Lesson 66: Structure and Function of the Pharynx and Esophagus

The pharynx

passage and that connects the nasal and oral cavities to the esophagus and larynx respectively.

role in both respiration and digestion

Anatomy of the pharynx

Oropharynx

Caudal to the oral cavity

From Palatoglossal folds and arches, to the hyoid apparatus.

The soft palate

separates the nasopharynx from oropharynx

Oral and oropharyngeal surface, nasopharyngeal surface

Pharyngeal constrictors

Sequentially contract to push food into the esophagus

Hyopharyngeus

Thyropharyngeus

Cricopharyngeus

Pharyngeal Dilators

Enlarges the pharynx during swallowing(the bolus will assist this act)

Stylopharyngeus

Pharyngeal shortners

Pterygopharyngeus,

Palatopharyngeus

Muscles of the pharynx

Motor innervation to the pharynx muscles

Pharyngeal plexus

Vagus nerve (CN X) –

Glossopharyngeal Nerve (CN IX)

Cranial root of the Accessory nerve (CN XI)

Sensory innervation to the lumen of the pharynx

Glossopharyngeal nerve (CN IX)

Vagus nerve (CN X)

Trigeminal nerve CN V (Maxillary branch)

Arterial: Branches of the external carotid a.(1)

Ascending pharyngeal a: Major blood supply

Branches from the lingual(3)and facial(2)aa.

Cranial laryngeal a.(4)

Cranial thyroid a. (5) – Pharyngeal branch

Venous drainage of the pharynx

Similar pattern with satellite veins

Pharyngeal plexuses and satellite veins finally drain into internal and external jugular veins

Horse: pharynx species differences

Long (capacious), but narrow pharynx, adapted for obligate nasal breathing.

Laryngeal opening is directed more towards the nasopharynx

longer soft palate that lies beneath the epiglottic cartilage, preventing oral breathing,

Horse guttural pouch

modified eustachian/auditory tubes connected to the nasopharynx

Ruminants species differences

adapted for regurgitation, remastication (Cud chewing), and re-swallowing of food, and to prevent Aspiration during regurgitation.

Well developed pharyngeal muscles, wider pharynx to allow swallowing of larger boluses, limited oral breathing in ruminants

Pigs species difference

Larger pharynx relative to head size

Pharyngeal diverticulum

The nasopharynx histology

(involved in respiration) has ciliated epithelium to trap and clear debris

The oropharynx and laryngopharynx histology

protective stratified squamous epithelium to withstand mechanical stress.

Tonsils - Lymphoid tissue of the pharynx

Either embedded within the mucosa(Mucosa-associated lymphoid tissue - MALT) or distinctly visible

Pharyngeal tonsils

Associated with nasopharynx,

Tubal (around opening of the auditory tube)

Palatine tonsils

Associated with oropharyngeal surface

Not clearly visible in dogs, cats, bovine. Pigs do not have palatine tonsils. Horse and Small ruminant – radially visible

Lingual tonsils

embedded in the mucosa of the tongue (root)

well developed in The horse, bovine and small ruminants

Palatine Tonsil, Horse.

5. Diffuse lymphatic tissue,

3. tonsillar crypts,

7. Lymphatic nodule,

11. Stratified squamous epithelium,

Horse palatine tonsils

Horses have scattered lymphoid tissue rather than a well-defined tonsillar structure.

However, they are visible on the ventro-lateral aspect of the oropharynx

Ruminants palatine tonsils

large palatine tonsils with deep crypts for antigen trapping, but they are embedded in the latero-dorsal wall (submucosa) of the oropharynx.

Not readily visible, but the opening leading to the tonsillar (lymphoid) tissue can be seen. swelling of the pharyngeal wall can be seen upon enlargement.

Palatine tonsils: dogs and cats

well defined tonsils with deep crypts in the palatine tonsils, making them more prone to tonsillitis

The tonsils in the dog and cat are covered by a mucosal fold(Semilunar fold) forming a tonsillar fossa/pouch.

not readily visible unless inflamed and enlarged.

Palatine tonsils: pigs

The palatine tonsils are absent.

However, they have numerous tonsils all along the oral surface of the soft palate called tonsils of the soft palate. Numerous openings are seen leading to the lymphoid tissue in the soft palate.

Transverse section through

oropharynx

a Oropharynx ;

b Nasopharynx;

1 Lingual tonsil ;

2 Palatine tonsil ;

J Tonsil of the soft palate;

4 Pharyngeal tonsil ;

5 Tubal tonsil

Structure of the esophagus

tubular and muscular hollow organ that connects the pharynx to the stomach, allowing the passage of food and liquids.

It Extends from Cricoid cartilage to the cardia of the stomach.

The cervical part: esophagus

Extends from the pharynx (Cricoid cartilage of the larynx) to the cardia of the stomach (just past the diaphragm).

Related to the trachea, hypaxial muscles, carotid sheath, recurrent laryngeal nerves etc.

Cranial/Upper Esophageal Sphincter(C/UES)– Cricopharyngeus m. and Thyropharyngeus m.

● Pharyngoesophageal limen (Red Arrow)

Lower Esophageal Sphincter (LES)/Cardiac sphincter

Esophagus: The Thoracic part

from thoracic inlet to the esophageal hiatus of the diaphragm; Within the mediastinum

Thymus, cranial vena cava, brachiocephalic trunk, trachea, lungs, base of the heart, aorta, and Vagus nn.

