1.2.1.1 Cranium and Face

Normal Craniofacial Development (Figs. 1.7, 1.8, 1.9 and 1.10)

(Kliegman and Nelson 2007) The human skull comprises three components of different origin: the chondrocranium, which forms from three parasagittal cartilages and three sensory capsules via endochondral ossification; the membrane (dermal) bones, ossifying directly from mesenchyme of the skin; and the branchial skeleton of the pharyngeal arches, forming via endochondral ossification. The parasagittal cartilages form the base and median elements of the skull, and the primitive sensory capsules are the origins for elements of the nose, orbit and temporal bone.

The membrane bones of the human skull include the cranial vault (calvaria) and the bones of the face. The bones of the calvaria are separate at birth but will fuse to form sutures, and the fontanelles between these bones will join later after the brain has finished growing.

From week 4 to week 10, the face develops from five facial swellings: paired maxillary swellings, paired mandibular swellings and an unpaired medial frontonasal process. The maxillary swellings enlarge in the fifth week; they lengthen medially and form the primordia of the cheeks and the lateral portions of the upper lip. The lateral portions of the maxillary and mandibular swellings fuse to produce the final shape of the mouth. The mandibular swellings enlarge to form the primordia of the lower lip and jaw in the fourth and fifth weeks. The buccopharyngeal membrane, which separates the ectodermal stomodeum from the endodermal foregut, breaks down on day 24.

In the fifth week, ectodermal thickenings, called nasal placodes, appear on the frontonasal process, which will give rise to the nose and philtrum. Each placode develops a nasal pit in its centre. In the sixth week, its lateral edge, the lateral nasal process, will form the sides of the nose; its medial rim, the medial nasal process, will fuse with its contralateral partner to form the bridge of the nose. During the seventh week, the inferior portion of the fused material forms the intermaxillary process that will join the maxillary swellings to form the philtrum of the upper lip.

The nasal pits enlarge and fuse to form the nasal sac, with the nasal fin developing from its floor to separate the nasal and oral cavities. The nasal fin thins to form the oronasal membrane. It finally ruptures, forming an opening into the oral cavity, called the primitive choana. The primary palate grows posteriorly from the intermaxillary process as a ridge to form the floor of the primitive nasal cavity.

In the eighth week, a pair of palatine shelves initially grows inferiorly from the maxillary swellings into the oral cavity, on either side of the tongue. The shelves rotate horizontally in the ninth week and fuse medially to form the secondary palate. The anterior portion of the secondary palate ossifies to form the hard palate, while muscles of the soft palate develop in its posterior portion. Meanwhile, the nasal septum grows inferiorly from the roof of the nasal cavity, fusing with the top of the hard palate to form two nasal passages that communicate with the pharynx through the definitive choanae.

Malformations of Lips and Palate (Figs. 1.11, 1.12 and 1.13)

Cleft lips occur about once in 100 births. Males dominate by 60–80%. The clefts vary from small notches in the red of the lip to larger gaps, including the floor of the nose and the alveolar process of the maxilla. They may appear uni- or bilaterally and are caused by a failure of the maxillary swelling and the nasal prominence to merge. The median cleft lip (Figs. 1.7, 1.8, 1.9, 1.10, 1.11, 1.12 and 1.13a) is an extremely rare malformation, probably induced by a deficiency of mesenchyme and an incomplete fusion of the medial nasal processes.

A split palate may occur solely or combined with a cleft lip. The cleft may be limited to the uvula but may extend across the soft and hard palate. The reason lies in an insufficient generation of mesenchyme, resulting in a disturbed fusion of the lateral maxillary shelves with the nasal septum and the posterior edge of the primary palate.

1.2.1.2 Pharyngeal Arches, Clefts and Pouches

During early development, five pharyngeal (branchial) arches are generated, which appear as bar-like ridges on the ventrolateral surface of the head and neck region. They are covered by ectoderm and are separated from each other by invaginations called pharyngeal clefts. The pharyngeal clefts have counterparts on the interior in the form of endoderm-lined pharyngeal pouches. Ectoderm and endoderm are isolated by a mesodermal core. Pharyngeal membranes separate the clefts from the pouches (Graham 2001).

The pharyngeal arches are numbered 1, 2, 3, 4 and 6; they develop in cranio-caudal sequence with the first pair appearing on day 22, the second and third pairs on day 24 and the fourth and sixth pairs on day 29. Each pharyngeal arch contains an arch cartilage, an arch artery, a mesodermal component as precursor for muscles and a specific cranial nerve.

The first branchial arch is divided into a maxillary and a mandibular process; the former develops to the palatopterygoquadrate bar cartilage, which will become the greater wing of the sphenoid and the incus; the latter contains Meckel’s cartilage, a precursor of the malleus and the fibrous core of the mandible. The jaws mainly consist of membrane bones formed by direct ossification; the maxillary process gives rise to the upper jaw, the maxilla, the zygomatic and the temporal squama, and the mandibular process generates the lower jaw.

The second arch cartilage, Reichert’s cartilage, forms the stapes, styloid process, stylohyoid ligament and parts of the hyoid. The third arch cartilage also contributes to the hyoid; the fourth and sixth arch cartilages form the larynx; and the epiglottis arises in the location of the fourth arch (Fig. 1.14).

The first arch is innervated by the trigeminal nerve, the maxillary swelling by V2 and the mandibular swelling by V3. The second arch is innervated by the facial nerve (VII), the third arch is innervated by the glossopharyngeal nerve (IX), the fourth arch is innervated by the superior branch and the sixth arch is innervated by the recurrent laryngeal branch of the vagus nerve (X).

1.2.1.3 Development of the Larynx

Normal Development (Fig. 1.15)

The respiratory system is an outgrowth of the primitive pharynx. Between the 20th and the 26th days of gestation, a ventral laryngotracheal groove in the primitive foregut differentiates into the laryngeal sulcus and the respiratory primordium. The tracheo-oesophageal folds between these tubular hollows later fuse to form the tracheoesophageal septum, separating the laryngotracheal groove from the foregut. From now on the foregut is divided into the ventral laryngotracheal tube and the dorsal oesophagus.

The larynx develops from the fourth and sixth branchial arches. The laryngotracheal opening lies between these two arches. The internal lining of the larynx originates from endoderm, whereas cartilages and muscles emanate from mesenchyme. The mesenchyme proliferates rapidly, and the sagittal slit of the laryngeal orifice changes into a T-shaped opening by the growth of three tissue masses: one is the hypobranchial eminence, which later becomes the epiglottis. The second and third growths are two arytenoid precursors. They grow between the fifth and seventh week, resulting in a temporary occlusion of the lumen. Recanalisation occurs by the tenth week and produces a pair of lateral recesses, the laryngeal ventricles that are bounded by folds of tissue that differentiate into the false and true vocal cords. Failure to recanalise may result in atresia, stenosis or web formation in the larynx.

