1.1 The ear
The ear is divided anatomically into outer, middle and inner sections, with the latter two embedded in the temporal bone (Figure 1.1). The sections contain the following structures:
- Outer ear: pinna, concha, and external auditory canal (EAC) or meatus
- Inner ear: the cochlea, vestibule and three semicircular canals
The external ear develops from six tubercles of the first branchial arch, as do the malleus, incus and tensor tympani. The stapes and stapedius (Figure 1.2) are derived from the second arch. The distal portion of the first pharyngeal pouch comes into contact with the epithelial lining of the first pharyngeal cleft, forming the EAC; the proximal portion of the first pharyngeal pouch forms the middle ear and the Eustachian (or pharyngotympanic) tube.
The tympanic membrane is formed from an ectodermal epithelial lining, an intermediate layer of mesenchyme and an endodermal lining from the first pharyngeal pouch.
The inner ear develops from the otic vesicle, an epithelial sac derived from the surface ectoderm of the neural tube.
External ear and acoustic meatus
The external ear or pinna is cartilaginous with closely adherent perichondrium (Figure 1.3). The blood supply of the cartilage of the pinna is entirely dependent on the perichondrium. The EAC is about 25 mm in length, cartilaginous in the outer one third and bony in the inner two thirds.
3Wax is made up of secretions produced from specialised sweat glands in the EAC called ceruminous glands, and skin cells. Skin and wax usually migrate radially outward from the tympanic membrane and then laterally along the EAC.
Figure 1.2: (a) Derivation of the tympanic membrane and (b) structures derived from the pharyngeal arches.
The tympanic membrane (eardrum) is composed of three layers (skin, fibrous tissue and mucosa), in keeping with its embryological origin. The normal appearance is pearly and opaque and its slight concavity results in the ‘light reflex’ (Figure 1.4), a characteristic triangular cone of light seen when light is reflected from its surface.
The pars tensa is the larger inferior section of the membrane and has a well-organised fibrous (middle) layer and an annulus or thickened ring at the periphery. In the smaller superior portion, the pars flaccida, the fibrous middle layer is poorly organised and the annulus is incomplete superiorly.
The middle ear (Figure 1.5) is an air-containing space, and developmentally a continuation of the Eustachian tube. 5It contains three small middle ear bones called ossicles – the malleus, incus and stapes. The tensor tympani attaches the malleus to cartilage of the Eustachian tube and dampens background sounds such as chewing. The 1 mm long stapedius muscle attaches the neck of the stapes to the pyramidal eminence of the posterior middle ear wall and prevents loud sounds causing acoustic trauma.
Figure 1.4: Left tympanic membrane. F, pars flaccida; H, handle of malleus; L, light reflex; T, pars tensa.
It also functions reflexively to 6dampen background low frequency sound such as the sound of one's own voice. The round and oval windows, lateral semicircular canal, basal turn of the cochlea and tympanic plexus of nerves are closely related to the medial aspect of the middle ear.
Temporal bone The pneumatised (air-filled) mastoid cells in the temporal bone are connected to the middle ear through the aditus. This reservoir of air helps prevent wide fluctuations of middle ear pressure. Chronic middle ear disease reduces the pneumatisation, causing a sclerotic (dense) mastoid.
Facial nerve The facial nerve has a long and tortuous course through the temporal bone, exiting through the stylomastoid foramen in front of the mastoid process (see p. for more on anatomy of the facial nerve).
The facial nerve controls the motor activity of most facial muscles, and it has intimate association with the middle and inner ear after it courses through the internal auditory canal in the petrous part of the temporal bone. It also contains sensory afferents, including the chorda tympani, which carries taste fibres from the anterior two thirds of the tongue. The facial nerve is posterosuperior to the medial wall of the middle ear.
Eustachian tube The Eustachian tube extends from the anterior wall of the middle ear to the lateral wall of the nasopharynx. About one third of the tube proximal to the middle ear is bony; the rest is composed of cartilage. At the lateral aspect of the nasopharynx is a raised mucosal elevation, the torus tubarius formed by underlying cartilage, which opens into the nasopharynx. The tube is shorter, wider and more horizontal in children, making them more prone to middle ear infections than adults. The opening of the tube during swallowing, with the action of the muscles of the palate, allows aeration of the middle ear and the equalising of pressures either side of the tympanic membrane.
