(1)
Faculty of Medicine of Montpellier, Montpellier, France
Abstract
The pharyngolaryngeal organisation can explain clearly how the two major complex functions of swallowing and speaking can be achieved. The first aspect is the aerodigestive crossway which requires a very precise regulation of both the air flow and the digestive tract, because only air can transit in superior and inferior pharynx and no solid nor liquid food can penetrate within the nasal fossa or the larynx. The reason for this complex organisation has to be found in the fact that the mouth is masticating not only food but also sounds coming from the larynx used to the language. This justifies for safe swallowing a great quantity of specific anatomical features like soft palate, epiglottis, junction of the base of the tongue with the arytenoid pad, mobility of the larynx,… To understand phonation mechanisms, some basic notions of acoustics, phonetics and linguistics are needed. The control of voice is given by the ear and the mandatory equipressure on both sides of the eardrum justifies the complex organisation of soft palate and tuba auditiva. The roles of the tongue and of the vocal cords are also very important to perceive the incredible possibilities of the language, which are directly related to the typically human specific brain cortical areas.
8.1 Introduction
Swallowing and speaking are two important functions using complex automated mechanisms that must be technically precisely understood. The concerned anatomical area is the pharyngo-larynx that embryologically drifts from the segmentation of the mesoderm surrounding the foregut, giving rise to four visceral arches limited externally by ectoblastic slots and internally by entoblastic pouches [1]. Each arch is colonised by a mixed nerve.
8.1.1 The Mandibular Arch
The first and also the largest one, the mandibular arch, builds the face and therefore most of the splanchnocranium. It has three floors, each colonised by a branch of the trigeminal nerve (V)—a mixed nerve—with a sensory root and a trigeminal ganglion (of Gasser), and a motor root, the masticatory nerve. Each of its branches carries parasympathetic ganglions originating from other nerves (indicated in parentheses):
on V1, the ciliary ganglion (from III) for iridian sphincter;
on V2, the pterygopalatine ganglion (from VII) for lacrimal and mucous glands;
on V3, the otic ganglion (from IX) for parotid gland;
on V3, the submandibular ganglion (from VII) for submandibular and sublingual salivary glands.
All these ganglions distribute their postganglionic fibres using the branches of the trigeminal nerve.
The upper part of the visceral arch forms the floor of the orbital cavity, and the lower part, the mandible, which is built from the draft of Meckel‘s cartilage that gives, in the back, the malleus and the incus ossicles of the middle ear. The muscles of this first arch are the masticatory muscles, the floor of mouth, the tensor tympani and the tensor veli.
8.1.2 The Hyoid Arch
The second one, the hyoid arch, gives birth to the styloid process of the temporal bone, the stylo-hyoid ligament, the little horn of the hyoid bone and the half top of it. The muscles of the hyoid arch are the stapedian, the stylohyoid, the posterior belly of digastric and the platysma of the face. The related nerve is the facial nerve (VII)—a mixed nerve—with a sensory root and its geniculate ganglion, and a motor root innervating all the muscles. We must stop talking about VIIbis or intermediate of Wrisberg, which was a mistake of the ancient anatomists, because it is indeed the sensory root of the facial nerve.
8.1.3 The Hyothyroid Arch
The third one, the hyothyroid arch, forms the lower half of the hyoid bone, its great horn and the upper part of the thyroid cartilage. The nerve is the glossopharyngeal (IX)—a mixed nerve—with two roots: firstly, a sensory root with its superior and inferior ganglions for the sensory innervation of the posterior third of the tongue, the mucosa of the promontorium in the middle ear, the palatine tonsils and the upper part of the pharynx, and secondly, a motor root for the following muscles: stylopharyngeal, superior and middle pharynx constrictors and levator veli.
8.1.4 The Thyro-Aryteno-Cricoid Arch
The fourth and last one, the thyro-aryteno-cricoid arch, represents the lower visceral hyo-laryngeal skeleton with the thyroid, arytenoids and cricoid cartilages.