The development of the larynx begins with the appearance of the mesenchymal-arytenoid swellings from the sixth branchial arches on the 32nd day of gestation on both sides of the opening of the laryngotracheal tube. These swellings approach each other in the midline and converge at the caudal end of the hypobranchial eminence to convert the vertical laryngotracheal opening into a T-shaped aditus. Midline compression of the tube by these swellings results in the fusion of the epithelial lamina, thereby closing the tube from the pharynx. If the closing does not occur, a posterior laryngeal cleft can result leading to severe aspiration in the newborn. The arytenoid swellings differentiate into the arytenoid and corniculate cartilages and the primitive aryepiglottal folds.

The epiglottal and cuneiform cartilages are formed by the hypobranchial eminence. Chondrification of both fourth branchial arches gives rise to the thyroid cartilage, whereas the cricoid cartilage derives from the chondral tissue of the sixth branchial arch. The laryngeal lumen obliterates to give rise to the epithelial lamina. The larynx recanalises by the tenth week of gestation.

The intrinsic muscles have gained their shapes and positions by the 40th day of gestation, and by the end of the eighth week, all components of the larynx are present including innervation and blood supply.

During the foetal period, the vocal processes develop from the arytenoids, and the thyroid cartilage laminae fuse in the midline. The epiglottal cartilage matures between the fifth and seventh months. During this period, the corniculate and cuneiform cartilages become evident. The foetal period ends with the cricoid cartilage changing from interstitial to perichondrial growth.

Malformations of the Larynx (Figs. 1.16, 1.17 and 1.18)

From the location of laryngeal malformations (Sidrah et al. 2007), one discriminates between supraglottal, glottal and subglottal anomalies.

The most abundant congenital anomaly of the larynx is the laryngomalacia, accounting for more than a half of all cases (Ahmad and Soliman 2007). The ratio between males and females is about 2:1. It is classified as Type 1, Type 2 or Type 3 on the basis of patterns of supraglottal collapse. In Type 1 laryngomalacia, redundant supraglottal mucosa prolapses; Type 2 is characterised by shortened aryepiglottic folds; and Type 3 displays posterior displacement of the epiglottis coincident with a deformation, due to an imbalance in its development. The epiglottis develops from the cartilages of the third and fourth branchial arches, and an overgrowth of the third arch portion results in an omega-shaped organ. In addition, an arytenoid prolapse may result from immature neuromuscular control.

The second-most common congenital laryngeal disorder, in about 15–20% of all congenital anomalies, affects vocal fold movement. It may occur unilaterally or, less frequently, bilaterally. Unilateral paralysis is usually idiopathic but may be secondary to peripheral nerve pathology. Strain injuries to the recurrent laryngeal nerve during birth may be one of the causes.

The glottal sulcus (or sulcus vocalis) is characterised by dysphonia due to hampered movement of the mucous membrane, absence of Reinke’s space and adhesion of the epithelium to the vocal ligament or the vocal muscle itself.

The congenital subglottal stenosis takes third place in laryngeal anomalies with approximately 15% of the cases, twice as often in boys than in girls. It may be subdivided into two types, the more abundant is membranous congenital subglottal stenosis, due to submucosal hypertrophy. The second, cartilaginous congenital subglottal stenosis, results from an abnormal growth of the cricoid cartilage.

Subglottal haemangioma accounts for 1.5% of congenital anomalies of the larynx, in girls twice as often than in boys. It results from a malformation of the mesenchymal vascular precursors.

Laryngoceles are rare congenital anomalies of the supraglottal larynx. They form as a result of air- or fluid-filled dilations of the laryngeal ventricle communicating with the laryngeal lumen. They may occur internally or externally or both.

About 25% of all laryngeal cysts are saccular cysts. In contrast to the laryngoceles, they do not communicate with the laryngeal lumen.

Laryngeal webs are rare congenital anomalies. They are due to an incomplete recanalisation of the laryngotracheal tube, which occurs in the third month of gestation. They appear mostly at the anterior level of the vocal folds.

Laryngeal or laryngotracheo-oesophageal clefts are posterior fusion defects between the airway and oesophagus during embryogenesis. These clefts may be minor and short or may even extend beyond the carina. They are classified according to their anatomical extent.

Laryngeal atresia is considered to be the rarest of the congenital anomalies of the larynx. It occurs when the recanalisation of the laryngotracheal tube during the third month of gestation fails.

1.2.1.4 Tongue Development

Normal Development (Figs. 1.19, 1.20 and 1.21)

Tongue development starts with a triangular elevation in the floor of the first pharyngeal arch during the end of the fourth week of gestation, which is called the median tongue bud (tuberculum impar). A pair of mesenchymal swellings in the ventromedial areas of the first pharyngeal arch forms the distal tongue buds (lateral lingual swellings) on either side of the tongue. They are covered by epithelium of ectodermal origin, overgrow the median tongue bud and fuse medially to form the midline sulcus. Sensory innervation of this part is by the lingual branch of the mandibular division of the trigeminal nerve, the nerve of the first pharyngeal arch.

Behind the foramen cecum, the second pharyngeal arch develops the copula in the midline. A second elevation, arising from the third and partly the fourth pharyngeal arch, forms the hypobranchial eminence, which will become the pharyngeal part of the tongue.

The copula is overgrown by the hypobranchial eminence in the fifth and sixth week. It will fuse anteriorly with the distal tongue buds, thereby creating the terminal sulcus.

The median and pharyngeal sections of the organ then become joined at the terminal sulcus. This posterior compartment of the tongue is innervated by the glossopharyngeal nerve, the nerve of the third pharyngeal arch, whereas the chorda tympani from the cranial nerve VII supplies the taste buds on the anterior two thirds. The growing tongue extends out into the oral cavity; its anterior part is covered by a layer of ectodermal epithelium. In contrast, the root of the tongue is covered with endodermal epithelium.

So far, only the epithelial and mucosal tissues of the tongue have been considered, which develop from the four pharyngeal swellings as described above. The muscular compartment of the tongue descends from myoblasts that differentiate after migrating from the myotomes of the occipital cervical somites. Following these myoblasts is the hypoglossal nerve, which generates the nerve supply for the tongue musculature.

1.2.1.5 Development of the Ear

Inner Ear, Normal Development (Figs. 1.22 and 1.23)

In the third week of embryonic development, the ectoderm on both sides of the rhombencephalon (hindbrain) begins to thicken and form the otic placodes. They shift caudally to the level of the second pharyngeal arch and invaginate during the fourth week to form the otic pits. The pits separate from the surface to form the otic vesicles, which are the precursors of the membranous labyrinth.

Each otic vesicle differentiates into three parts: a dorsomedial, elongated endolymphatic extension, origin of the endolymphatic duct and, at its distal end, the endolymphatic sac; a central partition, which will expand to form the utricle and the three semicircular ducts, arising from utricular diverticula; and a ventral, conical saccular region, which forms the saccule and the cochlear duct, as well as the ductus reuniens joining the saccule and cochlear duct. The duct elongates in the fifth week and starts to coil, with the spiral organ of Corti differentiating in the seventh week. By this time, the organ of Corti is innervated by the cochlear ganglion, which will elongate and wind up together with the organ of Corti.

At the end of the ninth month, the auricular pathway is completed; myelinisation, however, has not taken place, and axo-dendritic synapses are not yet established.

