Chapter 7E - Vestibular System
The vestibular system is important for normal function in several ways. It is a critical system for detecting the position and motion of the head, particularly angular motions such as rotation. It is through this system that the eyes are stabilized when the head is moving and also for adjusting neck and body muscle tone during movements.
General principles of the inner ear anatomy are introduced in the preceding section on hearing (figure 17). The vestibular organs are located in portions of the membranous labyrinth contained in the vestibule (the saccule and utricle) and in the semicircular ducts that fill the semicircular canals. There are receptive organs within the membranous labyrinth, the macula in the saccule and utricle and christae in the semicircular canals. Each of these receptive organs contains hair cells that were described in the section on hearing. These cells are polarized by the presence of the kinocilium such that bending the cillia toward the kinocilium results in increased neurotransmitter release from hair cells and increased activity in the vestibulocochlear nerve. Deflection away from the kinocilium results in the opposite. There is a constant background release of transmitter, meaning there is a tonic activity in the vestibular nerve (figure 18). Therefore, each nerve fiber can signal movement of the cilia toward or away from the kinocilium.
Maculae and christae have some structural similarities and some differences (figure 18C,D). Both types of organs include a thickened epithelium with the cilia of their hair cells being embedded in a gelatinous covering. In the case of the maculae, which are located in the utricle and saccule, this gelatinous covering is studded with calcium carbonate crystals (otoliths). These crystals are heavy and will distort the hairs by the effect of gravity. Additionally, they will lag behind if the head is accelerated in a linear direction. There are hair cells that are polarized in many different directions in each macula. Therefore, the macula can detect the orientation of the head in relation to gravity and the direction of linear acceleration.
In the case of the semicircular ducts, a dilated "ampula" is associated with each. The ampula contains a flame-shaped christae (figure 18C). In the christae, the gelatinous cap (the cupula) protrudes into the canal. Movement of the endolymph through the canal results in a deflection of the christae thereby deflecting the hairs. Each christae contains hair cells that are polarized in a single direction. So fluid movement in one direction will excite all of the hair cells and movement in the opposite direction will inhibit all of them. Each ear has three semicircular canals (containing semicircular ducts) that are at right angels to one another. These canals are not purely horizontal, coronal and sagittal. The horizontal canals are oriented at approximately 20 degrees, with the anterior side elevated. The posterior and anterior canals are oriented in a manner such that the anterior canal of one ear is basically in the same plane as the posterior canal of the other ear. This allows each ear to function as a backup for the other.
Angular acceleration of the head (i.e., rotation, lateral flexion or nodding) will cause fluid to tend to lag behind the movement of the head in relation to the canal that moves with the head, there will be relative movement of the fluid. This relative movement will be greatest if the head movement is directly in the plane of the semicircular duct. Therefore, the christae signal angular acceleration.
Of course, vestibular receptors would not be of much use if they were not connected to the brain. The vestibular part of the vestibulocochlear nerve consists of nerve fibers that have contacted hair cells of the macula and christae. These sensory fibers have bipolar cell bodies located in the vestibular ganglion. The central processes of these bipolar neurons enter the brain stem in the rostral medulla, where they synapse in the vestibular complex. Additionally, many vestibular nerve fibers terminate in the flocculonodular lobe of the cerebellum (the vestibulocerebellum) (figure 19).
The vestibular complex of the brain stem is so-called because it actually consists of several nuclei. For the purposes of this discussion, we will not consider them separately. The vestibular complex is located in the dorsal lateral portion of the rostral medulla and caudal pons and receives several types of input in addition to primary vestibular afferent information. For example, input from the spinal cord allows proprioception (particularly of the neck) to influence vestibular functions. The vestibular complex has a very important reciprocal connection with the flocculonodular lobe of the cerebellum (the vestibulocerebellum).
One of the most important roles of the vestibular complex is to keep the eyes focused on target when the head moves. This is termed the vestibulo-ocular reflex (VOR) (figure 20). When the head jerks in one direction the eyes move in an equal and opposite direction. The reflex is mediated via connections from the vestibular complex to the extraocular nuclei (i.e., those nuclei that control eye muscles). Many of these connections traverse the medial longitudinal fasciculus. The VOR must be adjusted from time to time in order to keep it accurate over the course of one's life, especially as the inner ear becomes less sensitive with age and disease. This "fine-tuning" of the reflex is dependent on activity of the vestibulocerebellum, which is critical in order to assure that the reflex continues to produce accurate eye movements.
The vestibular complex projects to other portions of the nervous system. The lateral vestibulospinal tract arises from neurons in the lateral vestibular nucleus (Deiter's) and projects through the ventral part of the lateral funiculus to activate extensor motor neurons at all levels of the cord. The medial vestibulospinal tract arises from the medial vestibular nucleus and projects through the descending medial longitudinal fasciculus (MLF) to enter the anterior funiculus of the spinal cord and affect neck muscle activity bilaterally. These vestibulospinal tracts adjust the muscle tone and position of the body in response to vestibular stimulation and are the physiological basis for vestibular righting reflexes and tonic neck reflexes.
In addition to projections to the spinal cord, there are projections from the vestibular complex to the reticular formation. Intense vestibular stimuli can produce vomiting and even influence blood pressure. Additionally, there are projections to the thalamus that relay to poorly-defined portions of the inferior parietal lobe that permit the conscious perception of movement.