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48 Rotational Vestibular Assessment
depolarization of type I and type II vestibular sensory hair cells occurs from the leading vestibular
labyrinth, which creates a subsequent increase in
afferent vestibular nerve firing rate. Conversely,
a concomitant hyperpolarization of type I and
type II sensory hair cells occurs in the opposing
(coplanar) labyrinth, which causes an opposite
but complementary decrease in vestibular nerve
firing rate in the trailing labyrinth. As a result,
the overall difference between opposing afferent
vestibular neural activity is significantly larger
than it would be if inhibition were not possible.
The maximal excitatory afferent neural drive
is approximately 400 to 500 spikes per second,
whereas the maximal inhibitory neural drive is a
complete saturation to 0 spikes per second (Gacek,
2005). This difference represents a nearly threefold higher excitatory drive than inhibitory drive.
Although this physiological design intentionally
creates a larger and more effective vestibular afferent imbalance during natural head movement,
the existence of such a high spontaneous resting
potential is not without a consequence. Specifically, such a high spontaneous firing rate creates
an opportunity for an inconsequential pathophysiologic neural imbalance following unilateral labyrinthine insult. The deleterious implications of
such a unilateral destructive event are significant
and will be discussed later.
Tuning of Vestibular Afferents
Tuning of vestibular afferents exist within the vestibular nerve; however, the tuning of these fibers
is different from that of the cochlear nerve. Evidence shows that vestibular afferents are broadly
tuned to acceleration stimuli and respond similarly over a wide range of angular and linear
frequencies (Leigh & Zee, 2006). Evidence also
shows afferent response gain and phase to be
relatively equal across a broad mid-frequency
range (e.g., from 0.1 to 1.0 Hz), with an increase
in sensitivity at higher frequencies and a decrease
at lower frequencies (Highstein, 1996). This sensitivity is likely governed by the type of firing
rate generated by the different types of afferents. Calyx nerve endings, associated with type
I hair cells and large diameter afferent fibers,
exhibit predominantly irregular discharge rates
and often show no firing while at rest (Baloh &
Honrubia, 2001). Bouton nerve endings, associated with type II hair cells and small afferent
diameters, predominantly have regular discharge
rates with little variability in their tonic neural discharge rate (Baloh & Honrubia, 2001). These regular afferents appear to be fundamentally important
to vestibular-ocular reflexes (VOR) because ablation of irregular afferents cause little, if any, effect
on the VOR (Minor & Goldberg, 1991; Goldberg,
2000). Irregular afferents, however, may be pertinent to an effective and efficient functioning of the
vestibulospinal reflexes (VSR), as well as responding to abrupt acceleration stimuli (Gresty & Lempert, 2001; Hain & Helminski, 2007). A summary
of the various properties associated with each type
of afferent nerve ending can be seen in Table 2–1.
Vestibular Afferent Neural Projections
Three sizes of afferent vestibular nerve fibers have
been identified. Some of the largest afferent nerve
fibers in the body (10 μm in diameter) are known to
terminate with the larger calyx nerve endings that
synapse with type I vestibular hair cells (Baloh &
Honrubia, 2001). The smallest afferent nerve fibers
(<2.5 μm in diameter) terminate with the smaller
bouton nerve endings and synapse primarily with
type II vestibular hair cells. Medium-sized afferent nerves (2.5 to 4.5 μm in diameter) terminate
on both calyx and bouton nerve endings, and synapse with both type I and type II vestibular hair
cells (Baloh & Honrubia, 2001).
Independent of sensory epithelium (cristae
ampullari or maculae), the location of each type
of afferent nerve fiber is also morphologically
dependent (see Figure 2–13). Large fibers terminate predominantly in the center of the cristae,
or toward the striola in the maculae, as well as
course centrally within each vestibular nerve
(Baloh & Honrubia, 2001). Small fibers terminate predominantly in the periphery of the sensory epithelium, and synapse with multiple type
II hair cells with smaller bouton nerve endings.
Small afferent nerve fibers are also noted to course
more peripherally within and along the perimeter of the vestibular nerve (Baloh & Honrubia,
2001). Medium-sized fibers are dispersed evenly
throughout the surface of the sensory epithelium
and throughout the diameter of the nerve (Baloh
& Honrubia, 2001) (see Figure 2–13).
