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28 Rotational Vestibular Assessment
in a potassium-rich fluid known as endolymph
(Lysakowski, McCrea, & Tomlinson, 1998). The
sensory end organs of the cochlear and vestibular systems are highly complex and are ultimately
responsible for hearing and sensing motion,
respectively. Within the vestibular membranous
labyrinth are five sensory end organs responsible
for sensing movement and postural orientation
both during motion, and at rest. These five vestibular sensory end organs synapse directly with
the eighth (VIII) cranial nerve in order to provide
a neural response that can be coordinated by the
central nervous system (Baloh & Honrubia, 1998).
The five vestibular sensory end organs, as well as
two vestibular branches of the VIII cranial nerve,
are collectively known as the peripheral vestibular system (Figure 2–1). Their anatomy and physiology are highly complex. Although much is
known regarding its form and function, much still
remains undiscovered. Ongoing research continues to uncover information regarding the molecular microstructure, proteomics, neural response
properties, and adaptation/compensation mechanisms of the vestibular system.
Each peripheral vestibular system is comprised of five sensory end organs and two vestibular branches of the VIII cranial nerve (CN).
The five sensory end organs of the peripheral vestibular system include two primary groups of vestibular sense organs; the cristae ampullari of the
semicircular canals, and the otoconia-rich matrix
of the otoliths, known as the maculae (Lysakowski
et al., 1998). There are three semicircular canals
and two maculae within each vestibular membranous labyrinth. The three semicircular canals are
responsible for sensing angular acceleration and
are identified and labeled with respect to their orientation in space: the horizontal, the anterior, and
FIGURE 2–1. Peripheral vestibular membranous labyrinth showing vestibular
sensory end organs (semicircular canals, saccule, and utricle) and the innervations
of the superior and inferior branch of the vestibular nerve. From Three Unpublished
Drawings of the Anatomy of the Human Ear by M. Brödal, 1946, Philadelphia, PA,
W. B. Saunders. In Cummings et al. (1998), Otolaryngology Head and Neck Surgery
(3rd ed.), St. Louis, MO, CV Mosby. Reprinted with permission.
2. Anatomy and Physiology of the Peripheral Vestibular System 29
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the posterior semicircular canal (Lysakowski et al.,
1998). The two maculae, the utricle and saccule,
are responsible for sensing gravity, linear acceleration, and static head tilt (Leigh & Zee, 2006). Fundamental to the function of all vestibular receptor
sensory organs is the vestibular hair cell, of which
there are two types, type I and type II (Baloh &
Honrubia, 2001). The hair cells produce the bioelectric response that synapses with cranial nerve
VIII. Along with the cochlear nerve, there are two
branches of the vestibular nerve, the superior and
inferior branch. The two vestibular branches are
named according to their anatomical orientation
to one another.
VESTIBULAR HAIR
CELL RECEPTOR
The basic element of all vestibular end organs is
the hair cell. Similar to the outer and inner hair
cells located within the organ of Corti, the vestibular hair cell transduces mechanical forces to nerve
action potentials (Baloh & Honrubia, 2001).
Endovestibular Potential and
Hair Cell Stereocilia Bundles
The apical surfaces of all vestibular hair cells bathe
in the potassium-rich (K+) endolymph, which,
in contrast to the +80 mV cochlear endolymph
potential, is much lower at +5 to 10 mV (Baloh
& Honrubia, 1998). Extending from the surface
of the cuticular plate of each vestibular hair cell
are the mechanosensing organelles, known as stereocilia bundles. Unlike the prestin-rich hair cells
and stereocilia bundles in the cochlea that actively
contract to electrical transduction, vestibular hair
cells and their respective stereocilia bundles contain the protein actin, and carry out flagella-type
movement (Baloh & Honrubia, 2001). The reason
for this is not completely understood; however,
Baloh and Honrubia (2001) offer a possible explanation.Although it remains unproven, Baloh and
Honrubia indicate that it is possible that hair cells
at the periphery of the vestibular organs actively
pull the cupula or otolithic membrane to influence
the response of the more centrally placed hair cell.
