gustatory cortex in the insula. Taste perception is initiated by the physical interaction of tastant molecules with specific receptor proteins present on the surface of taste receptor cells (Yarmolinsky et al., 2009).

Anatomically, the tongue is formed by the extrinsic (muscles with origin outside the tongue body) and intrinsic muscles (muscles with origin and insertion in the tongue body). These muscles act together in most tongue movements, which are not restricted to the protrusion–retrusion axis and involve intrincate three-dimensional changes in tongue shape (Sokoloff, 2004). The tongue is formed by four extrinsic muscles (genioglossus (XII), hyoglossus (XII), styloglossus (XII) and palatoglossus (X or XI)) and four intrinsic muscles (vertical, transverse, superior longitudinal and inferior longitudinal). The intrinsic muscles are all supplied by the hypoglossal nerve (XII), have no bone attachments and perform the more accurate movements (Sawczukl and Mosier, 2001). The genioglossus is an extrinsic muscle in that it originates from the mandible.

The neural supply to the tongue consists of three parts: the motor supply; a general sensory element, including the lingual nerve (V3) to the anterior two thirds and branches of the glossopharyngeal nerve (IX) to the posterior one third; and a small area near the base supplied by the internal laryngeal nerve (X). The sensation of taste is supplied by the chorda tympani (a branch of the facial nerve (VII)) to the anterior portion and by the glossopharyngeal (IX) and internal laryngeal (X) nerves to the posterior one third (German and Palmer, 2006).

The function of each muscle of the tongue is determined by the direction of the fibres. The posterior fibres of the genioglossi muscle move the root of the tongue forward and protrude the apex from the mouth. The anterior fibres move the tongue back into the mouth. The two muscles acting together draw the tongue downward so as to make its upper surface concave, forming a channel along which fluids pass toward the pharynx. The hyoglossi depress the tongue and move down its sides. The styloglossi draw the tongue upward and backward. The glossopalatini move the root of the tongue upward (Gray, 2000). The intrinsic muscles are particularly involved in altering the shape of the tongue, whereby it becomes shortened, narrowed or curved in different directions; (Gray, 2000).

1.6 CONCLUDING REMARKS

Mastication is one of the most important functions in the maintenance of general health. It initiates the digestive process, which is responsible for providing the nutrients necessary for cell activities. Prevention and treatment against common pathologies associated with the masticatory system (e.g. tooth deficiency, periodontal disease, temporomandibular dysfunction and dysfunction of the masticatory muscles, salivary glands) are important in order to improve one’s quality of life.

ACKNOWLEDGEMENTS

The author would like to thank Dr. Andries van der Bilt for revising the main text, the UNILAVRAS Dental School, Prof. Washington Loureiro Júnior and LL Comunication Co. for assistance on the illustrations. The author also would like to thank the CNPq (Brazilian Council for Scientific and Technological Development) for all research support and scholarship received.

REFERENCES

Alkan, A., Keskiner, I., Arici, S. and Sato, S. (2006) The effect of periodontitis on biting abilities. Journal of Periodontology, 77, 1442–1445

Anderson, L.C., Garrett, J.R., Johnson, D.A., Kauffman, D.L., Keller, P.J. and Thulin, A. (1984) Influence of circulating catecholamines on protein secretion into rat parotid saliva during parasympathetic stimulation. Journal of Physiology, 352, 163–171.

Bardow, A., Moe, D., Nyvad, B. and Nauntofte, B. (2000) The buffer capacity and buffer systems of human whole saliva measured without loss of CO2. Archives of Oral Biology, 45, 1–12.

Bialek, E.J., Jakubowski, W., Zajkowski, P., Szopinski, K.T. and Osmolski, A. (2006) US of the major salivary glands: anatomy and spatial relationships, pathologic conditions and pitfalls. Radiographics, 26, 745–763.

van der Bilt, A., Engelen, L., Abbink, J. and Pereira, L.J. (2007) Effects of adding fluids to solid foods on muscle activity and number of chewing cycles. European Journal of Oral Sciences, 115, 198–205.

van der Bilt, A., Olthoff, L.W., Bosman, F. and Oosterhaven, S.P. (1994) Chewing performance before and after rehabilitation of post-canine teeth in man. Journal of Dental Research, 73, 1677–1683.

van der Bilt, A., Olthoff, L.W., Bosman, F. and Oosterhaven, S.P. (1993) The effect of missing postcanine teeth on chewing performance in man. Archives of Oral Biology, 38, 423–439.

