TONGUE & PHARYNX OF HUMAN DIGESTIVE SYSTEM: Structure, location, function & diasorders.

The tongue and taste buds are an essential part of the mouth, involved in taste, chewing, talking and many other functions.

The tongue 

The tongue is a powerful, mobile, muscular organ attached to the mandible and hyoid bone. The body is a flat, oblong surface with a longitudinal ridge along the top. It lies on the floor of the mouth and a thin membranous frenulum runs along the under surface in the mid-line anteriorly. Posteriorly, the root is formed from muscle fibres passing downward towards the pharynx and the epiglottis forms its posterior border. The tongue is covered with a tough non-cornified stratified squamous epithelium continuous with the rest of the oral mucosa. On its upper surface it is thrown up into numerous ridges and papillae, creating a roughened surface to rasp and lick food. Papillae around the lateral and posterior edges contain numerous taste buds. These contain specialized sensory cells that communicate directly with nerve endings from sensory nerve dendrites. The sensory cells are surrounded and supported by adjacent epithelial cells. They express receptors for chemicals dissolved in saliva and each taste bud is sensitive to a single major modality. The hypoglossal(XIIth cranial) nerve innervates the tongue muscle. Sensory fibres travel in the glossopharyngeal(IXth cranial) nerve and in the chorda tympanibranch of the facial(VIIth cranial) nerve. Taste fibres terminate in the nucleus of the tractus solitariusin the mid-brain. The tongue also has a large representation in the somatic motor and sensory cortexof the brain.

Function 

The tongue moves in all planes and reaches throughout the mouth. It directs food between the teeth, retrieve spieces stuck between the teeth and clears away obstructions. It propels food and drink posteriorly to initiate the pharyngeal phase of swallowing. The tongue is also crucial to speech, varying its shape and selectively closing off and opening air channels. The major modalities of taste are sweet, sour, salt and bitter, and a fifth modality, called umami, typified by monosodium glutamate, is now also recognized. Taste receptors include G-protein-coupled receptors, ion channels and cold, heat and pain receptors. The flavour of food is a combination of taste and smell, which is sensed by a large family of G-protein-coupled olfactory receptors that bind to a myriad of different chemicals.

Common disorders 

The tongue may be paralyzed by damage to the hypoglossal nerve or a stroke affecting its central connections. In motor neuron disease, spontaneous fasciculations are readily seen in the denervated tongue muscle. The tongue may be affected by squamous cell carcinoma and herpes simplex infection (see Chapter 1). Occasionally the tongue may be pigmented, which is not pathological. Glossitis, manifest by a smooth, red, swollen, painful tongue occurs; for example, with B-vitamin deficiencies.



Dry mouth, or xerostomia, affects taste profoundly, as chemicals must be dissolved for the taste buds to function. Systemic diseases, such as uraemia, and drugs, such as metronidazole, may altertasteby interfering with the function of taste buds.

The pharynx 

The pharynx is an air-filled cavity at the back of the nose and mouth, above the openings of the larynx and oesophagus. The walls of the oropharynx are lined by the same non-cornified stratified squamous epitheliumthat lines the oral cavity. Superiorly, the floor of the sphenoidal air sinus and the skull base bound the nasopharynx. The soft palate can be drawn up, closing the connection between the nasopharynx and oropharynx. The oropharynx is bounded posteriorly by tissues overlying the bodies of the upper cervical vertebrae and laterally by the tonsils and the openings of the Eustachian tubes, which connect the pharynx with the middle ear. Inferiorly it narrows into the hypopharynx. Three straps of voluntary muscle surround the pharynx, overlapping each other and forming the superior,middle and inferior constrictors. The circular muscle of the upper oesophagus is continuous with the inferior constrictor. Motor and sensory fibres mainly travel in the glossopharyngeal(IXth cranial) and vagus(Xth cranial) nerves.

Function 

The pharynx is a conduit for air, food and drink, and swallowing requires coordinated action of the tongue, pharyngeal, laryngeal and oesophageal muscles, and is controlled by the brain stem, via the glossopharyngeal and trigeminal nerves. The tongue forces a bolus of food backwards into the oropharynx, initiating a reflex that raises the soft palate, sealing off the nasopharynx, and inhibits respiration. The superior and middle pharyngeal constrictors force the bolus down into the hypopharynx, and the glottis closes. The epiglottis is forced backwards and downwards, forming a chute over the larynx, opening onto the upper oesophageal sphincter. The sphincter relaxes, allowing the bolus to enter the oesophagus. It is then conveyed downwards by peristalsis. The glottis reopens and respiration recommences.

Common disorders 

The pharynx is critically important in ensuring that the upper airway is protected from aspiration of food, saliva and drink during swallowing and vomiting. Thus neurological disorders, including stroke, motor neuron disease, myasthenia gravisor reduced conscious level associated with intoxication, anaesthesiaor coma can cause aspiration into the lungs, and pneumonia. Upper respiratory tract infections often cause pharyngitis and may cause tonsillitis. Common pathogens include viruses, such as influenza and the Epstein–Barr virus, and bacteria, such as streptococci. Group A b-haemolytic streptococci may also cause rheumatic fever, a systemic autoimmune disorder that can affect the skin, heart and brain. Diphtheriais a serious cause of pharyngitis that is preventable by immunization.

STRUCTURAL ARRANGEMENT OF TONGUE & PHARYNX