The nerve is the vagus nerve, or pneumogastric nerve (X). Firstly, it has a sensory root and two jugular and plexiform ganglia with a sensitive area covering the mucosa of the larynx by the superior laryngeal nerve, the lower part of the pharynx, the auditory tube and a portion of the skin of the external auditory meatus. Secondly, it shows a motor root visible on the lower lateral side of the medulla oblongata, corresponding to the inferior recurrent laryngeal nerve. This nerve innervates all the intrinsic muscles of the larynx: cricothyroid, posterior and lateral cricoarytenoid, oblique and transverse interarytenoids and inferior thyroarytenoid or vocal muscle.
Below its visceral arch territory, the vagus nerve—also called pneumogastric—becomes a large parasympathetic nerve that descends in the thoracoabdominal cavity to end into the pelvis.
To retain all these anatomical data, it is necessary in a comprehensive approach to identify the technical problems and assess the relevance of the anatomical solution. In that view, the three following points have to be discussed: the aerodigestive crossroad, swallowing and phonation.
8.2 The Aerodigestive Crossroad
On the face, two types of holes are visible:
firstly, both nostrils marking the beginning of the respiratory airway whose goal is to let enter and exit the air into the nasal cavity, larynx, trachea, bronchi and lungs;
secondly, placed below, the oral mouth orifice which opens, after the double teeth harrow, into the large dining room whose ceiling is the oral palate covered by the mucosa. Its floor is made by the deformable tongue which represents the beginning of the digestive tract continuing in the oesophagus and the stomach (Fig. 8.1).
Fig. 8.1
General topography on a double median sagittal section of face. This section was made on a young medical student of 20 years old, given after death for anatomical studies by his parents. The three floors of the pharynx: I. Rhinopharynx, II. Oropharynx, III. Laryngopharynx. 1. Nasal fossa; 2. Choane limited by vomer; 3. Hard palate; 4. Velum palatinum; 5. Base of the tongue; 6. Vallecula epiglottica; 7. Tongue; 8. Chin (symphysis); 9. Buccal muscle floor; 10. Hyoid bone; 11. Epiglottis and laryngeal aditus; 12. Pre-epiglottic space (fat tissue); 13. Arytenoid pad (arytenoid cartilages and arytenoid muscles); 14. Thyroid cartilage (anterior part); 15. Glottis (rima glottidis); 16. Cricoid cartilage (posterior lamina); 17. Infraglottic space; 18. Trachea; 19. Oesophagus
The airway accepts and transports air only. The digestive tract lets solids or liquids go through the oesophageal tube after mixing and grinding in the oral cavity.
Technically, it is certain that the airway has exclusive transport of air and therefore cannot accept other materials. It should be logical to separate these two channels, considering that their final destination is totally different. Yet, by observing the region, we can see that the airway is first above within the nasal fossa and then anterior, at neck and chest levels, unlike the digestive tract that comes first in a lower position in the mouth and then is posterior at neck and chest levels. How to explain the crossroad of these two tracts at the level of the pharynx?
One can find the solution in the fact that the mouth dining room does not chew only solid or liquid food, but also sounds coming from the larynx and modulated or shaped by the oral-nasal resonators to generate an articulated speech. It is therefore the phonatory function which determines such a construction pattern.
From this main general principle, we can construct the complete system by introducing the absolute necessity of a strict control of the pharyngeal circulation with a major challenge: no liquid nor solid particles in upper or lower airways made by the reinforced gaping tracheal tube for easy air flow. Eventually, air can go into the oesophagus, which is a supple hose. The problem of the subsequent disposal for the air to go up is related to permissive or restrictive ethnic customs.
The master organ to avoid the digestive flow to enter into the nasal cavity is the soft palate. It can close the nasopharynx by its muscle contractions and thus plays a crucial role in both swallowing and phonation.