Table 2–1. Differences Between Vestibular Hair Cell and Afferent Nerve Types
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Type of Hair Cell Type I Type II Type I & Type II
Type of Nerve Ending Calyx Nerve Ending Bouton Nerve Ending Dimorphic Nerve Ending
Diameter of Afferent Fiber Large Medium to Small Intermediate
Firing Rate Irregular Regular Irregular (central &
peripheral) & Regular
(peripheral)
Loci of Innervation on
Central Region Periphery Throughout sensory
End-Organ Epithelium
Afferent Neural Fiber
Organization
Sensitivity to Linear
Stimulation (Otolith)
Central fibers within the
vestibular nerve
Responsive to quick
linear translations
Peripheral fibers of the
vestibular nerve
Responsive to
sustained centrifugal
force or head tilt
Sensitivity to High
Acceleration Angular
Stimuli (SCC)
Contributions to VOR Minimal (ablation of
Increased sensitivity
to high/abrupt angular
acceleration stimuli
Decreased sensitivity
to high/abrupt angular
acceleration stimuli
Significant Type I — low sensitivity
irregular type I hair cells/
afferent nerve fibers
has little impact on VOR
response)
Adaptation to Prolonged
Fast Slow Fast (Type I); Slow
Stimuli
Overall Gain Higher Lower
Overall Phasic
More phasic in response Less phasic in response
Responsiveness
epithelium
Central and peripheral
Quick linear (Type I)
Sustained (Type II)
Variable sensitivity from
central irregulars to
peripheral regulars
in central zone; high
sensitivity in peripheral
zone
Type II — significant
(Type
II)
Linearity Non-Linear Linear Non-Linear (Type I);
Spontaneous Rate Low High
Second-Order Neuron
Innervation
Suppressed by Galvanic
Innervate Large 2ndorder neurons
Innervate Small 2nd-
order neurons
Ye s No
Stimulation
Role(s) of TYPE I Irregular Afferents: VOR adaptation
and compensatory responses, especially for high Hz
and high velocity rotational stimuli. Modulation of VOR
during eccentric rotation. Cancellation of the VOR.
Role(s) of TYPE II Regular Afferents:
Compensatory responses to unilateral labyrinthine
lesions for low rotational stimuli. High-Hz, high-
velocity, t-VOR.
Generation of low-Hz, velocity storage component of
the VOR. Extend the linear range of the VOR at high
rotational accelerations
Source: Adapted from Baloh & Honrubia, 2001; Goldberg, 2000; Gresty & Lempert, 2001; Leigh & Zee, 2006; Minor & Goldberg, 1990; Ödkvist, 2001.
49
Linear (Type II)
50 Rotational Vestibular Assessment
Superior Branch of the Vestibular Nerve
The superior branch of the vestibular nerve
(SBVN) innervates the horizontal and anterior SCC,
the utricle, and the anterosuperior portion of the
saccule (see Figure 2–1). Upon entering the fundus (medial end) of the IAC, the SBVN begins its
course in the posterosuperior position in relation
to the cochlear, facial, and superior/inferior branches of the vestibular nerve. Upon exiting the porus
oticus, (lateral end of the IAC) the SBVN is oriented in the anterosuperior position (Gulya, 2007).
Innervation of the Horizontal Semicircular Canal.
Type I and type II hair cells located in the crista
of the horizontal semicircular canal (h-SCC) synapse with afferent projections of the SBVN (Gacek,
2005). Secondary to the arrangement of the kinocilium in both types of hair cells (located on the utricular side of the crista), endolymph flow toward the
utricle (also know as ampullopetal flow) causes a
depolarization of the sensory hair cells and a subsequent increase in the afferent neural firing rate.
Conversely, endolymph flow away from the utricle
(also known as ampullofugal flow) in the h-SCC
causes a hyperpolarization in the underlying hair
cells and a subsequent decrease in the afferent neural firing rate (Lysakowski et al., 1998) (see Figure
2–7).
Innervation of the Anterior Semicircular Canal.
