The anatomy and physiology underlying the human body’s sensation of balance is complex. Many systems are involved including the brain, the spinal cord, the eyes, the ears and the receptors in the skin, joints and muscles. Disruption to any of these areas through injury or disease can affect one’s feeling of being balanced.
The inner ear, which is also called the labyrinth of the ear, is made up of three primary structures which moderate balance and equilibrium; the semicircular canals along with the saccule and utricle. Collectively this system of the inner ear is termed the vestibular system or vestibular apparatus. The inner ear also contains the cochlea, which is the main structure involved in hearing.
The three semicircular canals work to detect rotational motion of the head. The canals are positioned at 90-degree angles to one another and are filled with fluid called endolymph. Hair cells are located at the base of each canal and project up into the endolymph. Movement of the head causes movement of the endolymph within the canals, which in turn causes the hair follicles to move accordingly and emit impulses about balance. Hair follicles in the saccule and utricle add to the balance information by providing feedback about the position of the head in reference to gravity (vertical orientation), as well as detecting linear motion of the head.
Sensory information from the inner ear is relayed to the brain via the vestibular portion of the eighth cranial nerve (CNVIII), which is also called the vestibulocochlear nerve. The cochlear portion of the nerve transmits information about hearing. Specific areas of the brain, in particular the cerebellum and brain stem as well as portions of the cortex, process sensory information coming from the inner ear. When both the right and left inner ears are sending the same information, the brain processes that the body is balanced. When the body or head moves, the sensory input from the ears is not identical so the brain perceives motion and the body adjusts accordingly.
The ears work in close relation with the eyes in order to maintain equilibrium and balance. The vestibulo-ocular reflex (VOR) is an automatic function of the eyes, which stabilizes images on the retina in response to the vestibular sensory input from the ears. This reflex causes the eyes to move in the opposite direction to the movement of the head in order for the eyes to remain fixed on a target. Thus, accurate vestibular input from the ears affects how the eyes adjust and to one’s sense of being balanced. The accurate relay of information from the eyes along the optic cranial nerve (CN II) to the brain is also required.
If one inner ear is affected by disease or injury then the sensory input being sent to the brain will falsely indicate movement from that vestibular system. In this case the eyes will adjust accordingly and move opposite to the perceived motion despite the head actually being still. An involuntary back and forth movement of the eyes results. This movement of the eyes is called nystagmus and when present, leads any health care professional to suspect a vestibular problem.
Two other reflexes, the vestibulocollic reflex and the vestibulospinal reflex also work to help the body maintain its balance and sense of equilibrium. The vestibulocollic reflex (sometimes called the vestibular-neck reflex) works in conjunction with the vestibular input and the neck muscles in order to stabilize the head. The job of the vestibulospinal reflex is to create compensatory body movements in response to the vestibular input in order to maintain one’s balance and avoid falling over.
The brain amalgamates the vestibular information from the inner ears with sensory information from the eyes, as well as the information coming from the receptors in the muscles and joints of the neck and the rest of the body to provide the body with its overall sense of balance within its environment.
Disruption along any portion of the anatomical pathway described above can affect one’s perception of balance or equilibrium. A problem with the inner ear portion of the pathway or the sensory information being relayed to the brain via the vestibulocochlear nerve is termed a peripheral vestibular disorder. If the problem affecting one’s balance is due to damage of a structure within the brain itself, which then affects the reception and integration of balance information, it is termed a central vestibular disorder.
