Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4612_Библиотеки_им_академика_М_И_Перельмана
.pdf
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
42
The auditory cortex completes advanced processing of auditory stimuli and, by many
interconnections to the remainder of the cortex, is responsible for speech understanding
The auditory cortex is asymmetrical in most humans, with the left being larger, which is
thought to be the basis for language dominance
The processing of auditory signals does not cease at the auditory cortex. There are many surrounding association areas and tracts (e.g., corpus callosum, cerebellum, insula) where auditory signals are
represented and transmitted, but these are beyond the scope of this chapter. Additionally, auditory
signals are not only processed in an afferent (ascending) manner, but there are efferent (descending)
tracts as well. For more information on these points, the reader is directed to Musiek and Baran (2020).
Vasculature of the Central Auditory Nervous System
Ascending from the neck are the vertebral arteries deep to the external carotid arteries. The latter
join together in the brainstem to form a basilar artery, which then leads into the circle of Willis. The
vertebrobasilar system is responsible for blood supply to the cochlear nucleus, SOC, IC, LL, and corpus
callosum. The circle of Willis supplies the internal and external capsules and thalamus. The internal
carotid branches into the middle cerebral artery (MCA), which supplies the temporal lobe, primary
auditory cortices, and insula.
Disorders of the CANS
Some disorders of the central auditory system are displayed in Appendix 2–D. These will present with
SNHL. Central disorders can present with abnormal MEMRs, rollover, asymmetrical hearing losses, or
sometimes normal hearing sensitivity due to the types and places of lesions, as well as the type of stimuli
presented (e.g., a person may effectively perceive tones but not words). Much like inner ear disorders,
there are no otoscopic findings that provide insight into possible etiologies of central disorders.
Vestibular System
The vestibular system is one of three systems (visual, proprioceptive, and vestibular) responsible for
maintaining normal posture, balance, and orientation. It reaches mature size by 17 to 19 weeks’ gestation and functions to transduce forces associated with head acceleration and gravity into awareness of
head and body in space and produce motor reflexes for postural and ocular stability. The following is
an overview of the structure and function of the vestibular system. Additional details are provided in
Chapter 7.
Peripheral Vestibular System
The peripheral vestibular system grossly is divided into two sections within the bony and membranous
labyrinths: the semicircular canals and the vestibule. The vestibular apparatus is located posterior to
the cochlea in the petrous portion of the temporal bone (Jacobson et al., 2021). This is shown in
Figure2–5.
CHAPTER 2 Anatomy, Physiology, and Relevant Pathologies
https://t.me/medicina_free
43
FIGURE 2–5. Anatomy of the peripheral vestibular labyrinth. The structures include the three semicircular
canals (horizontal/lateral, superior/anterior, and posterior) and two otolith organs (utricle and saccule).
Semicircular canal sensory epithelium contains the crista ampullaris and the otolith organs contain maculae
for the otolith organs. Source: From Lane JI, Witte RJ, Bolster B, Bernstein MA, Johnson K & Morris J. AJNR
Am J Neuroradiol. 2008 Sep;29(8):1436–1440; used with permission of Mayo Foundation for Medical
Education and Research, all rights reserved.
n
Endolymph fills the membranous labyrinth, created by the stria vascularis (continuous with
the scala media) and transmitted to the vestibular system by the ductus reuniens of Hensen.
n
Perilymph fills the space between the membranous and bony labyrinths, providing cushioning
between the two. It is supplied from the subarachnoid space via the perilymphatic duct.
