Vestibular Function
Sensory input from the vestibular system, along with visual and proprioreceptive cues, is integrated to provide balance and a sense of spacial orientation. The vestibular system primarily signals to the neural structures controlling eye movements and the muscles of posture.
Vestibular Anatomy
The organs of the vestibular system lie within the membranous labyrinth of the inner ear. The membranous labyrinth is suspended within the bony labyrinth of the skull by connective tissue. In addition to the cochlea, which functions in hearing, the membranous labyrinth contains the five organs of the vestibular system.
- The three semicircular canals (horizontal, posterior, and superior)
- The utricle
- The saccule
Mechanisms of Vestibular Function
The mechanism of the vestibular system function is similar to that of the auditory system. In general, a mechanical stimulus is converted into a nerve impulse that can be processed in the brain. Animations for the following descriptions of hair cell function can be found here.
Semicircular Canals
The semicircular canals are located at right angles to each other, like the three sides of a box coming together to form the corner. Each canal is most sensitive to angular rotation in the same plane. Additionally, the canals are paired contralaterally so motion excitatory to one will inhibit the canal on the opposite side of the head. The end of each semicircular canal is dilated to form the ampulla, within which lies the ridged layers of the crista ampullaris covered in sensory epithelium. When the head is rotated, the inertia of the endolymph leads to drag and relative motion of the fluid in the direction opposite of head motion. The moving fluid bends the cupula and the stereocilia extending up within the cupula from the sensory epithelium. Shear force on the hair cells causes K+ channels to open, depolarizing the cell and conveying a signal via CN VII.
The vestibular system consists of a push pull system where the nuclei of the vestibules on either side of the head play a crucial role. These nuclei are connected by a commisural fibers. By this connection, bilateral coordination of the vestibular system is done.
The commisural fibers transmit inhibitant impulses between the vestibule and the nuclei which are essential to maintain balance in the firing activity. This phenomenon is called inhibitory commisural mechanism.
Semicircular Canals
The semicircular canals are located at right angles to each other, like the three sides of a box coming together to form the corner. Each canal is most sensitive to angular rotation in the same plane. Additionally, the canals are paired contralaterally so motion excitatory to one will inhibit the canal on the opposite side of the head. The end of each semicircular canal is dilated to form the ampulla, within which lies the ridged layers of the crista ampullaris covered in sensory epithelium. When the head is rotated, the inertia of the endolymph leads to drag and relative motion of the fluid in the direction opposite of head motion. The moving fluid bends the cupula and the stereocilia extending up within the cupula from the sensory epithelium. Shear force on the hair cells causes K+ channels to open, depolarizing the cell and conveying a signal via CN VII.
The vestibular system consists of a push pull system where the nuclei of the vestibules on either side of the head play a crucial role. These nuclei are connected by a commisural fibers. By this connection, bilateral coordination of the vestibular system is done.
The commisural fibers transmit inhibitant impulses between the vestibule and the nuclei which are essential to maintain balance in the firing activity. This phenomenon is called inhibitory commisural mechanism.
Otolitic Organs
In the utricle and the saccule, otoconia rest in a layer of viscous gel. When the otoconia are displaced during linear acceleration, hair cells are activated and a neural signal is generated.
The following video summarizes these key points and provides a summary of vestibular system anatomy and function.
Balance and the Vestibular Ocular Reflex
Signals from the vestibular position are passed to brainstem via the vestibular nerve. The signals will then travel to the lateral muscles of one eye and the medial muscles of the other. The excitatory effect ensures that eyes compensate for the movement, allowing the visual field to remain stable, even though the head is moving.
Mathematical Model
The mechanics of the semicircular canals can be described by a damped oscillator. If we designate the deflection of the cupula with θ and the head velocity with q, the cupula deflection is approximately:
α is a proportionality factor, and s corresponds to the frequency. For humans, the time constants T1 and T2 are approximately 3 ms and 5 s, respectively. As a result, for typical head movements, which cover the frequency range of 0.1 Hz and 10 Hz, the deflection of the cupula is approximately proportional to the head-velocity. This is very useful, since the velocity of the eyes must be opposite to the velocity of the head in order to have clear vision.
Lee, Steve (2012). "Vestibular System Anatomy." Retrieved November 24, 2013, from http://emedicine.medscape.com/article/883956-overview.
Naunton, Ralph F. The Vestibular System. New York, 1975.
"Vestibular System: Structure and Function." Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy. The University of Texas Health Science Center at Houston, Web. 22 Nov. 2013.
"Vestibular System." CNS Clinic, Web. 25 Nov. 2013. <http://www.humanneurophysiology.com/vestibularsystem.htm>.
Feher, Joseph. Quantitative human physiology: an introduction. Access Online via Elsevier, 2012.
Vestibular Disorders Association. Web. 25 Nov. 2013. <http://vestibular.org/>.
Purves, Dale. Neuroscience. Sunderland, MA: Sinauer, 2008.
Naunton, Ralph F. The Vestibular System. New York, 1975.
"Vestibular System: Structure and Function." Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy. The University of Texas Health Science Center at Houston, Web. 22 Nov. 2013.
"Vestibular System." CNS Clinic, Web. 25 Nov. 2013. <http://www.humanneurophysiology.com/vestibularsystem.htm>.
Feher, Joseph. Quantitative human physiology: an introduction. Access Online via Elsevier, 2012.
Vestibular Disorders Association. Web. 25 Nov. 2013. <http://vestibular.org/>.
Purves, Dale. Neuroscience. Sunderland, MA: Sinauer, 2008.