Castelo, P.M., Pereira, L.J., Bonjardim, L.R. and Gavião, M.B. (2010) Changes in bite force, masticatory muscle thickness and facial morphology between primary and mixed dentition in preschool children with normal occlusion. Annals of Anatomy, 20;192(1), 23–26

de Almeida Pdel, V., Grégio, A.M., Machado, M.A., de Lima, A.A. and Azevedo, L.R. (2008) Saliva composition and functions: a comprehensive review. Journal of Contemporary Dental Practice, 1;9, 72–80.

de Matos, L.F., Pereira, S.M., Kaminagakura, E., Marques, L.S., Pereira, C.V., van der Bilt, A. and Pereira, L.J. (2010) Relationships of beta-blockers and anxiolytics intake and salivary secretion, masticatory performance and taste perception. Archives of Oral Biology, 55(2), 164–169

Denny, P.C., Ball, W.D. and Redman, R.S. (1997) Salivary glands: a paradigm for diversity of gland development. Critical Review of Oral Biology & Medicine, 8: 51–75.

Edgar, W.M. (1992) Saliva: its secretion, composition and functions. British Dental Journal, 172, 305–312.

Fontijn-Tekamp, F.A., Slagter, A.P., Van Der Bilt, A., Van ’t Hof, M.A., Witter, D.J., Kalk, W. and Jansen, J.A. (2000) Biting and chewing in overdentures, full dentures and natural dentitions. Journal of Dental Research, 79, 1519–1524.

Fontijn-Tekamp, F.A., van der Bilt, A., Abbink, J.H. and Bosman, F. (2004) Swallowing threshold and masticatory performance in dentate adults. Physiology & Behaviour, 15(83), 431–436.

German, R.Z. and Palmer, J.B. (2006) Anatomy and development of oral cavity and pharynx. GI Motility Online. 16 May 2006.

Gray, H. (1918) Anatomy of the Human Body. Lea & Febiger: Philadelphia. Bartleby.com, 2000. www.bartleby.com/107/ [accessed 16 October 2011].

Hatch, J.P., Shinkai, R.S., Sakai, S., Rugh, J.D. and Paunovich, E.D. (2001) Determinants of masticatory performance in dentate adults. Archives of Oral Biology, 46, 641–648.

Herring, S.W. (2007) Masticatory muscles and the skull: a comparative perspective. Archives of Oral Biology, 52, 296–299.

Hiiemae, K.M. and Palmer, J.B. (2003) Tongue movements in feeding and speech. Critical Review of Oral Biology & Medicine, 14, 413–429.

Honda, M.J., Fong, H., Iwatsuki, S., Sumita, Y. and Sarikaya, M. (2008) Tooth-forming potential in embryonic and postnatal tooth bud cells. Medical Molecular Morphology, 41, 183–192.

Humphrey, S.P. and Williamson, R.T. (2001) A review of saliva: normal composition, flow and function. Journal of Prosthetic Dentistry, 85: 162–169.

Johansson, A.S., Svensson, K.G. and Trulsson, M. (2006) Impaired masticatory behavior in subjects with reduced periodontal tissue support. Journal of Periodontology, 77, 1491–1497.

Julien, K.C., Buschang, P.H., Throckmorton, G.S. and Dechow, P.C. (1996) Normal masticatory performance in young adults and children. Archives of Oral Biology, 41, 69–75.

van Kampen, F.M., van der Bilt, A., Cune, M.S., Fontijn-Tekamp, F.A. and Bosman, F. (2004) Masticatory function with implant-supported overdentures. Journal of Dental Research, 83, 708–711.

Korfage, J.A., Koolstra, J.H., Langenbach, G.E. and van Eijden, T.M. (2005) Fiber-type composition of the human jaw muscles – (part 1) origin and functional significance of fiber-type diversity. Journal of Dental Research, 84, 774–783.