External Ear, Normal Development (Fig. 1.24)

Each of the adjacent ectodermal parts of the first and second pharyngeal arches differentiates into three auricular hillocks. They arise in the fifth week. In the seventh week, the auricular hillocks begin to enlarge, differentiate and fuse, producing the final shape of the ear, which is gradually translocated from the side of the neck to a more cranial and lateral site. The first pharyngeal arch gives rise to the tragus, the helix and the cymba conchae; the second pharyngeal arch forms the antitragus, the antihelix and the concha.

1.2.2.1 The Palate

The hard palate is generated by two types of bone, which are covered by a mucous membrane: the palatine processes of the maxillae and the horizontal parts of the palatine bones (Fig. 1.25). These bones continue into the soft palate, which contains a membranous aponeurosis. The soft palate, also called velum palatinum, is a movable, fibromuscular fold that is attached to the posterior edge of the hard palate. It separates the superior nasopharynx from the inferior oropharynx. Laterally, the soft palate is continuous with the wall of the pharynx and is joined to the tongue and pharynx by the palatoglossal and palatopharyngeal folds.

The components are as follows:

The levator veli palatini, extending from the cartilage of the auditory tube and petrous part of temporal bone to the palatine aponeurosis. It elevates the soft palate, drawing it superiorly and posteriorly and also opens the auditory tube to regulate air pressure in the middle ear. It is innervated by a pharyngeal branch of the vagus via the pharyngeal plexus.

The tensor veli palatini arises from the scaphoid fossa of the medial pterygoid plate, spine of sphenoid bone and cartilage of auditory tube to the palatine aponeurosis. It tenses the soft palate by using the hamulus as a pulley. It also acts on the membranous portion of the auditory tube in the same sense as the levator muscle. Innervation is through the medial pterygoid nerve (a branch of the mandibular nerve).

The musculus uvulae, which emanates at the posterior nasal spine and palatine aponeurosis and inserts into the mucosa of uvula. When the muscle contracts, it shortens the uvula and pulls it upwards. The pharyngeal branch of vagus innervates the muscle via the pharyngeal plexus.

The palatoglossus muscle between the palatine aponeurosis and the side of tongue. The mucous membrane covering the muscle forms the palatoglossal arch. The muscle elevates the posterior part of the tongue and draws the soft palate downwards onto the tongue.

The palatopharyngeus muscle, extending from the hard palate and palatine aponeurosis to the lateral wall of pharynx. Its mucous membrane forms the palatopharyngeal arch. The muscle tenses the soft palate and pulls the walls of the pharynx upwards, forwards and medially during swallowing. Both muscles are supplied by the cranial part of accessory nerve (CN XI) joining with the pharyngeal branch of vagus via the pharyngeal plexus.

The sensory nerves of the palate, which are branches of the pterygopalatine ganglion, are the greater (major) and lesser (minor) palatine nerves (Fig. 1.26). They accompany the arteries through the greater and lesser palatine foramina, respectively.

The palate has an abundant blood supply from branches of the maxillary artery.

1.2.2.2 The Pharynx

The pharynx is a fibromuscular tube that spans vertically from the base of the skull to the oesophagus. Being situated posterior to the nasal and oral cavities and posterior to the larynx, it is therefore divisible into the nasopharynx, oropharynx and laryngopharynx, which ends at the inferior border of the cricoid cartilage, where it becomes continuous with the oesophagus.

The anterior part of the nasopharynx communicates through the choanae with the nasal cavities. Its lateral walls contain the pharyngeal ostia of the auditory tube, bounded behind by the torus tubarius, a prominence of the mucous membrane caused by the medial end of the cartilage of the tube. On the posterior wall of the nasopharynx, an assembly of lymphatic tissue is located, known as the pharyngeal tonsil.

The oropharynx, or mesopharynx, lies behind the oral cavity, extending from the uvula to the level of the hyoid bone. It opens anteriorly, through the isthmus faucium, into the mouth. The anterior wall consists of the base of the tongue; the superior wall consists of the inferior surface of the soft palate and the uvula. Its entrance, the isthmus faucium, is formed by the palatoglossal and palatopharyngeal arches of each side of the oral cavity, between them the palatine tonsil is positioned.

The laryngopharynx extends from the superior border of the epiglottis to the inferior border of the cricoid cartilage, where it becomes continuous with the oesophagus. Its anterior wall is the rear of the epiglottis and the posterior aspects of the arytenoid and cricoid cartilages. The piriform recess is part of the cavity of the laryngopharynx, situated on each side of the inlet of the larynx.

The lateral and posterior walls (Fig. 1.27) of the three parts of the pharynx are formed by various muscles: two of them, the palatopharyngeal muscle with its origin in the soft palate and the salpingopharyngeal muscle, originating at the auditory tube, are longitudinally orientated and form the innermost muscular layer. They are covered by the three constrictors, the upper (superior), middle and lower (inferior) constrictor muscle.

Finally, the stylopharyngeus muscle, beginning at the styloid process, runs into a gap between the upper and the middle constrictor and ends at the thyroid cartilage. All of these muscles are of the striated type.

Altogether, the tubulo-muscular wall of the pharynx consists of four layers: a mucous membrane, the pharyngeal aponeurosis, the muscle layer and the buccopharyngeal fascia.

The motor nervous and most of the sensory nervous supply to the pharynx is by way of the pharyngeal plexus, which, situated mainly on the middle constrictor, is formed by the pharyngeal branches of the vagus and glossopharyngeal nerves and also by sympathetic nerve fibres.

Blood supply of the pharynx is ensured by pharyngeal branches of the ascending pharyngeal artery, ascending palatine artery, descending palatine artery and pharyngeal branches of inferior thyroid artery. Veins collect the blood into the pharyngeal plexus.

1.2.2.3 The Larynx

The larynx is located in the anterior neck, ventrally of the cervical vertebrae 3–6. It connects the pharynx with the trachea and regulates the flow of air to and from the lungs for respiration and vocalisation and guards the air passages against food and liquids entering it. Its ventral prominence is called Adam’s apple. The larynx extends from the tip of the epiglottis to the inferior border of the cricoid cartilage. Its interior can be divided into three parts, the supraglottis, the transglottis and the subglottis (see below).

The skeleton of the larynx is composed of nine cartilages, three single and three paired (Fig. 1.28):

First is the thyroid cartilage, of hyaline nature. Its superior margin and its superior horn are attached to the hyoid bone by the thyrohyoid membrane, centrally and laterally enhanced as the medial or lateral thyrohyoid ligament. Its inferior horn connects to the cricoid cartilage and takes part in the cricothyroid articulation.

The hyaline cricoid cartilage is situated below the thyroid cartilage. It is the only one that encircles the entire larynx. It is attached to the thyroid cartilage via the median cricothyroid ligament and to the first ring of the trachea via the cricotracheal ligament.

Two mostly hyaline arytenoid cartilages of pyramidal shape are positioned dorsally on the superior margin of the cricoid cartilage. They are connected to the vocal ligaments by their vocal process, and their muscular process serves for muscular attachment. Each of them has an elastic corniculate cartilage on its top. The latter connect to the cricoid cartilage via the posterior cricoarytenoid ligament.

