S11 Nasal Cavity, Sinuses and Ear - Learning Objectives
1. What are the three divisions of the ear? Describe the boundaries, contents, and functions of each part.
The ear is comprised of three parts: the external, middle, and internal ears. Most of the ear (with the exception of most of the external ear) occupy space within the temporal bone.
External ear: The external ear may be subdivided into the auricle (pinna) and the external acoustic meatus (EAM). The external ear gathers and modifies sounds, and then transmits sounds to the tympanic membrane (eardrum) via the external acoustic meatus. The external ear is largely a cartilaginous sound receptacle, capable of modifying sound by means of its shape, which can be altered by a series of extrinsic and intrinsic muscles controlled by the facial n. The EAM extends approximately 2.5 cm from the concha of the auricle to the tympanic membrane. The proximal third of the EAM is cartilaginous, and the remaining two-thirds is osseous. The tympanic membrane separates the external meatus from the tympanic cavity. Its fibrous basis is attached to the tympanic plate of the temporal bone and is covered laterally by epidermis and medially by the mucous membrane of the middle ear. The larger portion of the membrane is its tense part; the anterosuperior corner, or flaccid part, is bounded by anterior and posterior mallear folds. The membrane can be examined easily in vivo by an otoscope speculum.
Middle ear: The middle ear is mucosa-lined space within the petrous part of the temporal bone. The middle ear conducts sound energy from the tympanic membrane via three ossicles (malleus, incus, & stapes) to the cochlea of the inner ear. The tympanic cavity of the middle ear is conceptualized to have a roof & floor, medial & lateral walls, and anterior & posterior walls. Each wall and surface is associated with particular openings, ridges, prominences, muscle attachments, etc.
Roof (tegmen tympani):
- canal for tensor tympani m.
- atticus - part of the tegmen tympani that attaches to incus & stapes
Floor (jugular wall):
- tympanic canaliculus
- accessory mastoid air cells
Medial wall (superior-to-inferior):
- prominence of lateral semicircular canal (shared with posterior wall)
- facial prominence (shared with posterior wall) - bone outer border for facial canal
- oval window (fenestra vestibuli) - communication to vestibule of inner ear; receives footplate of stapes
- promontory & tympanic plexus
- round window (fenestra rotundum) - communication to cochlea; covered by a secondary tympanic membrane, which allows for cochlear fluid expansion
- entrance (aditus) to mastoid antrum - communication with the mastoid air cells
- prominence of lateral semicircular canal (shared with medial wall)
- facial prominence (shared with medial wall) - bone outer border for facial canal
- pyramidal eminence - contains the stapedius m. & transmits the tendon of stapedius m. anteriorly to the neck of the stapes
- opening for chorda tympani - from facial canal
- tympanic membrane with manubrium of malleus attached - tensor tympani m. attached to manubrium of malleus
- chorda tympani - surrounded by malleus (superolaterally), incus (superomedially), and tensor tympani m. (inferiorly)
- canal for tensor tympani m. - contains tensor tympani m.; tendon transmitted to manubrium of malleus
- opening to pharyngotympanic (auditory) tube - communication with nasopharynx
- also the posterior wall of carotid canal
Inner ear: The inner ear consists of a bony apparatus lined by a series of membrane-lined bony passages which facilitate hearing and balance. The osseous tissue of the bony apparatus is more dense than that of the surrounding petrous portion of the temporal bone.
- Vestibule - The central portion of the bony apparatus is the vestibule, which communicates with the middle ear via the oval window. Contains vestibular sacs (utricle, saccule, etc.), which detect linear acceleration of the head to promote balance
- Cochlea - the anteromedial portion of the bony apparatus; shaped like a snail shell (cochlos = snail in Greek); facilitates hearing
- Semicircular canals (anterior, posterior, lateral) - detect angular acceleration of the head to promote balance
2. How do the external ear and middle ears relate to the temporomandibular joint and nasopharynx?
The external and middle ears are posterior to the TMJ and the nasopharynx.
Two openings transmit nerves between the middle ear and the region of the TMJ: the petrotympanic fissure & tympanic canaliculus. The petrotympanic fissure transmits the chorda tympani (preganglionic parasympathetics of the facial n.) from the middle ear to the infratemporal fossa. The tympanic canaliculus transmits the tympanic n. (preganglionic parasympathetics of the glossopharyngeal n.).