The inner ear consists of:
- The vestibule and semicircular canals, responsible for balance
- The cochlea, responsible for hearing
They lie within a bony labyrinth (network of canals), which is one of the densest bones in the body and protects the closely adjacent membranous labyrinth and its sensitive neuroepithelium. The membranous labyrinth is hollow and filled with endolymph, a fluid with similar ionic concentrations to intracellular fluid. Perilymph, surrounding the membranous labyrinth, is a filtrate of blood and cerebrospinal fluid (CSF) and is similar to extracellular fluid. The endolymph transmits vibrations to the membranes via electromechanically sensitive cells called hair cells which generate action potentials subsequently transmitted to the vestibulocochlear nerve (cranial nerve VIII).
Vestibule This is the ‘entrance’ to the inner ear, via its oval window on the lateral (tympanic) wall, and measures 5 × 5 × 3 mm. It sits behind the cochlea and in front of the semicircular canals and consists of two membranous sacs: the saccule and the utricle. It contains receptors that sense gravity and acceleration.
Cochlea The snail-shaped cochlea is a system of three tubes coiled 2.5 turns and a bony core called the modiolus. It contains the cochlear duct of the membranous labyrinth, which has a pair of perilymph filled chambers, the scala vestibuli and the scala tympani either side (Figure 1.6). Inside the cochlear duct, in a structure known as the organ of Corti, there are nearly 20,000 hair cells, each connecting to its own nerve receptor. These are stimulated by endolymph movement generated by the footplate of the stapes at the oval window. The movement of endolymph is sensed by the stereocilia of the inner and outer hair cells.
Semicircular canals The three semicircular canals (superior, posterior and lateral) are at right angles to each other. Each canal has a widening called the ampulla which contains the embedded 8neuroepithelial hair cells. Rotatory motion, linear acceleration and deceleration result in movement of the surrounding endolymph and corresponding movement of the hair cells – the vestibular inputs are integrated with proprioceptive and visual inputs in the brainstem to maintain balance.
Auditory pathway The auditory pathway is responsible for hearing and extends from the cochlea to the cortex, with processing occurring at each stage:
- Cochlear (or auditory) nerve
- Cochlear nuclei of the pons and medulla oblongata
- Superior olivary nucleus of the brainstem (mostly pontine)
- Inferior colliculus of the midbrain
- Medial geniculate nucleus of the thalamus
- Auditory cortex of the temporal lobe
Vestibular pathway The vestibular pathway is responsible for balance and co-ordinates the perception of movement with changes in postural muscle tone (see ‘Physiology of balance’, p. ):
- Four vestibular nuclei of medulla (and pons)
- Branches to the eye muscle nuclei:
- CN III (oculomotor nerve innervating medial rectus)
- CN VI (abducens nerve innervating lateral rectus)
- Branches to the vestibulospinal tract (head and trunk movement co-ordination)
- Branches to the cerebellum
Physiology of hearing
Sound is a mechanical vibration which sets up oscillations of air molecules. The ear is structured to collect, amplify and transduce this mechanical energy into action potentials. Signals generated in the cochlea travel to the cochlear nuclei of the brainstem via the auditory nerve; from the nuclei they branch to the thalamus and then on to the auditory cortex of the temporal lobes.
The external ear acts as a ‘collecting device’ for these vibrations and propagates the sound towards the tympanic membrane. The conchae and the EAC act as acoustic resonators, affecting the sound pressure at the tympanic membrane. The EAC contributes substantially to an increase in sound pressure level at the tympanic membrane. Pressure changes in the EAC vibrate the tympanic membrane, which in turn causes movement of the ossicular chain in the middle ear.
The middle ear is both a coupler (transferring sound from air to fluid media) and a transformer (the three ossicles increase the sound energy transmitted to the cochlea to a greater extent than would occur from a direct coupling). The movement of the tympanic membrane results in a focused application of force by the ossicular chain at the oval window.