Type I and type II hair cells located in the cristae of
the anterior semicircular canals (a-SCC) synapse
with afferent projections of the SBVN (Gacek,
2005). Secondary to the arrangement of the kinocilium in both types of hair cells, (located on the
canalicular side of the crista and opposite that of
the h-SCC and utricle), endolymph flow away
from the utricle (also known as ampullofugal
flow) must occur to cause a depolarization of the
sensory hair cells and a subsequent increase in
the afferent neural firing rate. Conversely, endolymph flow toward the utricle in the a-SCC causes
a hyperpolarization in the underlying hair cells
and a subsequent decrease in the afferent neural
firing rate (Lysakowski et al., 1998).
Innervation of the Utricle. Type I and type II hair
cells located in the macula of the utricle synapse
with afferent projections of the SBVN (Gacek,
2005). As cited earlier, due to the co-arrangement
of the kinocilium on the utricle epithelium toward
the striola, a single translational force subsequently
produces both hyperpolarization, as well as depolarization of the underlying hair cells within a single utricular sensory end organ. Therefore, both
excitatory and inhibitory signals are sent via the
afferent nerve fibers from both utricles during a
single translational vector (Leigh & Zee, 2006).
Inferior Branch of the Vestibular Nerve
The inferior branch of the vestibular nerve (IBVN)
innervates the posterior SCC and the inferior portion of the saccular macula (Gacek, 2005) (see
Figure 2–1). Upon entering the fundus of the
IAC, the IBVN begins its course in the posteroinferior position in relation to the cochlear, facial,
and superior/inferior branches of the vestibular
nerve. Upon exiting the porus oticus, the SBVN
is oriented in the anteroinferior position (Gulya,
2007).
Innervation of the Posterior Semicircular Canal.
Type I and type II hair cells located in the crista
of the posterior semicircular canals (p-SCC) synapse with afferent projections of the IBVN (Gacek,
2005). Secondary to the arrangement of the kinocilium in both types of hair cells, (located on the
canalicular side of the crista and opposite that of
the h-SCC and utricle), endolymph flow away
from the utricle (also known as ampullofugal
flow) must occur to cause a depolarization of the
sensory hair cells and a subsequent increase in
the afferent neural firing rate. Conversely, endolymph flow toward the utricle in the p-SCC causes
a hyperpolarization in the underlying hair cells
and a subsequent decrease in the afferent neural
firing rate (Lysakowski et al., 1998).
Innervation of the Saccule. Type I and type II
hair cells located in the inferior macula of the
saccule synapse with afferent projections of the
IBVN (Gacek, 2005). As previously discussed,
due to the arrangement of the kinocilium on the
saccule epithelium away from the striola, a single
translational force subsequently produces both
hyperpolarization, as well as depolarization of
the underlying hair cells within a single saccular
2. Anatomy and Physiology of the Peripheral Vestibular System 51
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sensory end organ. Therefore, both excitatory and
inhibitory signals are sent via the afferent nerve
fibers from both sacculae during a single translational vector.
Vestibular Efferent Neural Projections
Peripheral vestibular efferent nerve fibers originate in the vestibular nuclei within the brainstem
and accompany the cochlear efferent fibers within
the olivocochlear bundle. From Scarpa’s ganglion,
the vestibular efferent fibers join each division
of the vestibular nerve and innervate type II sensory hair cells within the sensory epithelium of
the cristae and maculae. Although speculation
exists as to the exact function of the efferent system, its primary role remains elusive (Schwarz &
Tomlinson, 2005).
3
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Anatomy and Physiology of the
Central Vestibular System, Velocity
Storage, and Central Compensation
ROLE OF THE CENTRAL
VESTIBULAR SYSTEM:
AN OVERVIEW
The central vestibular system is complex in form
and in function. It is comprised of some of the
largest nuclei in the central nervous system, and
collectively integrates millions of neural signals
from the peripheral vestibular system, visual system, somatosensory system, cerebellum, and postural reflex system every second of the day. It is
responsible for generating and coordinating key
postural and visual reflexes for the maintenance
of balance and visual fixation. In addition, it has
an integral and complex sub-network of commissural neural fibers, known as the velocity storage
mechanism, which augments weak peripheral
inputs, and assists in the adaptation of peripheral
vestibular damage. This central mechanism also
allows for the compensation of vestibular pathology and rebalancing of asymmetric vestibular
inputs.
