The vestibular system is a group of linked structures in the body that subserves postural balance, spatial orientation and the control of eye movement during head motion. Knowledge of the different parts of the system is abundant, but there are some misconceptions about the system as a whole. One of the main reasons is failure to acknowledge that it is not just a peripheral system. It also consists of central and connecting structures. As such, it is a part of both the peripheral nervous system and the CNS.
The Peripheral Vestibular System
The peripheral vestibular system is the main sensory part of the system. It receives stimuli having to do with head movement and position in relation to gravity. It consists of the vestibular apparatus and the vestibular nerve.
Part of the vestibular system is in the labyrinth, but the whole labyrinth is not part of the vestibular system. The reason is part of it subserves hearing. The name for that portion of it is the cochlea (cochlear labyrinth). It is the snail-shaped spiral medial part of the structure. The cochlear nerve which is responsible for hearing originates from bipolar neurons of the spiral ganglion. Its dendrites make connections with hair cells in the organ of Corti – the part of the cochlea that senses sound. The axons of the nerve make central connections with the cochlear nucleus in the brain stem.
The labyrinth is a group of interconnected bony canals in the inner ear, of which a membrane lines the inner portion. The inner ear is located within the petrous part of the temporal bone of the skull. The bony part of the structure is termed the osseous labyrinth. The membrane lining the canals is termed the membranous labyrinth.
The vestibular apparatus is the part of the labyrinth that receives stimuli and sends signals to the brain having to do with balance, spatial orientation and eye control during movement of the head. Many refer to it as the vestibular system. But this is not accurate. The reason is it is only the peripheral and sensory part of the system. It consists of 3 semicircular ducts at near right angles to one another. The name for the membranous portion of these three structures is the semicircular canals or ducts. The one end of each which is larger than the other is the membranous ampulla; it contains the sensory end organs of he structure. The vestibular apparatus also consists of two sac-like divisions which contain their own sensory end organs. The utricle is the name of the larger and uppermost sac. The saccule is the name of the smaller and lowermost one. Both of these structures are termed otolith organs because they contain a chalky, lime-containing substance.
There is fluid between the osseous and membranous labyrinths. It bears the name perilymph. There is another type of fluid within the membranous portion of the structure known as endolymph. The endolymph plays an important role in the function of the vestibular apparatus and the cochlea. Both structures and types contain hair cells which are tiny projections from the surface of the membrane into the canal. When endolymph movement from head motion or sound bends the hair cells they evoke electrical signals which travel to the brain for interpretation. The dendrites of vestibular nerve and cochlear nerve fibers adjacent to the hair cell sensors receive the mechanical signals. They then convert them to electrical signals and transmit them to the brain.
The two types and five actual sensing structures of the vestibular apparatus have different functions. The utricle senses linear acceleration (change in the speed of movement in a straight line forward, backward, or to the side). The saccule senses head position relative to gravity. The semicircular canals sense head rotation in their respective planes.
The vestibular nerve is the branch of the vestibulocochlear nerve which is not involved in hearing. It arises from the vestibular ganglion which is located in the upper and outer part of the opening to the inner ear canal. The vestibular ganglion is a cluster of bipolar neuron cell bodies whose dendrites make contact with the hair cells of the semicircular canals and otolith organs. The axons of those nerve cells then transmit the resulting electrical signals to vestibular nuclei.
The vestibulocochlear nerve (8th cranial nerve) is a combination of vestibular nerve and cochlear nerve fibers which merge before entering the brain stem. Each retains its specific function within the brain though. Other names for it are the auditory nerve and the acoustic nerve.
The central vestibular system begins with the vestibular nuclei. They are the collection of neuron cell bodies at the junction of the medulla and pons that receive signals from the vestibular nerve. There are a total of four on each side of the brain stem, each of which makes connections to different parts of the brain.
The vestibular nuclei transmit some of the signals through tracts to the cerebellum and others to the MLF. Some signals pass via multipolar neurons from the MLF to the midbrain. Signals from the midbrain then pass to the nuclei of cranial nerves 3, 4, and 6. They are the nerves that control eye movement. Some signals also travel from the vestibular nuclei into the spinal cord via the vestibulospinal tracts.
The vestibulospinal tracts are the descending efferent nerves of the vestibular system. They are the axons of vestibular nuclei in the brain stem that connect with lower motor neurons in the spinal cord. There are two on each side of the spinal cord. One is termed lateral. The other is termed medial. By virtue of their connections with motor neurons they are involved in postural adjustments of the body to the influence of gravity.
A discussion of the various tracts between the vestibular nuclei and other parts of the nervous system is beyond the scope of this article. But suffice it to say, the central vestibular system processes and integrates various signals as they traverse its many pathways. It plays an important role in how the brain interprets and how the body responds to the world around us with respect to motion, position and gravity.