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60 Disorders of the Auditory System
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Figure 2–40. A block diagram illustrating the vestibular nuclei and their connections.
lor, sweating, nausea, and vomiting that
occur when function is lost either temporarily or permanently in one vestibular
end organ (see Figure 2–40). Finally, connections between the vestibular nuclei
and cortical centers located in the parietal
and temporal lobes make it possible for
the brain to interpret the pattern of electrical signals ascending from the brainstem
as motion (see Figure 2–40).
Function
Three primary vestibular reflexes act to
maintain our balance. The vestibulocolic reflex (VCR) acts on neck muscles to
stabilize the head based on the signals
received from the vestibular end organs.
The vestibulospinal reflex (VSR) serves to
stabilize the body, also based on signals
received from the end organs. Finally,
the purpose of the vestibulo-ocular reflex
(VOR) is to maintain clear vision during
head motion. It accomplishes this task by
generating compensatory eye movements
during head accelerations that act to keep
the fovea of the retina on an object of interest. For example, the VOR is recruited to
stabilize the environment when a person
is running on a treadmill. An intact VOR
enables that person to either read a magazine or watch a monitor at the same time.
For the sake of simplicity we describe
changes that occur in the VOR when a
person turns the head to the left. The horizontal VOR is shown in Figure 2–41.
When the head is turned to the left,
endolymph lags behind and bends the
stereocilia and kinocilia of the crista
2. Structure and Function of the Auditory and Vestibular Systems 61
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Figure 2–41. Dorsal perspective of the horizontal vestibulo-ocular reflex (VOR)
pathways. Key: HSC = horizontal semicircular canal, VNC = vestibular nuclear complex, VI = abducens nuclei, III = oculomotor nuclei, LR m.= lateral rectus muscle, MR
= medial rectus muscle.
m.
ampullaris in both lateral semicircular
canals. In this case, the left end organ is
housed in the leading ear for the head
movement and the endolymph moves
in a clockwise fashion, deflecting the stereocilia and kinocilia of the crista toward
the utricle. Because of the arrangement
of the stereocilia in the lateral canals, this
action results in an increasing firing rate
in the superior portion of the vestibu-
lar nerve on the left side. The stereocilia
and kinocilia of the crista ampullaris in
the right lateral canal, or the lagging ear,
are deflected away from the utricle driving the firing rate in the right vestibular
nerve below its tonic resting rate. The
two medial vestibular nuclei (MVN) each
receive input from the lateral semicircular
canals via the superior vestibular nerves.
In this case of a head turn to the left, the
62 Disorders of the Auditory System
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increased peripheral input from the left
end organ is received by the left MVN
and a corresponding decrease is received
by the right MVN. The left MVN routes
the increased activity to both ipsilateral
and contralateral secondary vestibular
neurons. Specifically, the left MVN sends
projections to the ipsilateral oculomotor nucleus (III) and sends decussating
projections to the contralateral abducens
nucleus (VI). On the right side, a similar
but opposite chain of events occurs. The
right MVN relays the decrease in neural
activity to the ipsilateral abducens nucleus
and also across the midline to the left oculomotor nucleus. The abducens and oculomotor nuclei send motor projections to
the effector organs of the horizontal VOR
(i.e., extraocular muscles).
In our example of a head turn to the
left, the right abducens nucleus and left
oculomotor nucleus relay the increased
neural input received from the vestibular nuclei through motor neurons to the
left medial rectus and right lateral rectus
muscles. This pattern of input to these
two extraocular muscles causes them to
contract, which results in a slow deviation
of the eyes to the right that is proportional
to the head turn.
Facilitating this rightward deviation
of the eyes is the corresponding decrease
in neural drive to the antagonist extraocular muscles (i.e., left lateral rectus
and right medial rectus). The decrease in
activity that has been relayed from the
vestibular nuclei of the lagging ear to the
left abducens and right oculomotor nuclei
decreases the neural drive to the oculomotor and abducens nerves and from there
to the right medial rectus and left lateral
rectus, resulting in a relaxation of these
muscles. The medial longitudinal fasciculus (MLF) is the brainstem pathway that
coordinates the outputs of the vestibular
nuclei to the oculomotor and abducens
nuclei, making it possible for these compensatory eye movements to be conjugate
(i.e., for the visual axes to be parallel).
In the situation where there is sustained rotation (i.e., if the person was in
a continuously rotating chair), this slow
deviation of the eyes in the opposite direction of the head turn will drive the eyes to
the lateral extremes in the orbits. Neurons
in the paramedian pontine reticular formation (PPRF) interrupt the flow of electrical activity from the vestibular periphery, which results in the eyes returning
quickly to midline. The neurons in the
PPRF go through a refractory cycle and
inputs from the vestibular system again
drive the eyes in the direction opposite
the head turn. This rhythmic, repetitive, sawtooth-shaped eye movement is
referred to as “nystagmus” and it will
continue as long as the asymmetry exists
between the two vestibular end organs (it
is the velocity of the slow eye deviation
that is measured in quantitative assessments of vestibular system function).