The middle ear communicates with the nasopharynx via the pharyngotympanic (auditory; Eustachian) tube. The pharyngotympanic tube connects from the chamber of the middle ear to the back of the nasopharynx, thus allowing equalization of pressures between these two cavities (as noted in the “popping” sensation we experience in our ears with alterations in altitude in air travel). It also allows for the drainage of materials from the middle ear. In young children (and sometimes in adults), pathogens may ascend the tube to the middle ear (and beyond) to cause otitis media (a middle ear infection). While the middle ear is an air-filled cavity, unlike the open external auditory canal, it is not in direct contact with the atmosphere outside the body.
3. What are the middle ear ossicles? How do the middle ear ossicles function in hearing?
The middle ear ossicles are the three smallest bones of the human skeleton. The ossicles form a chain (each connected by its own synovial joint to the next) of bones that transmit sound via vibration from the tympanic membrane (attached to the malleus) to the oval window (which receives the head of the stapes). Each ossicle has its own distinctive shape:
- hammer-shaped (head, neck, & manubrium)
- attached to tympanic membrane (manubrium)
- attachment for tensor tympani m. (manubrium)
- articulate with incus (head)
- close association with chorda tympani
- largest of the ossicles
- anvil-shaped (body, short limb, long limb, facet for malleus)
- articulates with malleus (facet for malleus)
- articulates with stapes (lenticular process of long limb)
- stirrup-shaped (head, neck, anterior & posterior crura, footplate)
- articulates with incus (head)
- attachment for stapedius m. (neck)
- sits in oval window (footplate)
- smallest of the ossicles
The middle ear ossicles work akin to a step-up electrical transformer: sound energy from the air is transmitted from the tympanic membrane through bone to the liquid contents of the cochlea, thus facilitating impedance coordination among these different media. As each medium (air, membrane, bone, and fluid) has a different innate resistance to conduct sound (acoustic impedance), the ossicles work to bridge the different acoustic impedances of air and cochlear fluid to facilitate hearing.
The middle ear ossicles have a fascinating evolutionary history. Well corroborated hypotheses of homology exist between the ossicles and lower jaw bones of ray-finned fishes and non-mammalian amniotes. The transition of these bones from elements of the jaw to elements of the middle ear is well documented via embryology, paleontology, and phylogenetics.
4. What are the two muscles of the middle ear? How are these muscles innervated?
The muscles of the inner ear that associate with the ossicles includes the tensor tympani m. and the stapedius m. The tensor tympani m. is attached to the manubrium (handle) of the malleus and pulls it medially to make the tympanic membrane more tense, thereby reducing the amplitude of its oscillations that could lead to damage to the inner ear when hearing loud sounds. It is innervated by the medial pterygoid br. of V3. The tendon of the stapedius m. attaches to the stapes at the neck, where it can pull the stapes posteriorly, thereby tilting the base of the stapes in the oval window. This will tighten the anular ligament and reduce the oscillatory range. Stapedius m. is innervated by the nerve to the stapedius of CN VII.
5. Where are the oval & round windows, and how do they function in hearing?
The oval and round windows are two communications on the medial wall of the middle ear with the inner ear. The oval window receives the footplate of the stapes. Vibrations conducted from the ossicles are transmitted to the membranous passages of the cochlear duct. The fluid of the cochlear duct continues the conduction of vibrations, and the round window allows for expansion of the vibrating fluid. Specialized sound receptors (hair cells) transform vibrations into neural action potentials.
6. Describe the pathways (and target ganglia & tissues) of preganglionic parasympathetics through the middle ear.
7. What is the greater petrosal n., and what is its relationship to the middle ear?
The greater petrosal n. carries preganglionic parasympathetic fibers from the geniculate ganglion to the nerve of the pterygoid canal. The greater petrosal n. never enters the middle ear, but it courses superior to the tympanic cavity in the vicinity of the stapedius m.
8. What are the conchae and meatuses of the lateral nasal wall? What spaces communicate via each meatus?
The superior and middle nasal conchae (turbinates) are scroll-shaped components of the ethmoid bone located on the lateral wall of the nasal cavity. The superior nasal concha is the smallest with the middle concha of intermediate size. The inferior nasal concha is the largest and most substantial of the nasal conchae, and are not components of another bone. Occasionally, there are highest (supreme) nasal conchae, within the sphenoethmoidal recess. The nasal conchae are thickly lined with nasal mucosa, and serve several functions: increase surface area of nasal cavity, humidification and filtration of inspired air, and direction of airflow.
