The balance system is designed to give you clear vision during motion, detect direction, and speed of movement. Its other job is to make tiny automatic adjustments to keep you steady on your feet. A basic understanding of how the balance system works helps in understanding why you can feel so sick when something goes wrong.

Your brain uses signals from three body systems to help keep you balanced:

• vestibular - inner ear balance mechanism
• visual - eyes
• proprioceptive - skin, muscles and joints

The balance system is an automatic and silent process. It works 24/7 to collect information about the world around you. You cannot feel your balance system working – it does its job without bothering to send information to the feeling and thinking parts of your brain.

Balance is sometimes called the 6th sense. The other 5 senses can be directed – for example, you can choose to smell a rose or shut your eyes. The balance system can neither be controlled consciously nor ignored. You only become aware of your balance system when you challenge it (such as by learning to ride a unicycle) or when something goes wrong.

Role of the brain

Information from the vestibular, visual and proprioceptive systems is sent to the central nervous system through the vestibular nerve (also known as the 8th cranial nerve or vestibulocochlear nerve). The brainstem sorts this information and combines it with information from two other parts of the brain, the cerebellum and cerebral cortex.

The cerebellum provides information about automatic movements learned through much repetition, such as balancing on a bicycle.

The cerebral cortex provides information about things you have already learned, such as walking with your feet a bit further apart (wider gait) when the sidewalk is slippery.

All of these signals are put together and interpreted by the brain. The brain then sends signals back to the eyes and other parts of the body to react and keep the body upright.

There is much redundancy built into the balance system. The brain does not need every single bit of the massive amount of information sent to it from the vestibular, visual and proprioceptive systems. Depending on what is going on, the balance system chooses which information to use and which to ignore. For example, when you are in a dark room and input from the eyes is reduced or unreliable, the brain uses more on information from the proprioceptive and vestibular systems. Likewise, if someone has no sensation in their legs and feels as though they are standing on cotton wool, input from the visual and vestibular systems will be used to keep them upright.

Role of the inner ear (vestibular system)

Together, 10 organs of balance – 3 semicircular canals and 2 otolithic organs in each ear – make up the inner ear balance mechanism (vestibular system). These structures sense head movement, for example when your head tilts, turns or changes speed. Information from these structures travels through the vestibular nerve (also called the 8th cranial nerve or vestibulocochlear nerve) to centres in the brain.

The vestibular system lies deep inside the inside the inner ear, just under the brain and flanked by the temporal bone. The inner ear is divided into two main parts – the bony labyrinth and the membranous labyrinth.

Each bony labyrinth in made up of these structures:

• vestibule (a central chamber)
• 3 semicircular canals (organs of balance)
• a snail-shaped cochlea (organ of hearing)

A corresponding part of the membranous labyrinth is housed within:

• otoliths (inside the vestibule)
• 3 semicircular ducts (inside the semicircular canals)
• cochlear duct (inside the cochlea)

The structures in the bony labyrinth (vestibule, semicircular canals and cochlea) are surrounded by a sodium-rich fluid called perilymph.  The membranous labyrinth contains potassium-rich fluid called endolymph. The functioning of the inner ear relies on a mechanism that controls the amount and make up of endolymph and perilymph. The walls of the membranous labyrinth are lined with the endings of the vestibular nerve. 

The superior, posterior and horizontal semicircular canals lie at right angles to one another. Together, they act like a gyroscope to detect different types of rotational head movement (for example when you bend down to pick something up or turn your head to look over your shoulder).

Each semicircular canal is a tube-like structure that ends in a fluid-filled space called the ampulla. The ampulla holds the cupula. A jelly-like cluster of sensitive hair cells called stereocilia are embedded within the cupula.

When the head is rotated, the inner ear turns with it. There is a brief lag before the endolymph (fluid) moves within the canals. This movement causes the top part of the hair cells (stereocilia) to bend. Receptors on these hairs sense the motion and and send information about head position to the brain through the vestibular nerve.

The otoliths are the other sensory organs in the inner ear. They differ in function from the semicircular canals. The otoliths are made up of two sac-like structures, the saccule and utricle. They sense gravity (such as going up or down in an elevator) and linear acceleration (such as gathering speed or braking in a car).

The otoliths take the form of a chandelier-like structure hanging from hairs on the ceiling of the inner ear. Similar to a chandelier, this structure is weighted by microscopic crystals of calcium carbonate (otoconia) on a jelly-like membrane. When the head is tilted, the crystals speed up with respect to the hairs and apply a shearing force on the hairs. This movement is detected by the hair cells and the information is sent to the brain.

