Figure 1: Vestibular System Anatomy
Figure 2: Anatomy of the Crista Ampullaris
initially perceives the change in motion with some disorientation and vertigo. That is not a sensation that we usually want to persist! After a brief period of constant velocity, the rider then experiences the sense of steady state, as the vestibular system adapts to the motion. Of course, once the ride stops the cupula will deflect in the opposite direction consistent with the properties of inertia, and the rider will once again experience a brief period of vertigo and disorientation. The semicircular canals are oriented as three coplanar pairs between the labyrinths, meaning that the right and left horizontal canals are one pair, the right posterior and the left anterior canals are another pair, and the left posterior and the right anterior canals are the third coplanar pair. This arrangement of coplanar pairing is responsible for the “push- pull” arrangement that drives CNS detection of head movement. When head movement occurs in a paired plane, the endolymph is displaced on each side in opposite directions relative to their ampullae. This causes an increase in neural firing in one canal and a decrease in firing in the opposite paired canal. The arrangement of coplanar pairing also allows for redundancy. In the presence of disease or dysfunction in one canal, the CNS will still receive information regarding head movement from the other co-planar canal in the opposite side. Comparison of the rate and magnitude in change of neural firing allows the CNS to detect velocity and direction of head movement so that it may effectively elicit compensatory motor responses to maintain postural control and gaze stabilization. Vestibular afferent neurons have a tonic (baseline) firing rate of about 90 to 100 pulses per second (pps). Excitation of vestibular afferents is correlated with the degree of head movement, such that a head velocity of 50° per second results in a change in firing rate of the afferent nerves of 50 pps. Specifically, that would translate to a firing rate of 150 pps (baseline plus 50 pps) on the excitatory side and 50 pps (baseline minus 50 pps) on the inhibitory side. However, the vestibular system does not respond as well to high-velocity head movement due to an asymmetry in the excitation-inhibition firing responses. The basis for this phenomenon, referred to as “inhibitory cutoff,” is that although increased firing on the excitatory side correlates with head velocity, hyperpolarization (decreased firing) on the contralateral side can be reduced only to zero. This is defined by Ewald’s Second Law , which states that responses to rotation that excite a semicircular canal are greater than responses to rotations that inhibit a canal (Herdman & Clendaniel, 2014). The firing rate of the vestibular nerve on the inhibitory side can be driven to 0 pps with head velocities of as little as 180° per second. Therefore, the inhibitory cutoff phenomenon will occur with head velocities greater than this. Under the conditions of normal activities that require rapid head motion, and certain sporting activities, the velocity of head movement can be as great as 550° per second, requiring the vestibular system to depend on the excitatory side to detect the velocity of head motion in order to elicit
into neural firing. When angular head movement occurs, the endolymphatic fluid within the semicircular canals flows in the direction opposite to the direction of the head movement, creating a unidirectional displacement of the mechanoreceptors in the ampullae (Herdman & Clendaniel, 2014). The mechanoreceptors within the ampullae are composed of two different types of hair cells: kinocilia and stereocilia. Kinocilia is the single, tallest hair cell, and the remaining smaller hair cells are called stereocilia . These hair cells sit on a bed of vascular and nerve fibers called the crista ampullaris and are embedded in a gelatinous cone called the cupula (see Figure 2). The hair cells are organized so that the kinocilia are in the same location relative to the canal on both sides. In the horizontal canals, the kinocilia are oriented in the ampullae away from the canal side. The kinocilia in the posterior and anterior canals are oriented toward the canal side. This important configuration allows these mechanoreceptors to be bidirectionally sensitive, with deflection of the stereocilia toward or away from the kinocilia determining whether hair cell discharge frequency increases or decreases vestibular nerve firing. When the stereocilia are deflected toward the kinocilia, there is a net increase in neural firing rate, or excitation. Conversely, when the stereocilia are deflected away from the kinocilia, there is a decrease in neural firing rate, or inhibition. Let’s consider how this works using the horizontal canals as an example. With lateral head rotation to the right, endolymph in the horizontal canals flows to the left. This results in the stereocilia being deflected toward the kinocilia on the right (toward the ampulla or away from the canal), causing excitation on that side. Conversely, on the left stereocilia are deflected away from the kinocilia (away from the ampulla and toward the canal), causing inhibition on the left side. In the anterior and posterior canals, the kinocilia are oriented toward the semicircular canal side, so that posterior rotation of the head causes excitation in the posterior canals and inhibition in the anterior canals (see Figure 2). In the semicircular canals, endolymph displacement is proportional to head velocity, thereby defining the role of the semicircular canals as “rate sensors.” This 1:1 ratio of velocity of head movement and endolymph flow, or gain, maintains a stable visual image on the retina during high-velocity movements. Without this, oscillopsia (blurred images) would result while the head is in motion. The anatomic organization of the ampullae underlies the dynamic characteristics of the semicircular canals’ response to prolonged rotation at constant velocity. At constant velocity of head rotation, steady state is achieved as inertia is eventually overcome, and the ampullae return to resting position. At this point excitation ceases to exist. These dynamics serve an important functional role in everyday life. Consider the example of riding in an amusement ride, where the rider
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