Behind the thyroid cartilage protrudes the epiglottis, a spoon-shaped elastic cartilage, which is connected to the thyroid cartilage by the thyroepiglottic ligament. It contacts the arytenoid cartilages via the quadrangular membrane, into which two elastic cuneiform cartilages are embedded.

The most prominent and most important ligaments of the larynx are the vocal ligaments, converging from the vocal processes of the arytenoids to the posterior surface of the thyroid. They serve as a margin for the conus elasticus, extending downwards to the cricoid cartilage.

Two pairs of joints affect the vocal ligaments: the cricothyroid joints allow tilting, and to a small extent gliding between the thyroid and cricoid cartilage, they thereby stretch or loosen the vocal ligaments. The cartilages move by action of the straight and oblique parts of the external cricothyroid muscle (Fig. 1.29). The muscle is innervated by the superior laryngeal nerve, which branches from the main trunk of the vagus nerve.

The cricoarytenoid joints permit gliding and rotation of the arytenoid cartilages, thus changing the positions of the vocal ligaments. Adductors of the vocal ligaments are the lateral cricoarytenoid muscle, the oblique and transverse arytenoid muscles and the vocalis muscle (by increasing its diameter in isometric contraction). The posterior cricoarytenoid muscle acts as an abductor of the vocal ligaments, whereas the aryepiglottic, thyroarytenoid and vocalis muscles are effective in reducing the tension of the vocal ligaments (Fig. 1.30). All of these muscles are innervated by the recurrent laryngeal nerve, a branch of the vagus nerve.

The internal cavity of the larynx (Fig. 1.31) is divided into three parts. It starts with the vestibule of larynx (supraglottis), the laryngeal inlet, which extends from the upper border of the epiglottis down to the ventricular folds. These are mucus membrane folds forming the lower free edge of the quadrangular membrane, they run from the thyroid cartilage above the vocal ligament to the arytenoid cartilages. They contain large sero-mucous glands, which serve to moisten the vocal folds. The vestibule continues into the ventricle of the larynx (transglottis), which extends between the vestibular and vocal folds.

The vocal folds extend from the angle of thyroid to the vocal processes of arytenoid cartilages. They are important for phonation by controlling the stream of air through the rima glottidis, the variable cleavage between them. They alter the shape and size of the wedge-shaped rima glottidis by movement of the arytenoids to ensure respiration or phonation.

Below the vocal folds, the subglottal space extends to the lower border of the cricoid cartilage.

On both sides of the laryngeal inlet, the piriform recesses ensure continuity between the pharynx and the beginning of the oesophagus, where they meet. They are bordered medially by the aryepiglottic fold, laterally by the thyroid cartilage and the thyrohyoid membrane (Fig. 1.32).

The epithelium of the vocal fold (Fig. 1.33) is of the non-keratinised stratified squamous type. It changes to a ciliated pseudostratified epithelium on the posterior glottis, ventricular folds and trachea. The lamina propria may be divided into three layers according to its histological composition: a superficial layer, pliable and flexible, also called Reinke’ space, of loose connective tissue. It is densely interwoven with the epithelium by digital projections containing small vessels. It continues into an intermediate and a deep layer. There is an increase in the presence of fibrous and interstitial proteins in the intermediate and, to a greater extent, in the deep layer of the lamina propria, both making up the vocal ligament. The intermediate layer is marked with a distinct elevation in the relative amount of elastin and collagen, and this increases even more within the deep layer (Jette and Thibeault 2011). The deep layer is adjacent to the vocal muscle, which may be considered as a medial part of the thyroarytenoid muscle.

The larynx as a whole is embedded into the vertically running muscle cords of the anterior neck (Fig. 1.34). Two longer muscles, the sternohyoid and the omohyoid, cover a group of shorter muscles ventrally, inserting directly at the thyroid. These extrinsic muscles may be divided into two groups: to the elevators (mainly suprahyoid muscles) of the larynx belong muscles that pull the hyoid upwards (the digastric muscle, the mylohyoid, the genioglossus, the stylohyoid and the stylopharyngeus) and, acting directly on the thyroid, the thyrohyoid muscle; depressors of the larynx are the sternothyroid, the omohyoid and the sternohyoid muscle, the latter two by pulling the hyoid downwards.

The entire larynx is innervated by the vagus nerve: the nerve separates a superior branch that leaves the main trunk high in the neck. Approximately at the level of the hyoid bone, this superior laryngeal nerve divides into an external and an internal branch. The only function of the external branch is the motoric innervation of the cricothyroid muscle.

The internal branch passes through a foramen in the thyrohyoid membrane together with the superior laryngeal artery and vein. It provides general sensation, including pain, touch and temperature for the tissue superior to the vocal folds.

The lower part of the larynx is supplied by the recurrent laryngeal nerve. It contains motor fibres to innervate all the intrinsic muscles of the larynx—except for the cricothyroid muscle—as well as both sensory and secretory fibres to the glottis, subglottis and trachea. The right recurrent laryngeal nerve leaves the vagus nerve, which parallels the internal jugular vein, near the point where the brachiocephalic trunk divides. The left recurrent laryngeal nerve emanates from the vagus nerve near the aortic arch. Both branches cross dorsally below the adjacent vessel and ascend laterally next to the trachea. They often terminate in forming an anastomosis with the ipsilateral internal branch of the superior laryngeal nerve.

The larynx has its arterial supply from the superior laryngeal artery, a branch of the superior thyroid artery, which accompanies the internal laryngeal nerve, and by the inferior laryngeal artery from the inferior thyroid artery, which runs parallel to the recurrent laryngeal nerve.

1.2.2.4 The Tongue

The relaxed tongue takes up most of the space inside the oral cavity. It basically comprises muscles surrounded by a mucous membrane. The posterior one third of the tongue, the root, is attached to the floor of the oral cavity. The mobile anterior two thirds of the tongue is called the body, and the tip is the apex.

The surface, or dorsum, contains numerous projections of the mucous membrane called papillae. They contain taste buds, which can sense five types of sensations: sweet, salty, sour, bitter and umami, which is a savoury meaty flavour. In addition, serous glands of the mucosa secrete some of the fluid of the saliva.

The inferior surface of the tongue is covered by a thin transparent membrane. A large fold of mucosa, called the frenulum, runs down the midline. The ducts of the submandibular salivary glands open at the base of the frenulum.

The muscles of the tongue are divided by the lingual septum. Four pairs are intrinsic, and four pairs are extrinsic (Table 1.2, Fig. 1.35).

