#5890 - Pharynx And Larynx - Cardiorespiratory system

Pharynx and Larynx

Pharynx

1. Function

The pharynx is a common cavity for food and ingesta. It lies behind the mouth and continues into the oesophagus. It can be split into three parts – the nasopharynx, the oropharynx and the laryngopharynx. It is part of both the respiratory and alimentary systems. Both systems have entrances to the pharynx but they are separated from each other by the soft palate. During exercise or times of respiratory distress, the mouth may function as an additional opening of the respiratory system and then the oropharynx also becomes an airway.

2. The soft palate.

The soft palate is a mass of soft tissue continuous with the palatine bone, known as the hard palate. It divides the pharynx, and is covered by respiratory and stratified squamous epithelium. The palatinus muscle can shorten the palate.

At rest, the soft palate lies on the tongue. During swallowing, the soft palate is raised which divides the pharynx into dorsal and ventral sections. The dorsal compartment is the nasopharynx and the rostral compartment is the oropharynx. The laryngopharynx is separated from the oropharynx by the epiglottis.

Two pairs of arches connect the soft palate to adjacent structures. The palatopharyngeal arches pass onto the lateral walls of the pharynx and meet above the entrance to the oesophagus. Together with the free margin of the soft palate they divide the pharynx into the dorsal and ventral compartments. The more rostral palatoglossal arches pass onto the sides of the tongue at its route, and mark the passage from the mouth to the oropharynx.

3. The nasopharynx

The nasopharynx is located dorsally to the soft palate. It extends from the choanae (opening of nasal cavity into the nasopharynx) to the intrapharyngeal opening (formed from the free edge of the soft palate and the palatopharyngeal arches). It is lined with respiratory epithelium, and its borders are defined by the base of the skull, the soft palate and the cranial cervical vertebrae. In the dorsal region of the nasopharynx are paired openings into the auditory (Eustachian) tubes. The lining of the middle ear cavity and auditory tube is continuous with that of the nasopharynx.

4. The oropharynx

The oropharynx is ventral to the soft palate. It extends from the oral cavity to the epiglottis. It is lined with stratified squamous epithelium, and its borders are defined as the tongue, palate and palatoglossal arches. Tonsils are present on the lateral walls of the oropharynx and are covered by flaps of mucosa.

5. The laryngopharynx

The laryngopharynx extends from the intrapharyngeal opening to the opening of the oesophagus and larynx. The opening of the larynx is located on the floor of the pharynx. Caudal and dorsal to the laryngeal opening is the opening to the oesophagus. It contains the epiglottis of the larynx. It is wide but narrows before joining the oesophagus at a boundary that is well defined by a mucosal fold in the dog, but is more difficult to see in other species.

6. Musculature

The wall of the pharynx is covered in striated muscles. These fall into three groups – constrictor, dilator and shortener.

Muscles that constrict the pharynx run dorsally to its roof. They form a series of arches that enclose the lumen on its lateral and dorsal aspects. There are rostral, middle and caudal constrictor muscles.

Rostral constrictor muscles include the pterygopharyngeus and palatopharyngeus muscles. They originate from the pteryoid region of the skull (pterygopharyngeus) and the aponeurosis of the soft palate (palatopharyneous) and embrace the pharynx at the level of the palatopharyngeal arches.

The middle constrictor, the hyopharyngeus muscle, arises from the hyoid bone.

The caudal constrictor muscle arises in two parts, from the thyroid (thyropharyngeous) and cricoid (cricopharyngeus) cartilages.

When the three contrsictor muscles contract in succession, they move boluses of food distally into the oesophagus.

The dilator muscle of the pharynx, the stylopharyngeus caudalis muscle, encloses it laterally and dorsally. It originates from the hyoid apparatus and widens the rostral pharynx.

The palatopharyngeous muscle also closes the pharyngeal arch.

The palatinus muscle shortens the soft palate. The tensor veli palatini and levator veli palatini tense the soft palate by exerting lateral traction and raise the soft palate respectively.

7. Innervation

The nervous supply for these muscles comes from the pharyngeal branch of the vagus nerve (CN X) and from the cranial root of the accessory nerve (CN XI). The glossopharyngeal nerve supplies taste to the pharynx. The tensor veli palatini is supplied by the mandibular nerve (a branch of the trigeminal nerve, CN V).

Larynx

1. Function

The larynx forms the connection between the pharynx and the tracheobronchial tree. It lies behind the pharynx and behind the mouth, suspended from the cranial base by hyoid apparatus. Because of its connection with the tongue and hyoid apparatus, the larynx shifts its position when the animal swallows.

The larynx is shifted forwards and the epiglottis tilted backwards when the animal swallows, partially covering the entrance to the larynx. It deflects fluids into the piriform recesses of the pharyngeal floor, and food boluses are passed quickly over the larynx by pharyngeal muscles.

A deeper protection is provided by the glottis, which is closed by adduction of the vocal folds. On inspiration, the vocal folds may be abducted to widen the glottis, but this occurs only when breathing is vigorous. This is achieved by the dorsal cricoarytenoideus, and subsequent adduction is achieved by the lateral cricoarytenoideus.

The larynx allows passage of air to the lungs and increases the intra-abdominal pressure. It is also used for vocalisation.

2. Cartilages

The forms of the laryngeal cartilages vary between species, but the major and concistently present include the median epiglottic, thyroid and cricoid cartilages and the paired arytenoid cartilages.

The epiglottic cartilage is the most rostral. It is composed of elastic cartilage. It is composed of a thin stalk and a large leaflike blade. The stalk is attached to the root of the tongue, the basihyoid bone and the thyroid cartilage. The larger blade-like part lies behind the soft palate and points dorso-laterally at rest. During swallowing, the blade is tilted backwards to partially cover the entrance to the larynx.

The thyroid cartilage is composed of hyaline cartilage and forms most of the floor of the larynx. It is composed of two lateral plates that meet ventrally. The rostral and caudal extremities of each plate articulate with the thryrohyoid bone and arch of the cricoid cartilage respectively.

The cricoid cartilage is also composed of hyaline cartilage, and is shaped like a signet ring. It has an expanded dorsal ‘seal’ and a narrower ventral arch. The dorsal lamina carries a median crest and two facets on its rostral rim for the arytenoid cartilages. The arch carries a facet on each side for articulation with the thyroid cartilage.

The arytenoid cartilages are also composed of hyaline cartilage and are paired. A caudal facet articulates with the rostral part of the cricoid cartilage. The vocal process projects ventrally from this into the laryngeal lumen. The vocal folds attach to this process. A muscular process also extends laterally, and a corniculate process dorsomedially. The corniculate process is composed of elastic cartilage.

Cuneiform processes support the mucosal folds from the epiglottis to the arytenoid cartilages. It is not present in all species and can be free or fused with the epiglottic cartilages.

The interarytenoid cartilage is a nodule of hyaline cartilage located between the arytenoid cartilages dorsally.

3. Musculature

In addition to the extrinsic laryngeal muscles that connects the larynx to the pharynx, tongue, hyoid bone and sternum, intrinsic musculature is present that connects the laryngeal cartilages.

The cricothyroideus muscle runs between the lateral surfaces of the thyroid lamina and cricoid arch ventral to the cricothyroid joint. It moves the cricoid and arytenoid cartilages caudally to the tense the vocal folds. It is innervated by the cranial laryngeal nerve, a...