The auditory tube (also known as the pharyngotympanic tube, Eustachian tube, Latin: tuba auditiva) is a tunnel that connects the tympanic cavity to the nasopharynx and equalizes pressure on both sides of the tympanic membrane.
Structure of auditory tube
The auditory tube is 3,5-4 cm long and around 2 mm in diameter. The opening of the auditory tube in the middle ear is located in its anterior wall. From here, the auditory tube extends forward, medially, and downward to enter the nasopharynx, opening in the lateral wall of the nasopharynx just posterior to the inferior meatus of the nasal cavity.
By joining the two structures, the auditory tube acts as a pathway for upper respiratory infection to spread into the middle ear. The tube is shorter and straighter in children than in adults, so middle ear infections are more common in children.
The main functions of the auditory tube are as following:
- Pressure equalization on both aspects of the tympanic membrane;
- Mucociliary clearance;
- Protection from the influences of the nasopharyngeal environment and loud sounds.
The auditory tube consists of two parts:
- Bony part (the one-third nearest the middle ear);
- Cartilaginous part (the remaining two-thirds).
The bony part starts in the anterior wall of the tympanic cavity and is gradually getting narrower. The opening of the bony part of the auditory tube is clearly visible on the inferior surface of the skull at the junction between the squamous and petrous parts of the temporal bone, immediately posterior to the foramen ovale and foramen spinosum.
The cartilaginous part of the auditory tube is formed of a triangular plate of elastic fibrocartilage. Its apex is attached to the superior margin of the medial end of the bony part, but its base lies directly under the mucous membrane of the nasal part of the pharynx. The cartilage lies in a groove between the petrous part of the temporal bone and the great wing of the sphenoid. The groove ends opposite the middle of the medial pterygoid plate.
The auditory tube has two openings - the pharyngeal orifice of the auditory tube in the lateral wall of the pharynx and the tympanic orifice of the auditory tube that opens in the tympanic cavity anterior wall.
On the border between both of the parts of the auditory tube is the narrowest place called isthmus tubae auditivae. The auditory tube is lined with mucosa that is the continuation of the pharynx mucosa.
The air pressure in the external ear and the nasopharynx is equal to open-air pressure. This connection between the middle ear and the nasopharynx serves primarily to equalize the pressure. Equalization occurs while the auditory tube stays closed for most of the time. When we yawn or swallow, the auditory tube opens, and it allows air to flow and pressure to equalize. Having the auditory tube protects the tympanic membrane from rupturing quickly under pressure.
Muscles of the auditory tube
All in all, there are four muscles associated with the auditory tube:
- Levator veli palatini - during swallowing, the tube opens due to the contraction of this muscle. This muscle is the elevator muscle of the soft palate.
- Salpingopharyngeus - it starts from the superior margin of the medial cartilage of the auditory tube. During swallowing, this muscle opens the pharyngeal orifice of the auditory tube and allows the equalization of pressure.
- Tensor tympani - it allows involuntary control known as a tympanic reflex. The reflex contracts the muscles of the middle ear. Contractions pull the manubrium of the malleus inwards and tighten it. The tightening stops the vibration from disturbing the perilymph.
- Tensor veli palatini - this muscle tenses the soft palatine and assists levator veli palatini. It assists in allowing air pressure to equalize between the tympanic cavity and the external ear during swallowing.
Vasculature and innervation of the auditory tube
Blood supply and venous drainage
The blood supply to the auditory tube is provided by the following arteries:
- Ascending pharyngeal artery (a branch of the external carotid artery);
- Middle meningeal artery (a branch of the maxillary artery);
- Artery of the pterygoid canal (a branch of the maxillary artery).
The venous blood is drained from the auditory tube via the pterygoid plexus of veins in the infratemporal fossa.
The lymph from the auditory tube is drained into the deep cervical lymph nodes.
The nerve supply to the auditory tube is provided by the tympanic plexus. This is an indirect way for the tympanic nerve (a branch of the glossopharyngeal nerve (CN IX) to supply the auditory tube.