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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4455_Библиотеки_им_академика_М_И_Перельмана
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Figure 2–34. A left lateral view of the human brain with the circled
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area indicating the main auditory regions. Key: A = frontal lobe, B =
parietal lobe, C = occipital lobe, D = temporal lobe, E = cerebellum, 1 =
temporal pole, 2 = sylvian fissure (arrows marking the beginning and
end of this fissure), 3 = supramarginal gyrus, 4 = angular gyrus, 5 =
superior temporal gyrus. From The Auditory System: Anatomy, Phys-
iology, and Clinical Correlates
J. A. Baran, 2020, San Diego, CA: Plural Publishing. Copyright © 2020
Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 29), by F. E. Musiek and
Figure 2–35. A photo of the human brain
(horizontal section) that defines the primary
auditory cortex (within the dotted lines). Key:
INS = insula, HG = Heschl’s gyrus, PT = pla-
num temporale. [Note: the arrows are pointing
to the temporal sulcus (anterior) and Heschl’s
sulcus (posterior)]. From The Auditory Sys-
tem: Anatomy, Physiology, and Clinical Correlates
and J. A. Baran, 2020, San Diego, CA: Plural
Publishing. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 280), by F. E. Musiek
50
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the left Heschl’s gyrus is larger than the
right (Campain & Minckler, 1976; Musiek
& Reeves, 1990). The planum temporale is
located immediately posterior to Heschl’s
gyrus and immediately anterior to the
supramarginal gyrus. It also is typically
larger on the left side than the right side
(Geschwind & Levitsky, 1968). The insula
is a cortex medial to the mesial temporal
lobe. It has a series of long and short gyri
with adjacent sulci, and like the two structures discussed previously, it also tends to
be larger on the left side than the right side
(Mesulam & Mufson, 1985). In addition, the
insula is viewed as a key auditory structure (Bamiou, Musiek, & Luxon, 2003).
There is also a different view of the
anatomy of the auditory cortex termed
the core–belt arrangement (Kaas, Hackett, & Tramo, 1999). The core is the main
auditory region, which is surrounded by
a belt and parabelt. There is an outflow of
fibers from the core to the belt and parabelt regions. Primarily, the connections to
other parts of the brain are via the parabelt region (see Musiek & Baran, 2020).
There are intra- and interhemispheric
connections involving the cerebral auditory
areas. The interhemispheric connections are
presented later. The main intrahemispheric
connection is via the arcuate fasciculus,
which is part of the longitudinal fasciculus.
This tract courses from the area around the
supramarginal gyrus (often referred to as
Wernicke’s area) to the frontal lobe picking
up neural connections along the way. This
is how Heschl’s gyrus conveys impulses
to the frontal lobe (Musiek, 1986).
Function
The tonotopic arrangement of Heschl’s
gyrus has traditionally been viewed as
low frequencies in the lateral aspect and
high frequencies in the medial-posterior
aspect. However, recent work, mostly
from functional imaging studies, has
shown a much more complex tonotopic
configuration (Figure 2–36). Our “interpretive view” of the tonotopic organization of Heschl’s gyrus reveals the high
frequencies at the anterior and posterior
ends of this gyrus with the mid and low
frequencies in between the ends of the
gyrus (interpretation based upon the
research reports of Dick et al., 2012; Herdener et al., 2013; Humphries, Liebenthal,
& Binder, 2010; Langers, 2014; Saenz &
Langers, 2014; for further discussion see
Musiek & Baran, 2020). Generally, as
intensity increases, most fibers within the
gyrus fire at a higher rate and more fibers
respond. Some fibers within the gyrus,
however, do “roll over” with intensity
increases. At the cortex, there also may be
some interesting interactions for intensity
increases alterations with inhibitory and
excitatory fibers. In other words, inhibition may result in decreases rather than
increases in firing rate and the numbers of
fibers activated. On the other hand, activation of excitatory fibers could result in
increases in intensity representations (see
Musiek & Baran, 2020).
The auditory cortex responds better
to modulated tones than to steady-state
tones (Evans & Whitfield, 1964). The cortical neurons also respond better to slow
as opposed to fast modulation rates (<50
per sec). Most auditory cortex fibers can
respond to periodicities (pure-tone cycles)
only up to 100 Hz. However, individual
abrupt stimuli like click stimuli yield
faster responses (Phillips & Hall, 1990).
The overall capacity of cortical neurons to
temporally process sounds is not as fast
as is observed for more caudal auditory
structures.
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Figure 2–36. An interpretive construction of the tonotopic arrangement of
auditory cortex based on a review of studies by Dick et al. (2012), Herdener
et al. (2013), Humphries et al. (2010), Langers (2014), Langers and van Dijk
(2012), and Saenz and Langers (2014). (Courtesy of B. St. George, Neuroaudiology Lab, University of Arizona, 2020.) From The Auditory System:
Anatomy, Physiology, and Clinical Correlates
E. Musiek and J. A. Baran, 2020, San Diego, CA: Plural Publishing. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
(2nd edition, p. 289), by F.
The auditory cortices play an important role in localization. Time and intensity differences are utilized to trigger
localization processes, and there appears
to be stronger cortical activity for contralaterally directed signals compared to
ipsilaterally directed signals (see Musiek
& Baran, 2020).