Semicircular Canals
n
Detect angular motion
n
Three on each side: work as antagonistic pairs
1. Anterior/superior: 45° to frontal plane (angled forward); 90° to posterior canal
2. Posterior: 45° to frontal plane (angled backward)
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
44
3. Horizontal/lateral: 20° to 30° to horizontal plane; 90° to anterior/superior and
posterior canals
n
Canal extends into middle ear space through the medial and posterior walls (Furman &
Lempert, 2016)
AUDIOLOGY NUGGET
An easy way to remember the pairs of semicircular canals that are excited and
inhibited together is through the acronyms RALP and LARP (not live-action
role-playing). RALP stands for right anterior, left posterior, meaning if one is
excited, the other is inhibited. Similarly, LARP stands for left anterior, right
posterior, which correlates to the excitation and inhibition patterns of the two
semicircular canals.
n
Each semicircular canal contains a sensory organ called the crista ampullaris; these sit
perpendicular to each other and are in the enlargement at the base of the canal called the
ampulla. There are four parts to the crista ampullaris:
1. Cupula
●
Same specific gravity as surrounding endolymph
●
Gelatinous composition
●
Extends to “roof” of semicircular canals and separates endolymph of semicircular canals
from endolymph of utricle
●
Prevents endolymph from moving freely in the canal, but is elastic so it will bend slightly
in either direction
2. Type I and Type II HCs
●
Resting charge of –40 mV
●
Stereocilia contain tiplinks
●
Calcium (Ca+) and potassium (K+) channels
●
Type I more responsive within a small area of sensory epithelium
●
Flask-shaped
●
Crest of crista ampullaris and striola of macula
●
Calyx (calyces; large synapses that look like the calyx flower) to afferents
●
Type II react to stronger stimuli over entire sensory epithelium
●
Cylindrical
●
Small bouton afferents
●
Efferents synapse directly to cells
3. 20 to 100 stereocilia and one kinocilium (longest stereocilia) on surface of HCs
●
Project through crista and embedded in cupula
CHAPTER 2 Anatomy, Physiology, and Relevant Pathologies
https://t.me/medicina_free
●
Arranged in “haystacks” as opposed to stereocilia of the IHCs and OHCs
●
Second law of Ewald (1882) — see Chapter 7 for a review of the laws of Ewald
4.
Afferent nerve endings projecting to the vestibular nuclei in the brainstem
KNOWLEDGE CHECKPOINT
If a person turns their head to the left, endolymph will flow to the right in
both lateral canals. This will be excitatory (utricopetal) in the left semicircular
canals and inhibitory (utricofugal) in the right semicircular canals. The utricle
is located anterior to the semicircular canals, so if the endolymph is moving
forward, it will deflect the cupula forward, or toward the utricle. On the other
hand, if the endolymph is moving backward, it will deflect the cupula backward,
or away from the utricle.
n
Physics of the semicircular canals
Friction, viscosity, mass, and acceleration determine fluid lag and angle of displacement
●
Greater mass means that there needs to be a greater force/acceleration to move the fluid
and/or cupula
45
Increase of cupula stiffness or increase of endolymph viscosity: lower canal sensitivity and
shorter postrotary sensations
Increase of absolute endolymph mass: higher canal sensitivity and longer postrotary
sensations
Change of endolymph specific mass compared to that of the cupula: sensitivity of the
canals for gravity and linear accelerations is induced
Vestibule
n
Detect linear motion (up/down, front/back, head tilt, gravity)
n
Two organs on each side
Utricle
1.
●
Oriented horizontally
●
Superior to the saccule, anterior to the semicircular canals
2. Saccule
●
Oriented vertically
●
Inferior to the utricle
n
Each organ contains a macula
Utricular macula detects linear horizontal acceleration and is not sensitive to constant
velocities
Saccular macula detects linear vertical acceleration and is not sensitive to constant velocities
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
46
The macula is similar to cupula (or tectorial membrane of the cochlea), but does not extend
to the roof of the vestibule; instead, it has otoliths/otoconia that sit on top of the maculae
Otoliths provide a weightbearing system (utricle and saccule are also referred to as otolithic
organs)
●
Calcium carbonate crystals
●
Sit on top of gelatinous material of macula
●
Gravity causes displacement, which causes deflection of hair cells
There is an imaginary center line of the utricular and saccular maculae called the striola
●
Utricular striola: Hair cells have short stereocilia, polarized toward center
●
Saccular striola: Hair cells have long stereocilia, polarized away from center (Furman &
Lempert, 2016)
AUDIOLOGY NUGGET
An easy way to remember the motions detected by the otolithic organs is as
follows: When writing an “S” for saccule, your pen moves from top to bottom.