TRANSITION BETWEEN
PERIPHERAL AND CENTRAL
VESTIBULAR ANATOMY
The transition zone between the peripheral and
central vestibular system occurs at the neuroglialneurilemmal junction within the IAC near the
lateral aperture (or otic porus) (Nager, 1993). This
transition zone is significant as myelination converts from peripheral neuroglial cells, known as
Schwann cells, to central neuroglial cells, known
as oligodendrocytes. Upon exiting the medial
aperture of the IAC, the now fused superior and
inferior braches of the vestibular nerve course
through the cerebellar pontine angle cistern where
most afferent nerve fibers enter the brainstem at the
rostral medulla (Gacek, 2005). The vestibular nerve
courses medially, passing between the inferior cerebellar peduncle and the descending tract of the trigeminal nerve prior to entering the vestibular nuclei
(Lysakowski et al., 1998). Here, ascending and
descending vestibular neural fiber tracts innervate
53
54 Rotational Vestibular Assessment
the central vestibular nuclei. A smaller subset of
afferent fibers directly innervates the cerebellum
(Gacek, 2005). From the vestibular nuclei, an extremely
complex arrangement of afferent and efferent projections coordinates postural stability and visual
stabilization. Projections from the vestibular nuclei
are numerous and innervate many central nervous
system ganglia and neural centers. On a gross anatomical level, the vestibular nuclei integrate signals
form the peripheral vestibular end organs with
those from the spinal cord, cerebellum, visual system, and the contralateral vestibular nuclei. More
specifically, the vestibular nuclei project to the ocular motor nuclei (oculomotor, abducens, and trochlear nuclei), reticular and spinal centers concerned
with skeletal movement, the vestibular regions of
the cerebellum (flocculus, nodulus, ventral paraflocculus, and ventral uvula), as well as the thalamus and hippocampus. In addition, each vestibular
nucleus has major projections to the opposing contralateral vestibular nuclei (Goldberg & Hudspeth,
2000). An overview of the vestibular nuclei and the
key central structures involved in vestibular processing is illustrated in Figure 3–1.
FIGURE 3–1. Central vestibular system, showing the four vestibular nuclei (inset box) and their respective ori-
entation within the brainstem. The primary ocular motor nuclei (Oculomotar, Trochlear, and Abducens) are also
respectively identified, in addition to the primary vestibular brainstem tracts (Vestibulospinal Tract and the Medial
Longitudinal Fasciculus or MLF). The superior and inferior branches of the vestibular nerve are seen innervating
the vestibular nuclei. From Neuroscience for the Study of Communication Disorders (4th ed.) by A. C. Bhatnagar,
2013, Baltimore, MD, Lippincott, Williams & Wilkins. Reprinted with permission.
3. Anatomy and Physiology of the Central Vestibular System, Velocity Storage, and Central Compensation 55
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FUNDAMENTAL ROLE OF
STIBULAR NUCLEI
VE
The fundamental role of the vestibular nuclei is
the integration of head, neck, and trunk movements in order to coordinate an appropriate and
compensatory eye movement or postural response
(Lysakowski et al., 1998). The coordinated eye
movement occurs in the opposite direction of head
movement, but at a velocity that is (ideally) equal
to head movement. This ensures that the visual
system can maintain stabilization of an image on
an extremely small area of the retina known as the
fovea. In fact, clarity of an image significantly deteriorates even if the intended image was to “slip”
1° to 2° from the center of the fovea (Wong, 2008).
For best visual clarity, it is imperative that images
be held stable within 0.5° of the center of the fovea
(Wong, 2008). If this process does not occur, or
fails to occur efficiently, an inadvertent slip of the
image will occur on the retina and cause significant
blurring or even “visual jumping” of the image
and visual scene during active head movements
(this is known as oscillopsia) (Leigh & Zee, 2006).
A primary and fundamental basis of vestibular
anatomy and physiology is critical to the understanding of this compensatory process. The neural
foundation of the compensatory eye movement is
a three-neuron reflex pathway known as the vestibular-ocular reflex (VOR) (Leigh & Zee, 2006).
This reflex is discussed in length in Chapter 4.