This, of course, is analogous to the cochlear amplifier and the effect of cochlear outer hair cells on
inner hair cells. In addition, there is evidence to
suggest that deflection of the stereocilia exhibits
non-linear transduction potentials with respect to
their degree of displacement (Baloh & Honrubia,
2001). Small stereocilia displacements produce
a more linear response in the hair cell receptor
potential, whereas large displacements produce
potentials that become non-linear near saturation
of the hair cell receptor response (Baloh & Honrubia, 2001). This is likely a key component of the
cupular pendulum model, which is discussed in
subsequent chapters of this text.
Vestibular Hair Cell Types
There are two types of labyrinthine hair cells in
the vestibular system, type I and type II hair cell
receptors (Figure 2–2). Type I vestibular hair cells
are globular in shape and are often marked by
a single afferent synapse with a terminal nerve
ending known as a chalice, or calyx (Lysakowski
et al., 1998). This calyx-type nerve ending completely surrounds the basal portion of the hair
cell. Type II vestibular hair cells are cylindrical
in shape and are often marked by multiple, and
much smaller, efferent, and afferent nerve endings
known as boutons (Lysakowski et al., 1998). There
is a clear morphologic organization of type I and
type II sensory hair cells across all sensory epithelium. In general, type I hair cells, with their large
calyx nerve endings and larger diameter afferent
nerve fibers, are predominantly located in the
center of the various vestibular epithelia, whereas
the type II hair cells are more common along the
periphery (Baloh & Honrubia, 2001). This holds
true for both the cristare ampullari, as well as
the maculae.
In the human vestibular end organ, there are approximately 23,000 hair cells (type I and type
II) in the SCC cristae, and about 52,000 in the
two maculae (Baloh & Honrubia, 2001). Type I
and type II hair cells are present in nearly a 1:1
ratio within the vestibular end organs (Harsha, Phillips, & Backous, 2008). The functional
(physiological) differences between type I and
type II hair cells occur largely because of the
different afferent innervation to each hair cell
type, rather than the morphological differences
30 Rotational Vestibular Assessment
FIGURE 2–2. Type I and type II vestibular hair cells and their
respective innervations. From Baloh and Honrubia’s Clinical Neurophysiology of the Vestibular System (4th ed.) by R. W. Baloh, V. Hon-
rubia, and K. A. Kerber, 2011, New York, NY, Oxford University Press.
Reprinted with permission.
between each hair cell. These innervation differences have a significant implication with respect
to the tuning of vestibular afferents, which is discussed in greater detail later.
SEMICIRCULAR CANALS
The semicircular canals (SCC), and their respective
cristae ampulla, respond to angular acceleration.
The horizontal canal, also known as the lateral
canal, is responsible for sensing angular movement of the head or body in the horizontal, or yaw,
plane. The anterior aspect of the horizontal SCC is
inclined approximately 30° upward from the true
horizontal plane, connecting the external auditory
canal to the lateral canthus, known as Reid’s base-
line (Della Santina, Potyagaylo, Migliaccio, Minor,
& Carey, 2005) (Figure 2–3). The anterior SCC, also
known as the superior SCC, is primarily re-sponsible for sensing angular movements in the roll
(side-to-side) plane. The anterior SCC is oriented approximately 90° from the horizontal SCC
(Della Santina et al., 2005). The posterior SCC, also
known as the inferior SCC, is responsible for sensing angular movements in the anterior-posterior,
or pitch, plane. The posterior SCC is oriented approximately 92° from the horizontal canal. The
posterior and anterior SCCs are oriented roughly
24° from one another (Schwarz & Tomlinson, 2005).