8.3 Swallowing
This is a feature that allows sending back in the oesophageal tube the content of the oral cavity. It is absolutely necessary to avoid nasal reflux—exactly what the soft palate does by obstructing the nasopharynx. It should also prevent the introduction of oral contents into the larynx that is exclusively reserved for the passage of air [2–6].
Swallowing is a complex function that requires a full automation and involves a series of phenomena perfectly adjusted.
8.3.1 The Tongue
It occupies the whole oral cavity and is able, by its mobility, to move the content of the cavity backward and to clean its peripheral parts by passing along the dental arches. The floor of the cavity is closed by a muscular diaphragm made of the wide mylohyoid muscle doubled superficially with the anterior belly of the digastric muscle. A contraction lifts all these muscles and pushes the tongue and the buccal content backward. The content falls into the pharynx through a narrow space (isthmus of fauces) bounded laterally by the lower pillars of soft palate, between which are two lymphoid masses, the tonsils. Those vigilant guardians against infectious agents are not always fully effective, but knowing better now the important role of immune lymphoid posterior circle described by Waldeyer leads oral surgeons to remove them much less frequently than before.
8.3.2 The Closure of the Larynx
After the first fast phase and while the soft palate has closed the nasopharynx to prevent ingress of liquid or solid, the larynx also must be closed [7]. This is done by a double movement: climb of the larynx and retropulsion of the vertical base of the tongue which appends to the arytenoid pad.
To understand it, it is needed to remember that the larynx is formed by three articulated cartilage pieces:
the thyroid cartilage (from the Greek “the shield”) shaped like a screen rearwardly open;
the cricoid cartilage, a circular ring connecting to the trachea, which is a reinforced hose by cartilaginous rings and movable vertically (Figs. 8.2, 8.3 and 8.4);
Fig. 8.2
Axial sections of a newborn head seen from below. (a) Mandible level: 1. Mandible; 2. Masseter; 3. Geniohyoid muscle; 4. Digastric; 5. Parotid gland; 6. Rhinopharynx with epiglottis; 7. SCM muscle; 8. Semi-spinalis of the head; 9. Oblique inferior. (b) Thyroid cartilage level: 1. Infrahyoid muscles; 2. Vocal cord and vocal process of arytenoid cartilage; 3. Thyroid cartilage; 4. Superior part of cricoid cartilage; 5. Intervertebral disk; 6. Splenius capitis; 7. Trapezius. (c) Cricoid cartilage level: 1. Glottis; 2. SCM; 3. Inferior part of cricoid cartilage; 4. Peripharyngeal haematoma; 5. Vertebral artery. (d) Trachea level: 1. Trachea; 2.Thyroid gland; 3. Common carotid artery and internal jugular vein; 4. Oesophagus
Fig. 8.3
Axial sections of new born skull base. (a) Inferior section: 1. Basisphenoid synostosis; 2. Cochlea (same size as adult); 3. Vestibule; 4. Lateral SCC; 5. Brain stem; 6. Posterior SCC (adult size). (b) Superior section: 1. Cochlea; 2. Stapes; 3. Superior SCC; 4. Brain stem; 5. Cerebellar tonsils
Fig. 8.4
Parasagittal sections of face and cervical spine of a newborn. (a) Parasagittal section of skull base: 1. Eye ball, optic fascicle and muscles; 2. Pons; 3. Clivus; 4. Larynx with epiglottis. (b) More lateral parasagittal section: 1. Spheno-occipital synostosis; 2. Velum palatinum; 3. Hyoid bone. (c) Craniocervical junction: 1. Cochlea (adult size); 2. Lateral occipital; 3. Spinal cord. (d) Occipital foramen: 1. Internal carotid artery; 2. Cochlea; 3. Brain stem and cerebellum; 4. C2
the two arytenoid cartilages, placed astride on the upper edge of the cricoid and in a square form. A sagittal process (vocal process) is included in a long muscle called vocal muscle or muscular vocal cord made of fine twisted fibres fixed in the angle of the thyroid shield (Fig. 8.5). The arytenoids can move laterally or rotate on their vertical axis.
