The Balance System




The sensation of balance is complicated.  Humans need a sophisticated balance mechanism.  As we walk on two legs with only a narrow base and a height that is many times that base width (just think of London buses), then it is essential that our brain knows in effect where we are in relation to the ground at all times. 


When we move our head and eyes, the world in effect remains as it is when our head is still.  If for some odd reason, we choose to stand on our hands or head, the world still remains the right way up.  Why is this?  If we spin round and round in circles, then we all know from childhood games that we will emerge from such play with feelings of dizziness and a tendency to fall over.  What we do under these circumstances is overwhelm the balance mechanism temporarily leading to our staggering or falling.  Some of us have better balance mechanisms than others.  This is in part genetic or it may come about by training.  Ballet dancers, skiers and mountain climbers for instance have particularly well-trained balance mechanisms that allow them to perform feats that most normal people are unable to achieve.


The Structures Involved


Our sensation of balance requires the integration of three different systems.  The first, and probably most important, is the so-called vestibular apparatus.  The second, our eyes and their ability to move.  The third comprises the position sense receptors of the whole of our axial skeleton but also position sense receptors in our pelvis, hip, knee, ankle and foot joints.


Neural signals arise from each of these “systems”.  The signals arrive in the brain stem by passage along the peripheral nerves and then the spinal cord. Within the back part of the brain or the brain stem, these signals merge sending what is in effect a corporate message into the brain which allows us to recognise that we are sitting, standing, lying, moving, twisting, bending or any other movement that we care to challenge our biped state.


Most of the discussion will now focus on the vestibular apparatus although there will be some discussion about eye movements as well.



The Vestibular Apparatus


The vestibular apparatus consists of five sensory organs within a structure called the labyrinth.  The labyrinth is a system of cavities or semi-circular canals sitting within the petrous temporal bone of the skull.  These cavities contain the sensors for both the auditory, that is hearing, and vestibular (balance) systems.


There are two otoliths as well as three semi-circular canals on either side of the head. 


The otoliths have two functions.  They are able to sense the head’s linear motion.  What this means is the movement of the head forward or to the side.  They are also able to sense the position of the head in relation to gravity.  In simple terms, these organs are able to tell us which is up and which is down.  If we did not have otoliths, we would be in serious difficulty knowing what to do when we try to bend down or stand up as we would not know for certain where gravity was acting.


The semi-circular canals are able to sense angular acceleration due to head rotation and hence it is vital that the otoliths and the semi-circular canals work closely together in order to maintain our equilibrium.



The Otoliths


The otoliths are spherical.  They are called the utricle and the saccule.  Part of these structures is covered with hair cells which project into a jelly-like substance.  Within this jelly, there are calcium carbonate crystals that allow for movement of the hair cells. 


In simple terms, motion is picked up by the otoliths.  The hair cells are bent by the calcium crystals and these cause chemical changes in the neural fibres that go to the hair cells which causes a nerve impulse to be generated in the 8th cranial nerve that transmits these impulses into the central nervous system.

The Semi-Circular Canals


We have three semi-circular fluid-filled canals on each side of the head.  The three canals are nearly at 90º to each other.  One canal is horizontal and the other two are known as the anterior and posterior canals.  At the end of each canal, there is a swelling called the cupula.  The cupula once again contains hair cells. 


With the head movement, the fluid in the semi-circular canals will lag behind that motion because of the inertia in any fluids.  This will cause a distortion within the cupula which in turn will cause the hair cells to bend in one direction or the other.  This will then generate neural impulses in the vestibular division of the 8th cranial nerve.


As each canal sits at 90º to the other, then there will be more motion in one canal compared to the others on each side according to the rotation of the head.  As often happens in the body, the canals are arranged in pairs so that each canal on one side of the head has a twin canal on the other side.  When one of the canals is activated, then the canal that is its partner on the other side is maximally inhibited.  Although not so important in this discussion, for completeness I would mention that the anterior canal of one side has the posterior canal on the other side as its partner canal.



Eye Movements


The movement of the eyes is very important in the balance mechanism. The nerves going to the eye muscles are activated by structures known as the eye nerve nuclei within the brain stem. These nuclei are very closely opposed to the balance mechanism nuclei within the brain stem.  There are a lot of connections and pathways between the eye muscle nuclei and the balance mechanism.  As is explained later, when our balance mechanism does not work properly, we become much more reliant on so-called visual cues that enable us to continue with a reasonable sense of balance unless challenged by darkness or rapid head movements when visual cues are diminished or removed.  This then exposes the balance mechanism to a much greater challenge when individuals often fall over. 


Older people who have visual disturbance will have much greater difficulty with their balance mechanism.  This and a general slowing of neural responses with age and wear and tear on the balance mechanism itself makes it much more likely that older people will fall.  When this is added to a whole range of reduced autonomic (automatic) neural deficiency and problems with arthritis and reduced position sense responsiveness in the axial skeleton, it is actually quite surprising that all of us do not keep falling down all the time as a consequence of the ageing process.



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