Our inner ear is the powerhouse of our hearing and vestibular (balance) senses. Hearing is part our everyday life: we play the music we love, chat with friends and family, and are aware of changes in our surroundings through sounds. On the other hand, our real “sixth sense”, the vestibular sense, often remains elusive to us, as we are not aware of the extra information processing that our inner ear does in the background.
The word vestibular comes from vestibule, which is the anatomical term given to the central part of the bone enclosing and protecting all inner ear organs. The vestibular system works as a gravity sensor contributing to the coordination of movement and balance. It detects rotational and translational movements caused by our own motion or external forces (think of the feeling of a car lurching forward after pressing the gas or going up or down in an elevator). The brain processes this information from vestibular inputs to determine where and how we move through space in order to, for example, stabilize our vision.
To better understand this connection between vision and the vestibular system, try recording a video with your phone while running. The movie will be very shaky due to constant up and down motion. When we walk or run, our eyes do not have this problem because an involuntary vestibular reflex prevents “shaky” eye movements when our head is bouncing and keeps our visual field stable. We should not assume that vestibular sensing is completely synonymous to what we understand as “keeping our balance”; vestibular inputs are a major contribution to remaining steady and not falling, but there is also integration and processing of information coming from our eyes, muscles and the cerebellum for this task.
Anatomy and Function
The inner ear is a highly refined structure divided into two compartments: the hearing organ (the cochlea) and the balance organs (the semicircular canals and the otolith organs). The cochlea looks like a snail shell (cochlea coming from Greek word for snail), and turns sound into hearing. The semicircular canals and the otolith organs detect rotational and translational movement. The name otolith (otolith is Greek for “ear stones”) refers to calcium carbonate crystals called otoconia present in these organs. The key cells for sensory reception in the hearing and balance organs are the hair cells, which are called so because they have cilia that resemble hair on their surface. These cilia are referred to as the hair cell bundle due to their grouped staircase arrangement, and provide hair cells with mechanical sensitivity. Mechanical forces physically bend the hair cell bundle, opening ion channels that transform the mechanical signal into an electrical one (electrical current). Most ion channels open and close in response to electrical or chemical changes, but these channels are special because they are activated mechanically.
The cochlea transforms a sound (air vibration) into electrical signals. The majority of sounds are a combination of tones with different amplitudes (how loud it is) and frequencies. The cochlea divides the sound into its frequency components and then mechanically stimulates different groups of hair cells based on sound frequency. How far the hair cell bundle bends depends on how loud the sound is. The electrical signals generated in the cochlear hair cells following mechanical stimulation trigger the firing of nerve impulses that carry sound amplitude and frequency information to the brain. You can see the whole process in the following movie:
Head rotations, head tilts, and gravitational acceleration (e.g. accelerating in a translational movement) cause mechanical stimulation of hair cells in the balance organs. The three semicircular canals detect rotational movements. They are filled with fluid, and fluid motion caused by head rotations generates a mechanical force stimulating hair cells. There are three canals in different orientations because three orthogonal planes are the minimum required to discriminate 3-D movements. The otolith organs, which are the utricle and the saccule, detect head tilts and gravitational acceleration. The hair cells of these organs have their bundles embedded into a gel layer with otoconia on top (the calcium carbonate crystals mentioned before).
During a translational movement, the head experiences a linear and gravitational acceleration, creating inertia to shift the gel layer due to the weight of otoconia and therefore mechanically stimulating hair cells. The utricle and the saccule have different anatomical orientations to discriminate horizontal and vertical movements respectively (the feeling of a car lurching forward after pressing the gas or going up or down in an elevator). Again, electrical signals generated in vestibular hair cells following mechanical stimulation trigger firing of nerve impulses that carry all this information to the brain.
The final, important question that needs answer is why we have two ears. Although the sound delay difference between the two is used in the brain to determine the location of a particular sound, the two vestibular inputs carry the exact same information because of symmetry and are compared in the brain for validation.
The highly refined structure and complex sensory system described above is the result of millennia of evolution, with vestibular organs preceding specialized hearing structures. Life began in the oceans, and early animals must have perceived sound (vibrations) in water through bone conduction (transmission of the sound vibration through bone). Their sensing, if any, was limited to expansive low frequency vibrations. Today, bony fish have hair cells under the skin to detect pressure changes in water, and in some cases just a big singular otolith to serve as a hearing bone. Amphibians were the first to venture onto land, but with restrictions as they are born and reproduce in water. This water-air transition required better detection of airborne sounds, and over time parts of the skull and rear bones of the jaw of amphibians developed into a specialized sound-conducting pathway (around 360 million years ago). This was an early version of a tympanic middle ear structure.
Toads and frogs today have a specialized hearing organ called the amphibian papilla. Following evolution, reptiles began to lay eggs on land enabling them to explore and move around without restriction. Consequently, their airborne hearing sensitivity improved, though the diversity of hearing capabilities of reptiles is quite vast. Birds and reptiles (specially Crocodilia) have similar hearing organs due to common ancestors, and in fact, the avian and reptilian basilar papilla is considered homologous to the mammalian cochlea. Interestingly, fish, amphibians, reptiles, and birds have similar vestibular structures compared to mammals, but with the addition of a third otolith organ: the lagena. The lagena seems to supplement the function of the saccule, and in some species, may have some role in hearing too. The inner ear refined structurally a step further after reptilian jaw bones gradually moved and evolved into the inner ear bones of mammals. This resulted in the cochlea, which is a masterpiece for hearing. Humans hear frequencies in a smaller range than some animals, but we are able to distinguish close sound frequencies with higher precision.
Hearing loss and vestibular disorders
All this evolutionary refinement in the mammalian inner ear has come at a price. Do you think chickens go deaf as we do, or that they experience vestibular related dizziness as they age? The truth is that they do not, even when they experience severe trauma. We as mammals lose hair cells as we age and after acoustic trauma to the inner ear; this is why we go deaf as we grow old. Once a hair cell is gone, it is unlikely to regenerate. The loss of hair cells translates into deafness and vestibular problems, such as vertigo and dizziness, that increase the risk of falling. We are all exposed to loud noise at concerts, restaurants, the movies, shopping malls, gyms, and at celebrations that include music. You can turn down the music volume of your head phones and avoid loud noise (using ear plugs for example) to preserve your hearing. Birds and other species can regenerate their hair cells, so even if they experience trauma in hearing or vestibular organs, they grow new cells and recover without permanent injuries. It is not fully understood why mammals have lost the capability of hair cell regeneration. Extensive scientific research is aiming to find a way to induce regeneration since it could become a breakthrough therapy for hearing loss and vestibular disorders.
Written by Vicente Lumbreras, Ph.D. (Postdoctoral Scholar, University of Chicago). Follow Vicente on IG @vicentedarunner.