Slightly on the left side (more left in the caudal mediastinum)

Esophagus: motor innervation

Somatic (Voluntary) Control – For striated muscle portions → pharyngeal branches of the Vagus nerve (CN X).

recurrent laryngeal nerve, a branch of the vagus, controls most of the esophageal musculature in species with extensive skeletal muscle (e.g., ruminants, dogs)

Esophagus: sensory innervation

Afferent (sensory) fibers from the Vagus nerve and sympathetic fibers detect stretch, pain, and temperature changes, sending signals to the brainstem.

Esophagus: Autonomic (Involuntary) Control – For smooth muscle portions

Parasympathetic innervation via the Vagus nerve (CN X) enhances peristalsis and mucus secretion.

Sympathetic innervation from the cervicothoracic ganglion and middle cervical ganglion regulates esophageal tone and sphincter function

Arterial blood supply: Cervical Esophagus (Neck Region)

Supplied by the esophageal branches of the thyroid arteries (e.g., cranial and caudal thyroid arteries)

Arterial blood supply: Thoracic Esophagus (Chest Region)

Receives blood from the bronchoesophageal artery, a branch of the thoracic aorta or intercostal arteries

Arterial blood supply: Abdominal Esophagus (Near the Stomach)

Supplied by branches of the left gastric artery, which arises from the celiac artery

Venous drainage of the esophagus

The cervical and thoracic esophagus drains into the jugular veins, bronchoesophageal veins, and the azygos vein

The abdominal esophagus drains into the portal vein, linking it to the hepatic circulation.

Anatomy of esophageal blood supply

Esophagus Mucosa

The innermost layer, lined with stratified squamous epithelium to resist mechanical damage from food

Esophagus Submucosa

Contains glands that secrete mucus to lubricate food passage

Lamina propria submucosa, Muscularis mucosae

Esophagus Muscularis

Composed of skeletal and/or smooth muscle, depending on the species

esophagus Adventitia

The outermost connective tissue layer that provides support and attaches the esophagus to surrounding structures.

Serosa (abdominal part only)

The esophageal wall layers

Esophagus, Mid-region, x.s., Dog. The glands of the dog’s esophagus are predominantly mucous. They are located in the submucosa throughout the length of the esophagus of the dog. Stratified squamous epithelium

Esophagus, Cranial, l.s., Cat. few mixed glands are present in the submucosa. In the cat, horse, and ruminant, esophageal glands occur only near the junction of the esophagus and pharynx. Stratified squamous epithelium

Esophagus, Cranial, x.s., Horse. The papillated stratified squamous epithelium shows a distinct keratinized layer

Esophagus, Cranial, l.s., Pig. The stratified squamous Epithelium is keratinized in pig, horses and ruminants. Note the abundance of mucous glands in the submucosa.

Esophagus: Muscular coat; composed of skeletal and/or smooth muscle, depending on the species

Ruminants and dogs: Entirely skeletal muscle, allowing voluntary control.

Horses, pigs, and cats: The upper part is skeletal muscle, transitioning to smooth muscle towards the stomach

Esophageal obstructions: Most probable locations

Thoracic inlet, Base of the heart and just cranial to the Esophageal hiatus

Megaesophagus

Carnivores in general are prone

Usually occurs just before the diaphragm

Benign Esophageal stricture

May occur during healing process, adhesion of the mucous membrane.

Mechanism of swallowing: Oral Phase (Voluntary Phase)

conscious control and involves food prehension, ingestion, mastication, and the formation of a food bolus.

tongue pushes the bolus towards the oropharynx → soft palate elevates to close off the nasopharynx, preventing food from entering the nasal cavity

Mechanism of Swallowing: Pharyngeal Phase (Involuntary Reflex Phase)

rapid and involuntary, controlled by the swallowing reflex in the brainstem

The bolus stimulates sensory receptors in the oropharynx, triggering the swallowing reflex.

hyoid apparatus and larynx move forward and upward to help close the airway → epiglottis moves/folds backward to cover the laryngeal opening → pharyngeal muscles (constrictors) contract sequentially to propel the bolus into the esophagus

Mechanism of Swallowing: Esophageal Phase (Involuntary Phase)

upper esophageal sphincter relaxes, allowing the bolus to enter the esophagus → Peristalsis (wave-like contractions) moves the bolus down the esophagus → lower esophageal sphincter relaxes, allowing the bolus to enter the stomach

Neural control of swallowing

Afferent (Sensory) fibers in the glossopharyngeal nerve, the pharyngeal and esophageal branches of the Vagus nerve and the cranial laryngeal nerve /activate the solitary tract nucleus (NS)

Brain Components (Integration) solitary tract nucleus (NS), a sensory nucleus → central pattern generator (CPG) for swallowing (reticular formation – Brainstem) → nucleus ambiguus (NA) a motor nucleus

Motor Component (Execution) of swallowing

Via the “CPG for swallowing”, the neurons in the nucleus ambiguus (NA), the motor neurons of the swallowing muscles via Vagus nerve, generate sequential swallowing activity.

Interconnections between the left and right CPGs coordinate their activity.

Nucleus ambiguous (Motor), Solitary tract nucleus (Sensory): Cranial nerves involved

Trigeminal nerve (CN V) – mastication

Facial nerve (CN VII) – lip closure

Glossopharyngeal nerve (CN IX) – pharyngeal sensation and contraction

Vagus nerve (CN X) – pharyngeal and esophageal control

Hypoglossal nerve (CN XII) – tongue movement