The piston-like vibration of the stapes in the oval window leads to a pressure differential between the scala vestibuli and the scala tympani, which is essential to the mechanical excitation of the cochlear hair cells. The transformer effect of the middle ear is primarily due to:
- 10The area of the tympanic membrane being greater than the area of the stapes footplate (Figure 1.7), such that pressure at the footplate is in effect 14 times higher than the pressure on the tympanic membrane
- A lever system (Figure 1.8), whereby the displacement of the incus is less than that of the malleus as the distance of the long process of the incus is shorter than the manubrium of the malleus from the fulcrum. This results in the incus a applying greater force than the malleus in the ratio 1.3:1.0
The acoustic reflex Stapedius has a protective role by reducing the intensity of sound signals reaching the inner ear; this is called the acoustic (or attenuation) reflex. When high intensity sound is transmitted to the cochlear nuclei of the brainstem, interneurons to the pontine motor nuclei of the facial nerve initiate a reflex after just 40–80 ms. The stapedius muscle (the effector organ of the reflex arc, innervated by the facial nerve), contracts to pull the stapes away from the oval window of the cochlea.
Figure 1.7: The ratio of the area of the tympanic membrane and stapes footplate (T/S) results in an increase in pressure at the oval window.
11The overall effect of the acoustic reflex is to make the ossicular chain rigid, thereby attenuating transmission of lower frequency sound by up to 40 dB. Sounds of long duration are suppressed at high levels, whereas short duration bursts of sound energy are transmitted relatively unimpeded by middle ear muscle activity. This attenuation is also initiated pre-emptively, such as just before speaking.
The physiology of the human tensor tympani remains obscure. In contrast to findings in most animals, it does not respond to sound unless the sound is strong, sudden and causes a ‘startle’ response. It acts by tightening the ear drum by pulling the manubrium of the malleus inwards.
Displacement of perilymph within the scala vestibuli by the motion of the stapes imparts a travelling wave of vibration to the basilar membrane (Figure 1.9) of the cochlear duct. The travelling wave builds up to a maximum depending on the pitch, and then falls to nothing. The wave peaks near the base of the cochlea for high-pitched sounds (where it is stiffer), and near the apex for low-pitched sounds. The fluid wave causes a shearing force on the stereocilia of the hair cells that bends them and induces a receptor potential (Figure 1.9). This causes the release of neurotransmitters. There are two types of hair cell:
- Inner hair cells are responsible for the majority of the acoustic nerve signal
- Outer hair cells amplify sound-induced vibrations by vibrating at the frequency of the acoustic signal (known as mechanical feedback amplification)
12Neurotransmitters released at the base of the hair cell generate excitatory postsynaptic potentials (EPSPs) in primary afferent nerve fibres of the auditory nerve. All-or-nothing responses propagate through these axons to second order fibres in the brainstem. About one fifth of a second after detection, electrical signals reach the auditory cortex of the temporal lobes and sounds are perceived.
Figure 1.9: Stereocilia of cochlea hair cells. Displacement towards the tallest row of stereocilia is depolarising. Differences in receptor potential in each state are shown in blue below the cells.
Cells in the central auditory system are exquisitely sensitive to small differences in intensity and time differences of the sound arriving at both ears, giving rise to the ability to localise sound.
Balance is maintained by co-ordination of information from three main sensory systems (Figure 1.10):
- The vestibular system
- The eyes
- Proprioception, i.e. sensory information from muscles, joints, tendons and ligaments
The signals from these systems are integrated in the brainstem, cerebellum and cortex. Disorders affecting any of these structures or their physiology (e.g. cardiac, respiratory, metabolic diseases) can affect balance.
Vestibular labyrinth In the inner ear the vestibular labyrinth detects acceleration of the head in any direction, whether in a straight line (linear) or turning (angular). Similar to the cochlear system of sensing hearing, the mechanical stimuli are transduced into electrical impulses which travel along the vestibular nerve to the brainstem.
