5.1: Anatomy of the ear Flashcards
Which structures comprise the external ear?
Pinna (auricle)
External auditory meatus
Tympanic membrane
Which structures comprise the middle ear and where is it? Name one other name for the middle ear
Middle ear/Tympanic cavity: In the petrous temporal bone and includes the..
Tympanic membrane 3 auditory ossicles Epitympanic recess (space superior to the tympanic membrane) Muscles Mastoid air cells Eustachian tube
Which bone is the inner ear located in? Describe its two part structure and its contents (plus their individual functions)
In petrous temporal bone
- Bony labyrinth:
- Cochlea: Hearing
- Vestibular apparatus (vestibule + semicircular canals): Balance
- filled with perilymph - Membranous labyrinth: lies within the bony labyrinth and consists of the cochlear and vestibular labyrinths, and is filled with endolymph
- Cochlear labyrinth: has the cochlear duct (which contains the spiral organ of corti)
-Vestibular labyrinth: Vestibular utricle and saccule (which communicates with the semicircular canals/ducts)
Which bone of the skull is most involved in the ear’s structure?
The temporal bone
What are the three auditory ossicles and where are they?
Which pharyngeal arches do they arise from and what is their general function?
Lie in the upper part of the tympanic cavity
Malleus and Incus: first arch
Stapes: second arch
Amplify, concentrate and conduct sound waves/vibrations from the tympanic membrane to the inner (sensory) part of the ear
Where embryologically does the pinna arise from?
Six hillocks of His (from pharyngeal arches 1 and 2)
What is the pinna composed of and what is its function?
Cartilage and skin, collects and amplifies sound
What reflex response may occur when clearing the ears with a cotton-wool bud and why?
Since the nerve supply to part of the ear canal and posterior part of tympanic membrane is via the auricular branch of CN X, irritation of this branch is referred along its other branches and may trigger a gag or cough reflex
Which nerves supply the sensory innervation to the external ear? Identify the two major ones
CN X and Facial N are two major ones
Pinna:
Auriculotemporal branch off of V3 - area anterior to the external acoustic meatus
Greater auricular nerve C2 and C3 - posterior pinna and lower 1/2 of anterior pinna
Small area of concha (close to opening of EAM): Auricular branch of CN X
Walls of ear canal: Auricular branch and auriculotemporal branch, facial nerve
*and Lesser occipital nerve C2 apparently?
Describe the structure of the external auditory canal
Outer 1/3 cartilage, Inner 2/3 bone (tympanic plate). Lined with skin and hairs and runs obliquely
What is cerumen and what is responsible for its production?
Cerumen (earwax/modified sebum) is the combination of secretions from the ceruminous glands in the outer 1/3 of the ear canal (cartilaginous portion) + dead skin cells
Describe the structure, shape and appearance of the tympanic membrane
Structure: has 3 layers
- Outer: stratified squamous epithelium
- Middle: Fibrous
- Inner: mucous membrane
Concave laterally (shallow cone with its apex pointing medially) and lies obliquely. It is translucent and allows for visualization of some structures in the middle ear (notably the malleus)
Describe how skin cells migrate out of the ear
Skin cells migrate from the middle of the TM and outwards through the ear canal
What is the sensory nerve supply to the outer and inner surfaces of the tympanic membrane?
Outer: auriculotemporal (branch of V3) + some from auricular branch of VAGUS
Inside: glossopharyngeal IX
Describe how vibrations from the eardrum are transmitted to the inner ear
*hint: includes details of the structures which transmit these vibrations
Vibrations pass from the eardrum/tympanic membrane to the attached malleus, which articulates with the incus through the incudomalleolar joint. The incus articulates with the stapes via the incudostapedial joint. From here, vibrations pass to the base of the stapes which attaches to the oval window and into the bony labyrinth of the internal ear
What is the oval window?
Membrane covered opening from the middle ear to the cochlea of the inner ear
What is the function of the middle ear? What would happen if you didn’t have one?
Converts sound waves in air from the external auditory meatus to vibrations in fluid of the inner ear
You would lose ~30 dB because..
Most sound waves (~30dB) which hit the inner ear fluid are reflected (and not transmitted to the inner ear), but the middle ear gains this 30 dB back by using its
a) relative size of tympanic membrane and oval window is 14:1
b) Lever action of the ossicles (x1.3)
What are the muscles of the middle ear and what is their function? Which ossicles do they attach to and which nerves innervate them?
They reduce the vibrations of the ossicles they attach with to prevent damage to the eardrum and oval window respectively
1. Tensor tympani; attaches to malleus, innervated by tensor tympani branch off of V3
- Stapedius; attaches to stapes, innervated by VII
Describe the structure and location of the eustachian tube, what does it communicate with posteriorly?
Extends from the anterior wall of the tympanic cavity -> passes down, forwards and medially to the nasopharynx.
Anterior 2/3 cartilage
Posterior 1/3 bony
Posteriorly communicates with the mastoid antrum
What is the function of the eustachian tube? How does it open?
It’s usually closed, but opens intermittently by the pull of attached palate muscles when swallowing, yawning, chewing or valsalva
When open it equalizes the pressure between the middle ear and the nasopharynx (to prevent sucking in of middle ear)
What does the communication between the middle ear and the mastoid air cells allow for?
Equalization of middle ear pressure with the pressure in the external auditory canal, as mastoid air cells act as a ‘buffer system’ of air – releasing air into the tympanic cavity/middle ear when the pressure is too low
This is necessary for efficient transfer of sound energy to the internal ear
What is the function of the inner ear?
- Converts mechanical signals (vibrations in fluid) to electrical signals -> auditory pathway -> brain
- Maintains balance