Fig. 8.5
Serial coronal sections of larynx from front to back (fresh cadaver). (a) 1. Hyoid bone body; 2. Mylohyoid muscle; 3. Hypoglossal nerve (XII) and lingual artery; 4. Hyoglossal muscle; 5. Lingual nerve (V3). (b) 1. Thyroid cartilage; 2. Ventricle fold; 3. Ventricle (Morgagni); 4. Infraglottic space; 5. Vocal cord; 6. Cricothyroid muscle; 7. Cricoid cartilage; 8. Glottic space. (c) 1. Thyroid cartilage; 2. Ventricular band; 3. Vocal cord; 4. Elastic conus; 5. Cricoid cartilage; 6. Infraglottic space; 7. Trachea. (d) 1. Arytenoid cartilage; 2. Thyroid cartilage; 3. Interarytenoid muscles; 4. Cricoarytenoid joint; 5. Cricoid cartilage; 6. Infraglottic space
This laryngeal set is suspended to the hyoid bone, which is attached to the skull base by the stylohyoid ligament, doubled by the stylohyoid muscle, with a viscoelastic suspension formed by the digastric muscle. Its posterior belly is inserted on the medial side of the mastoid process and, after reflection of its intermediate tendon by the means of a fibrous pulley placed on the middle part of the hyoid bone (little horn), its anterior belly is fixed on the back of the chin. This realises an elastic suspension system that is able to raise the larynx.
The tongue, based on the floor of the mouth, is propelled upward and backward by the mylohyoid diaphragm inserted on the hyoid bone and an oblique line of the internal aspect of the mandible body.
Several important details demonstrate the intelligence and reliability of the whole system.
As we saw, the laryngeal continence is achieved by juxtaposing the base of the tongue and the arytenoid pad formed by the two arytenoid cartilages joined by the thick interarytenoid muscles. This looks like a mouth, and between the lips, there is the interposition of the epiglottis. This soft and flexible fibrocartilage is like a petal with its petiole fixed in the angle of the thyroid cartilage by the thyroepiglottic ligament. It folds down passively and is “stuck” between the two lips. Its front part has the form of a tile, with two oblique slopes, and plays an important role in covering the laryngeal inlet, mainly in diverting the digestive flow laterally into the piriform sinuses, also called pharyngolaryngeal gutters (Fig. 8.6).
Fig. 8.6
Dissection of posterior muscles of larynx seen from behind. (a) Section of pharyngeal constrictors: 1. Vertical basis of tongue; 2. Posterior aspect of epiglottis; 3. Pirifomis sinus; 4. Arytenoid pad; 5. Posterior part of the cricoid cartilage covered by mucosa. (b) Median opening of mucosa: 1. Interarytenoid muscles with corniculate tubercle on the top (Santorini); 2. Anastomosis between superior and inferior laryngeal nerves; 3. Posterior cricoarytenoid muscle (posticus). (c) Posticus reclined: 1. Lateral border of isthmus of fauces; 2. Uvula; 3. Reclined posterior cricoarytenoid muscle; 4. Posterior aspect of cricoid cartilage; 5. Oesophagus. (d) Interarytenoid muscles reclined: 1. Posterior concave aspect of arytenoid cartilage; 2. Cricoarytenoid joint
It has to be added that the middle part of the larynx is occupied by a sagittal slit bordered by the two vocal muscles, the glottic slit or glottis. It can, by the lateral mobility of the arytenoids, either open for breathing by vocal cord abduction activated by the posterior cricoarytenoid muscle or close completely for laryngeal continence by adduction done by interarytenoid muscles.
Finally, the technical secret of this last phase of pharyngeal swallowing is to achieve a lateralisation of the digestive tract (epiglottis and piriform sinuses) and a medialisation of the airway (glottic slit). In that way, the larynx is also a sphincter (Figs. 8.7 and 8.8).