14Utricle and saccule These are called the otolithic organs: they contain crystals surrounded by less dense endolymph. The difference in flow response of the crystals and endolymph is sensed by hair cells during linear acceleration (Figure 1.11), such as side-to-side or up-and-down movement. When the head is tilted from side to side, gravity will cause a shearing force between the otolithic membrane and the surface of the maculae, resulting in a bending of the stereocilia.
The deflection of the stereocilia in the direction of the longer stereocilia causes the transduction channels to open, allowing hair cell depolarisation. Conversely, movement of the stereocilia in the opposite direction causes hyperpolarisation. The hair cells then generate vestibular nerve action potentials, which sends information about head position to the brainstem and spinal cord. This is relayed to eye muscles (utricle) and posture muscles (saccule).
Ampullae of the semicircular canals These detect angular acceleration, for example the movement experienced on a merry-go-round (Figure 1.12). The ampulla contains the saddle-shaped crista, on which the hair cells sit. The stereocilia of the hair cells protrude into a gelatinous material called the cupula.
With a turn of the head, the inertia of the endolymph in the semicircular canal causes the cupula to move, deflecting the stereocilia and stimulating transduction. Each semicircular 15canal is paired with another in a parallel plane on the opposite side of the head. One gives an excitatory response and the other an inhibitory response in a given plane.
Vestibular reflexes The vestibular system is involved in two reflexes:
- The vestibulo-ocular reflex co-ordinates and stabilises eye movement with head movement, so that objects can remain in focus and in fixed view. The interneurons are between the vestibular nuclei and the oculomotor and abducens nuclei
- The vestibulospinal reflexes co-ordinate head and body movement with posture. The interneurons are between the vestibular nuclei and the vestibulospinal tract of the spinal cord, where they synapse with efferents to neck and posture muscles
The external nose consists of a bony and mainly cartilaginous skeleton (Figure 1.13). The two nasal cavities are separated by a bony and cartilaginous septum and terminate at the posterior choanae (from the Greek word for funnel), which lead to the nasopharynx.
The nasal vestibule is the most anterior part of the nose. Each is formed by nasal cartilages, connective tissue and hair-bearing skin. The main cartilages are the bilateral greater alar cartilages (lower lateral cartilages) and the cartilaginous nasal septum.
The junction between the nasal vestibule and the nasal cavity is the narrowest part of the nasal airway, known as the internal nasal valve.
Lateral nasal wall On the lateral wall of the nose are three turbinates (inferior, middle and superior) or bony ridges 17which increase the surface area of the nasal mucosa; they have a rich nerve and blood supply, and are therefore sensate. The ostia (openings) of the sinuses, apart from the sphenoid sinus and the opening of the nasolacrimal duct, are located on the lateral wall of the nose in the meatus that lie inferior to the turbinates (Table 1.1).
Blood supply The nose receives blood from both the internal and external carotid arteries. There is a rich vascular anastomosis between vessels from the two systems: for example, in Little's area, an anteroinferior part of the nasal septum, four arteries meet to form Kiesselbach's plexus (Figure 1.14).
- Anterior ethmoidal artery
- Posterior ethmoidal artery
The ethmoidal arteries are branches of the ophthalmic artery. They descend into the nasal cavity through the cribriform plate.
The external carotid branches supplying the nose are the:
- Sphenopalatine artery
- Greater palatine artery
- Superior labial artery
- Lateral nasal arteries
The paranasal sinuses (Figure 1.15) are divided into groups named according to the bones in which they lie:
Maxillary sinuses (or antra) These are the largest of the paranasal sinuses and are located in the maxillary bones. The superior wall is the floor of the orbit, the sinus floor is formed by the alveolar process of the maxilla and can sometimes be perforated by the apices of the molar teeth. This sinus can be involved in orbital blowout fracture and root canal and dental infections.
Frontal sinus This is in the frontal bone, superior to the eyes, and forms the roof of the orbit; its posterior wall is the bony anterior cranial fossa.