Collectively, the SCCs exhibit an orthogonal (mutually perpendicular) relationship to one
another, such that the three-dimensional orientation is similar to the meeting of two conjoining
walls and floor at the corner of a room meeting,
forming three surfaces at right angles to one
2. Anatomy and Physiology of the Peripheral Vestibular System 31
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FIGURE 2–3. Orientation of the horizontal semicir-
cular canal with respect to the horizontal plane. The
horizontal canal is oriented at an approximate angle of
+30° from the horizontal plane, which forms an imaginary line from the tragus to the lateral ocular canthus,
known as Reid’s baseline. From Barin, K. & Durrant, J.
(2000). Applied physiology of the vestibular system. In
P. R. Lambert & R. F. Canalis (Eds.). The Ear: Compre-
hensive Otology, Philadelphia, PA, Lippincott Williams
& Wilkins. Reprinted with permission.
another (Figure 2–4). Due to the slightly less than
perfect orthogonal relationship between the three
canals, as well as the functional implication that
head or body movements occur rarely, if ever, in a
FIGURE 2–4. Orthogonal or coplanar relationship of
the semicircular canals showing the orthogonal planar
relationship of the right anterior and the left posterior
canals (RALP plane), the left anterior and the right posterior canals (LARP), and the right and left horizontal
canals. From Barin, K. & Durrant, J. (2000). Applied
physiology of the vestibular system. In P. R. Lambert
& R. F. Canalis (Eds.). The Ear: Comprehensive Otol-
ogy, Philadelphia, PA, Lippincott Williams & Wilkins.
Reprinted with permission.
single plane of motion, excitation of a single crista
ampullaris from a distinct vestibular labyrinth is
highly unlikely (Barin & Durrant, 2000). Therefore, it is almost certain that a neural synapse
occurs from two, if not all three cristae ampullari,
during normal daily activities.
end of each horizontal SCC articulates directly
with the vestibule, however, the open ends of the
anterior and posterior SCCs first form a common
crus before communicating with the vestibule.
The opposite end of each SCC crus, known as the
ampulla, is a distended or bulbous portion and
Semicircular Canal Ampullaris (Cupula)
is approximately twice the diameter of each SCC
arching crus (Gulya, 1997). Each ampullated end
Each SCC crus arches approximately 240° (Gulya,
1997), and is marked by two distinct ends, a closed
amupullar end and an open end. The open end of
each SCC crus articulates with the common vestibule of each labyrinth and allows for free endolymph flow in and out of each SCC. The open
is “occluded” by a fluid-tight partition known as
the cupula (Figure 2–5). The cupula is a gelatinous
mass with approximately the same density as
the surrounding endolymph. The cupula is held
tight against the entire ampullar lumen, forming
a fluid-tight diaphragm. Although the cupula is
A
B
FIGURE 2–5. A. Semicircular canal cupula forms a fluid-tight partition against the surrounding walls of the
ampullar ending of each semicircular canal. Embedded within each cupula are a number of stereocilia bundles
that deflect in response to angular acceleration of the head in space. From Principles of Human Physiology (6th
ed.) (p. 323) by C. L. Stanfield, 2017, Pearson Education. Reprinted with permission. B. Deflection of the fluid-tight
cupula and underlying stereocilia bundles via head rotation creates either an excitation or inhibition response.
From Barin, K. (2009). Clinical neurophysiology of the vestibular system. In Katz et al. (Eds.). Handbook of Clinical
Audiology (6th ed.). Baltimore, MD: Lippincott Williams & Wilkins.
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2. Anatomy and Physiology of the Peripheral Vestibular System 33
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not structurally attached to the ampullar lumen,
the opening is “sealed” by the cupula due to cellular turgor pressure (Lysakowski et al., 1998). The
density matching of the cupula to the surrounding
endolymph is critical so as not to exert a resting
force on the sensory epithelium embedded within
the cupula due to a potential negative gravity vector that could be applied during certain orientations of the head in space.