The importance of understanding the VOR in
association with its peripheral and central structures is vital for understanding and interpreting
most vestibular testing. Three primary central
vestibular structures are essential to generating
the VOR, namely the vestibular nuclei, the cerebellum and central commissural neural tracts.
and 3–2). The vestibular nuclei complex encompasses the superior nucleus (of Bechterew), the
lateral nucleus (of Deiters), the medial nucleus (of
Schwalbe), and the inferior nucleus (or descending
nucleus) (Leigh & Zee, 2006). Several other minor
nuclei cell groups, most notably the Y-group, have
also been identified as being a part of the vestibular nuclei complex.
Superior Vestibular Nucleus
The superior vestibular nucleus (SVN) is located
dorsally and rostrally in the vestibular complex
(Lysakowski et al., 1998). The SVN is topically
organized. That is, most first-order afferent nerve
fibers of large to medium diameter innervate the
center, whereas smaller diameter fibers innervate
the periphery of the SVN (similar to the innervation pattern of the peripheral end organs and vestibular nerve branches) (Lysakowski et al., 1998).
The majority of afferent fibers within the SVN
originate from the crista of the SCCs (see Figure
3–2). A few fibers from the otolith macula have
been identified in the lateral aspect of the SVN.
Other than vestibular afferents, another major
group of fibers terminating in the SVN originate
from the cerebellum. (Baloh & Honrubia, 2001).
Efferent second order neurons from the SVN
predominantly ascend through the medial longitudinal fasciculus (MLF) and the ascending tract
of Dieters to innervate the ocular motor nuclei
(Leigh & Zee, 2006). In light of these innervations,
the SVNs primary role is that of maintaining and
coordinating visual stabilization (ocular reflexes)
in response to head movement, thus forming a
critical neural center to the VOR arc.
Lateral Vestibular Nucleus
The lateral vestibular nucleus (LVN) is located
inferior to, and at the caudal end of the SVN.
VESTIBULAR NUCLEI
The vestibular nuclei are subdivided into four
distinct large nuclei, which are anatomically segregated with respect to their functionality and
labeled with regard to their mutual anatomical
position with respect to one another (Figures 3–1
Primary afferents that terminate within the LVN
originate primarily from the cerebellum, predominantly the vermis and fastigial nuclei (Lysakowski
et al., 1998). Few neurons within the rostroventral
portion of the LVN receive vestibular afferents
from the cristae as well as the maculae, suggesting
some involvement in maintaining the VOR (Leigh
& Zee, 2006) (see Figure 3–2). A lesser number of
56 Rotational Vestibular Assessment
A B
FIGURE 3–2. Central vestibular nuclei (VN), showing the four vestibular nuclei [Superior VN (S); Lateral VN
(L); Medial VN (M); Descending or Inferior VN (D)]. The primary afferent vestibular nerve branch divisions are differentiated between the Superior Nerve Branch (A), and Inferior Nerve Branch (B). The Superior Nerve Branch
carries neural information the vestibular nuclei from the Anterior SCC (AC), the Horizontal SCC (HC), and the
Utricle (UT). The Inferior Nerve Branch carries neural information from the Posterior SCC (PC) and the Saccule
(SA). The superior and inferior branch of the vestibular nerve are seen innervating the vestibular nuclei. From
Baloh and Honrubia’s Clinical Neurophysiology of the Vestibular System (4th ed.) by R. W. Baloh, V. Honrubia,
and K. A. Kerber, 2011, New York, NY, Oxford University Press. Reprinted with permission.
afferent fibers have been identified to originate
from the spinal tract as well as the contralateral
vestibular nucleus through commissural fiber
tracts (Barin & Durrant, 2000; Leigh & Zee, 2006).
Most of the efferent fibers from the LVN
project primarily through the lateral and medial
vestibulospinal tracts, suggesting, “an important
station for control of vestibulospinal reflexes, particularly those involving the forelimbs” (Baloh &
Honrubia, 2001, p. 56).
bia, 2001). The MVN is the largest and most complex nucleus within the vestibular nuclei complex.
