Philosophers are known to coin thought provoking sayings that leave individuals in awe. One such question that has stood the test of time is “If a tree falls in a lonely forest, but there are no animals around, does it make a sound?” In 1884, the authors of Scientific American answered a different form of this question stating that sound is the perception of vibratory impulses transmitted via air or other media. So although the tree will generate vibration, if there are no ears to perceive it, then there is no sound. However the reader chooses to interpret this question, there is one undeniable fact: the ear is an extremely important component in the perception of sound.
The human ear is one of the most intricate pieces of machinery found in the human body. It is responsible for transmitting and converting vibrational energy so that sound can be appreciated. The ear is structurally divided into three parts: the outer, middle and inner ear. This article will focus on the structure and function of the middle ear. Special attention will also be given to the neurovascular supply of this part of the ear and its components. Additionally, relevant neighbouring structures and some clinically relevant points will also be discussed.
Walls & Boundaries
In order to appreciate its boundaries, imagine that the specimen is viewed from a coronal section transecting the external acoustic meatus. The middle ear, also known as the tympanic cavity or the tympanum, is a pneumatized (air filled) region of the temporal bone that lies just medial to the tympanic membrane (ear drum) and lateral to the promontory (caused by the turns of the cochlea) of the ear. The tegmen tympani, which is a laminar projection of the petrous part of the temporal bone, forms the roof of the tympanic cavity. The thin bony structure that forms the roof of the jugular foramen also forms the floor of the tympanum. The floor also has a microscopic canal, known as the tympanic canaliculus, which permits the passage of the tympanic part of CN IX (glossopharyngeal). There is an imperfection in the posterior wall of the tympanum known as the aditus.
Just inferior to the aditus is a pitted conical structure called the pyramid. It extends into the tympanic cavity and provides a point of insertion for the tendon of the stapedius muscle. The roof and floor of the tympanum abuts anteriorly, thus reducing the size of the anterior wall. The anterior wall is pierced inferiorly by a bony auditory tube and superiorly by the canal for the tensor tympani muscle. Furthermore, the inferior aspect of the anterior wall of the tympanum also provides a posterior limitation for the carotid canal. As a result, the sojourning internal carotid neurovascular structures provide tympanic branches that pierce this common wall to supply structures of the middle ear.
Not only does the tympanic membrane serve as a physical boundary for the middle ear, but it is an important part in the auditory conduction pathway. It is among the first to begin vibrating once it is struck by the sound waves collected by the external auditory meatus. This thin, fibrous structure has an external surface that is exposed to the external environment and lined by stratified squamous epithelium, and an inner surface (facing the tympanum) that is lined by low columnar epithelium. The tympanic membrane is an oval structure that is situated at an angle relative to the external acoustic meatus.
When viewed laterally with an otoscope, the important landmarks can be described with reference to the face of a clock. At the center of the clock (where the hands would be attached) is a depression in the tympanic membrane known as the umbo. This concavity is created by the attachment of the malleus to the center of the tympanum. The handle of the malleus then extends from the umbo towards the one o’clock position, where it becomes the lateral process of the malleus. The light from the otoscope is reflected from the umbo in the five o’clock direction and is described as the cone of light.
The anterior and posterior mallear folds are fibrous extensions from the lateral process of the malleus that project in the one and eleven o’clock directions, respectively. The superior part of the tympanic membrane, between the mallear folds, is known as Shrapnell’s membrane or pars flaccida and is relatively loose. The remainder of the membrane is held tight by the tensor tympani muscle, and is therefore known as the pars tensa. The perimeter of the tympanic membrane is thicker than the central portion; this allows it to rest adequately in the groove of the tympanic plate.
There are three auditory ossicles located in the middle ear known as the malleus, incus and stapes. Although they are the three smallest bones in the human body, they are extremely important components of the auditory pathway. Through synovial articulations, these bones are able to transmit mechanical energy to the internal ear from the tympanic membrane.
The stapes is the most medial in the chain of bones. It has a foot-plate that sits over the oval window (fenestra vestibuli), two crura (anterior crus and posterior crus) that converge to a narrow neck supporting its head. The small head of the stapes articulates with the bulbous tip of the incus, known as the lentiform nodule.
The incus has two limbs and a cuboidal body. Anteriorly, it adjoins with the mallear head and its long limb courses into the tympanic cavity alongside the malleus and articulates to the stapes. The short limb of the incus extends posteriorly, inferior to the aditus.
