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
What are the two openings from the inner to the middle ear and which structures do they connect?
- Oval window: Connects the vestibule to the middle ear at the stapes footplate
- Round Window: inferior to the oval window, this opening connects scala tympani (lower bony passage of the cochlea) with the middle ear
Describe how vibrations from the stapes travel from the inner ear to the brain
Vibrations travel from the stapes at the oval window to the perilymph in the scala vestibuli (upper bony passage of the cochlea)
Vibrations are then transmitted to the endolymph in the membranous labyrinth within the cochlear duct and spiral organ of corti on the basilar membrane
Hair cells on the basilar membrane are stimulated and signal impulses along CN VIII to the brain
Describe the structure of the vestibular apparatus
3 semicircular canals (at right angles to each other) that contain semicircular ducts containing balance receptors (respond to rotational acceleration in 3 planes!). They form a swelling on the end of each canal end known as an ampulla.
The ampulla connects with the utricle which connects with a saccule
What stimulates the inside of the ampulla (SCC)? What is another term for the ampulla?
Ampulla/Cupula is connected to the SCC, and is stimulated by the starting/stopping of rotatory movements; as this causes movement of the endolymph which bends the hair processes and stimulates the hair cells
What stimulates the inside of the utricle and saccule (vestibule)?
The utricle and saccule both contain a macula. The macula of the utricle is stimulated by changes in the position of the head (i.e tilting); as the weight of otoliths (calcium carbonate crystals) in gelatinous mass bend the hair processes
Which nerve(s) carry impulses from the inside of the ampulla and utricle/saccule to the brain?
Which structure do these fibres pass through to REACH the inner ear and which other nerve accompanies part of their course?
Vestibular branch of CN VIII
These fibres reach the inner ear by passing through the internal acoustic meatus and are accompanied by the facial nerve for part of this course
The cochlea is shaped like a ___ and contains the ____. The spiral canal begins at the ___ and makes 2.5 turns around a central bony core called the ____.
The cochlea is shaped like a shell and contains the cochlear duct. The spiral canal begins at the vestibule and makes 2.5 turns around a central bony core called the modiolus.
How do the cochlea and the vestibule communicate with the brain (and which parts of the brain do they communicate with)?
Cochlea: The bony labyrinth at the base of the cochlea communicates with the subarachnoid space via the cochlear aqueduct
Vestibule: communicates with the posterior cranial fossa via the cochlear aqueduct
What structure is found on the vestibule lateral wall? What is the vestibule continuous with anteriorly and posteriorly?
The oval window is found on its lateral wall, with the stapes attached to the lateral aspect of this membrane
Vestibule is continuous with the cochlea anteriorly, the SCC posteriorly
The external acoustic meatus consists mostly of which specific bone? Which structure does it fuse with medially?
Tympanic plate (1/4 component of the temporal bone). Medially, it fuses with the petrous part of the temporal bone
What lies below the irregular inferior surface of the external acoustic meatus?
The carotid canal for the ICA
Where are the facial and vestibulocochlear nerves in relation to the external acoustic meatus?
The inner part of the EAM which forms the front of the posterior cranial fossa is pierced by the internal acoustic meatus; transmitting the facial and vestibulocochlear nerves
How might disease of the middle ear easily spread to cause mastoiditis?
Through the cavity of the mastoid antrum
Describe how the structure of the auricle helps its function
Its elastic cartilages are arranged in a number of curved ridges; including the outer rim (helix) and a small flap (the tragus) which guard the EAM
How might the middle ear be liable to a respiratory tract disease?
As it communicates anteriorly with the nasopharynx via the eustachian tube, which is part of the upper resp tract
Which nerve might a middle ear infection cause a lesion to and why?
The facial nerve, as it lies in the facial canal and is separated from the middle ear cavity by a very thin bony partition
What substance is responsible for separating the bony and membranous labyrinth of the inner ear?
Perilymph
What kind of epithelium lines the external surface of the eardrum?
Stratified squamous
Which structure is separated by the tympanic cavity by a thin, bony floor?
Superior bulb of the Internal jugular vein
What embryologically does the middle ear, eustachian tube derive from, external acoustic meatus and tympanic membrane derive from?
1st pharyngeal arch invaginates outwards and becomes the eustachian tube and middle ear
1st pharyngeal cleft (outside) invaginates inwards and becomes the EAM and TM
Which part of the tympanic membrane is most likely to retract and get sucked into the tympanic cavity?
The Pars flaccida 1 (top thin portion)
Which part of the brain and skull lies superior to the middle ear?
Temporal lobe, middle fossa
What does the anterior wall of the middle ear separate the cavity from and which structures are in communication with the middle ear via this wall?
Separates the cavity from the ICA and contains two openings for the Eustachian tube and tensor tympani muscle
What represents the bulging of the inner ear into the middle ear?
The promontory (has a plexus with glossopharyngeal n)
What are the three branches of the facial nerve given off in the middle ear? List them in order and the structures they innervate
- Chorda tympani: special sensory anterior 2/3 tongue, visceral motor to salivary glands
- N to Stapedius: stapedius muscle of middle ear
- Greater Petrosal nerve: mucous (retropharyngeal, nasal and palatine) and lacrimal glands
What lies on the lateral and medial wall of the middle ear?
Lateral wall: tympanic membrane and bony portion of the epitympanic recess
Medial wall: bony wall of the inner ear!
What do the inferior and superior portions of the posterior wall of the middle ear communicate with?
Superior portion: Aditus to mastoid antrum
Inferior portion: separates cavity from mastoid air cells
What is another name for the ‘bony roof’ of the tympanic cavity and which cavity does it separate it from?
Bony roof = Tegmen Tympani
Separates the tympanic membrane from the cranial cavity
Label the following structure
GW labelled image
Need pro
What is the mastoid antrum and what is its important posterior relation?
It’s a prolongation of the cavity of the middle ear within the petrous portion of the temporal bone. It communicates posteriorly with the mastoid air cells. Behind the mastoid antrum and air cells is the sigmoid venous sinus (a dural sinus) and the cerebellum.
Therefore, infections in the region of the mastoid may spread to the brain
What does the Mastoid antrum communicate with anteriorly?
The epitympanic recess of the middle ear
Which ganglion do the greater petrosal and chorda tympani nerves contain preganglionic parasympathetic fibres for?
Greater petrosal: Sphenopalatine ganglion
Chorda tympani: submandibular ganglion
Why might sucking a sweet during take-off and landing of a plane relieve ear discomfort experienced during air travel?
Swallowing opens the eustachian tube; allowing equalisation of pressure in the middle ear and nasopharynx. Since pressure changes are quite noticeable during take-off and landing, the decreased air pressure inside the middle ear will cause discomfort unless the eustachian tube is open!