Crista Ampularis and the Semicircular
Canal
Beneath the cupula, and forming the floor of the
ampulla, is the crista ampullaris, which houses
the neurosensory epithelium containing the sensory hair cells and stereocilia bundle that are ultimately responsible for creating the neural synapse
to the VIII CN (see Figure 2–5). Stereocilia bundles
project through the cuticular plate of each sensory
hair cell, and are embedded into the gelatin mass
of the surrounding cupula. A single stereocilia
bundle is comprised of approximately 100 to 200
stereocilia and a single kinocilium that are linked
together with microtubules known as tip-links
(Gacek, 2005). Orientation of the kinocilium, in
relation to the stereocilia bundle, is dependent on
each SCC. The kinocilium of the horizontal SCC
is located on the side of the crista closest to the
vestibule, whereas the kinocilium of the anterior and posterior SCC is located on the side of
the crista closest to the canal (Lysakowski et al.,
1998). Orientation of the kinocilium is critical, as
deflection of the kinocilium toward or away from
the stereocilia will determine the type of polarization, or bioelectric response that is applied to the
underlying vestibular hair cell (see Figure 2–6).
Deflection of the stereocilia bundle toward the
kinocilium causes a depolarization of the underlying hair cell and results in a transduction of potassium cations into the hair cell. As described by
Harsha, Phillips, and Backous (2008), this influx
causes a positive deflection of the resting membrane potential of the hair cell, and a subsequent
opening of the voltage-gated calcium channels
at the basal-lateral aspect of the cell. Subsequently,
there is an influx of calcium and an increase in
Neurosensory Epithelium
the release of excitatory neurotransmitter glutamine, which leads to an increase in the firing rate
of the afferent vestibular neurons. Deflection of
the stereocilia bundle away from the kinocilium
causes a hyperpolarization of the underlying
hair cell and results in an inhibitory bioelectric
response (Barin & Durrant, 2000). A subsequent
decrease in the action potentials of the synapsing afferent vestibular nerve fibers ensues (Figure
2–6).
Stereocilia deflection toward or away from
the kinocilium occurs in response to angular
accelerations within the plane of a particular SCC.
Free-flowing endolymph lags behind and exerts
a hydrodynamic pressure against the cupula.
Consequently, the cupula is deflected by the lagging endolymph flow in the direction opposite
head rotation. The resulting displacement of the
embedded stereocilia bundle, either toward or
away from the kinocilium, polarizes the underlying sensory hair cells. Depending on the induced
polarization, an excitatory or inhibitory neural
synapse is applied to the afferent vestibular nerve
fibers (see Figure 2–6).
Coplanar Semicircular Canal Physiology
Each of the three semicircular canals is oriented
in such a way that the polarization of a particular crista ampullaris from one otic capsule has an
antagonistic, yet complementary, polarization from
a SCC in the opposing otic capsule. As a result,
when depolarization (excitation) of a particular
SCC occurs from the right otic capsule, hyperpolarization (inhibition) occurs from the complementary SCC in the left otic capsule (Baloh &
Honrubia, 1998) (Figure 2–7). The horizontal semicircular canals are complements of one another.
However, due to the orthogonal relationship of
the SCCs, the posterior and anterior canals of
opposing otic capsules are complements of one
another. The anterior canal in one otic capsule is
oriented in the approximate plane as the posterior
canal in the opposite otic capsule. This orientation
is often referred to as coplanar (Baloh & Honrubia, 1998) (see Figure 2–4). Specifically, the right
anterior semicircular canal is aligned with the left
posterior canal, which is often referred to as the
A
B
C
FIGURE 2–6. Semicircular canal cupular deflection causes a subsequent deflection of the underlying stereocilia
embedded within the cupula. A. At rest (no head movement) the underlying resting neural potential (firing rate)
is approximately 90 spikes per second. B. Deflection of the kinocilium away from the stereocilia bundle causes
depolarization of the hair cell and a subsequent increase in the resting neural firing rate above the resting neural rate
of 90 spikes per second (excitation). C. Deflection of the kinocilium toward the stereocilia bundle causes polarization
of the hair cell and a subsequent decrease in the resting neural firing rate below the neural resting rate of 90 spikes
per second (inhibition). 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.