Koussoulakou, D.S., Margaritis, L.H. and Koussoulakos, S.L. (2009) A curriculum vitae of teeth: evolution, generation, regeneration. International Journal of Biological Sciences, 5, 226–243.

Lindemann, B. (2001). Receptors and transduction in taste. Nature, 413, 219–225.

Llena-Puy, C. (2006) The role of saliva in maintaining oral health and as an aid to diagnosis. Medicina Oral Patología Oral y Cirugía Bucal, 11, E449–455.

Madeira, M.C. (2003) Face Anatomy: Anatomic-Functional Bases for Dental Practice, 4th edn. Sarvier Medical Books: São Paulo.

Nagler, R.M. (2004) Salivary glands and the aging process: mechanistic aspects, health-status and medicinal-efficacy monitoring. Biogerontology, 5, 223–233.

Pedersen, A.M., Bardow, A., Jensen, S.B. and Nauntofte, B. (2002) Saliva and gastrointestinal functions of taste, mastication, swallowing and digestion. Oral Diseases, 8, 117–129.

Pereira, L.J., Duarte Gaviao, M.B. and Van Der Bilt, A. (2006) Influence of oral characteristics and food products on masticatory function. Acta Odontologica Scandinavica, 64, 193–201.

Pereira, L.J., Gavião, M.B., Bonjardim, L.R., Castelo, P.M. and van der Bilt, A. (2007) Muscle thickness, bite force and craniofacial dimensions in adolescents with signs and symptoms of temporomandibular dysfunction. European Journal of Orthodontics, 29, 72–78.

Pereira, L.J., Gazolla, C.M., Magalhães, I.B., Ramos-Jorge, M.L., Marques, L.S., Gameiro, G.H., Fonseca, D.C. and Castelo, P.M. (2011) Treatment of chronic periodontitis and its impact on mastication. Journal of Periodontology, 82, 243–250.

Sawczukl, A. and Mosier, K.M. (2001) Neural control of tongue movement with respect to respiration and swallowing. Critical Review of Oral Biology & Medicine, 12, 18–37.

Sokoloff, A.J. (2004) Activity of tongue muscles during respiration: it takes a village? Journal of Applied Physiology, 96, 438–439.

Tenovuo, J.O. (1997) Salivary parameters of relevance for assessing caries activity in individuals and populations. Community Dentistry and Oral Epidemiology, 25, 82–86.

Turner, R.J. and Sugiya, H. (2002) Understanding salivary fluid and protein secretion. Oral Diseases, 8, 3–11.

Van Eijden, T.M. and Turkawski, S.J. (2001) Morphology and physiology of masticatory muscle motor units. Critical Review of Oral Biology & Medicine, 12, 76–91.

Wilding, R.J. (1993) The association between chewing efficiency and occlusal contact area in man. Archives of Oral Biology, 38, 589–596.

Yarmolinsky, D.A., Zuker, C.S. and Ryba, N.J. (2009) Common sense about taste: from mammals to insects. Cell, 16(139), 234–244.

Yousem, D.M. and Chalian, A.A. (1998) Oral cavity and pharynx. Radiology Clinics of North America, 36, 967–981.

2

Oral Receptors

Lina Engelen

2.1 INTRODUCTION TO ORAL RECEPTORS

2.1.1 Babies Sense the World Around Them Through the Mouth

Young children and babies are known to put everything they can get hold of into their mouths. There is a good reason for this. At a young age, humans have not yet developed the sensitivity of their fingertips, and while at birth the eyes can only focus on objects at about 30 cm, the mouth is already a well developed sensor. Babies have a strong urge to explore and learn about the world around them, and one of the ways they can experience how an object feels is to put it in the mouth and manipulate it with the lips and tongue. In this way the very young child learns what soft, hard, rough, cold and warm is and feels like. The mouth can do this long before the fingers are able to pick up on these sometimes very subtle differences. Most objects also have a flavour, which is interesting for the small baby to experience when their whole repertoire of food experiences usually is restricted to milk, with a hint of what mum had the day before (Beauchamp and Mennella, 2009).