Ethmoid sinuses These are formed from several discrete air cells within the ethmoid bone between the nose and the eyes. They are further divided according to their drainage into anterior and posterior groups (Table 1.1). The lateral wall forms the (paper-thin) lamina papyracea, which separates the sinus from the orbital cavity. Ethmoidal infection, especially in children but also in adults, can breach the lamina and involve the orbit, with the potential to affect vision by compressing the orbital contents.
19Sphenoid sinuses These are in the sphenoid bone at the centre of the skull base under the pituitary gland. The lateral walls are related to vital structures, including the internal carotid artery, cavernous sinus and cranial nerves II–IV. The transnasal approach to the pituitary gland is through the sphenoid sinuses.
Figure 1.15: Paranasal sinuses. Frontal sinus (FS); ethmoid sinuses (ES); uncinate process (UP); inferior turbinate (IT); concha bullosa (CB); maxillary sinus (MS).
Physiology of smell
The olfactory system (Figure 1.16) allows distinction between large numbers of different smells.
The olfactory area is a region of specialised sensory epithelium in the roof of the nasal cavity, with a surface area of 20200–400 mm2. This area is increased by receptor cells’ cilia, which project into the mucus lining of the nasal epithelium. Other types of cells of the olfactory epithelium include columnar supporting cells and basal cells. The basal cells continually divide to produce new olfactory receptor cells which, because of their short lifespan, need to be continually replaced. This is an unusual characteristic, because most other nerve cells cannot be regenerated.
The process of sniffing ensures maximum exposure of odours to the olfactory area, via turbulent airflow. Odours that reach this area are absorbed into the water fraction of the mucus and in turn react with the lipid bilayer of the receptor cells 21at specific sites. This causes K+ and Cl− to flow out leading to depolarisation of the sensory cells. A slow compound action potential (i.e. the sum action potential of the multiple primary afferents) is generated from the olfactory mucosa. Depending on the chemical nature of the stimulus, the threshold varies: the threshold for perceiving a smell is lower than that required to identify a smell. There is also marked adaptation of the olfactory response, with an increase in threshold following exposure, but recovery occurs quickly.
Each receptor cell is connected by non-myelinated nerve fibres to the olfactory glomeruli of the olfactory bulb. Each glomerulus receives about 25,000 fibres and fires in an ‘all-or-nothing’ fashion into the mitral or tufted cells of the olfactory bulb. These bulbar cells have (approximately 100,000 axons projecting along the olfactory tract [as the olfactory nerve (CN I)] to synapse at five cerebral regions:
- Piriform cortex
- Periamygdaloid area
- Olfactory tubercle
- Entorhinal cortex
Unlike other sensory pathways to the cerebral cortex, the olfactory pathway does not relay to the thalamus. However, fibres do leave the olfactory cortical areas and relay in the thalamus on their way to the hypothalamus or other areas, where they perhaps play a role in the regulation of the intake of food and other behaviours that depend on olfactory information.
1.3 Head and neck
Knowledge of gross and microanatomy of the head and neck is key to understanding the normal physiology of taste, phonation and swallowing. Structures located in or passing through the neck include the jugular veins, vagus nerve and carotid arteries, part of the oesophagus, the larynx and vocal cords, seven cervical vertebrae and enclosed spinal cord, along with 22sternocleidomastoid and hyoid muscles anteriorly and trapezius and other nuchal muscles posteriorly.
The neck contains various layers of fascia that divide it into different compartments:
- Investing fascia is the outermost layer just deep to the platysma
- Prevertebral fascia is in front of the prevertebral muscles
- Pretracheal fascia encloses the thyroid gland and allows gliding movement during swallowing
- The carotid sheath envelopes the common carotid artery, internal jugular vein and vagus nerve
The neck is anatomically divided into anterior and posterior triangles and extends from the skull base above to the upper border of the sternum below.