Table 1.2

Internal and external muscles of the tongue

Muscles	Origin	Insertion	Function
Internals
Superior longitudinal muscle	Submucosal fibrous layer and septum	Margins of the tongue and mucous membrane	Curls the tongue upwards and shortens it
Inferior longitudinal muscle	Root of the tongue and hyoid bone	Apex	Curls the tongue downwards and shortens it
Transverse muscle	Septum of the tongue	Lateral margins of the tongue	Narrows and protrudes the tongue
Vertical muscle	Submucosal fibrous layer of the dorsum of the tongue	Inferior surfaces of the borders of the tongue	Flattens and broadens the tongue
Externals
Genioglossus muscle	Mandible	Entire dorsum of the tongue and hyoid bone	Protrudes the tongue and assists with other movement
Hyoglossus muscle	Hyoid bone	Inferior and lateral parts of the tongue	Depresses and shortens the tongue
Styloglossus muscle	Styloid process of temporal bone	Posterior parts of the tongue	Retracts the tongue and curls its sideways
Palatoglossus muscle	Palatine aponeurosis	Posterolateral parts of the tongue	Elevates the posterior part of the tongue and depresses the soft palate

All muscles of the tongue, except for the palatoglossus, are innervated by the hypoglossal nerve (CN XII). The palatoglossus is innervated by the pharyngeal plexus (CN X).

The somatosensory innervation of the anterior two thirds of the tongue comes with the mandibular nerve via the lingual nerve; the visceral sensory innervation is by the facial nerve via the chorda tympani. The posterior 1/3 part of the tongue has somatosensory and visceral innervation from the glossopharyngeal nerve. The somatosensory innervation of the root is by the vagus nerve (Fig. 1.36).

The tongue gains its blood by the lingual artery, a branch of the external carotid artery. It is drained by lingual veins, which continue into internal jugular vein.

1.2.2.5 Swallowing

Swallowing is a complex series of sequential neuromuscular events that are integrated into a smooth and continuous process, which is divided into three stages: oral, pharyngeal and oesophageal.

The oral phase of swallowing can be further subdivided into the oral preparatory and the oral transport phase. In the oral preparatory phase, the lips, tongue, mandible, palate and cheeks act in common with salivary flow to form food into a consistency and position appropriate for the subsequent phases of swallowing. Once the food bolus is prepared, the oral transport phase occurs, as the musculature of the lips and cheeks contract, followed by tongue contraction against the hard palate. The soft palate elevates as a consequence of contraction of the tensor veli palatini, levator veli palatini and palatopharyngeus muscles. Thereby a reflux of food into the nasal cavity is prevented.

The anterior two thirds of the tongue are critical in the oral phase of deglutition. The posterior one third of the tongue, the tongue base, plays an important role in propelling a food bolus posteriorly towards the pharynx.

The nerves involved so far are the trigeminal nerve (CN V) to control general sensation to the face and motor supply to the muscles of mastication, the facial nerve (CN VII) to supply taste to the anterior two thirds of the tongue and motor function to the lips, the glossopharyngeal nerve (CN IX) to provide general sensation to the posterior third of the tongue and the hypoglossal nerve (CN XII) to enable movements of the tongue.

Once the food bolus touches the palatoglossal folds, the pharyngeal phase of swallowing reflexively begins.

When the swallowing reflex is initiated, the following reactions take place: velopharyngeal closure to prevent reflux of material into the posterior choana. This is affected by contraction of the levator veli palatini muscles, which elevate the soft palate against the posterior nasopharyngeal wall. Medial contraction of the lateral pharyngeal wall musculature and a slight anterior movement of the posterior pharyngeal wall create Passavant’s ridge, against which the velum is approximated during the initiation of the pharyngeal phase of swallowing. The pharyngeal constrictor muscles contract in a superior-to-inferior direction. The epiglottis inverts to cover the larynx and prevent aspiration of contents into the airway. This retroversion of the epiglottis directs the food bolus laterally towards the pyriform sinuses. The vocal folds adduct to prevent aspiration.

With contraction of the superior pharyngeal constrictor muscle, laryngeal elevation occurs. The larynx elevates following the anterior movement of the hyoid bone and tongue base owing to contraction of the mylohyoid, geniohyoid, stylohyoid and anterior digastric muscles. This anterior movement of the larynx combined with the contraction of the middle and inferior constrictor muscles forces the food bolus inferiorly, initiating the final portion of the pharyngeal phase, which is the entry of the food bolus into the cervical oesophagus.

The mylohyoid nerve, branch of CN V3, supplies the mylohyoid and the anterior digastric muscles. The stylohyoid muscle is innervated by branches of the facial nerve (CN VII), and the geniohyoid muscle receives fibres from the first cervical nerve, which joins the hypoglossal nerve. The pharyngeal constrictors have their nervous supply through the glossopharyngeal (CN IX) and the vagus (CN X) nerves.

1.2.2.6 The Ear

Outer Ear

The pinna or auricle (Fig. 1.37) is a prominent skin-covered flap located on the side of the head and is the external visible part of the ear. It is shaped and supported by cartilage except for the earlobe. The outer verge of the ear is called the helix, and the inner elevated rim is the antihelix, which originates from the fusion of two crura, between which is a triangular depression, the fossa triangularis. The deepest depression, which leads to the ear canal, is known as the concha. It is overlapped by the tragus, a small cartilaginous flap that can be pushed down to block the opening to the ear canal.

The pinna collects sound waves and directs them to the external ear canal. Its shape also partially shields sound waves that approach the ear from the rear, therefore enabling a person to tell whether a sound is coming directly from the front or the back.

The external auditory canal (meatus acusticus externus) begins at the bottom of the concha and ends at the tympanic membrane. It is approximately 2.5–3 cm long and slightly S-curved. It is supported by cartilage at its first third and by the bone for the rest of its length. It exhibits two narrowings, one near the inner end of the cartilaginous portion and another, the isthmus, within the osseous part. The whole tube is lined by the skin and contains glands that produce secretions that mix with dead skin cells to produce cerumen (earwax).

Middle Ear

The outer ear ends, and the middle ear (Fig. 1.38) begins, at the tympanic membrane, commonly known as the eardrum. It lies in the tympanic cavity within the temporal bone. The cavity connects to the nasal part of the pharynx via the auditory tube. It is therefore filled with air, normally of the same atmospheric pressure as the outer ear. This ensures that the tympanic membrane can swing freely between both ear parts. Within the tympanic cavity, the vibrations of the tympanic membrane are transmitted to the oval window, the beginning of the inner ear, by a chain of ossicles: the malleus, which has a handle that attaches to the inner surface of the eardrum and a head that is suspended from the wall of the tympanic cavity; the incus, which is connected by its body to the head of the malleus and by its long arm to the stapes. Both its body and its short arm are fixed to the wall of the tympanic cavity by ligaments. The third ossicle, the stapes, has an arch and a footplate. The arch connects to the incus, whereas the footplate is held by a ring-like piece of tissue in the oval window, which is the entrance into the inner ear.

Two muscles exert influence on the movements of the middle ear bones: the tensor tympani (innervated via a branch of the mandibular nerve), whose tendon inserts on the medial part of the malleus, pulls the malleus medially, tensing the tympanic membrane, damping its vibration and thereby reducing the amplitude of sounds. The other one, the stapedius muscle (innervated by a branch of the facial nerve), inserts into the posterior neck of the stapes and reflexively lessens its vibrations by pulling its head backwards.