This correlates with the vertical acceleration detected by the saccular maculae.
When writing a “U” for utricle, your pen moves from left to right. This corresponds to the horizontal acceleration detected by the utricular maculae.
Otolithic Organ Function
n
Head accelerates
Lower part of utricular or saccular (basement) membrane immediately follows
Otoconia lag behind, deflecting stereocilia toward or away from kinocilium
Hair cells de- or hyperpolarize (depending on direction of deflection)
n
Head decelerates to stopping
Basement membrane decelerates to stopping
Otoconia remain in motion, deflecting the stereocilia in the opposite direction
Hair cells de- or hyperpolarize (depending on direction of deflection)
Peripheral Vestibular Vasculature
The peripheral vestibular system is first supplied by the AICA (or sometimes the basilar artery), which
then branches to the labyrinthine artery. This divides into the anterior vestibular artery and the common
cochlear artery, which then branches into the posterior vestibular artery. The anterior vestibular artery
supplies blood to the utricle, ampullae of the lateral and anterior semicircular canals, and some to
the saccule. The posterior vestibular artery supplies blood to other areas: the inferior saccule and the
ampulla of the posterior semicircular canal (Jacobson et al., 2021).
CHAPTER 2 Anatomy, Physiology, and Relevant Pathologies
https://t.me/medicina_free
47
Cranial Nerve VIII: Vestibular Portion
The vestibular portion of CN VIII comprises approximately 15,000 bipolar neurons; their cell bodies
make up Scarpa’s ganglion. These first-order neurons will synapse with vestibular nuclei in the brainstem, which are located on the floor of the fourth ventricle after traveling through the posterior portion
of the internal auditory canal. The nerve is divided into two branches: inferior and superior. The
inferior vestibular nerve carries input from the saccule and posterior semicircular canal. The superior
vestibular nerve carries input from the horizontal and anterior semicircular canals, as well as the utricle
and some of the saccule. Figure 2–3 shows the orientation of the vestibular portion of CN VIII in the
internal acoustic meatus.
Central Vestibular System
In the central vestibular system, there are four primary vestibular nuclei located in the dorsal medulla
and pons (Furman & Lempert, 2016). Note that no primary afferents cross midline, unlike in the
auditory system.
1. Superior — receives primary inputs from the vestibular end organs (semicircular canals) and
commissural fibers from the contralateral superior vestibular nucleus
2. Inferior — receives inputs from the vestibular end organs (saccule), cerebellum, and contralateral
vestibular nuclei
3. Medial — receives main input from vestibular end organs (semicircular canals), reticular
formation, contralateral medial vestibular nucleus, cerebellum, and spinal cord
n
Sends efferent signals to the CN III (oculomotor) and VI (abducens) via the medial
longitudinal fasciculus (MLF)
4. Lateral — receives main input from the utricle, cerebellum, and spinal cord as well as some fibers
from the frontal and parietal cortices
The central vestibular system is responsible for communicating with the central visual and proprio-
ceptive systems to make motor controls based on the head’s movement in space (Furman & Lempert,
2016). There are three main outputs from the vestibular nuclei:
1. CN III and VI — two main routes
n
MLF: direct pathway good for quick communication
n
Reticular formation: maintains spontaneous activity, is the integration circuit for many
sensory systems, and is responsible for compensatory eye movements
2. Spinal cord — via the reticulospinal or vestibulospinal pathways
3. Cerebellum — via the vestibulocerebellar pathway through the cerebellar peduncle
There are four main reflexes that follow the abovementioned pathways:
1. Vestibulo-ocular reflex (VOR)
2. Vestibulo-spinal reflex (VSR)
3. Vestibulo-collic reflex (VCR)
4. Cervico-ocular reflex (COR)
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
48
Vestibulo-Ocular Reflex (VOR)
Under normal circumstances, the visual and vestibular systems act synchronously to keep a target
image steadily focused on the fovea (back portion) of the eye, referred to as the foveal reflex. However,
head movements can be very fast while the “recording” of the foveal images can be slow. This would
create blurred/unfocused images to be perceived. The visual system alone cannot keep a target image
focused during movement, which is where the vestibular system plays a role.