The importance of the mouth as a receiver for tactile and chemical stimuli remains into adulthood. The oral area is one of the most sensitive parts of the body. The lips and tip of the tongue are even more sensitive than the finger tips (Bukowska et al., 2010). Considering the sensitivity and the number of fine muscles in the oral region, as in the finger tips, it is not strange that many of our tokens of affection are directed to and from these body parts. But perhaps the most important role of the mouth is to ingest food. We all enjoy food, not only for its stomach filling properties, but also because of the pleasure it brings us to experience the various tastes, textures and temperatures of food and drink.

In this chapter we will discuss the basic functions and mechanisms of the tactile, gustatory and olfactory receptors present in the mouth and in what way they influence how we process and perceive food. For more detailed information, please refer to the suggested reviews/literature mentioned throughout the text.

2.1.2 Receptors

Receptors throughout the body provide the central nervous system with vital information about the body and its environment. Thus the posture of the body, its supply with nutrients and oxygen, the state of the cardiovascular and digestive systems, as well as the body temperature and ion concentrations are constantly monitored by sets of sensory cells. Information about objects in the environment, their shape, colour, chemical composition, their distance and movement are collected and conveyed to the central nervous system. This steady and complex flow of coded information is then integrated into a perception and used to generate suitable actions. Each sensory cell detects specific stimuli using highly specialised structures that operate as receptors for adequate stimuli. The receptor must be selective as well as sensitive. The oral receptors are the first step in perceiving food and manipulating food safely and effectively in the mouth.

Humans have four classes of receptors, each of which is sensitive primarily to one modality of physical energy – mechanical, thermal, chemical and electromagnetic. In the mouth all types, except the photoreceptors sensitive to electromagnetic energy, are present. The mechanoreceptors mediate sensations of touch and proprioception; the thermoreceptors sense the temperature of the body and objects that we come in contact with; nociceptors signal sensations of pain; and chemical receptors respond to taste and smell. All these types of receptors contribute to the total sensation and perception of food that we ingest.

2.1.3 Innervation and Transduction

The oral region is innervated by three cranial nerves that carry sensory information from the oral region to the brain: trigeminal (V), facial (VII) and glossopharyngal (IX) Table 2.1.

Table 2.1 Summary of receptor types and channels for the sensory modalities taste, tactile and olfaction.

The trigeminal nerve consists, as the name implies, of three branches that together innervate most parts of the orofacial region. The trigeminal nerve is responsible for the sensation of tactile, proprioceptive, temperature and painful stimuli. The chorda tympani branch of the facial nerve is of greatest importance for sensations of taste as this branch innervates the taste buds of the anterior two thirds of the tongue. The glossopharyngeal nerve (IX) innervates the taste buds of the posterior third of the tongue. The signals evoked by receptors innervated by these cranial nerves are conducted to the cortex in slightly different ways.

Tactile, proprioceptive, nociceptive and thermal information from the receptors in the mouth is conveyed to the central nervous system by the trigeminal somatic sensory system (Figure 2.1). The oral receptors initiate action potentials upon stimulation. These receptors are part of the first order neurons of the trigeminal nerve with cell bodies in the trigeminal ganglia. The trigeminal nerve enters the brainstem at the level of the pons to terminate on second order neurons in the trigeminal brainstem complex. This complex has two major components: the principal nucleus (responsible for processing mechanosensory stimuli) and the spinal nucleus (responsible for processing thermal and painful stimuli). The second-order neurons of the trigeminal brainstem nuclei give off axons that cross the midline and ascend to third order neurons in the ventral posterior medial (VPM) nucleus of the thalamus by way of the trigeminal lemniscus. The axons arising from neurons in the VP complex of the thalamus project mainly to cortical neurons located in the primary somatosensory cortex (SI). Somatic sensory information is distributed from the SI to ‘higher-order’ cortical fields, such as the adjacent secondary somatosensory cortex, which sends projections to limbic structures, such as the amygdala and hippocampus. On all levels neurons also receive parallel information.

Figure 2.1 Trigeminal pathway from receptor to higher brain centres.

