It's all about the ear!
The human ear is magic. Not only does it allow us to hear beautiful music and screaming babies (!), it also helps to keep us upright. The machinery necessary for equilibrium is housed in the business end or the inner ear. This also mediates hearing, along with the outer ear (the bit we can see, and the auditory canal, up to the eardrum), and the middle ear (the air-filled cavity that houses the auditory bones).
The workings of the ear are a lovely example of mechanics in human biology, and how deceptively simple mechanisms can translate into something truly amazing: I am going to give an overview of how this all happens, but hopefully without getting too anatomical. Firstly, though, I am going to show you a standard anatomical representation of the ear, in all its glory:
The workings of the ear are a lovely example of mechanics in human biology, and how deceptively simple mechanisms can translate into something truly amazing: I am going to give an overview of how this all happens, but hopefully without getting too anatomical. Firstly, though, I am going to show you a standard anatomical representation of the ear, in all its glory:
Now, this is my own highly artistic 'interpretation' of the ear, which includes a brief summary of how we hear sounds (balance will come later):
The main functions of the outer and middle ear are to collect soundwaves and amplify the signal by vibration of the tympanic membrane and ossicles, before passing it on to the inner ear, where things get serious. With regards to hearing, the most important component of the inner ear is the cochlea. This is a coiled tube that looks like a snail’s shell, which is 35mm in length and consists of two and a half turns. It contains two membranes that divide it into three fluid-filled compartments (see the cross-sectional diagram below). Reissner's membrane separates the scala vestibuli from the scala media, while the basilar membrane separates the scala media from the scala tympani. These compartments are adjacent to the auditory ossicles, which pass on the vibrational energy to the fluid within.
The organ of Corti... what's that all about?
Well, this is where the soundwaves that have been propagated by the outer and middle ear are finally converted into electrical impulses that are sent to the brain so that we can hear (without signals being sent to the brain we have no conscious awareness or perception). The organ of Corti contains thousands of tiny hair cells, the receptors for the auditory system (the equivalent of rods and cones in the eye), which are covered by the tectorial membrane.
Depending on the frequency and amplitude of the original soundwaves (remember, these were initially converted into vibrational energy - also with a characteristic frequency and amplitude - by the tympanic membrane and the auditory ossicles, before being transmitted as a waveform within the compartments of the cochlea), movement of the tectorial membrane due to the flow of the cochlear fluid causes the hair cells to also move, but in a very specific way: for example, the axis or direction of movement will determine the type of electrical firing of the hair cells. This is then relayed to the brain by the cochlear nerve and translated to tell us how loud the sound is (coded for by the amplitude of the waves), or its pitch (a function of the frequency of the waves). How wonderful is that?!
Depending on the frequency and amplitude of the original soundwaves (remember, these were initially converted into vibrational energy - also with a characteristic frequency and amplitude - by the tympanic membrane and the auditory ossicles, before being transmitted as a waveform within the compartments of the cochlea), movement of the tectorial membrane due to the flow of the cochlear fluid causes the hair cells to also move, but in a very specific way: for example, the axis or direction of movement will determine the type of electrical firing of the hair cells. This is then relayed to the brain by the cochlear nerve and translated to tell us how loud the sound is (coded for by the amplitude of the waves), or its pitch (a function of the frequency of the waves). How wonderful is that?!
From hearing to equilibrium
There are three semicircular canals in each ear, all perpendicular to each other, so they occupy all three dimensions in space. These canals are also fluid-filled and at one enlarged end, contain hair cells. The tips of the hair cells are embedded in an overlying gelatinous structure called the cupula, while their bases are connected to the vestibular nerve.
The other main components of the equilibrium system are the utricle and saccule. These are also located in the inner ear, close to the semicircular canals, and contain hair cells. As in the semicircular canals, the tips of these hair cells are embedded in an overlying membrane, while their bases are in contact with the vestibular nerve. However, they differ in that the overlying membrane contains little crystals of calcium carbonate (also known as otoliths or, more whimsically, ear dust - I do like that!).
The canals and the otolith organs (i.e., the utricle and saccule) are able to detect changes in the rotation or acceleration of the head, due to the inner movement of fluid that exerts mechanical pressure on the hair cells. In particular, the otolith organs respond to linear acceleration - the saccule to vertical and the utricle to horizontal - while the semicircular canals are responsive to rotational changes. This ear-based equilibrium system largely determines how we orient ourselves in space, although visual cues also play a major role, along with other sensory information from joints and skin receptors, including those that are responsive to touch or pressure.
The other main components of the equilibrium system are the utricle and saccule. These are also located in the inner ear, close to the semicircular canals, and contain hair cells. As in the semicircular canals, the tips of these hair cells are embedded in an overlying membrane, while their bases are in contact with the vestibular nerve. However, they differ in that the overlying membrane contains little crystals of calcium carbonate (also known as otoliths or, more whimsically, ear dust - I do like that!).
The canals and the otolith organs (i.e., the utricle and saccule) are able to detect changes in the rotation or acceleration of the head, due to the inner movement of fluid that exerts mechanical pressure on the hair cells. In particular, the otolith organs respond to linear acceleration - the saccule to vertical and the utricle to horizontal - while the semicircular canals are responsive to rotational changes. This ear-based equilibrium system largely determines how we orient ourselves in space, although visual cues also play a major role, along with other sensory information from joints and skin receptors, including those that are responsive to touch or pressure.
Wrapping up...
I do admire the ear very much, even though it's not the prettiest organ, and isn't really much to look at. I think the teeny-tiny little hair cells are amazing, and I love the nuts and bolts of the process; it's very physical and hands-on, but also quite fragile. I especially admire the flow of energy through the system, how a sound floating in the air can be harnessed to cause the mechanical movement of membranes and bones and fluid inside our heads, and the resulting conversion to electricity and perception and enjoyment of a song - seriously, how clever is the ear?!