n
Compensatory eye movements (nystagmus) are initiated by stimulus exciting the vestibular
end organs, which then triggers a response in the VOR arc
n
Eye movements can be conjugate (both eyes moving in the same direction) or vergent (eyes
moving in opposite directions)
n
Nystagmus — defined as involuntary eye movements with clearly defined slow and fast
components; it is normal to have nystagmus in some cases, but pathological in others
The slow component (tracking/pursuit) is the compensatory movement while the fast
component is generated by the central nervous system
The direction of the fast component (saccade) defines the nystagmus (i.e., right-beating
nystagmus is when the fast phase is to the right)
●
When the head is turning to the left, the slow phase of nystagmus will be to the right
as the eyes try to keep the visual target centered on the fovea. When this is no longer
possible, the eyes will snap back to center, moving quickly to the left. This is the fast
phase of nystagmus, which is left-beating in this case. If the head is turning to the right,
the individual will experience right-beating nystagmus.
Nystagmus normally occurs when the head is rotating, can be visually evoked (watching a passing
picket fence), under caloric stimulation (differing temperatures of water or air being forced
into the outer ear), or when gaze is focused at the extreme lateral (i.e., endpoint nystagmus)
It is pathological when these eye movements occur spontaneously, result from changes in
head position, or result from changes in gaze position
AUDIOLOGY NUGGET
When a patient undergoes vestibular evaluation, pathological and normal
nystagmus is being recorded. Under caloric testing, cool and/or warm stimuli
(sometimes the testing is completed monothermally, sometimes bithermally,
and in some cases, ice water is used) are applied to each ear and the resultant
nystagmus is evaluated for direction and strength. To remember the direction
in which the nystagmus should occur, the acronym COWS is used; this stands
for cold-opposite, warm-same, meaning that cold stimuli should result in nystagmus beating toward the contralateral ear and warm stimuli should result in
nystagmus beating toward the ipsilateral ear. Caloric results are compared to
normative values as well as between ears to determine if a “weakness” or reduced
degree of nystagmus is produced in a certain testing condition, which can be
indicative of peripheral vestibular pathology.
CHAPTER 2 Anatomy, Physiology, and Relevant Pathologies
https://t.me/medicina_free
Table 2–4 provides additional details about types of nystagmus (Jacobson et al., 2021)
n
For the VOR arc to be complete, extraocular muscles are included. These work in antagonistic
pairs, similar to the semicircular canals. The muscles and their innervations are shown in
Table2–5.
n
The VOR is a three-neuron arc: vestibular afferents project to the vestibular nuclei, which go
to the ocular nuclei, which go to the extraocular muscles
For example, if the head is turning left, the left horizontal semicircular canal is excited,
which sends excitatory signals to the medial vestibular nuclei, to the ipsilateral abducens
nucleus, which then goes to the ipsilateral (left eye) medial rectus via the medial
longitudinal fasciculus and oculomotor nucleus as well as directly to the contralateral (right
eye) lateral rectus. Inhibitory signals from the right horizontal semicircular canals go to the
vestibular nuclei, then ipsilateral abducens to the contralateral (left eye) lateral rectus); it
also sends projections to the ipsilateral MLF, oculomotor nucleus, and ipsilateral (right eye)
medial rectus. These steps are shown in Figure 2–6.
