Any talk of a sixth sense prompts thoughts of the supernatural. Everyone knows we only have five senses, right? Any ten-year old will tell you that they are sight, sound, touch, taste and smell. We can no more have a sixth sense than we can have a fourth physical dimension.
So what would you call balance, if not a sense? A sense of balance is not just something you may need if you fancy a career as a political commentator with the BBC. Where would we be without our sense of balance? Unable to descend a flight of stairs without needing an ambulance at the bottom!
We all know how we experience the five “obvious” senses, even those unfortunate to have lost one or more of them. Few people, apart from those with a reason to know, can even say where their organs of balance are in their bodies.
The answer is that they are right between their ears. In fact, they are within their ears. Most people’s understanding of how their ears work stops at their eardrums. We can all understand that sound waves in the air make our eardrums vibrate. Exactly how that results in our brains accurately deciphering sounds is a mystery best left to the medics. However, digging a little deeper (pardon the expression) we find that the inner workings of our ears incorporate a separate organ that is absolutely nothing to do with hearing.
After a sound wave sets the ear drum vibrating, the vibrations are carried through the middle ear by a linked set of tiny bones. Infections in the middle ear result in partial, and usually reversible, deafness. If we live long enough, the little joints between these bones get a bit stiff – just like the rest of our bodies – and this accounts for our tendency to become a little “Mutt & Jeff” in our dotage.
All being well however, the vibrations reach a little window of membrane into the inner ear, or cochlea. This bony structure is reminiscent of a snail’s shell and contains a fluid which now also starts to vibrate. Tiny hairs lining the inside of the cochlea are disturbed by these vibrations in the fluid. At the base of these hairs are tiny emitters, which send nerve impulses in response to the way the fluid is making them move. These impulses are interpreted as sounds when they are processed by our grey matter.
So where does balance come into the picture? Well the reason the balance organs are part of the inner ear is that they use a very similar mechanism that is just specialised in a different way.
The same fluid that fills the cochlea also fills three bony “semicircular canals” (forming what is sometimes called the vestibule or labyrinth). As our heads move, so these bony structures also move, but the fluid inside moves after a slight lag. There is another slight lag when our heads stop moving before the fluid also stops, and the more violent the movement, the longer the lag. Imagine swirling some water around in a glass.
The canals are also lined with tiny hairs, or cilia, but this time the nerve impulses they create take a different path to a part of our brains that enables them to be perceived in terms of movement rather than sound.
Stop and think for a moment about what is going on within the head of a gymnast on the asymmetric bars, or a high diver executing a double back somersault with a twist. Everything is happening so fast they couldn’t make sense of it by relying simply on the blur that their eyes are showing them. Just like the rest of us, they are relying on their sense of balance. They know exactly how far they are from their equilibrium, something that they need to recover at some point, and something a tightrope walker simply can’t afford to lose.
The balance organs in our ears can’t give us our sense of balance all on their own. We actually use three interlocking frames of reference absolutely automatically, just as we have done from the moment we were born. The first is to do with our bodies: we know how gravity is acting on our bodies, and where the pressure points are that are holding us up – typically the soles of our feet, or the “seat of our pants”. We also know, without thinking about it, how our joints are oriented and how tense or otherwise all of our muscle groups are. Taken altogether, this is called our somatosensory system.
The second frame is provided by sight. The perceived motion of our visual field provides constant feedback about our movements; for example, turning our heads to the left gives us a simultaneous movement of our entire visual field to the right.
The third and final component is provided by the balance organs in our inner ears. These directly sense precise changes in the orientation of our heads, and directly sense momentary accelerations and decelerations. They provide finely tuned information about our movements without reference to our eyes or any other parts of our bodies. Referring to one of the names for the balance canals in our ears, this is called the vestibular system.
All of this happens without us thinking about it, but when these frames of reference are lost, or lose their coherence, the results can be very unpleasant.
Our bodies are very adaptable, and many of the key systems within it are able to cover for one another to some extent. It seems that we can manage – not perfectly, but we get by – with any two of the three balance frames of reference. For example, those who have lost their sight are obliged to make do with just the feelings in their bodies and with their vestibular systems. Similarly, people whose vestibular function is lost, either through disease or accident, can get by even if they may be more than a little unsteady (especially in the dark!).
Adaptation takes time however, and the most unpleasant problems result when one of the three systems is temporarily and repeatedly interrupted or distorted. Because the three inputs are normally combined by our brains, seamlessly and automatically, we have problems making sense of our situation when one of the inputs clashes with the other two. Given time we can adjust, but if the problem is intermittent that isn’t possible and the resulting problems may be disabling.
Astronauts have their very own special form of travel sickness, as their weightlessness makes their somatosensory system disagree with the input from their eyes and their balance canals. Perhaps the worst sensations of vertigo, resulting in severe sickness and visual disturbance, are experienced by those with sudden problems in their vestibular systems – such as those with Menieres Syndrome or labyrinthitis. The distress caused by these illnesses is compounded by the fact that it nearly always involves some degree of hearing loss because of the dual function of the inner ear.
Luckily for most of us, these are not problems we are likely to encounter although overdoing it on alcohol is a common way of becoming unbalanced, literally as well as emotionally. The principal reason for this seems to be that once our blood stream contains enough alcohol, the density of the fluid in our inner ears is altered. Given the sensitivity of these organs, it’s no surprise that they then start to send misleading information to that part of our brains that’s still awake about how our heads are moving. Next time you see someone staggering down the street it may well be that they’ve had “one over the eight”. But spare a thought for those unfortunate enough to lose their “sixth sense” through no fault of their own.