34
2. Anatomy and Physiology of the Peripheral Vestibular System 35
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Discharge neural firing rate from
the left h-SCC decreases
Left Neural Rate Right Neural Rate
Head turn to right
Resting Neural Rate
Approximately 90 s/s
Discharge neural firing rate from
the right h-SCC increases
Utricle Utricle
EXCITAT ION INHIBITION
FIGURE 2–7. Image depicting left and right deflection of the cupulae and underlying stereocillia
within the horizontal semicircular canals in response to a right head turn. Green arrows depict
the direction of endolymph flow. Deflection of the right cupula is toward the utricle (ampullopetal
stimulation), which causes deflection of the stereocilia toward the kinocilium, and a subsequent
increase in the neural firing rate. Deflection of the left cupula is away from the utricle (toward
the canal (or ampullofugal stimulation), which causes deflection of the stereocilia away from the
kinocilium, and a subsequent decrease in the neural firing rate. Adapted from Barin, K. & Durrant,
J. (2000). Applied physiology of the vestibular system. In P. R. Lambert & R. F. Canalis (Eds.).
RALP plane of excitation/inhibition. Conversely,
the left anterior semicircular canal is aligned with
the right posterior canal, which is often referred
to as the LARP plane of excitation/inhibition. The
same is true of the opposing right and left horizontal semicircular canals (see Figure 2–4). It is
with similar polarizations, and (3) It offers a physiological redundancy that allows for detection of
head movement despite a diseased labyrinth due
to the inhibition (decrease) of neural activity from
the intact labyrinth (Gacek, 2005). Each of these is
discussed in greater detail later.
this coplanar orientation that is often referred to
as a push-pull arrangement insomuch that one
SCC is always excited as its opposing complement
is always inhibited (see Figure 2–7). The advantages of this push-pull arrangement are threefold:
Cupular Physiology and the
Cupular Pendular Model of
Semicircular Canal Function
(1) It allows for a more effective functional recovery, or compensation, if damage is to ever occur in
one labyrinth, (2) It also allows for a greater neural
disparity between the two labyrinths than if each
were operating independent of one another or
The primary function of the cupula and the embedded neural sensory epithelium of the semicircular canals are to mechanically integrate angular
head movement, specifically acceleration, into an
36 Rotational Vestibular Assessment
afferent neural response, which will ultimately
be encoded into an eye velocity response. The
mechanical properties of the cupula have been
likened to a dampened torsional pendulum
within the ampulla whose displacement from the
hydrodynamic pressure form the free-flowing
endolymph within the membranous canal can be
mathematically described and predicted (by the
pendular model of cupular mechanics) (Baloh &
Honrubia, 1990; Leigh & Zee, 2006). In response to
head accelerations, the viscoelastic cupula is displaced (deflected) within the ampulla, and subsequent shearing of the embedded stereocilia occurs.
Any changes in head acceleration will continuously alter the mechanical properties of the “pendular” cupula, causing a continuously changing
neural signal being delivered to the afferent nerve
fibers. Because of this, the cupula has often been
referred to as an accelerometer, as it detects and
monitors changes in head or body accelerations.