Fig. 8.7
Schematic drawing of swallowing. (a) Sagittal section: 1. Retrotraction of soft palate by levator veli contraction; 2. Rigidification of anterior third of soft palate by tensor veli contraction; 3. Tongue retro pulsion by mouth floor muscle contraction; 4. Epiglottis between the tongue vertical base and the arytenoid pad; 5. Up and down movements of larynx. (b) Posterior view: 1. Narrowing of isthmus of fauces; 2. Passive flexion of epiglottis deviating the digestive flow; 3. Lateralization of digestive flow; 4. Ascending arytenoid pad
Fig. 8.8
Serial anatomical axial sections of larynx seen from above (up to down). (a) 1. Left carotid fork; 2. Oropharynx; 3. Intermediate tendon of the digastric muscle; 4. Submandibular gland; 5. External carotid artery; 6. Sternocleidomastoid muscle (SCM); 7. Internal carotid artery; 8. Cervical vertebra. (b) 1. Hyoid bone body; 2. Epiglottis; 3. Left common carotid artery; 4. SCM. (c) 1. Junction between hyoid body and great horn; 2. Epiglottis with its specific shape (roman tile) and hyoepiglottic ligament; 3. Right carotid fork; 4. Large internal jugular vein. (d) 1. Superior thyroid incisure; 2. Thyroid cartilage; 3. Superior horn; 4. Ary-epiglottic fold; 5. Inferior pharyngeal constrictor muscle; 6. Right common carotid artery and internal jugular vein; 7. SCM. (e). 1. Piriform recessus; 2. Thyroarytenoid muscle; 3. Vestibule of larynx; 4. Corniculate cartilage (Santorini); 5. Inferior pharyngeal constrictor muscle. (f) 1. Arytenoid pad (oblique and transverse arytenoid muscles); 2. Piriformis sinus; 3. Ventricle of larynx; 4. Arytenoid cartilage with vocal process and ligament; 5. Glottis (rima glottidis). (g) 1. Piriformis sinus; 2. Glottis; 3. Vocal muscle; 4. Arytenoid cartilage base with vocal process. (h) 1. Trachea; 2. Lateral lobe of thyroid gland; 3. Cricoid cartilage base; 4. Common carotid artery; 5. Oesophagus
It may happen that this sphincter function has imperfections in the case of “false road” with laryngeal vestibule intrusion of solid or liquid particles. In this case, an effective cough reflex happens that corresponds to the high sensitivity to foreign bodies of the laryngeal mucosa innervated by the superior laryngeal nerve. For example, banquets often end in joy with many alcoholic toasts. Laughing loudly and drinking at the same time most often end by fits of coughing to clean the laryngeal vestibule.
In another domain, surgery of the larynx, primarily for cancer, has made much progress but remains subject to a number of anatomical constraints:
to avoid postoperative swallowing problems, it is necessary to preserve the tongue base and a part of the interarytenoid pad;
the laryngeal cartilages are avascular formations unaffected by the tumoral process that primarily affects the mucosa. In case of resection of the epiglottis, swallowing disorders are minor;
the glottis represents a vascular and lymphatic barrier differentiating tumours above and below glottis;
in the case of total resection of the larynx, reconstruction of continuity of the digestive tract is possible, but this is not always the case for the airway, which requires a stoma by anastomosis of the trachea to the skin. The absence of glottal voice means no voice at all, but it is possible with a specific rehabilitation to restore an “oesophageal voice” by modelling belching sounds in the oral cavity and by the use of an amplifier. The larynx prostheses are for the time being not clinically useful.
8.4 Phonation
Swallowing, practised hundreds of times a day, is a quick and fully automated procedure. But the main function of the larynx is to emit sounds, which will then become phonemes and words by modulation into the supraglottic resonators.