TABLE 2–4. Types of Nystagmu s
49
NYSTAGMUS
TYPE INDUCED SPONTANEOUS NORMAL ABNORMAL NOTES
Postrotary X X
Optokinetic X X
Caloric X X
Galvanic X X
Endpoint X X
Gaze X X
Positional X X
Hallpike X X
Fistula X X
Vestibular type X X Fast and slow phase;
can be suppressed with
fixation; horizontal
Ocular type X X Seen in blindness and
miners
Central type X X Irregular fast and slow
Source: Adapted from Balance Function Assessment and Management, Third Edition (pp. 1–717) by Jacobson, G. P., Shepard,
N. T., Barin, K., Burkard, R. F., Janky, K., & McCaslin, D. L. Copyright © 2021 Plural Publishing, Inc. All rights reserved.
phases; oblique, vertical;
cannot be suppressed
with fixation
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
50
TABLE 2–5. Extraocular Muscles
MUSCLE INNERVATION MOVEMENT EXCITATORY INPUT INHIBITORY INPUT
Medial rectus CN III Adduction Ipsilateral horizontal
SCC
Lateral rectus CN VI Abduction Contralateral horizontal
SCC
Contralateral horizontal
SCC
Ipsilateral horizontal
SCC
Superior rectus CN III Elevation Ipsilateral anterior SCC Contralateral posterior
SCC
Inferior rectus CN III Depression Contralateral posterior
Ipsilateral anterior SCC
SCC
Superior oblique CN IV Extorsion Ipsilateral posterior SCC Contralateral anterior
SCC
Inferior oblique CN III Intorsion Contralateral anterior
SCC
Ipsilateral posterior
SCC
FIGURE 2–6. VOR pathway of right head turn. Black solid lines indicate excitatory signaling; gray dashed
lines indicate inhibitory signaling. Key: LR: lateral rectus; MR: medial rectus; MLF: medial longitudinal
fasciculus, CN VIIIvs: superior vestibular portion of the vestibulocochlear nerve.
CHAPTER 2 Anatomy, Physiology, and Relevant Pathologies
https://t.me/medicina_free
51
The VOR can be evaluated clinically via ocular vestibular evoked myogenic potentials (oVEMP),
calorics, or video head impulse testing (vHIT; Jacobson et al., 2021). See Chapter 7 for additional
details of these tests.
Vestibulospinal Reflex (VSR)
The VSR is responsible for maintaining head and body posture (general control of posture) during
daily activities (Jacobson et al., 2021). It serves to stabilize the head relative to inertial space by generating a command to move the head in the opposite direction of the existing head-in-space velocity and
consists of a three-neuron arc:
1. Vestibular afferents
n
The lateral VSR comes from afferents stemming from the posterior semicircular canal and
utricle while the medial VSR comes from afferents of the saccule and horizontal and superior
semicircular canals (in fact, a bit from everywhere). The reticulospinal VSR has afferents from
the entire peripheral vestibular system.
2. Vestibular nuclei
3. (Cervical) spinal cord
The VSR is clinically evaluated with posturography, specifically the sensory organization test (SOT).
Vestibulocollic Reflex (VCR)
The VCR maintains head position and horizontal gaze relative to gravity. This reflex is evaluated via
cervical VEMPs (cVEMP).
Cervico-Ocular Reflex (COR)
The COR is responsible for eye movements in relation to head or torso movements. It is thought to
come into play when there are disturbances in VOR function. There are no clinical tests designed to
specifically examine the COR (Jacobson et al., 2021).
Higher-Order Vestibular Processing
This section provides a brief overview of the central vestibular system. For more detailed information,
see Chapter 7. Five main areas of the cerebellum receive vestibular input:
1. Lobules I to V of anterior lobe — proprioceptive (neck) and vestibular signals for VSR
2. Deep cerebellar nuclei — communication point and tract for VSR
3. Lobules VI to VII (oculomotor vermis of posterior lobe) — visual-vestibular processing
4. Lobes IX and X (nodulus and central vulva) — inertial motion processing
5. Central paraflocculus and flocculus — compensatory eye movements (Furman & Lempert, 2016)
From the cerebellum (and vestibular nuclei), projections run through the thalamus to the cortex,
primarily to the temporal and parietal lobes. Most areas of the thalamus and cortex receive and integrate input from the visual, vestibular, and somatosensory systems. The parietoinsular vestibular cortex
(PIVC) is critical for shaping perception of self-movement, spatial orientation, and body representation.
The dorsal medial superior temporal cortex (MSTd) plays a role processing optic flow information
Соседние файлы в папке Библиотека им академика М.И. Перельмана