It has been reported that the SCCs are exquisitely
sensitive to head accelerations as small as 0.1° per
second squared. This equates to the completion of
a single 360-degree rotation in approximately 90
seconds (Harsha et al., 2008). In addition, the operating frequency range of the sensory epithelium
of the semicircular canals is between 0 to 20 Hz,
which is well beyond the 1 to 6 Hz functional range
of everyday life activities (Schubert & Shepard,
2008). Under constant and sustained velocity
(unchanging acceleration), however, the viscoelastic properties of the cupula become significant and
cause it to return to its resting position with an
exponentially decaying time course (in accordance
with the pendular model). Although cupular time
decay has not directly been measured in humans,
it is estimated to be around 6 seconds (Leigh &
Zee, 2006). In other words, it takes approximately
6 seconds for the viscoelastic cupulae to return to
their resting position following an abrupt acceleration, despite ongoing constant velocity. This pendular model of cupular mechanics is well known
and reflects how the vestibular system responds
to accelerations and not velocity. Consequently,
the pendular model of cupular mechanics also
forms the mathematical prediction construct for
understanding normal vestibular physiology, as
well as vestibular pathology (Leigh & Zee, 2006).
OTOLITH RECEPTORS
The otolith sensory receptors, collectively known
as the maculae, are comprised of the utricle and
the saccule. The utricles are located directly behind
the ocular orbits, whereas the saccules are positioned essentially behind the maxillary sinuses
(Figure 2–8). The orientation of the utricle and
saccule within each otic capsule is roughly in the
horizontal and vertical planes, respectively (Figure 2–9). The maculae are anatomically oriented
such that each otolith’s epithelium is positioned at
approximately 90 degrees (or right angles) to one
another. As the semicircular canals are responsible for sensing angular acceleration, the otolith sensory receptors are responsible for sensing
translational accelerations in the linear plane. Specifically, each otolith receptor is described as being
a curved-shaped sac that senses gravitational, linear, tangential, and centripetal forces during head
movement (Leigh & Zee, 2006).
Utricle Anatomy
The utricle is an oddly shaped elliptical tube that
is tilted backward and downward by 25 to 30
degrees, and laterally by about 10 degrees (Figure 2–10). This orientation is nearly identical to
the orientation of the horizontal SCC. Since normal head position tilts the stereotactic plane by
about 25 degrees with the chin downward, both
of these structures are normally positioned in the
plane of their maximum sensitivity during daily
life activities (Schwarz & Tomlinson, 2005). This is
in accordance with Ewald’s first law, which states
that maximum afferent excitation (and inhibition)
will occur in the spatial plane of the particular
semicircular canal being stimulated. Although
Ewald’s law describes semicircular canal function,
a similar (although more complex) pattern can be
derived from utricular stimulation.
The utricle contains the sensory epithelium
that transduces horizontal linear accelerations
into neural afferent signals. Also located within
the utricular sac is the open end of the h-SCC crus,
as well as the common crus opening of the a-SCC/
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FIGURE 2–8. Anatomical location and orientation of the maculae with
respect to the head in the upright position. Dashed lines further indicate the
relative orientation of each semicircular canal. Inset image depicts the otoconial layer with respect to the surface of each maculae. From The Physiology
of the Vestibuloocular Reflex (VOR) by B. Cohen and T. Raphan, 2004, New
York, NY, Springer. Reprinted with permission.
p-SCC. Here, endolymph flows freely between
the nonampullated ends of each SCC crus and the
utricle. Within the utricular space on the anteroinferior wall lays the utriculo-endolymphatic valve
from which extends the endolymphatic duct, which
terminates in the endolymphatic sac. The valve
is believed to act in a passive manner to release
excess endolymphatic pressure (Gulya, 1997).
Saccule Anatomy
The saccule is a flattened sac that lies in the vertical parasagittal plane, with its lower end deflected
laterally by about 18° (Schwarz & Tomlinson, 2005).
It lays inferiorly and at an approximate right-angle
to the utricle (see Figures 2–8 and 2–10). Of particular note to saccular anatomy is its proximity to the
cochlea. Of all the vestibular sensory end organs,
the saccule endolymphatic space is the only one
to communicate directly with the cochlea via the
ductus reuniens (see Figure 2–1). The saccule and
utricle are connected via the utricular and saccular
duct. The saccule contains the sensory epithelium
that transduces vertical linear accelerations into
neural afferent signals.
Otolith Orientation
The orientation of the otolith receptors in space
is critical to understanding their physiological
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