Anterior triangle of the neck
The boundaries of the anterior triangle (Figure 1.17) are:
- Lateral: anterior border of the sternocleidomastoid (as described by anatomists although surgeons clinically use the posterior border of sternocleidomastoid muscle)
- Superior: the lower border of the body of the mandible
The anterior neck is subdivided into four smaller triangles by the digastric muscle above and the superior belly of the omohyoid below. These four smaller triangles contain the following structures:
- The muscular triangle: the anterior neck muscles, larynx, thyroid, trachea and oesophagus
- The carotid triangle: the carotid sheath
- The submandibular triangle or digastric triangle: the submandibular gland, facial artery and vein, hypoglossal nerve, hypoglossus, mylohyoid muscle and nerve and glossopharyngeal nerve
- The submental triangle: the submental lymph nodes and anterior jugular vein
Posterior triangle of the neck
The posterior triangle contains the accessory nerve, the inferior belly of the omohyoid, the occipital artery, the external jugular vein, the lymph nodes and the cutaneous branch of the cervical plexus.
The floor is formed by the prevertebral fascia overlying the prevertebral muscles. Its boundaries are:
- Anterior: posterior border of the sternocleidomastoid muscle
- Posterior: anterior edge of the trapezius muscle
- Inferior: the middle third of the clavicle
Anatomical levels of the neck
The anatomical landmarks of the triangles are subdivided into levels I–VI for oncological purposes (known as the Memorial Sloan–Kettering group). These are listed in Table 1.2 and shown in Figure 1.18.24
There are five clinically distinct potential neck spaces:
- parapharyngeal space
- submandibular space
- carotid sheath space
- pretracheal space
- retropharyngeal space
Their anatomical boundaries and infections within them are outlined in Table 1.3.
Pharynx and oral cavity
The pharynx, or throat, is the passageway leading from the mouth and nose to the oesophagus and larynx. It is subdivided into three regions:
The boundaries and relationships of the pharynx and oral cavity are shown in Figure 1.19.
This is bound superiorly by the skull base and inferiorly by an imaginary line level with the soft palate. Anteriorly are the posterior choanae and laterally the Eustachian tube openings. The posterior nasopharynx contains pharyngeal mucosa and, in children, adenoid tissue.
This extends from the level of the soft palate to the base of the vallecula, i.e. the level of the hyoid bone. It contains the palatine tonsils bilaterally.
Larynx and trachea
The larynx or ‘voice box’ has a cartilaginous skeleton consisting of three single (thyroid, cricoid and epiglottic) and three paired (arytenoid, corniculate and cuneiform) cartilages (Figure 1.20). The larynx extends vertically from the tip of the epiglottis to the inferior border of the cricoid cartilage – the only complete cartilaginous ring above the trachea. Its interior can be divided into the:
- Supraglottis: consisting of the whole epiglottis, aryepiglottic folds, vestibular folds, ventricles and arytenoids
- Glottis: the vocal folds and the area 1 cm inferior to them (Figure 1.21)
- Subglottis: below the glottis to the lower border of the cricoid cartilage
Functionally, the larynx is essential in breathing, phonation and protecting the airways against aspiration.29
Figure 1.20: Vocal fold and laryngeal anatomy. (a) Endoscopic view of the larynx from above with the vocal folds in abduction (opened position). The trachea and subglottis are visualised through the rima glottidis. (b) Vertical cross-section through the larynx showing the false vocal folds, true vocal folds and the ventricles. A, arytenoid cartilage; E, epiglottis; F, false vocal fold; P, pyriform fossa; *, true vocal fold; R, right side; S, subglottis; T, trachea; V, ventricle.
The vocal folds (or cords) are adapted for phonation. They the only part of the larynx lined (superiorly) by stratified squamous epithelium; the rest of the respiratory tract is lined by pseudostratified ciliated columnar epithelium. It is the thinner stratified squamous epithelium that makes the glottis an efficient vibratory organ. The layered anatomy of the vocal folds is crucial to the physiology of phonation (see p. ). The epithelial layer vibrates or glides over a loose layer of superficial lamina propria (Reinke's space) supported by a firm body (vocalis/thyroarytenoid muscle).
