Hello everyone and welcome to this week's blog post. In this blog post, I will be discussing balance. I will first explain how humans can stay upright. As part of this, I will discuss the various sources of information the brain receives, how the brain processes this information and what the body does with this information to remain upright. I will then explore some of the diseases associated with balance problems.
How do humans stay upright?
We often take being able to stand upright for granted, yet without the ability to balance, we would find it very difficult to cope in our environment. Balance involves the brain constantly receiving information from various structures, understanding the information and then acting on this information to adjust the body's position.
The first part of our balance system is to do with receiving information. This information comes from 3 main sources: the visual system, the vestibular system and the proprioceptive system.
The visual system
The visual system involves what we can see through the eyes. The eyes have a tissue called the retina that contains two different types of sensory receptors (rods and cones). These sensory receptors work together to allow us to see objects at different light levels. Rod cells contain the pigment rhodopsin, which is stimulated by low light intensities, allowing us to see objects in low light levels. Unfortunately, this means that they can't detect colour or create precise images of the surroundings (thus explaining our monochrome and blurry vision during the night).
In contrast, cone cells use the pigment iodopsin to detect light. As there are three different cone cells (which respond to one of red, green or blue light), we can see in colour at higher light intensities. Cone cells also allow us to see much finer details than rod cells
These receptors stimulate the optic nerve, which carries these messages to the brain and allows it to construct an image of our surroundings. The brain can then use these visual cues to judge balance - for example, it could see that buildings appear tilted to the left and adjust our stance accordingly.
The vestibular system
In addition, the brain also uses information received from the vestibular system. This system, which is located in the inner ear, is made up of the utricle, saccule and three semicircular canals.
These three semicircular canals are filled with a fluid (called endolymph) and lined with lots of tiny hairs (called cilia). When our heads move, it causes the fluid inside these canals to slosh around, thus stimulating these hairs. This information is then relayed to the brain via the vestibular nerve. The fact that there are 3 canals in each ear helps to improve the number of planes the brain can detect movement in since the canals are all fixed at right angles to each other. This allows us to detect motion in three different planes, including nodding, shaking the head from side to side and rotating.
In addition, the saccule and the utricle (known together as the otolith organs) also assist with the vestibular system. This is because they can detect gravity, allowing the brain to judge movement in a straight line. They can do this as they contain small calcium carbonate crystals, which are displaced when one moves forwards and backwards or upwards and downwards. When displaced, these crystals put pressure on cilia, which relay this information via the vestibular nerve to the brain.
Proprioceptive system
Furthermore, the brain also receives information from the proprioceptive system. This system is where the brain receives information from muscles, joints and sensory receptors in the skin. For example, when you lean forwards, the pressure on the front of the sole of your foot increases. This causes sensory receptors in the skin to be stimulated, sending information to the brain. The brain can then process this information to judge how you have moved. In addition, sensory receptors next to joints and muscles can also help the brain decide how your body has moved, thus allowing it to judge your position in 3D space.
Processing of the information
There are three key parts of the brain involved with the processing of this information. All three systems send information to the cerebellum and the brainstem. These regions of the brain are responsible for sorting this sensory information and sending signals to the muscles to make adjustments to your stance. In addition, the cerebral cortex can also become involved when perception, high-level thought and memory are required.
Sometimes, the signals we receive from the three systems associated with balance can contradict each other, causing confusion and disorientation. In these cases, the brain sorts through this information and uses past experiences to calculate which scenario is the most likely. Moreover, the brain can also cause you to look away (to remove confusing visual cues) or touch something (to gain useful proprioceptive information) to aid in this calculation.
Outputs from the brain
Once the brain has determined the body's position in space and how it's moving, it then sends signals to a range of muscles around the body to allow you to stay upright. Most of these signals are sent by the brainstem. These muscles then make changes to your posture to allow you to stay upright.
In addition, the brain also sends signals to the eyes - this is known as the vestibulo-ocular reflex. This reflex causes the eyes and head to coordinate their movement, thus stabilising gaze and allowing us to follow movement with our eyes, without the image becoming blurry.
When a new response to a stimulus is learnt, a new pathway forms in the brain. Over time, if this pathway is used more often, it strengthens allowing complex movements to become 'automatic' after enough practice. This is how we are able to constantly stand up once we have learnt to as a baby.
Are there any diseases associated with balance?
There are many diseases associated with balance. One of the main problems with diagnosing these diseases is the complexity of the balance system. Since there are so many different systems involved with balance, this makes it harder to identify the faulty one and treat the cause of imbalance.
One of these diseases is called Meniere's disease. According to the Mayo Clinic, Meniere's disease "is a disorder of the inner ear that can lead to dizzy spells (vertigo) and hearing loss." While we don't know the cause of the disease, we suspect that it is caused by an abnormal amount of fluid in the semi-circular canals in the inner ear. This could be because of viral infections, genetic reasons, an abnormal immune response or even the improper fluid drainage. Unfortunately, there is not currently a cure for Meniere's disease. However, medicine (e.g. antihistamines and prochlorperazine) can be used to control the nausea, vomiting and vertigo caused by the disease.
Additionally, labyrinthitis can also affect your balance. Labyrinthitis, which is also known as vestibular neuritis, is a disease caused by an infection of the inner ear. This disrupts the vestibular system, thus causing dizziness and vertigo. Sufferers also report feeling sick and a ringing sound in their ears. Labyrinthitis will usually resolve itself within a few weeks. In the meantime, those suffering from the disease are advised to avoid bright lights and noise, drink lots of water and lie in a dark room if they are feeling dizzy.
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