Physiology of the Vestibule (The Balancing Part of the Ear)
In our journey to start with the 8th cranial nerve (the Vestibulocochlear Nerve), I started with explaining the Cochlea. I will not be doing any explanation on the anatomy of the inner ear, because I discussed this extensively in the past. You can read about it in The Anatomy, and Physiology of the Ear. In my previous post, I looked into the anatomy of the cochlea, and you can read the post Physiology of the Cochlea (The Anatomy of the Inner Ear) if you missed it. Moving forward, I will be explaining extensively, the Vestibule. Grab your popcorn, and let's have a great ride together.
While explaining the inner ear, I have been making mention of hearing, well, while hearing is one of the functions of the ear, it isn't the only function of the ear. The ear while performing the function of hearing, performs the function of balancing. The balancing aspect of the ear, is known as vestibular sensation. The Vestibular sensation/apparatus has to do with two aspect, the vestibules and the semicircular canal.
The Vestibule of the Ear
The vestibule is the bony structure that connects the cochlea and semicircular canals. It is made up of the Outer Bony Labyrinth which is composed of perilymph (if you remember in my previous post, I explained that the perilymph is rich in sodium concentration but low in potassium concentration), and an inner membrane labyrinth which has structures in it, namely the Saccule and the Utricle which are made up of endolymph (from our previous post, is rich in potassium concentration and low in sodium concentration), and the maculae. In other words, the Saccule and the Utricle makes up the vestibule, and are responsible in detecting changes in equilibrium. The Maculae is on the floor of the utricle, with about 2mm hair cells (stereocilia) pointing upwards, and on the wall of the saccule, is the maculae with its hair cells pointing out.. The Maculae in the Utricle responds to sound in linear acceleration of the head (head movement), in an horizontal plain, as well as displacements of the head. While the Maculae in the Saccule responds to sound in from vertical linear acceleration of the head, and low frequency vibrations coming to the ear.
The Maculae is made up of supporting cells, with the vestibular dark cells above the supporting cells. On top of the vestibular dark cells are the hair cells type 1 and type 2. Above the hair cells are the otolithic membrane which have calcium carbonate crystals (CaCo3) between them known as otoconia/otoliths. These structures are rich in endolymph, and the structure in the Maculae secrete out potassium.
Type 1 hair cells have a bulb shape at the end, towards the basolateral membrane, they have calyx like afferent neurons with larger diameter than type 2, while type 2 have cylindrical base towards the basolateral membrane with a smaller and bulbous like.
In the maculae within the Utricle, when there is the linear acceleration of the head, the otoconia/otoliths which is made up of calcium carbonate crystals (CaCo3) move forward, due to its heavy structure. The Otoconia pulls the gelatinous membrane downwards causing the hair cells to move towards the direction of the otoconia. Remember in my last post, I mention that the hair cells longest hair cell is known as kinocilium.
As the hair cells in the masculae of the Utricle move forward, the smaller hair cells move to n reach the kinocilium. When there is a forward linear movement of the head, the action potentials of in the Masculae becomes high. This usually happens when a person is in a accelerating car and all of a sudden, the accelerating car stops or apply an opposite force, causing the head to push forwards. In an opposite direction of the head where the stereocilia moves away from the kinocilium, such as in cases where a car from rest accelerates and make the head push backwards, there is a decrease in action potential. When the head is a stationary state with no forward or backward linear acceleration, there is no action potential (or constant action potential) in the masculae. , .
The hair cells in the masculae of the Saccule are affected by gravitational forces which pulls the otoconia downwards, when there is an upward vertical movement, in cases such as an elevator going upwards, or the upward movement of a plane, the gravitational force makes the stereocilia of the hair cells to move towards the kinocilium, thereby causing an action potential. In cases of a downward movement in a roller coaster, or a sudden fall in an elevator, the otoconia pulls upwards, taking away the sterocilia from the kinocelium in the otolitic membrane thereby reducing the action potential.,.
Today's post isn't lengthy, so let me do a quick recap. The ear is responsible for hearing but it also serves as a means for balancing. The Cochlea plays a major role in hearing while the vestibular apparatus plays the role of balancing. The vestibular apparatus is made up of two components, the vestibules, and the semicircular canal. The vestible is made up of Otolith organs which are the Utricle and Saccule. They are responsible in detecting changes in equilibrum, which are linear acceleration or deceleration vertically, and linear acceleration and deceleration horizontally.