Due to endolymph-cupular dynamics, the vestibular system is less sensitive to angular accelerations of less than
0.05 Hz and greater than 3 Hz (Baloh &
Honrubia, 2001). For example, if rotation in the yaw plane (i.e., rotating about
a superior–inferior axis) is sustained at
a constant velocity, fluid motion will
approximate the speed of the canals and
the cupulae will drift back to their neutral position (Goldberg & Fernandez,
1971). In this case, it would seem intuitive that the subject being rotated would
no longer perceive the sensation of rotary
motion once the cupulae return to their
static state. However, horizontal VOR
responses to a sustained rotational stimu-
2. Structure and Function of the Auditory and Vestibular Systems 63
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lus persist approximately three times longer than the “drive” from the periphery.
This prolongation of the VOR after cessation of the neural drive from the peripheral end organs has been termed “velocity storage” and is under the control of
a neural integrator (NI) that is located in
the brainstem and cerebellum. The NI acts
to adjust the output of vestibular afferent
activity, allowing the VOR response to be
extended one order of magnitude beyond
what the canal responses produce, thereby
enhancing the low-frequency response of
the VOR. In order for the velocity storage
mechanism to function, several anatomic
structures must be intact. These include
the peripheral end organs, the vestibular
portions of the eighth cranial nerve, the
vestibular nuclei, the commissural fibers
connecting the vestibular nuclei (i.e.,
“direct” and “indirect” pathways), and
the connections between vestibular nuclei
and the cerebellum. The velocity storage
mechanism manifests itself in many of the
quantitative tests that are used clinically.
Impairment in the velocity storage system
has been shown to result in stereotyped
abnormalities on quantitative tests of vestibular system function and may contribute to balance impairments in unsteady,
fall-prone patients (Jacobson, McCaslin,
Patel, Barin, & Ramadan, 2004).
Vascular Supply for the
Vestibular System
The labyrinthine artery supplies the
peripheral vestibular end organs and is
known to have a variable origin (Baloh &
Honrubia, 2001). It arises most commonly
from a branch of the AICA, but occasionally can be a direct branch of the basilar
artery or the superior cerebellar artery.
Once the labyrinthine artery enters the
inner ear, it bifurcates into the common
cochlear artery and the anterior vestibular artery. The common cochlear artery
has two branches: the main cochlear
artery and the vestibulocochlear artery,
which turns into the posterior vestibular
artery. The main cochlear artery nourishes
structures within the cochlea (e.g., basilar membrane). The posterior vestibular
artery supplies the ampulla of the posterior semicircular canal and the majority of
the saccule (Schuknecht, 1993). The anterior vestibular artery supplies the vestibular portion of the eighth cranial nerve, the
ampullae of the anterior and lateral semicircular canals, the macula of the utricle,
and a small part of the saccule (Baloh &
Honrubia, 2001). The different sources of
blood supply to the different end organs
can be a source of the varying findings
often encountered during quantitative
vestibular testing.
summaRy
This chapter presented an overview of
some key aspects of the auditory and vestibular systems in regard to their structures and functions. The efferent pathways also were discussed as well as the
vascular anatomy of the peripheral and
central auditory systems and the vestibular system. This chapter serves as an orientation and reference for the subsequent
chapters on disorders of hearing that, by
their nature, require grounding in structure and function of the auditory system.
Acknowledgments. The authors grate-
fully acknowledge the contributions of
Devin McCaslin, PhD, Senior Associate
64 Disorders of the Auditory System
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Consultant and Associate Professor, Division of Audiology, Department of Otorhinolaryngology, Mayo Clinic Rochester,
and Gary Jacobson, PhD, Professor and
Director of the Division of Audiology,
Vanderbilt Bill Wilkerson Center, Department of Hearing and Speech Sciences,
Vanderbilt University for the vestibular
portion of this chapter.
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3
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Audiologic, Vestibular, and
Radiologic Procedures
intRoduction
The informal assessment of audiologic
difficulties can be traced back to a date
as early as 377
reported clinical observations of hearing
loss (Vogel, McCarthy, Bratt, & Brewer,
2007). The actual examination of the type
of hearing impairment can be dated to the
late 1700s when test measures such as tuning fork tests were first employed. More
precise methods of audiologic assessment were introduced in the 1920s with
the advent of the Western Electric 1-A
audiometer, which permitted better definition of the type of hearing loss and also
provided a means to evaluate the extent
and configuration of the hearing loss.
Advances in technology have changed
dramatically over the years; however,
many of the fundamental principles of
audiologic assessment remain the same.
Therefore, for readers who are not familiar with traditional audiologic assessment
measures, a review of the fundamentals of
peripheral assessment certainly deserves
b c when Hippocrates first
attention. New advances in our understanding of the role that the central auditory nervous systems (CANS) plays in
hearing, coupled with the development
of specialized electrophysiologic and
electroacoustic equipment that permits
the assessment of CANS integrity and
processes emerged in the 1970s. These
advances have opened up new assessment opportunities for the audiologist
and provided better diagnostic information for the patient. Therefore, a review of
central auditory assessment procedures
is also essential. It is beyond the scope of
this chapter to provide all of the details
necessary for a thorough understanding
of audiologic test procedures; instead, this
chapter provides a general overview of
relevant and more commonly used evaluation measures. In addition to the review
of both peripheral and central auditory
test procedures, a brief overview of various vestibular and radiologic assessment
procedures is provided in this chapter as
these are often used in conjunction with
audiologic testing to determine the anatomic site of lesion and/or the presence of
69
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