Role of the eyes (visual system)

The vestibular system (inner ear balance mechanism) works together with the visual system (eyes, muscles and parts of the brain that coordinate vision). The visual system senses the direction and speed of head motion in relation to the world around you. It also keeps images steady when the head moves.

Role of the skin, muscles and joints (proprioceptive system)

The proprioceptive system (skin, muscles, and joints) lets your body sense itself in relation to the world around you. For example, sensors sensitive to stretch or pressure communicate to the brain how your legs and feet are positioned compared with the ground as well as how your head is positioned compared with your torso.

Two main reflexes play an important role in sending muscle commands back to the eyes and body from the brain:

• The vestibulospinal reflex (VSR) works with the proprioceptive system to stabilize your body in space using gravity as a reference point. Damage to the VSR can cause imbalance and staggering.
• The vestibulo-ocular reflex (VOR) is an “eye driver” that aims and steadies your gaze. For example, it lets you focus on the words on this page even if you nod your head up and down or side to side. And when you walk, the VOR keeps your vision steady while your head bobs ups and down. A damaged VOR sends faulty signals to the brain and may cause movement-related dizziness, blurry vision, unsteadiness, and nausea.

When something goes wrong with the balance system

Many body systems send, process and receive vital information about balance. It is not surprising, then, that a wide range of symptoms and disorders can happen when one or more parts of this communication chain breaks down. The complexity of the balance system, coupled with the number of things that can go wrong with it, often makes diagnosis challenging.

Glossary

Ampulla – widened ending of the semicircular canals

Bony labyrinth – rigid outer wall of the inner ear within the temporal bone. It consists of three parts: vestibule, semicircular canals and cochlea. Within the bony labyrinth is a membranous labyrinth.

Brainstem – part of the brain that integrates and sorts sensory information

Central nervous system – part of the nervous system that consists of the brain and spinal cord

Cerebellum – (Latin for “little brain") part of the brain that coordinates and regulates movement

Cerebral cortex – part of the brain that plays a key role in higher-level thinking and memory

Cochlea – inner ear hearing apparatus. It looks a bit like a snail. This fluid-filled (endolymph) coil acts like a telephone by converting vibration (sound waves) into electrical impulses that are relayed to the brain via the vestibulo-cochlear nerve.

Cochlear duct – an endolymph filled cavity within the cochlea

Cupula – sensory receptor within the ampulla comprised of a gelatinous cluster of tiny sensitive hair cells

Endolymph – (Latin for “inner fluid") potassium-rich fluid contained in the membranous labyrinth

Endolymphatic sac – blind pouch at the end of the endolymphatic duct; it is a poorly understood structure that may regulate fluid exchange within the ear

Labyrinth – part of the inner ear that contains the organ of hearing (cochlea) and the organs of balance (semicircular canals and otoliths). It has two parts, one inside the other, the membranous labyrinth contained within the bony labyrinth

Membranous labyrinth – a complex system of communicating ducts (within the semicircular canals) and sacs (otoliths) filled with endolymph and cushioned by perilymph within the bony labyrinth

Otoconia (also called canaliths) – tiny crystals of calcium carbonate within the inner ear. Their movement tells the brain about the body’s movement up or down, to the left or right, or backwards or forwards.

Otoliths – collective term for two sac-like structures in the inner ear (saccule and utricle) that sense gravity and linear acceleration

Perilymph – (Latin for “outer fluid") - sodium-rich fluid contained within the space separating the membranous labyrinth from the bony labyrinth

Proprioceptive system – components of skin, muscles, joints, and tendons that connect with the brain through the nervous system to provide a subconscious awareness of body position and movement

Saccule – one of two otolith organs, the saccule provides information about vertical acceleration (such as when in an elevator).

Semicircular canals – three interconnected curved tubes in the labyrinth that detect rotation movement. The canals join at their wide ends (ampulla) and are filled with endolymph.

Utricle – one of two otolith organs, the utricle is sensitive to changes in horizontal movement

Vestibular system – sensory system composed of the sensory organs in the inner ear (semicircular canals and otolith), the vestibular nerve and parts of the brain that interpret and respond to information derived from those structures. Also known as the peripheral balance system.

Vestibule – central chamber of the labyrinth (inner ear) containing endolymph and patches of sensory hair cells

Vestibular nerve (also referred to as the vestibulo-cochlear nerve, auditory nerve, vestibular-cochlear nerve or eighth cranial nerve) – carries information to the brain from both parts of the inner ear – the organ of hearing (cochlea) and the organs of balance (semicircular canals and otoliths)

Visual system – the part of the nervous system that allows us to see. It includes the eyes and connecting pathways to the brain.