The function of the external ear is to gather and amplify sound and to transmit this to the middle ear. It consists of the:
- Pinna (auricle)- the cartilagenous irregular structure that is visible from the skin surface. The irregular structure introduces sound delays that help to localise sound.
- Tragus- the little flap of cartilage that protects the external meatus anteriorly
- Concha– the ring of cartilage that surrounds the external meatus (the inner cartilagenous structure within the pinna.
Vascular supply- Posterior auricular artery (direct branch of the carotid), the anterior auricular branch of the superficial temporal artery and the occipital artery.
Nerve supply- the great auricular nerve (C2-3) (lower 2/3rds); the auriculotemporal nerve (CN V3) (superior 1/3rd- anteriorly); lesser occipital nerve (C2) (superior 1/3rd- posteriorly); Auricular branch of vagus (CN X- Arnold nerve) (external auditory canal floor and concha).
- External acoustic meatus (external auditory canal)– around 25mm in length; cartilagenous for outer third (where it is also lined with hair and wax glands), bone for inner 2/3rds; there is an antero-inferior recess medially which has the propensity for collecting wax and other debris within it (risk of deafness/infection)
Nerve supply– the auriculotemporal nerve (CN V3) (sensation to anterior wall and roof); auricular branch of CN X (sensation to posterior wall and floor)
Arterial supply– posterior auricular artery, deep auricular branch of maxillary artery; superficial temporal artery.
The function of the middle ear is bony conduction of sound. It is located in the petrous part of temporal bone and is filled with air normally. It begins at the membrane and ends at the oval window and contains several important structures:
- Tympanic membrane– oval window of a tri-layered membrane (skin, fibrous tissue and mucosal membrane) separating the external and middle ear. The manubrium of the malleus can be seen attached to the membrane medially, creating a concave shape (umbo) that can be seen as a light reflex on examination. This connection transmits sound from air to bone conduction.
- pars flaccida is the area of the membrane above the umbo
- pars tensa is the area below
Nervous supply– Auriculotemporal nerve (CN V3); auricular branch of CN X (Arnold); tympanic branch of CN IX.
Blood supply– Stylomastoid branch of the posterior auricular; deep auricular; anterior tympanic branch of maxillary artery.
- Tympanic cavity– covered in mucoperiosteum (mucosal membrane) and filled with air.
- Ossicles (Malleus, Incus, Stapes)- transmit and amplify (10x) sound waves from air to perilymph.
- Auditory (Eustachian) Tube– Allows communication between the middle ear and nasopharynx to equalise pressure across the tympanic membrane. Contraction of the tensor veli palatini and salpingopharyngeus (on swallowing) can open and dilate the tube.
- Stapedius muscle (neck of stapes- posterior region of the middle ear)- CN VII innervation- prevents loud noises from damaging the ear
- Tensor tympani (manubrium of malleus- auditory tube)- mandibular branch CN V3- also dampens sound by tensing the tympanic membrane.
Primary functions are to conduct sound from the middle ear to the CNS (bony conduction-sensorineural conduction) and to assist with balance. Consists of an osseus labyrinth (bony cage- filled with perilymph) in which the membranous labyrinth is situated (the inner ear organs filled with endolymph).
- The Oval window– the opening in the lateral wall of the osseus labyrinth that is covered by a membrane- to which the stapes is attached and vibrates against. This in turn, moves the fluid within the vestibular apparatus.
- Vestibule– contains the saccule and the utricle that detect movement of endolymp in the vertical and horizontal planes, respectively, by the movement of hair cells that line these structures, detected by the superior division of the vestibular nerve. (primary organ of directional sensation)
- Semicircular/membranous canals– 3 bony semicircular canals, each at 90° to the others, connecting to and sitting behind and superior to the vestibule.
- Superior canal- vertically positioned (attaches to upper vestibule and upper post canal)
- Lateral canal- positioned horizontally (attaches to upper/lat vestibule and upper post vestibule)
- Posterior canal- (attaches to lower vestibule and crus commune)
Important in balance.
- Cochlea– organ of hearing- snail shell shape
- Round window– NOT THE SAME AS OVAL WINDOW- the round window compensates for the change in pressures of the fluid filled inner ear and communicates from the cochlea to the middle ear in the scala tympani (inferior chamber of the cochlea- in contrast to the oval window that transmits stapes vibrations to the cochlea).
The nervous intermedius-
The nervus intermedius must be considered as part of the facial nerve (VII), but it carries taste fibres and preganglionic, parasympathetic motor fibres. Together, VII and nervus intermedius pass through the inner ear to reach the medial wall of the middle ear. At this point, where they turn at right-angles backward, is the geniculate ganglion which is the cluster of cell bodies for the primary, taste-sensory neurones. The greater petrosal nerve (parasympathetic, and some taste) leaves the ganglion to pass through the petrous temporal bone and eventually reach the pterygopalatine ganglion to synapse and then be distributed with branches of the maxillary nerve.
The facial nerve and remainder of the nervus intermedius pass posteriorly in the middle ear to reach the posterior wall and turn inferiorly. The nervus intermedius sends its taste and parasympathetic fibres into the chorda tympani that emerges from the ear to run with the lingual nerve. The facial nerve supplies stapedius and then emerges from the stylomastoid foramen to be distributed to the muscles of facial expression.