An Introduction to balance
This tutorial will prepare you for a study of common ear-­‐related balance disorders. It will show some of the anatomy and physiology of the part of inner ear that detects our movements and helps us stand and move without falling over. It is an introductory tutorial and it sets out to give you a basic understanding only. When you understand the concepts within it I will write a second tutorial that explores things more deeply. Finally when you have learned and understood the second tutorial I will send a third tutorial on balance disease. You will find that it is much easier to understand balance disorders such as Benign Paroxysmal Positional Vertigo, Vestibular Neuritis and Ménière’s disease when you understand the underlying anatomy and physiology of balance. SOME TERMINOLOGY Discussions on balance use words that may be unfamiliar to you. Here is a short list of the words that will be used in this tutorial together with an explanation of their meaning. You should read and learn this list before you continue with the tutorial. Input Information from our senses that is travelling to the brain. Examples of inputs are: vision, pressure sensation, joint position sensation and vestibular sensation from the inner ear. Integration This is a process that happens within the brain that compares all of the sensory information that is coming into the brain. These calculations help the brain understand what is happening to the body and help it plan a response. Neuroepithelium A specialized epithelium that has hair cells that are in contact with nerve fibers. The hair cells detect movement and make the nerve fire. Output When we think of balance an output is a motor command. The brain tells a muscle to contract or relax and this enables the brain to keep control of balance. Proprioception All of your joints have nerve sensors in them that tell the brain about what the joint is doing. They measure the stretch of the joint so that the brain knows how far the joint has moved and what direction it has moved in. Reflex An action that happens without conscious control. They are fast and automatic because they don’t involve the higher (thinking / conscious) areas of the brain. A good example of a very simple reflex is the knee jerk reflex. Rotation A turning movement. Nodding up and down or shaking the head from side to side cause rotation of the head. Somatosensation This word is used to describe a number of different sensations such as pressure, pain, and heat. Nerve endings in your skin detect these and send the information to the brain for analysis. There are also nerve endings in muscles, joints, bones and internal organs but in this tutorial we are only really thinking about pressure sensors in skin and muscles. Translation A movement in a straight line such as forwards or backwards, from side to side or up and down. There is no rotation only a straight movement like being in a car on a straight road. Vestibular This is usually used to mean something to do with the balance organ. E.g. vestibular hair cells are cells within the balance organ; the vestibular nerve is the nerve that sends information from the balance organ to the brain. Vestibular The part of the labyrinth that specializes in sensing rotation and apparatus translational (forwards/ backwards, side to side, and up and down) movements. Vestibular This is a small area in the brainstem. Sensory information from nucleus the eyes, ears, skin and joints come to this nucleus. It is important to remember that although it is called the vestibular nucleus it gets inputs from a lot of other places as well. BALANCE IN GENERAL There are two main functions of the balance system: firstly it allows us to resist gravity, stand up and move without falling over; secondly it helps to keep our vision clear when we are moving. The system works by having sensors that measure our movement. These send information into the brain where calculations are made about what the movement was, how far, how fast and in what direction it was. The brain uses these calculations to build a picture of the body in space. It also uses the calculations to make corrections in the body’s position so that it remains stable and doesn’t fall over. Once the brain has made its calculations it sends out a command to the muscles. In the terminology of physiology there is an input (sensation), integration (brain calculations) and an output (muscular activity). This diagram shows a map of the balance system. The inputs are from the balance organ (vestibular), the eyes and from sensors that tell us what are joints are doing (proprioception) and where we feel pressure on our skin (somatosensation). The part of the brain that compares all this information is the vestibular nucleus although there are other areas as well. The output is a motor command that tells a muscle to contract or relax. Think of a woman waiting to cross a road. It is a windy day and there is a sudden gust of wind that comes from her right side. The wind pushes her towards the left side. If she did not have a balance system she would fall over onto her left side. However, this does not happen to the woman because her balance system first detects the movement of her body to the left and then makes a set of movements that prevent the fall. This happens very quickly and without the woman having to think about it too much. These quick actions that happen without us being conscious of them are called reflexes. The vestibular apparatus also gives us the ability to keep our eyes fixed on an object while we are moving. Do the following experiment. Look at the X. Keep your eyes fixed on it and then turn your head to the