Inner Ear Anatomy
This page gives you the fundamentals of anatomy. It is important for you to know this before you try to understand vestibular physiology.
The inner ear is a complex three dimensional shape with semicircular canals, dilations called the utricle and saccule and a spiral portion known as the cochlea. All of these organs are housed inside a bony shell known as the bony labyrinth and this is within the temporal bone. The cochlea is the site where sound is transformed into neural energy for hearing. The rest is concerned with balance.
The Right Membranous Labyrinth
The diagram above shows the membranous labyrinth and it is this that contains the neuroepithelia that detect motion and sound. Surrounding the membranous labyrinth is the bony labyrinth.
Despite the complexity of their shapes, the bony and membranous labyrinths can be simplified as in the following diagram.
The yellow central portion in the diagram represents the membranous labyrinth. It contains endolymph and all of the neuroepithelia required for hearing and balance. Surrounding this membranous labyrinth is a fluid filled space. This space separates the membranous labyrinth from the bony labyrinth. The space is filled with perilymph and can be considered to act in the same way as CSF - as a cushion for the delicate structures it protects.
The endolymph is produced by 'Dark Cells' within the membranous labyrinth. The dilated portion in this diagram represents the endolymphatic sac (ES). This is thought to regulate the volume and composition of the endolymph although how this is done is open to speculation.
If the volume, composition or specific gravity of the endolymph is changed, as in Ménière's Syndrome or alcohol consumption, the function of the balance and hearing epithelia within it are affected. Both Ménière's and Alcohol intoxication are explained in other tutorials.
The Right Membranous Labyrinth
The inner ear is supplied by the Superior Vestibular Nerve, the Inferior Vestibular Nerve and the Cochlear Nerve. All of these nerves travel from the inner ear towards the brain stem within the Internal Acoustic Meatus. Along with these in the IAM is the Facial Nerve and the vascular supply.
The Right Membranous Labyrinth - Nerve supply
In this diagram we visualise the inner ear. The cochlea is supplied by the cochlear nerve. The utricle, some of the saccule, the lateral semicircular canal and the superior semicircular canal are all supplied by the superior vestibular nerve.
The posterior canal is supplied via the inferior vestibular nerve. This nerve also supplies part of the saccule.
The Right Membranous Labyrinth - Blood supply
The diagram below outlines the blood supply for the inner ear. The anterior vestibular artery supplies the utricle, superior canal and the lateral canal. The posterior vestibular artery supplies the posterior canal. Both of these are branches of the common cochlear artery which ultimately is derived from the anterior inferior cerebellar artery.
The blood supply is driven from the anterior inferior cerebellar artery, usually. Regardless of its source, it is an end artery system. This means that, if the blood supply is blocked, such as by an embolus, the parts of the inner ear supplied will become ischaemic and non-functional.
The diagram below shows two situations where the blood supply is interrupted. In the first case the symptoms are of cochlear and vestibular disease - sensory deafness and vertigo. In the second, only the supply to the vestibular is interrupted partially. This patient will not have sensory deafness but will experience vertigo.
Knowing this is important because, when a patient presents with sudden sensory deafness, vertigo, or both, we should consider a vascular cause a possibility. Apart from managing the acute symptoms and trying to reverse them, we should counsel the patient about vascular risk factors such as obesity, smoking, raised blood pressure, and hyperlipidaemia. There is some evidence to suggest that patients with sudden sensory deafness, for example, are at a higher risk of stroke, so managing these risks is important.
Internal Acoustic Meatus
The IAM is the bony conduit in the petrous temporal bone through which the vestibular, cochlear and facial nerves leave the posterior fossa. Note that there are two vestibular nerves on each side, a larger superior and a smaller inferior nerve. The diagram shows the right IAM seen from the pons.
The Right Internal Acoustic Meatus
In the diagram above imagine standing at the brainstem and looking outwards towards the Right ear.
S= superior, I= inferior, A= anterior, P= posterior.
VII is the Facial Nerve, C is the Cochlear Nerve,
SV is the Superior Vestibular Nerve, and IV is the Inferior Vestibular Nerve.
The vertical line represents Bill's Bar while the horizontal line is the Crista Falciformis. The Nervus Intermedius travels with the Facial Nerve and is not shown.