Its anatomical separation from the SVN is less
distinct than any other neighboring nuclei within
the complex. It is here where small and medium
afferent vestibular fibers from the cristae of the
SCCs (see Figure 3–2) as well as from the fastigial
nucleus and flocculus of the cerebellum have been
identified (Baloh & Honrubia, 2001). Moving ventrally within the MVN, small and medium fibers
from both otolith maculae (saccule and utricle) are
also found (Lysakowski et al., 1998). Within the
Medial Vestibular Nucleus
caudal and ventral portion, a preponderance of
afferent fibers from the fastigial nuclei and noduThe medial vestibular nucleus (MVN) is located
beneath the floor of the fourth ventricle, caudal to
the SVN and medial to the LVN (Baloh & Honru-
lus of the cerebellum have been identified. Other
afferents of lesser number are found to originate
from the contralateral MVN as well as the reticu-
3. Anatomy and Physiology of the Central Vestibular System, Velocity Storage, and Central Compensation 57
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lar formation and nucleus prespositus hypoglossi
(NPH) (Baloh & Honrubia, 2001; Leigh & Zee,
2006). The MVN-NPH complex plays a critical
role in the transformation and maintenance of
spatial eye position known as the neural integrator (Hain & Helmisnki, 2007).
Primary efferent fibers from the MVN project
through the MLF, the spinal cord, the contralateral MVN, and the cerebellum. In light of these
projections, it is suspected that the MVN is a critical center for the coordination of the VOR, central
compensation, as well as head and neck movements (Leigh & Zee, 2006).
Inferior Vestibular Nucleus
The inferior vestibular nucleus (IVN) is located
lateral to the MVN and inferior to the LVN. Primary afferent fibers from the cristae and maculae
terminate in the lateral aspect of the IVN whereas
afferent fibers from the cerebellum (flocculus,
nodulus and uvula) terminate throughout the
IVN (Baloh & Honrubia, 2001) (see Figure 3–2).
The IVN has been identified as being a primary
receptor of vestibular afferents from the otolith
maculae (Lysakowski et al., 1998).
The majority of efferent fibers from the IVN
terminate in the cerebellum and the reticular formation with secondary projections to the vestibulospinal pathways (Baloh & Honrubia, 2001;
Lysakowski et al., 1998). In light of these efferent projections, the IVN is likely critical for the
coordination of postural control through the vestibulospinal reflexes, and the cervicocollic (neck)
reflexes.
Y-Group
The Y-group is located caudal and lateral to the
SVN, and is bound dorsally by the LVN and ventrally
by the inferior cerebellar peduncle (Lysakowski
et al., 1998). Many of the afferents originate from
the cerebellar flocculus and the saccular macula.
Efferent projections from the Y-group nuclei predominantly innervate the cerebellar flocculus and
appear to mediate and coordinate vertical eye
movements (Lysakowski et al., 1998).
A discussion of vestibular physiology without
detailing the cerebellum’s relevant contributions
would be a significant shortfall. The cerebellum is
well known to be a major recipient and propagator of neural fibers both from and to all four vestibular nuclei (Gacek, 2005). In fact, the cerebellum
even receives a group of afferent fibers directly
from the periphery without first innervating the
vestibular nuclei (Barin & Durrant, 2000). The cerebellum is foremost an adaptive processor that
monitors vestibular input and readjusts central
vestibular processing (output) if necessary (Hain
& Helminski, 2007). Interestingly, if the cerebellum were removed, vestibular responses (reflexes)
would still occur; however, the responses would
be poorly calibrated, ineffective, and extremely
inefficient (Hain & Helminski, 2007).
Cerebellar Divisions
The cerebellum consists of the cerebellar cortex
and the cerebellar white matter. The cerebellar
cortex is comprised of ten distinct lobules that
extend outward from the apex of the roof of the
fourth ventricle and medially from the midline
of the cerebellum known as the vermis; a narrow
midline zone coursing the entire sagittal cerebellar plane (Arslan, 2001). These ten lobules are
categorized into three primary lobes; the anterior
lobe, the posterior lobe, and the flocculonodular
lobe. The anterior lobe is comprised of lobules I
to V and the posterior lobe is comprised of lobules VI to IX (Figure 3–3A). The flocculonodular
lobe is separated by the posterolateral fissure and
is comprised solely of lobule X (Figure 3–3B).
Beneath the cerebellar cortex is the cerebellar
white mat-ter from which axons project ventrally
to innervate four deep, bilaterally paired cerebellar nuclei; most important of which are the fastigial nuclei. All ten lobules of the cerebellar cortex
project axons to the deep cerebellar nuclei with
the exception of the flocculonodular lobe, whose
nerve fiber projections course directly to the vestibular nuclei (Brodal, 2004). The axonal projec-
CEREBELLUM
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