The chorda tympani (part of the intermediate branch of the facial nerve) enters the brain stem at the level of the pontomedullary junction, while the glossopharyngeal nerve enters at the level of the rostral medulla (Figure 2.2). Gustatory fibres innervating the taste buds collect in the solitary tract, where after the axons synapse on second-order neurons in the nucleus of the solitary tract (NST) of the medulla. Axons of these neurons project onto the ventral posterior medial nucleus (VPM) of the thalamus. Neurons in the VPM nucleus in turn project into areas of the insular cortex and frontal operculum. There are also connections from the NST to motor systems and parts of the digestive tract, such as the glands that secrete saliva and other glands that secrete digestive fluid. Connections to the pancreas make taste stimulation affect the excretion of insulin.

Figure 2.2 Pathway of chorda tympani (CN VII) and glossophayngeal (CN IX) afferents from receptor to higher brain centres.

The representations from each sensory modality (taste, vision, olfaction and touch) are brought together in multimodal regions, such as the orbitofrontal cortex. The signals are integrated into a complete picture, the perception (Rolls and Rolls 2005; Verhagen and Engelen 2006).

2.2 TASTE

The sense of taste, also known as the gustatory system, is primarily involved in feeding, with the main function of identifying food that is rich in nutrients and avoiding toxic substances.

Loss of ability to taste can lead to severely decreased food intake and malnutrition, which in turn can lead to decreased quality of life (Millen 1999). With age, olfactory sensitivity decreases more rapidly than gustatory sensitivity (Murphy 1986). Even though we often talk about the taste of food, the actual gustatory sensation, including the five basic taste modalities (sweet, sour, salt, bitter and umami), is only a part of the perception we actually are talking about. The rest of the perception comes from the touch (mechanical), trigeminal and olfactory stimuli, together forming the perception of flavour. Flavour reflects the unification of these qualities into a single percept during eating (see also Chapter 10, Section 10.8).

2.2.1 Taste Receptors

Taste receptors are located on the apical region of taste cells. The taste cells are clustered in taste buds that contain several types of cells (type I–IV), including supportive cells, progenitor cells and cells that express proteins etc. All taste buds have similar structures and there does not seem to be any variation in sensitivity. Taste buds appear at 7–8 weeks gestation. At this time, however, they are not yet mature and this happens at a later stage. After birth the number of taste buds increases further and they continue to mature. It has been shown that the ability to discriminate between taste stimuli (sweet, bitter and sour, not salt) is innate, but the preference can be modified by post-natal experience (Beauchamp and Mennella, 2009; Cowart et al., 2004). One taste bud can contain up to 100 taste cells representing all five basic taste qualities. Each taste bud has a pore that is in contact with the oral cavity and through which the tastants can reach the taste receptors. The receptors are transmembrane proteins that admit the ions that give rise to the sensations of salty and sour and bind to the molecules that give rise to the sensations of sweet, bitter and umami. A single sensory neuron can be connected to several taste cells in each of several different taste buds. One or more taste buds reside in taste papillae, which are spread over the tongue, soft palate, upper oesophagus and epiglottis. There are four different types of papillae on the tongue: filiform, fungiform, foliate and circumvallate. Filiform papillae do not contain any taste receptors and are therefore not important for taste perception. They are mechanical and probably involved in the transport of food and perhaps in mechanoreception. Foliate papillae are located on the sides of the tongue, fungiform papillae on the middle and circumvallate are large papillae on the dorsal side of the tongue, on the border with the pharyngeal part of the tongue, and they all contain taste receptors, see Figure 1.5 in Chapter 1.

2.2.2 Taste Molecules and Modalities

The substances that the gustatory system can detect are water soluble chemicals that are detected during direct contact with the gustatory (taste) receptors in the oral cavity.

Chemical constituents of food interact with receptors on the taste cells. Most taste stimuli are hydrophilic molecules that are soluble in saliva. The gustatory system distinguishes five basic stimulus qualities: salt, sweet, sour, bitter and umami (monosodium glutamate). Recent evidence indicates that fat may represent an additional taste quality (Khan and Besnard, 2009) and the taste system may also be responsive to other classes of compounds (e.g. calcium salts), but less is known about the underlying mechanisms for detecting these nutrients (Bachmanov and Beauchamp, 2007).