The malleus is the largest, and most lateral of the bony chain. It is a club-shaped structure whose head rests in the epitympanic recess. The other features of the malleus (the lateral process and handle) were previously described.
Although the auditory ossicles and the tympanic membrane are responsible for the transmission of vibration, their activities are modified by two muscles. The stapedius muscle and the tensor tympani muscles are extremely important modulators of the hearing pathway. Stapedius originates from the internal concavity of the pyramid and inserts into the neck of the stapes. The muscle acts by pulling back the neck of the stapes, which indirectly pulls back the footplate from the fenestra vestibuli. Tensor tympani originate in a canal superior to the bony part of the pharyngotympanic tube and inserts into the handle of the malleus. The action of the muscle increases the concavity and rigidity of the tympanic membrane.
The outer ear is related laterally with respect to the middle ear, while the inner ear has a medial relation. Superior to the tympanum, above the tegmen tympani, is the temporal lobe of the cerebrum. In addition to serving as the medial wall for the tympanum, the promontory also serves as a landmark for the fenestra vestibuli and the fenestra cochleae (round window). The former is located posterosuperiorly, while the latter is located posteroinferiorly. Also, above the promontory is the canal for CN VII (facial), which is inferior to the prominence, created by the lateral semi-circular canal.
The pharyngotympanic tube (Eustachian tube) is a medial continuation of the middle ear. In children, the tube is more horizontal, while in adults, it is oblique. It permits communication between the tympanum and the nasopharynx. The mastoid antrum is related posteriorly to the epitympanic recess of the tympanum. Communication between the two spaces is permitted by the aditus.
The blood supply to the middle ear is primarily derived from the maxillary division of the external carotid artery, which is complemented by branches of the internal carotid artery. The deep auricular artery (from the maxillary artery) contributes to the tympanic arterial network in the floor of the tympanum and also supplies the meatal (external) side of the tympanic membrane. There is also an anastomosing branch of the meningeal accessory artery (branch of maxillary), called the tubal artery that travels through the pharyngotympanic tube and communicates with the other arteries of the middle ear.
The anterior tympanic artery (arising from the maxillary artery) provides blood supply to the auditory ossicles. It is also involved in a circular anastomosis with the posterior tympanic branch of the stylomastoid artery (arising from either the occipital or posterior auricular divisions of the external carotid artery) that also supplies the tympanic membrane. The stylomastoid artery goes on to perfuse the mastoid air cells and the posterior tympanum. Additionally, it gives a branch to the stapedius muscle, called the stapedial artery, and also a branch to the mucosal (internal) side of the tympanic membrane. The tensor tympani muscle is supplied by the superior tympanic artery, which is a branch of the middle meningeal branch of the maxillary artery. The middle meningeal artery also provides petrosal and superior tympanic branches that supply the tympanum.
The tympanic plexus is formed by contributions of the inferior tympanic artery, a branch of the ascending pharyngeal artery, and the caroticotympanic arteries. The crura of the stapes are wrapped by anterior and posterior crural arteries, which are derived from the tympanic plexus.
The tympanic veins drain deoxygenated blood to the superior petrosal sinus and the pterygoid venous plexus.
The auriculotemporal nerve (branch of CN V3) and the vagus nerve (CN X) provide general sensory innervation to the meatal side of tympanic membrane. The mucosal side of the tympanic membrane transmits its general sensory impulses via the tympanic branches of the glossopharyngeal nerve (CN IX). The tympanic cavity is also innervated by CN IX via the tympanic plexus.
Sympathetic innervation is achieved by the caroticotympanic nerves from the carotid sympathetic plexus. Stapedius receives motor innervation from a branch of the facial nerve, while tensor tympani is supplied via the medial pterygoid branch of CN V3.
Otitis media is a common pathology observed in paediatric cases. The aetiology may be viral or bacterial. Irrespective of the causative agent, the infection may migrate along the pharyngotympanic tube. Therefore it isn’t uncommon to see patients presenting with both otitis media and pharyngitis or tonsillitis. As was previously mentioned, the position of the pharyngotympanic tube in the paediatric setting is more horizontal than in the adult setting. This angle may facilitate the spread of pathogenic agents to and from nasopharynx, resulting in a greater incidence of otitis media in the paediatric arena.
In the event that the stapedius muscle is paralyzed, patients may experience a decrease in their ability to tolerate sounds of a particular frequency. This condition, known as hyperacusis, arises since stapedius would no longer be able to decrease transmission of vibration from the stapes to the fenestra vestibuli by contracting and removing the footplate from the oval window.