The internal branch of the superior laryngeal nerve supplies sensory innervation to the glottis and larynx above this level. The external branch of the superior laryngeal nerve is motor and supplies the cricothyroid muscle. The bilateral recurrent laryngeal nerves supply sensory innervation to the subglottis and motor innervation to the rest of the pairs of intrinsic muscles of the larynx:
- Posterior cricoarytenoids (the only abductors)
- Lateral cricoarytenoids
- Transverse arytenoids
The course of each recurrent laryngeal nerve is long: the left recurrent laryngeal nerve loops around the aortic arch and travels in the tracheo-oesophageal groove; the right recurrent laryngeal nerve passes under the right subclavian artery and then 31upwards into the tracheo-oesophageal groove. Both enter the larynx at the inferior cornu of the thyroid cartilage.
Thyroid and parathyroid glands
The thyroid gland (Figure 1.22) is a butterfly-shaped bilobed gland with a strand of thyroid tissue connecting the lobes called the isthmus. The gland is surrounded by its own capsule and pretracheal fascia. Between the two layers of the capsule or just outside the posterior side of the lobes are the superior (arising from the fourth pharyngeal pouch) and inferior (arising from the third pharyngeal pouch) parathyroid glands. The firm attachment of the thyroid gland to the underlying pretracheal fascia means that it moves during swallowing.
Embryology The thyroid gland descends from the foramen caecum in the tongue base to its final position in front of the trachea. The thyroglossal duct normally involutes completely, however a thyroglossal cyst (see section 7.4) can develop anywhere along the duct, most commonly below the hyoid bone in the midline.
32Neurovascular supply and lymphatic drainage The thyroid is supplied by the superior thyroid artery, a branch of the external carotid artery; the inferior thyroid artery, a branch of the thyrocervical trunk and in less than 10% of the population the thyroid ima artery, a branching directly from the brachiocephalic trunk. The recurrent laryngeal nerve and the inferior thyroid artery are closely related to the inferior pole of the gland and meticulous and careful dissection is required to avoid injury to the nerve during thyroid surgery. Lymphatic drainage frequently passes to the lateral deep cervical lymph nodes and the pre- and paratracheal lymph nodes.
The ear, nose and throat are innervated by cranial nerves (Table 1.4). Assessing these nerves’ function is a key aspect of clinical examination.
Physiology of taste
There are five primary submodalities of taste: sweet, sour, salty, bitter and umami. The tip of the tongue is the most sensitive to sweetness and saltiness. The lateral aspects of the tongue are most sensitive to sourness, and the back of the tongue is most sensitive to bitterness. Umami is sensed throughout.
The taste buds are situated predominantly on raised tongue protrusions called papillae (Figure 1.23), of which there are four types, as described in Table 1.5.
There are also taste buds on the palate and lips. Taste buds are a collection of 50 to 100 elongated epithelial cells called taste receptor cells (TRCs) embedded in the papillary epithelium. They are of epithelial origin and undergo constant renewal. There are three types of cells in taste buds:
- Type I TRCs have tall microvilli and are thought to be support cells (i.e. glial-like)
- Type II TRCs have short microvilli and sense sweet, bitter and umami tastes
36From each cell type, processes extend up into the pore region of the bud, and nerves enter and leave the taste bud through its base.
Figure 1.23: The tongue contains outward protrusions called papillae, of which there are three main types. Taste buds are situated in various locations on the papillae.
Taste stimuli are transmitted to the brainstem via two embryologically separate pathways:
- the chorda tympani nerve, a sensory branch of the facial nerve (VII) which runs in close proximity to the annulus of the tympanic membrane, then traverses the middle ear between the incus and malleus and joins the lingual nerve to supply the anterior two thirds of the tongue
Physiology of voice (phonation)
Voice production requires:
- An air source (the lungs)
- A vibratory source (the vocal folds)
- A resonating chamber (the pharynx, oral cavity and nose)
Any disorder of these components can contribute to changes in the voice.