left. The X stays in focus and your eyes move to the right. This simple experiment shows you that you can look at the X and keep it in focus while moving your head. We will look at how this happens in the next tutorial but for now just remember that your vestibular apparatus allows you to keep things in focus while your head moves around. This reflex is called the vestibulo-­‐ocular reflex or VOR. It permits such everyday actions as reading, walking and seeing clearly, and running towards a football while keeping your eyes on it. ANATOMY 1 – THE BIG PICTURE The inner ear is describes as being like a labyrinth. This is a good term because it consists of narrow passages connected to each other. X On the left is a picture of Wakehurst Labyrinth, which is near London. It is a path that winds this way and that and eventually leads to the centre. On the left is a picture of a human membranous labyrinth. It has many twists and turns in it. The shape of the membranous labyrinth is complicated but it can be simplified. Just think of it as having two balloon-­‐like portions (the utricle and saccule) and three tubular structures (the semicircular canals). Of course these are all connected to each other in a complicated way but the beautiful complexity of the organ is not important to us now. Here is a simplified diagram for you to study. This is a line drawing of the membranous labyrinth of the inner ear. It is called a labyrinth because of its winding character. Sac – saccule, Ut – utricle, C – cochlea, ES – endolymphatic sac, L – lateral semicircular canal, P – posterior semicircular canal, S – superior semicircular canal. The membranous labyrinth contains the cells that detect movement and sound. This tutorial will not talk further about sound detection but will concentrate on balance. However, it is important to remember that the areas that detect sound are joined to the areas that measure movement in the membranous labyrinth. They have the same fluids in them and the same blood supply. This means that some diseases that cause balance problems also cause hearing problems and tinnitus. The fluid in the membranous labyrinth is called endolymph. This fluid is very important for the normal function of the cells inside the membranous labyrinth and diseases that affect endolymph can cause problems with balance and hearing for example Ménière’s disease. ANATOMY 2 – THE HAIR CELL We have seen what the anatomy of the membranous labyrinth looks like. Now we need to study the cells inside the labyrinth that change movement into nerve impulses. The cells are called hair cells because they have a small group of hair-­‐like structures sticking out of the top. This tutorial does not explain how the hair cell works but it will describe the microscopic anatomy of the cell. At the top of the hair cell there is a set of small hair like structures that are called cilia. They are arranged in a way so that they start off short and then get longer and longer. The longest cilium is called the kinocilium. All of these hairs are surrounded by endolymph. They are also stuck into a layer of gel. We will look at this a little later. At the base of the hair cell is a nerve fiber. This is a fiber that will join the vestibular nerve and travel to the vestibular nucleus in the brain stem. Any movement of the cilia at the top of the hair cell will cause information to be sent to the brain via this nerve. A human hair cell. The hair cells are grouped together into small patches called neuroepithelia. The neuroepithelia detect two types of movement: rotational movements and translational movements. The neuroepithelium that detects rotational movement is called the crista and it is inside the ampulla at the end of each semicircular canal. Translational movements are detected by the macula in the utricle and saccule. This diagram shows the membranous labyrinth. Ampullae are dilations at one end of each semicircular canal. They contain the neuroepithelium that detects rotational movements. It is called the crista. The ampulla of the lateral semicircular canal is outlined with dotted lines so that we can see the macula of the utricle through it. The neuroepithelium in the utricle and saccule measure translational movements. SSC – superior semicircular canal PSC – posterior semicircular canal The physiology of the crista and macula will be discussed in the next tutorial. Here we will look at their anatomy. Both the crista and the macula are made up of a group of hair cells attached to a nerve fibre. They are called hair cells because they have a clump of hair like projections at the apex (top) of the cell. The crista is inside the ampulla of the semicircular canal. You can see that it is a patch of hair cells with their cillia stuck in a structure called the cupula. The cupula is made of gel. Together the crista and cupula detect rotational movements. In the macula we also see a patch of hair cells with their cilia stuck in a layer of gel. However, in the macula there is also a layer of crystals. These are called otoconia and they are very important in helping the macula detect translational movement. They are also importamt in the most common balance disorder – benign paroxysmal positional vertigo (BPPV). SUMMARY The inner ear contains one of the senses that contribute to our sense of balance. It is also extremely important in keeping our vision clear when we move around. Although we should never forget that vision, proprioception and somatosensation are all important inputs to balance we will focus our learning on the inner ear, how it works and what happens when it is diseases. The physiology of the labyrinth is the subject of the next tutorial.