There are four meatuses or recesses associated with the nasal conchae, the:
- spheno-ethmoidal recess is located superior to the superior nasal concha,
- superior nasal meatus can be found between the superior and middle nasal conchae,
- middle nasal meatus is located between the middle and inferior nasal conchae,
- inferior nasal meatus can be identified below the inferior nasal concha.
The spheno-ethmoidal recess is located above the superior nasal concha, and the opening of the sphenoidal sinus is located here.
The superior nasal meatus is located between the superior and middle nasal conchae. The openings of the posterior ethmoidal cells are located in the superior nasal meatus.
The middle nasal meatus is located between the middle and inferior nasal conchae. The openings of several sinuses are found in the middle nasal meatus (middle ethmoidal cells, frontal sinus, maxillary sinus, and anterior ethmoidal cells).
Within the middle nasal meatus, there are several important structures. The ethmoidal bulla is a noticeable projection just inferior to the middle nasal concha, caused by the middle ethmoidal cells. Inferior to the bulla is a slit-like opening, the semilunar hiatus. The anterosuperior portion of the semilunar hiatus expands into a funnel-shaped opening, the ethmoidal infundibulum. The frontonasal duct of the frontal sinus typically drains into the ethmoidal infundibulum, while the semilunar hiatus hosts openings for the anterior ethmoidal cells and maxillary sinus.
The inferior nasal meatus can be identified below the inferior nasal concha, and contains the opening of nasolacrimal duct, which conducts tears from the orbit.
9. Describe the innervation and vasculature of the nasal cavity.
Nasal septum (medial wall of nasal cavity):
The nasal septum receives vasculature from three general sources:
- superiorly, from the anterior ethmoidal a. (via anterior septal brs.) and the posterior ethmoidal a.,
- posteriorly from the sphenopalatine a. (via posterior septal brs.), and
- anteriorly from the superior labial a. (via nasal septal brs.) and greater palatine a.
These arteries come from the ophthalmic a. (anterior & posterior ethmoidal aa.), maxillary a. (sphenopalatine & greater palatine aa.), and the facial a. (superior labial a.). The arteries of the nasal septum anastomose on the anterior aspect of the septum, and area known as the Kiesselbach’s area/plexus. A preponderance (>90%) of episodes of epistaxis (‘nose bleed’) originate in Kiesselbach’s area.
The nasal septum is innervated by branches of V1 and V2.
V1 supplies the septum through:
- medial nasal brs. (from internal nasal brs. of the anterior ethmoidal n.), whereas
V2 supplies the septum through:
- nasopalatine n., and the
- posterior superior medial nasal brs.
Most of the nasal septum is innervated by nasopalatine n. A small part of the nasal roof and superior portion of the septum is innervated by the posterior superior medial nasal brs. The anterior portion of the nasal septum is innervated by medial nasal brs. (of internal nasal brs. of the anterior ethmoidal n.).
One of the more obvious contributions of neurovasculature to the nasal septum is the collection of olfactory nn. (CN I). The olfactory nn. can be located descending inferiorly from the cribriform foramina of the ethmoid bone.
Lateral wall of nasal cavity:
The lateral nasal wall is served by branches of the same arteries which serve the nasal septum, namely the:
- anterior ethmoidal (via anterior lateral nasal brs.) & posterior ethmoidal aa.,
- sphenopalatine a. (via posterior lateral nasal aa.), and the
- facial a. (via lateral nasal br.).
The lateral nasal wall is innervated by branches of V1 and V2 that also serve the nasal septum.
V1 supplies the lateral wall through:
- lateral nasal brs. (from internal nasal brs. of the anterior ethmoidal n.), whereas
V2 supplies the lateral wall through:
- posterior superior lateral nasal brs.,
- posterior inferior lateral nasal brs. (from greater palatine n.),
- nasal brs. of the anterior superior alveolar n., and
- internal nasal br. of the infra-orbital n.
The mid- to posterior aspects of the lateral nasal wall are innervated by posterior superior lateral nasal brs. and posterior inferior lateral nasal brs. (from greater palatine n.). Posterior superior lateral nasal brs. innervate the superior and middle nasal meatuses, whereas the posterior inferior lateral nasal brs. innervate the middle and inferior nasal meatuses. The anterior aspect of the lateral wall is innervated superiorly by lateral nasal brs. (from internal nasal brs. of the anterior ethmoidal n.) and inferiorly by nasal brs. (of the anterior superior alveolar n.) and internal nasal brs. (of the infra-orbital n.).