The vestibular system is located in the inner ear of humans and other mammals, and is one of our sensory systems; it contributes to our sense of balance, spatial understanding, as well as sensations of speed and inertia. The senses enabled by this system are collectively named equilibrioception.
A rough illustration of the basic structure of the vestibular system can be seen on the right. The anatomical region shown, in combination with various brain areas and muscle groups, work together to provide the function of equilibrioception. This essay aims to explain the functions and abilities of the vestibular system, and relate these back to the anatomical structures, to fully describe the workings of this system.
The semi-circular canals detect rotational movement. Our bodies have 3 degrees of rotational freedom: roll, pitch, and yaw. The canals measure head rotation because they are fluid-filled tubes (the fluid within the labyrinth is called endolymph), and when the head tilts or turns, the liquid inside flows and triggers hair cells which line the inside of the canal, in a sensory epithelial structure called crista, which consists of the hair cell bundles and cupula. The sensory hair cells are in the ampullae of the semi-circular canals, embedded/immersed in a jelly-like substance called cupula, and are surrounded by afferent nerve complexes called ‘calyx’. The endolymph has a low viscosity, because it needs to flow easily, but to avoid over-sensitivity and reactions being too quick, the force of the liquid flow must be able to activate the thicker gelatinous cupula substance in order to stimulate the hair cells. Depending on how many hair cells are stimulated, and for how long, this sends signals of conditional strength to the brain, via vestibulocochlear nerve, giving information on head position. The cupula is also essential because when there is a head rotation, the endolymph initially lags due to inertia, so the cupula is antagonistically deflected in the counter direction to head movement, but as the fluid acceleration adjusts with the rate of rotations of the canals, the cupula can regain resting position, as do the hair cells. This compensatory system also occurs when the head movement ceases, as the endolymph continues to flow, the cupula and hair cells are again deflected to the opposite side. This system allows us to control a steady flow of movement, and maintain an understanding of which direction we are looking.
Because the semi-circular canals work antagonistically, each of them is also functionally paired with mirrored canals in the opposite ear, and their neural pathways also connect to help build a full picture of head position. Once the position/angle of the head is understood by the brain, a mechanism called the ‘righting reflex’ is stimulated, which aims to correct the orientation of the body when it is detects it has taken out of its normal upright position. The vestibular system senses that the head is displaced, so acts to restore this by moving the head back into position, and the rest of the body follows. Through this mechanism, a steady posture is maintained. This reflex is also enabled by the otolith organs, which detect changes in acceleration and gravity, and will be explained next.
The other 3 degrees of freedom our bodies have to move in, is the linear freedoms: X, Y, and Z axes. These translational freedoms are measured and detected by the otolith organs. Specifically, the saccule identifies linear accelerations and head movements on the vertical plane, whereas the utricle measures these on the horizontal plane. These structures work in a similar way to the canals, using the movement of endolymphatic fluid to measure head position. The brain interprets the signals from these organs by comparing the input information from the saccule and utricle, with the visual input from the eyes, allowing discrimination between a tilted head, and movement of the whole body.
However, these mechanisms (detecting the 6 degrees of movement freedom) can occur without any visual cues.
Once the sensory cells of the vestibular system have been stimulated, there are well-developed neural projection pathways which process this information. Key connections are seen from the canals, utricles, and saccules to the vestibular nuclei, which then directs these signals to the cerebellum, cerebral cortex, spinal cord, thalamus, and reticular formation. To respond to the information in the signals, the central nervous system sends descending signals to the ocular motor nuclei and extra-ocular muscles, to ensure the body makes compensatory movements in response to external forces, but also self-induced movements.
This system is evident in the case of the vestibulo-ocular reflex (VOR), which is an automatic reflex of the eye muscles, in response to changing head movements. The function of this reflex is to maintain stable images on the retina. So, if a person is staring at a page and their head tilts right, the eye muscles will innately move left to the same degree, so that the eyes position on the page can remain centre of the visual field. VOR is entirely mechanical, and does not depend on a visual input, and has been shown to occur even with eyes closed, in darkness, or in blind patients. The importance of VOR is seen when this system is damaged in some way: in individuals diagnosed with nystagmus, the eyes make constant, minute, repetitive movements, and do not respond to head rotation, making it extremely difficult to complete tasks such as reading, but also effects balance and depth perception in severe cases.
In conclusion, the vestibular system is an extremely well evolved and finely tuned to its purpose, with many anatomical structures working with push-pull mechanisms to allow compensatory movements in response to stimulus. The sense of equilibrioception is an essential ability to maintain normal activities, as perception of movement, acceleration, balance, and spatial position, are key to completing many basic tasks such as walking and reading. The structures of the inner ear (semi-circular canals, saccule, utricle), as well as the neural projection pathways, work together to ensure that we rapidly right ourselves in response to any bodily movements or external forces. An example of these senses working is, if you stand blind folded in an elevator, you will still be able to sense when the lift begins, and in which direction you are going. This is one of many daily instances of the vestibular system working that often are not consciously noticed, but are essential nonetheless.