Inner Ear

The oval window, where the footplate of the stapes is fixed, is the beginning of the inner ear, a system of osseous cavities within the petrosal part of the temporal bone, called the labyrinth (Fig. 1.39). It consists of three components, the vestibule (vestibulum), the three semicircular canals and the cochlea.

The Organs of Equilibrium: Vestibulum and Semicircular Canals

The organ of equilibrium (Fig. 1.40) consists of five compartments, two saccular organs, the utriculus and the sacculus, and three semicircular canals, orientated at right angles to each other, corresponding to the three dimensions of space (Speckmann et al. 2013). The inner membranous compartments of these cavities contain endolymph and do not directly contact their osseous walls but are separated by a small space filled with perilymph. Via the ductus endolymphaticus, the five membranous compartments are continuous to the endolymphatic sac, which in turn has contacts to the dura mater. Both of them have absorptive and secretory functions and regulate the volume and composition of the endolymph. This endolymph is not identical with the endolymph of the cochlear duct (see below) but differs in its ion content.

The sensory element of the sacculus, as well as of the utriculus, is named the macula (Fig. 1.41). It is a plain area containing roughly 16,000 or 30,000 hair cells. They are covered by a gelatinous layer, in which otoliths, small crystals of calcium carbonate, are embedded. This layer has a higher density than the endolymph, and when the otolith membrane is moved, induced either by bending the head or by linear acceleration or deceleration, the ciliary hairs of the sensory cells are declined and evoke depolarisations or hyperpolarisations that trigger the spontaneous activity of the vestibular nerve fibres. The macula utriculi is nearly horizontally orientated and reacts mainly to changes of horizontal movements (e.g. to increasing or reducing speeds of a car); the macula sacculi, on the other hand, is in an approximately vertical position and therefore registers vertical accelerations (e.g. to movements of a lift).

The semicircular ducts (Fig. 1.42), the membranous components of the semicircular canals, start and end within the recessus ellipticus of the vestibulum. The end of one haunch of each of them is enlarged to an ampulla and is the domicile of the sensory organ. Each ampulla contains a so-called crista ampullaris, a specialised connective tissue with approximately 7000 hair cells. They are covered by a dome of jelly-like material (cupula ampullaris), which does not touch the cells directly: only the sensory hairs of the cells contact the cupola through a small gap. The cupola has nearly the same density as the endolymph, so it exerts no direct pressure on the sensory cells. Inertness of the endolymph together with the cupola evokes an opposite movement within the semicircular ducts, contrary to turning movements of the head, and induces a shear force on the hairs of the sensory cells, which causes depolarisation or hyperpolarisation, depending on the direction of turning.

The Cochlea (Fig. 1.43)

The vestibulum continues into the cochlea, which winds by 2½ turns around a section of spongy bone called the modiolus. The modiolus is shaped like a screw whose threads, the lamina spiralis ossea, form a spiral platform that supports the membranous parts of the cochlea. The cochlea measures about 35 mm in length. It contains three fluid-filled chambers separated by membranes. The upper chamber, scala vestibuli, and the bottom chamber, scala tympani, are filled with perilymph. They communicate with each other at the top of the modiolus, the helicotrema and the scala tympani ends at the round window, which again continues into the middle ear but is closed by the secondary tympanic membrane. Between these two perilymphatic ducts, the triangular scala media or ductus cochlearis is expanded. Its basilar membrane extends laterally from the osseous spiral lamina to the outer wall of the cochlea. It separates the scala media from the scala tympani and carries the organ of Corti. Its inclining roof, a more subtle membrane, is called Reissner’s membrane and serves as boundary against the scala vestibuli. The lateral wall of the scala media consists of a specialised epithelium, the stria vascularis (as it contains blood vessels); it generates the endolymph, with which the scala media is filled.

The basilar membrane and its medial continuation, the osseous spiral lamina, carry the organ of Corti, which changes pressure waves into nervous impulses.

The organ of Corti (Fig. 1.44) is composed of a series of epithelial structures. As a central part, the inner and outer rods or pillars of Corti flank a triangular tunnel, the tunnel of Corti. On both sides of the tunnel, the inner and outer hair cells are located. These are short columnar cells; their free ends are level with the heads of Corti’s rods. The approximately 3500 inner hair cells are arranged in a single row on the medial side of the inner rods. They do not reach the basilar membrane but are positioned within supporting phalangeal cells. This facilitates abundant contacts of nerve endings with the cell bodies. The name ‘hair cells’ originates from ciliary structures on their tips, which contact the tectorial membrane. The outer hair cells (approx. 12,000) are nearly twice as long as the inner ones. They are supported by outer phalangeal cells, the cells of Deiters. A space exists between the outer rods of Corti and the adjacent hair cells; this is called the space of Nuel. The Deiters’ cells are neighboured by five or six rows of columnar cells, the supporting cells of Hensen, followed by another group of columnar cells, the cells of Claudius. Covering the sulcus spiralis internus and the spiral organ of Corti is the tectorial membrane, which is attached to the limbus laminae spiralis close to the inner edge of the vestibular membrane. It has contact with the cilia of the inner as well as of the outer hair cells.

The so-called cochlear transduction process of sound, from air waves to nervous impulses, is the result of several steps.

Vibrations of the eardrum are transmitted through the three ossicles of the middle ear, which induce pressure waves within the scala vestibuli. These pressure waves generate a travelling wave on the basilar membrane of the scala media. The basilar membrane vibrations cause shear movements between the tectorial membrane and the stereocilia of both types of hair cells, resulting in deflection of the stereocilia and activation of ion channels. Thereby the mechanical stimulus is transduced, and receptor potentials are generated. In inner hair cells, these receptor potentials induce neurotransmitter release and action potential generation in the synapses of auditory nerve fibres.

The receptor potentials of the outer hair cells, however, initiate a contraction in the longitudinal axis of the cells, which influences the basilar membrane’s motions (Fettiplace and Hackney 2006; Ashmore 2008). The cells act in a sense of a ‘cochlear amplifier’, a mechanism that increases both the amplitude and frequency selectivity of basilar membrane vibration for low-level sounds. These activities of the outer hair cells can be measured as otoacoustic emission (OAE). Efferent nerve fibres contact the outer hair cells by crossing the tunnel of Corti medially as radial fibres. They are thought to adjust the resting membrane potential of these cells, thereby regulating the amount of feedback provided to the basilar membrane.

1.2.2.7 Auditory Pathway and Vestibular Tracts

Auditory Pathway

About 90% of the afferent nerve fibres that leave the organ of Corti come from the inner hair cells. They are Type I nerve fibres, i.e. thick, myelinated and fast conducting. The remaining 10% are afferent fibres from the outer hair cells; they are slow-conducting Type II fibres and cross the tunnel of Corti as basilar tunnel fibres. The cell bodies of all of these fibres form the spiral (or cochlear) ganglion, embedded within the central part of the modiolus. Their axons continue as the acoustic nerve (pars cochlearis of the vestibulocochlear nerve, cranial nerve VIII) and enter the brainstem (Fig. 1.45) (ten Donkelaar 2011).

