Compartments Of The Ear

Compartments of the ear External ear Muscles of the auricle  Intrinsic: between cartilaginous parts of the auricle.  Extrinsic: anterior, posterior and superior auricular muscles that attach to skull or scalp.  Both groups are innervated by facial nerve. Sensory innervation of the auricle  Superficial surfaces are supplied by: o Great auricular nerve (cervical plexus). o Lesser occipital nerve (cervical plexus). o Auriculotemporal nerve (mandibular).  Deeper surfaces are supplied by: o Branches of facial nerve. o Auricular branch of vagus nerve. External acoustic meatus  ~2.5cm long.  Walls contain cartilage at first, but then bone towards the inner area of the external compartment (temporal bone).  Covered by skin containing: o Hair. o Modified sweat glands that produce cerumen (trap and deter insects). Middle ear   When there is an imbalance of pressure in the middle ear and external ear (two sections adjacent to the tympanic membrane), there will be conformational changes to the tympanic membrane. To relieve this, the air must be equilibrated using the pharyngotympanic tube (e.g. yawn or chew, blow a closed off nose). Tympanic membrane  When the tympanic membrane vibrates, it transmits the vibrations down across the three bones (malleus, incus and stapes), finally to the oval window.  A problem posed is that sound must travel from an air medium to an aqueous medium. o Normally (e.g. speaking to someone with head under water), there is a dulling of the sound. o However, certain properties amplify the sound 20 times between the tympanic membrane and the oval window.  Pressure difference: pressure across oval window is about 17 times that of pressure at tympanic membrane because of its much smaller size.  Ossicular lever action: lever action of malleus, incus and stapes (the ossicles) provides the remainder of the amplification. Acoustic reflex  Actions of two muscles: o Tensor tympani is supplied by the mandibular branch of trigeminal nerve. It pulls malleus medially. o Stapedius is supplied by a branch of facial nerve. It pulls the stapes posteriorly.  Overall, these muscles decrease the amount of sound energy transmitted to the cochlea, serving to protect the inner ear.  Acoustic reflex protects inner ear from loud sounds. o The muscles are triggered by sounds that exceed 85 – 95dB. o Latency 25 – 100ms (i.e. doesn’t work on sudden loud sounds). o Adapts, especially to high frequencies. Inner ear  The bony labyrinth. o Made of the petrous part of the temporal bone, referred to as the otic capsule (differs with surrounding bone because it is much harder).  o Filled with perilymph. The membranous labyrinth. o Enclosed in the bony labyrinth. o Filled with endolymph. Transmission of sound Cochlea  Divided into three fluid filled compartments (scala).  Perilymph. o Scala vestibule. o Scala tympani.  Endolymph o Scala media.    As sound waves travel across the scala vestibuli, the pressure forces the vestibular membrane down. Since the scala media is relatively incompressible, the force is transmitted to the basilar membrane, causing it to move down. The spiral organ (or organ of Corti) on the basilar membrane contains hair cells which detect a change and send impulses to the brain. o Three rows of outer hair cells. o One row of inner hair cells. o The tectorial membrane only has one point of connection (pivot) and so is able to move around with relative ease.