Anatomy
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. Vertigo is the chief complaint of patients when there is dysfunction or disease within the inner ear portion of the balance pathway or along the inner ears’ connections to the brain.
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 the position of the head in space. 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 the inner ear sensory information. 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 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 cranial nerve called the optic nerve (CN II) to the brain is also required.
For the healthcare professional, assessing reflexive eye motion is important in order to determine whether the vestibular system is working properly. 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. Nystagmus can be caused by several reasons other than vestibular problems, however in the case of accompanying vertigo, nystagmus leads the health care professional to the suspicion that the vestibular system is the culprit.
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 to provide the body with its overall sense of balance within its environment.
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