Each of the nerve fibres diverges, one branch projects rostrally to the dorsal cochlear nucleus, the other projects caudally to the ventral cochlear nucleus. The cochlear nuclei contain second-order neurons, which generally project to higher centres by an ipsilateral or, after decussating, by a contralateral pathway.

The ventral cochlear nucleus projects to the superior olivary complex, whereas fibres of the dorsal cochlear nucleus bypass the superior olivary complex and directly enter the lateral lemniscus to reach the inferior colliculus.

The superior olivary complex consists of the medial nucleus of the superior olive, the lateral nucleus of the superior olive and the medial nucleus of the trapezoid body. Both nuclei of the superior olive receive fibres from the ipsilateral and contralateral ventral cochlear nucleus. The medial nucleus analyses time differences of neuronal signals; the lateral nucleus evaluates differences in intensities. On their way to the nuclei of the superior olive, the contralateral fibres pass through the trapezoid body as a passive relay. In mammals as well as in man, its nuclei, however, are thought to play a role in distinguishing inter-aural intensity differences.

The superior olivary complex sends outputs to the cranial nerves V and VII for reflex contractions of the tensor tympani and stapedius muscles to dampen loud sounds.

The fibres connecting the olivary complex with the inferior colliculus form a lateral tract in the brainstem, called the lateral lemniscus. Within this tract a mass of grey matter is embedded, called the nuclei lemnisci lateralis dorsalis and ventralis. They serve as synaptic relay stations for some of the fibres of the lateral lemniscus.

The end point of the lateral lemniscus is the inferior colliculus. It serves as an auditory relay and reflex centre, where information derived directly from the dorsal cochlear nucleus and from the olivary complex may be compared. Moreover, it receives inputs from the somatosensory system. The inferior colliculi of both sides contact each other by commissural fibres.

The inferior colliculus projects to the medial geniculate nucleus or medial geniculate body. This nucleus is part of the thalamus and serves as a thalamic relay on the way to the auditory cortex. From their neuronal morphology, a number of subdivisions can be distinguished. They have different afferent and efferent connections, and they are thought to be involved together in the direction and maintenance of attention. The fibres leaving the medial geniculate body join the internal capsule as the radiatio acustica and terminate in the primary auditory cortex, the Brodmann’s areas 41 and 42 within the superior temporal gyrus.

Nervous activation of individual parts of the auditory pathway may be recorded as the auditory brainstem response (ABR) upon a stimulus that evokes a response of the cochlea. The first two waves correspond to true action potentials of the cochlear nerve. Later waves may reflect postsynaptic activities (afferent and efferent) in brainstem auditory centres:

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  Wave I is a response of the auditory nerve action potential in the distal portion of the cochlear nerve (cranial nerve VIII).

  
  
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  Wave II is generated by the proximal part of the cochlear nerve as it enters the brainstem.

  
  
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  Wave III arises from second-order neuron activities in or near the cochlear nucleus.

  
  
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  Wave IV is said to arise from the superior olivary complex, but additional contributions may come from the cochlear nucleus and nucleus of lateral lemniscus.

  
  
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  Wave V is believed to originate from the vicinity of the inferior colliculus.

  
  
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  Waves VI and VII are suggested to have their origins in the medial geniculate body of the thalamus.

Beginning in the auditory cortex, several descending pathways exist. The centrifugal fibres run close to, but usually not within, the tracts containing the auditory afferent pathways. They project to the medial geniculate bodies, the inferior colliculi and other midbrain nuclei; from the inferior colliculi, they descend to the superior olivary complex. From here the olivo-cochlear bundles travel down to the inner and outer hair cells of the cochlea. They carry information from the superior olivary complex, the nuclei of the lateral lemniscus, the reticular formation and the inferior colliculus. These bundles have two compartments: a lateral one, containing unmyelinated fibres, which end on the afferent fibres of the inner hair cells, and a medial one with myelinated fibres, which medially crosses the tunnel of Corti and terminates directly on the bodies of the outer hair cells.

The pathway of the acoustic startle reflex is part of the auditory pathway from the ear up to the nucleus of the inferior colliculus. Mainly from there but also from the dorsal nucleus of the lateral lemniscus and from parts of the superior olivary complex, it follows connections to nuclei of cranial nerves and motor centres in the reticular formation. The motoneurons of the ventral root are activated, and moving of the body is initiated, by the reticulospinal tract in the anterolateral part of the spinal cord. The inferior colliculi also project to the superior colliculi, important relay stations in the visual pathway, coordinating optical reflexes.

Learned reflexes (e.g. turning upon hearing one’s name) are conditioned reflexes. They include the auditory pathway and other complex parts of the central nervous system.

The Vestibular Tracts

The axons emanating from the ganglia of the vestibular nerves (Fig. 1.46) project to the ipsilateral vestibular nuclei, a group of four major and several minor nuclei in the rhombencephalon (hindbrain). There they join afferent fibres from the cerebellum, from proprioceptors of the deeper neck region, as well as afferent fibres from the optical system via the superior colliculi. One of their projections is to the ventral posterior nucleus (the somatosensory nucleus) of the thalamus. These projections are bilateral (crossed and not crossed). The thalamus forwards the vestibular information to vestibular areas in the cerebral cortex. They are not as precisely circumscribed as the areas of the other sensory systems and tend to overlap each other.

Ascending fibres of the vestibular nuclei join the medial longitudinal fascicle and enter the interstitial nucleus of Cajal (Nieuwenhuys et al. 2008), which serves as a coordination centre for eye and head movements. Over this route, the vestibular nuclei also project to ipsilateral and contralateral nuclei of ocular muscles: nuclei of the oculomotorius, abducens and trochlear nerve. These nerves stimulate the six external muscles of the eye, located in three perpendicular planes, roughly colinear with the planes of the semicircular canals. Excitatory subunits connect a single semicircular canal to the eye muscles, initiating a compensatory eye movement in the plane of the semicircular canal, whereas their antagonists are inhibited.

The vestibulo-collic reflex produces head movements in the planes of the stimulated canals. More than 30 neck muscles are innervated from the upper cervical cord and receive excitatory or inhibitory inputs, or both, from all six semicircular canals. The motoneurons are contacted by the vestibulospinal system, including lateral, medial and crossed tracts.

Descending fibres form the vestibulospinal tracts and join the reticulospinal tract, by which they reach the motor neurons of skeletal muscles, predominantly activating the extensor and inhibiting the flexor muscles.

One main target of the vestibular tracts is the cerebellum. By mossy fibres, they reach a special region, the vestibular cerebellum, mainly consisting of the flocculonodular lobe, combining the nodulus and the flocculus. The efferent fibres of this region act synergistically on the oculomotor and spinal motor systems to maintain balance in upright movements (Trepel 2004).

1.2.2.8 Neuroanatomical Basics of Language

The cerebrum is divided into four main parts: the frontal lobe is separated from the parietal lobe by the central sulcus and houses capabilities such as planning, motivation, working memory and motor functions including speech production. The parietal lobe is the main sensory part of the brain. The gyri immediately adjoining the central sulcus are the precentral gyrus, origin of the pyramidal tract on the motoric site, and the postcentral gyrus as the primary sensory centre on the parietal side. The parietooccipital sulcus separates the parietal lobe from the occipital lobe, where among others the visual areas are domiciled. The lateral sulcus (or fissura sylvii) separates the temporal lobe on the one hand from the frontal and parietal lobe on the other. The auditory centre is located in the temporal lobe, where acoustic radiation, emanating from the medial geniculate body, terminates.

