Anatomy And Physiology

Understanding the vestibular system in humans

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The vestibular system is a critical component of the human body. It provides a sense of balance, gives humans an awareness of motion, and performs other critical functions by working with other systems such as the visual system. But what is the vestibular system? How does it work, and what can happen when it does not function properly?

Anatomy of the vestibular system

The vestibular system is located in the inner ear. It is a cavity in the temporal bone of the skull which has several components: two called otolith organs that detect linear movements, and three called semicircular canals, which detect rotations.

The otolith organs are called the utricle and the saccule. The utricle is a cavity lined with a membrane that is parallel to the ground while standing upright. This membrane contains cells called hair cells, which have small, thin projections (similar to hairs) called stereocilia. The bases of these hair cells are connected to the vestibular nerve, which carries information back to the brain, while the stereocilia are directed into the utricle from the membrane. Resting on this membrane, among the stereocilia, are small crystals called otoliths, which are free to move around on the membrane. The cavity of the utricle above the membrane is filled with a viscous fluid called endolymph. The saccule is a similar cavity, except that its membrane is oriented vertically.

The semicircular canals are three ring shaped cavities that are filled with endolymph and the ends of which open into the utricle. Endolymph is able to flow freely out of the utricle, through the semicircular canal, and back into the utricle at the other end of the canal. The semicircular canals are oriented perpendicularly to each other, with one in the horizontal plane similarly to the membrane of the utricle, and two in the vertical plan, with one extending up/down and forward/backward, and the other extending up/down and side to side. At one end of each of the semicircular canals is a membranous bag called the ampulla. Inside of the ampulla are many hair cells like the ones in the utricle and saccule.

Functions of the vestibular system

The otolith organs detect motion in a straight line by the inertia of the otolith crystals. When the body begins to move forward, the head and therefore the utricle also moves forward. The otoliths are free to move in the utricle, so they slide towards the back of the head in the same way that objects can slide off the dashboard of a car when the car accelerates. The movement of the otoliths causes the hairlike projections of the hair cells to bend. When these stereocilia bend, the hair cells themselves begin to change their electrical activity, which sends a signal through the vestibular nerve to the brain, allowing the body to respond appropriately to the motion. The same function is mirrored in the saccule, but in the vertical direction.

The semicircular canals detect rotations around the three different axes that they are centered on. For example, the horizontally oriented semicircular canal detects the rotation that would occur when sitting in a chair and spinning in circles, while the one that is oriented up/down and front/back would detect the rotational motion of a somersault. This detection occurs by the inertia of the endolymph. When a rotation begins, the semicircular canal begins to move with the body while the endolymph lags behind. The difference in movement between the endolymph and the semicircular canals bends the ampulla, which bends hair cells and causes information to be sent along the vestibular nerve to the brain.

An important function of the vestibular system is the perception of gravity, which helps maintain body posture. While standing, the otoliths in the vertically oriented saccule are pulled downwards by gravity, while the otoliths of the horizontal utricle are unaffected by gravity. If the body begins to tilt, the otoliths in the utricle start to roll “downhill,” informing the brain of the loss of posture. With this information, the brain can rectify the loss of posture. Another important function is called the vestibulo-ocular reflex. In order to keep the eyes fixed on an image during head movement, the brain uses input from the semicircular canals to move the eyes opposite to the direction of head rotation. This, combined with visual input, allows the brain to maintain focus on a stationary object while the head is rotating.

Vestibular dysfunction

The vestibular system plays critical roles in the maintenance of gaze and posture, so any dysfunction in the system can have debilitating effects on someone suffering from this kind of problem. When there is damage to the vestibular system in the inner ear due to infection, trauma, or other causes, the input to the brain can become unmatched between sides of the head, with one dominating the other. This kind of damage can cause a persistent signal which informs the brain that the body is spinning in circles, even while standing still. Symptoms that can be caused by this include vertigo and nystagmus.

Vertigo is a sensation of dizziness or lightheadedness when there is no reason for it, such as recently spinning in circles. People suffering from vertigo often begin to feel nauseous and can even vomit. This sensation can make balancing difficult for the person. Vertigo is often accompanied by nystagmus, which is the result of the vestibulo-ocular reflex responding to improper signals from the vestibular system. Nystagmus, unlike vertigo, can easily be observed by looking at the individual’s eyes. Nystagmus is a “repetitive, uncontrolled movement” of the eyes. The continual movement of the eyes can make it difficult to focus on anything in particular, and the combination of this with vertigo can have more serious effects than vertigo alone.

The vestibular system is arranged in such a way that it is able to detect acceleration in the horizontal and vertical planes, as well as any rotations of the head. Nerves connected to the vestibular system carry this information to the brain where it is used to keep the body upright and to keep the eyes focused on images while the head is moving. The importance of this system means that any dysfunction can have very negative consequences, including vertigo, nausea, vomiting, and poor vision due to nystagmus. These conditions can make simple activities very difficult, and should be reported to a medical professional for treatment when they are noticed.

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