Phonation In order to phonate, the recurrent laryngeal nerves set the vocal folds into the adducted position. However, as the vocal processes of the arytenoid cartilages (forming the posterior one third) are more bulky than the membranous vocal folds, a slight gap exists between the vocal folds. The lungs then expel air, and this airstream passes through the gap. According to the Bernoulli principle, (Figure 1.24) there is a drop in pressure at the site of the glottis and this causes the mucosa of the vocal folds to be drawn into the gap and block it. Subsequently, the sub-glottic pressure rises and causes another stream of air to pass through the glottis, followed again by a drop in pressure and glottic closure. Repeated cycles of this process set up a vibratory pattern in the vocal folds, and the resulting sound is what we interpret as voice. As the sound passes through the resonating chamber of the pharynx and oral cavity containing the palate, tongue, teeth and lips, this voice is further modulated into speech.
Pitch The tension and length of the vocal folds together with the tracheal air pressure are important in determining the pitch of the voice. Vocal fold length is altered by the cricothyroid 38and thyroarytenoid muscles. Shortening of the vocal folds leads to the tension being readjusted by the vocalis muscle. An increase in tension with maintenance of vocal fold length, as with raising the voice, leads to a rise in pitch. An increase in volume is attained by a rise of air pressure associated with a reduction in the elasticity of the glottis.
Swallowing is the mechanism that transmits liquids or solids from the mouth to the stomach, via the pharynx and oesophagus, without entering the respiratory tract. Although it is initiated voluntarily there are involuntary components, with complex neuromuscular involvement. Swallowing has three stages:
- Oral (Figure 1.25)
- Oesophageal (Figure 1.26)
Figure 1.25: Oropharyngeal phases of swallowing. (a) With a food bolus in the mouth, the airway is open and upper oesophageal sphincter closed. (b) Behind the bolus the tongue blocks the mouth and the soft palate closes the nasopharynx to block the airway. The epiglottis blocks the larynx, and the upper oesophagal sphincter opens. (c) Pharyngeal muscles contract and the bolus passes into the oesophagus. The upper oesophageal sphincter then closes.
This begins when fluid or food is placed into the mouth. As well as closure of the lips it requires closure of the oropharyngeal 40sphincter so that material is retained in the mouth until ready to progress into the pharynx. Solids require preparation to form a bolus, with coordinated action from the lips and buccal, mandibular and tongue movements for chewing to break up the particles and mix them with saliva. The cranial nerves involved are:
- CN V (trigeminal nerve): sensation and mastication
- CN VII (facial nerve): lip and buccal movements
- CN X (vagus nerve): oropharyngeal sphincter)
- CN XII (hypoglossal nerve): tongue movements
Figure 1.26: Oesophageal phases of swallowing. Contraction of circular muscles pushes the food bolus down; contraction of longitudinal muscles shortens the oesophagus ahead of food.
The bolus is assembled between the tongue and the hard palate. Co-ordinated contraction of the tongue, in an anterior to posterior direction, propels the bolus posteriorly into the oropharynx with relaxation of the palatoglossal sphincter. The soft palate is pulled posterosuperiorly to close the nasopharynx to stop respiration and prevent nasal regurgitation.
This is completed within 1 second and involves the tongue base acting as a piston, pumping the bolus towards the entrance of 41the oesophagus, as well as the elevation of the larynx by the suprahyoid muscles. This gives rise to a negative pressure in the entrance of the oesophagus. There is associated relaxation of the upper oesophageal sphincter, allowing the bolus into the oesophagus. Movement of the leading edge of the bolus into the oesophagus triggers the pharyngeal constrictor muscles to contract from above downwards, propelling the bolus into the oesophagus. During this phase both the laryngeal inlet and the nasopharynx are closed to prevent aspiration and nasal regurgitation. The sensory input travels via the glossopharyngeal and vagus nerves.
The oesophageal stage lasts 8–20 seconds; it begins with the bolus entering the oesophagus and ends when it has passed through the lower oesophageal sphincter into the stomach. Peristaltic waves can be primary, passing down the oesophagus, or secondary arising locally in response to distension of the oesophagus, helping transport the bolus through the oesophagus. Tertiary oesophageal contractions are irregular and non-propulsive, involving long segments of the oesophagus and frequently developing during emotional stress. The lower oesophageal sphincter is regulated by the vagus nerve.42