The whole cerebrum has been mapped by Korbinian Brodmann (1868–1918) according to the cytoarchitectonic patterns of its various parts. These areas are widely used to describe functional compartments of the brain.

An important part involved in language (Fig. 1.47) is Wernicke’s area, located in the posterior part of the superior temporal gyrus in Brodmann’s area 22 and part of area 39. It is predominant in the left hemisphere of right-handed persons and plays a key role in understanding words. Spoken words arrive at Wernicke’s area directly via the auditory cortex in areas 41 and 42 (Shalom and Poeppel 2008). Visual inputs reach this area from the eye via the visual cortices in Brodmann’s areas 17, 18 and 19 and the ‘read and write centre’ in the angular gyrus (Brodmann’s area 39). Wernicke’s area projects to Broca’s area and has connections to the prefrontal cortex by longitudinal association fibres (Friederici 2009).

Vocalising language requires motor outputs from the lower regions of the precentral primary motor cortex (motor areas for the face, mouth, tongue and larynx). The plan and coordination of these motor outputs originate in Broca’s area, which lies in the Brodmann’s regions 44 and 45. It joins the lower part of the precentral cortex, receives inputs from Wernicke’s area via the arcuate fasciculus and projects to the above-mentioned corresponding primary motor areas of both hemispheres (to the contralateral side via the corpus callosum), as well as to the basal ganglia, which are thought to play a role in giving timing cues for the correct motor activity during speech (Fig. 1.47).

Recently, Hickok and Poeppel proposed a dual pathway of language processing (Fig. 1.48): on the basis of the findings that speech recognition is bilaterally organised in the superior temporal gyrus, they postulate a bilateral ventral stream comprising the primary auditory area, Heschl’s gyrus, and neighbouring areas of the superior temporal gyrus (including Wernicke’s area), adjacent areas of the middle and the inferior temporal gyrus. These areas are important in speech perception. The speech production is generated by a unilateral dorsal stream in the left hemisphere including interconnections between the supramarginal gyrus and the dorsal premotor portion (Brodmann’s area 6) as well as the area of Broca together with the lower motor gyrus of Brodmann’s area 4. The Broca region also connects to the anterior temporal lobe (anterior part of Brodmann’s area 21), which is supposed to play a role in processing complex elements of language (Hickok 2009).

1.3.7.1 Localisation

For detecting the spatial origin of low-frequency (about <800 Hz) sound, the auditory system evaluates inter-aural time differences (ITD), e.g. a sound coming from the right side arrives earlier at the right ear than at the left ear. Contrarily, for high frequencies (about >1600 Hz), the inter-aural level differences (ILD) are predominantly evaluated by the brain. The ILDs are mainly due to the head shadow effect, as the level of sound from the right causes a higher sound pressure level on the right ear than on the left ear.

Localisation in the median plane is possible as the human outer ear (pinna and the ear canal) forms acoustic filters that are sensitive to the direction of the incoming sound. Different resonances of these filters cause direction-specific patterns into the frequency responses of the ears, which are evaluated by the brain. The combination of these patterns with other direction-selective reflections at the head, shoulders and torso forms the so-called head-related transfer functions (HRTF) which are specific for each individual. In closed rooms, not only the direct sound from a sound source reaches the ears but also sound that has been reflected from the walls. When localising a sound source under such conditions, the auditory system analyses only the original direct sound (which arrives first at the ear); not the reflected sound (which arrives later). This feature is called the ‘law of the first wave front’, as the analysis of the reflected sound is suppressed.

1.3.7.2 Speech Intelligibility in Noise

Hearing in noise is one of the more challenging tasks in today’s acoustic environment, as background noise is nearly always present in daily listening situations.

The presence of noise may cause substantial masking of acoustic signals and in particular the masking of speech, the most common being the cocktail party effect. For speech understanding in noisy environments, binaural hearing and proper functioning of the inner ear are crucial. The brain’s comparison of the right and left auditory inputs will then result in a significant internal noise reduction called binaural release from masking.

Speech intelligibility in noise is most often quantified by the signal-to-noise ratio, i.e. the ratio between the sound level of the signal (speech) and that of the noise at the point where the listener achieves 50% speech intelligibility. The signal-to-noise ratio is an important measure for assessing the benefit of hearing aids or cochlear implants. Besides advantages of localisation abilities, the improvement of hearing in noise is the main argument for a binaural supply with hearing prostheses.

1.4.2.1 Fundamental Concepts

Our perception of sound is based on logarithmic scales. Concerning loudness, this has led to the introduction of the decibel scale, which is a logarithmic measure of physical sound pressure levels. In pitch perception, we use the octave scale, which is a logarithmic measure of frequency.

Our ‘hearing range’ is often visualised in a graph of sound pressure levels in decibels (dB) versus frequency in Hertz (Hz) organised into octaves (the interval of one octave represents a doubling of frequency, so that 250 Hz, 500 Hz, 1 kHz and 2 kHz, e.g. are octave steps). Section 1.3 Fig. 1.50 shows the hearing range of a normal-hearing subject, which is determined by the frequency-dependent threshold of hearing (in dB SPL, dB sound level pressure; lower limit) and the loudness discomfort level (also in dB SPL; the blue line marked 100). Adolescents have the best hearing, in a frequency range from 20 Hz to 20 kHz, with a hearing range at 1 kHz from approximately 0 to 110–125 dB SPL. Normal thresholds of hearing differ at each frequency and are worse at the low and high frequencies, as displayed in Sect. 1.3 Fig. 1.50, which shows the standardised normal threshold (the lower red line). For this reason, the hearing thresholds of an individual subject are usually not plotted in dB SPL but in dB HL, or dB Hearing Level, which is relative to the standardised normal-hearing threshold. This standardised normal threshold is 0 dB HL at all frequencies. Thus, hearing thresholds in dB HL are 0 for a normal-hearing person. In clinical practice, however, a normal range of <20 dB HL is typically used. A further decibel scale, dB SL, or dB Sensation Level, is sometimes used, especially in research, to present sounds at a set level above each subject’s threshold. Any individual’s threshold (in dB SPL or dB HL) can be represented as 0 dB SL. 20 dB SL is therefore 20 dB above the individual’s threshold. The loudness discomfort level now equals approximately 100 dB HL for all frequencies. For diagnostic purposes, hearing thresholds and loudness discomfort levels are only measured in the frequency range from 0.25 to 8 kHz. The resulting graph, called an audiogram, is displayed in Fig. 1.52. Note that in an audiogram, hearing levels are presented upside-down. For counselling purposes, the speech dynamic range is indicated, which is the mean sound pressure level of normal conversational speech as a function of frequency, together with its range of 30 dB, expressed in dB HL. During normal running speech, the speech sound levels will be within this area 90% of the time.