(From Dr. Glasser’s Lecture, 11 July 2000, by Brian Buschman)
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The vestibular system is primarily for sensing change in position and very little is for conscious perception. There are two parts to the vestibular system, static and dynamic functions.
Static vestibular function is sensed by the macula which contains the utricle and the saccule which are parts of the membranous labyrinth and detects changes from head tilting and liner acceleration. The sensory organ is made of hair cells that are all lined up in a row with the same orientation relative to a cilium on one end. They also have hair cells that get progressively longer across the top of the cell as they get closer to the side with the cilium.
The hair cells are enclosed in a gelatinous membrane that contains calcium carbonate inclusions called otoliths, hence the thing is called the otolithic membrane. When the head is tilted it causes movement of the otolithic membrane which bends the hair and causes the hair cells to depolarize if the hairs bend towards the cilium or hyperpolarize if they bend away from the cilium. This causes either an increase or decrease in the firing rate of the bipolar cells that innervate the hair cells.
The system is innervated by CN VIII and goes to the lateral vestibular nucleus ipsilaterally. We learned previously the lateral vestibular nucleus functions in the extensor bias of the antigravity reflex. This will cause increased firing on one side while it leads to decreased firing on the other side. This difference is because when you tilt your head it will have opposite effects on both sides.
The system also to the vermin, fastidual nucleus and the F. N. lobe of the cerebellum with reciprocal output back to the lateral vestibular nucleus.
Dynamic vestibular function involves hair cells in the crista ampularis of the semicircular ducts and works to detect angular acceleration. They are also enclosed in a gelatinous membrane but this one is called the cupula. Once again it depolarizes and hyperpolarizes in the same manner as the static system.
The dynamic system also uses CN VIII, the vestibular nerve but it goes to the other three vestibular nucli (medial, superior and inferior). The principle output from these nucli is to the MLF which we need to remember functions to coordinate the movements of the eyes, head and neck.
The semicircular ducts work in combination with one of the ones on the opposite side. The anterior duct works with the posterior duct of the other side and the two horizontal ducts work together. The three ducts are arranged in a 90 degree array to allow for maximal detection in three dimensions (I.e. God knew what he was doing when he put it together.)
Most vestibular study has been done on the horizontal ducts because it works in a horizontal plane. The others require NASA’s multi-access trainer to play with. If you spin someone to the right it will cause one side to hyperpolarize and one to depolarize. The resultant eye movements are called a nystagmus where there is a slow drift opposite the body’s rotation to keep the eyes on a fixed point and then a quick snap to lock onto a new point. The direction of the nystagmus is named after the direction of the snap. A rotation to the right will cause a right nystagmus.
If you suddenly stop the spin the endolymph will move in the opposite direction and result in a post-rotatory nystagmus which is a short nystagmus in the opposite direction. If you spin someone and stop them the post rotatory nystagmus will cause them to fall or past point in the direction opposite to the original spin.
One way to test vestibular function other then spinning someone is to put either warm or cold water in the patient’s ears. If you put cold water in one ear you will get a nystagmus to the opposite side. Warm water will cause the nystagmus to the same side. (COWS = cold opposite, warm same.) Clinically this tests the function of a few cranial nerves in the unconscious patient. It’s also important to know that in the unconscious patient this will cause the opposite effects.
Seasickness is a mismatch between the visual and vestibular systems.
Vertigo is associated with a legion to the CN VIII.
F.N. lobe problems are associated with balance.
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