8.4.1 Basic Notions on Sounds
To understand the intelligence of phonatory larynx, it is appropriate to treat some basics in acoustics, the physical science of sound; in phonetics, the science of phonatory organs; and linguistics, the study of language [8–12].
Sounds are produced by mechanical vibrations, which are oscillating air molecules without moving them. It is therefore an energy transport without material moving. The classical representation of the phenomenon is the stone thrown into water creating a series of waves gradually damped swinging a floating object without moving it.
The speed of sound depends on the density of the propagation medium and temperature (in air, 330 m/s at 0 °C and 340 m/s at 20 °C).
The frequency is the number of oscillations per time unit and is measured in Hertz: oscillation number/second.
The wavelength, λ, is the distance travelled by a wave during a period (full cycle) and uses metric units.
The variation of energy that decreases with distance from the source is embodied in a logarithmic function in decibel expressing a ratio of two lengths.
The intensity of the sound wave expressed in decibels can also be measured in Watts/cm2.
The pitch defining treble and bass sounds depends on the frequency.
The human ear can hear sounds between 25 Hz and 20 kHz; below are infrasound and beyond ultrasound.
The Fletcher diagram defines an audible area (audiogram) reporting the frequencies, expressed in Hz, into dB with a perception threshold and pain threshold.
It is possible to study for sounds’ various representations: temporal with shape, amplitude and period, spectral with the different frequencies and vectorial.
Joseph Fourier (1768–1830) has specifically developed a spectral analysis of sounds. We can thus identify a zone of greater sensitivity of the ear between 500 and 5,000 Hz. With age, the presbyacousy characterises a loss of treble. An acoustic trauma in the same frequency band (e.g. use the jackhammer at 100 Hz without protection for years) can remove the corresponding frequency band of the audiogram.
We can distinguish pure tones that have single frequency, such as the one given by diapason, and complex or musical sounds that have fundamental frequency, the slower at spectral analysis which defines the note, mixed with other frequencies that are harmonics, integral multiples of the fundamental say pairs (asymmetric signals) and odd (symmetric signals). The noise is a more or less a structured combination of sounds.
The sound is thus defined with three parameters: height (high shrill or low), intensity (weak or strong) and timbre (harmonic mixed with the fundamental frequency for defining a musical instrument or the voice).
Producing a sound in a column of air was used since the beginning of humanity. It may be done by a vibration transmitted in the column of air (excitatory) contained in a space (resonator) that modifies it more or less with tightening, creating areas of compression optionally followed by decompression more or less explosive according to the aperture of the tube.
8.4.2 The Articulatory Phonetics
Phonetics studies how human speech sounds are produced.
The exciter is the centrepiece of the larynx: the glottic area with a posterior triangular space with a posterior base formed by the two vocal muscles surrounded by the mucosa (vocal fold) and fixed on the back onto the arytenoid cartilages.
They are topped with two ligaments, vestibular or ventricular bands, in the same orientation and circumscribing a mucous cavity, the ventricle described by Morgagni. The elastic cone of larynx contributes to the formation of sounds and is placed under the subglottic space.
Below is the subglottic wind supply with trachea, bronchial tubes and lungs activated by diaphragm bellows and the dynamics of abdominal wall muscles, so important for the control of the voice.
Above stands the voice channel going from glottis to lips and bordered by supraglottic resonators:
pharynx,
nasal fossa surrounded by aerial sinusal cavities (maxillary, frontal, sphenoid sinuses and ethmoidal cells),
mouth with the tongue,
bony palate forward with alveolar ridges and teeth,
soft palate on the back with uvula and azygos muscles,
upper and lower lips.
All these accidents that punctuate the voice channel will change the column of air that passes through it by generating “formants” which characterise the wealth of articulated speech.
The articulation point is the point where there is an obstacle to the passage of air which is used to define different types of articulation in relation with the place of articulation:
lips (bilabial or labial);
teeth (dental and interdental) or both (labiodental);
gingival margin of the upper incisors (alveolar pre-mid-post);
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