Some sensory systems have a single basic type of receptor cell that uses one transduction mechanism (e.g. the auditory system). However, taste transduction involves several different processes, and each basic taste uses one or more of these mechanisms. Taste stimuli may either, pass directly through ion channels (salt and sour), bind to and block ion channels (sour and bitter), bind to and open ion channels (some sweet amino acids), and bind to membrane receptors that activate second messenger systems that, in turn, open ions channels (bitter, sweet, umami). The detection thresholds for substances that evoke these sensations vary greatly.

2.2.2.1 What Substances Give Rise to the Different Sensations?

Sour

The sour taste is brought about by acids. The ‘functional’ part of acids in the sensation of sourness is the hydrogen ion but, depending on the associated anion, there is considerable variation in the degree of sourness of different acids. The thresholds for citric acid has been reported to be around 0.5 to 1.5 mM (Brosvic and McLaughlin 1989) and others have reported a detection threshold for acids around 0.6 mM on average (Paulus and Reisch 1980). The search for transduction mechanisms of sour-sensitive cells has proved difficult due to the fact that the pH of the stimulus affects many channels and proteins and therefore it is difficult to localise the actions of pH changes (Frings, 2009). However, at present the best potential candidate for a specific proton sensor in sour taste receptors is an ion channel from the TRPP (Transient Receptor Potential Polycystic) family, TRPP3 when coexpressed with PKD1L3 (Huang et al. 2006) (Figure 2.3). Much remains to be discovered about the actual transduction machinery for acid stimuli, however. Attention has been focused on a search for transducer proteins activated by extracellular protons, whereas the actual target appears more likely to be an intracellular site (Roper 2007).

Figure 2.3 Transduction models for the chemosensory membrane of taste cells. The receptor families T1R and T2R mediate sensitivity to sweet, umami and bitter by activating phospholipase β2 (PLCβ2) through a GTP-binding protein (G). The resulting release of Ca2+ from intracellular stores opens TRPM51 channels which generate a depolarising receptor potential. Sour taste is mediated by a combination of TRPP32 and PKD1L33 channels. Salt taste may, in part, be mediated by ENaC4 channels.

(Reproduced by permission of Professor Stephan Frings.)

1 transient receptor potential M5.

2 transient receptor potential P3.

3 polycystic-kidney-disease-like ion channels.

4 Na+-permeable epithelial sodium.

Salt

The principal stimulus for salty is a common ion, Na+. Table salt, NaCl, is the prototypic salty tastant. Na+ and Cl− are essential nutrients, vital for maintaining blood volume, blood pressure, regulating body water and, in the case of Cl−, maintaining acid/base homeostasis (i.e. Cl− shift). The detection threshold for NaCl is 1 to 15 mM on average in humans depending on the stimulus volume (Brosvic and McLaughlin, 1989; Slotnick et al., 1988). The activation of salt receptors probably involves the activation of sodium receptors, but the strength and taste quality is also modified by the anion present. Hence, salt detection is thought to be dependent on cation ion channels. The exact molecules that mediate salt taste are presently not clearly defined. The best candidate seems to be the epithelial sodium channel ENaC. Na+ or K+ simply diffuses from the surface of the cell through the channel into salt-sensitive cells, and can hence depolarise the taste cell. Cl- ions are thought to traverse the paracellular route across the taste epithelium together with Na+ (Roper, 2007).

Sweet

Most ripe fruit and vegetables taste sweet and contain the most valuable nutrients. Hence perception and the innate preference of sweet taste is likely to have evolved to ensure that we pick and ingest the most nutritious plant parts. Sugar tastes sweet, but there are many other substances that are molecularly very different to sucrose that evoke the same sensation, such as other saccharides, various amino acids, peptides and proteins, as well as artificial sweeteners (Roper 2007). Due to the diversity of the sweet tasting substances it is difficult to give a detection threshold, but the thresholds for sucrose have been reported to be both around 2–5 mM (Zhang et al., 2009) and 14–22 mM (Brosvic and McLaughlin, 1989).

Umami