A number of evoked potentials are
relevant to the function of the auditory
cortex and subcortex. The MLR is generated from the thalamocortical pathway
and auditory cortex. The late potentials
(N1, P2) also are generated by the auditory
cortex. Although the event-related potentials (P300, MMN) likely have numerous
generators, the auditory cortex certainly
plays a role in these responses as well
(McPherson, 1996; Musiek & Baran, 2020).
The Corpus Callosum
Structure
The corpus callosum (CC) is the largest commissure in the brain connecting
the two hemispheres and is responsible
for transferring information from one
hemisphere to the other (Figure 2–37; see
Musiek & Baran, 2020). The CC is heavily myelinated and in adult humans is
about 6.5 cm in length and about 1 cm
in thickness. Its fibers run from cortex to
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Figure 2–37. A view of the human corpus callosum. Key: 1 = splenium, 2 =
sulcus or isthmus, 3 = trunk or body, 4 = genu, 5 = anterior commissure. From
The Auditory System: Anatomy, Physiology, and Clinical Correlates (2nd
edition, p. 35), by F. E. Musiek and J. A. Baran, 2020, San Diego, CA: Plural
Publishing. Copyright © 2020 Plural Publishing, Inc. All rights reserved.
cortex. Some auditory fibers connect to
the same locus in the other hemisphere
(homolateral fibers) and some connect to
other regions (heterolateral fibers). The
CC is organized to transfer information
from every main area of the hemispheres;
hence, it has different anatomic regions
(see Figure 2–37). The posterior sixth of
the CC is the splenium where visual fibers
from the occipital lobes cross. Just anterior
to the splenium is the isthmus or sulcus
(a thinned region) where auditory fibers
from the temporal lobe reside. Proceeding anteriorly is a division known as the
trunk or body where somatosensory and
motor fibers from the parietal lobe cross.
Anterior to the trunk is the genu for frontal lobe and olfactory fibers. Beneath the
genu is the anterior commissure for which
there is much controversy regarding the
fiber types, but they could be auditory
or olfactory.
Function
The key function of the CC is the transfer
of impulses between the two hemispheres.
There are both excitatory and inhibitory
fibers within the CC. The heavily myelinated fibers that are excitatory have an
interhemispheric transfer time (ITT) of 3
to 6 msec and the inhibitory fibers may
have an ITT of more than 100 msec. The
ITT in humans changes with age. The best
ITT is found in teenagers, with individuals who are older and younger yielding
increased ITTs (Salamy, 1978).
The transfer of information from one
hemisphere to the other can be critical to our
perception. Each hemisphere is dominant
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for certain processes, and it is the timely
exchange of these processes that allows
the brain to work efficiently. A relevant
example of this exchange of hemispheric
information is dichotic listening for speech.
When speech is presented in the dichotic
mode, the right hemisphere receives input
from the left ear and the left hemisphere
receives input from the right ear. If a verbal response is required, the information in
the right hemisphere must be transferred
to the speech hemisphere, which is the left
hemisphere. If this function is compromised, a left ear deficit in dichotic listening
will result. This deficit takes place because
the ipsilateral pathways to the cortex are
suppressed during dichotic listening, leaving only the contralateral system to function for speech perception (Kimura, 1961;
Musiek, Kibbe, & Baran, 1984).
Recent research also has demonstrated
that the CC may play a role in modulating the functions (inhibitory-excitatory)
in each hemisphere (Williams, PascualLeone, & Fregni, 2010). Although it is
difficult at this time to theorize how this
may influence various auditory processes,
it indeed seems to be an exciting avenue
for future research. Needless to say, the
CC plays important roles in audition. It
allows the efficient exchange of information across all sensory systems and therefore is critical to both auditory and speech
perception processing.
the effeRent system
Structure
Coursing along a similar tract as the afferent auditory system is the descending
or efferent auditory system (EAS). This
system is smaller and less well under-
stood than is the afferent system. The
EAS starts at the auditory cortex where it
likely has several areas of input including
fibers from secondary auditory regions. It
courses caudally through the internal capsule and on to the MGB where some reciprocal routes to the cortex are noted. The
EAS fibers descend to the IC where more
reciprocal connections involving the MGB
and cortex are present. It then descends
farther along the LL and into the area
around the SOC where it becomes known
as the olivocochlear bundle (OCB).
The OCB can be divided into two
main descending tracts, the lateral olivocochlear (LOC) and the medial olivocochlear (MOC) systems. The LOC is primarily an ipsilateral route and the MOC
is primarily a contralateral route to the
cochlea. These ipsilateral and contralateral routes descend from the OCB area to
the cochlear nucleus, exit the brainstem
via the internal auditory meatus, and
run along the vestibular tracts out to the
cochlea. The LOC fibers finally connect to
the afferent fibers leaving the IHCs and
the MOC fibers connect directly to the
OHCs (Sahley, Nodar, & Musiek, 1997).
Function
The function of the rostral portion of the
EAS remains somewhat of a mystery. The
number of reciprocal connections as well
as some available physiologic data indicates that both excitation and inhibition
influences can be exerted on the incoming acoustic signal (Mitani, Shimokouchi,
& Nomura, 1983). It also seems likely that
the rostral and caudal portions of the EAS
work together and that the rostral portion also has some influence on the OCB.
More research on the rostral EAS is certainly indicated as it may play a subtle,
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but important, role in hearing that is yet
to be discovered.
The caudal EAS, which we will refer
to as the OCB, was studied more than 60
years ago by Galambos (1956). Galambos
demonstrated that, if the OCB was stimulated electrically in animals, it resulted
in reduced firing rates of the AN fibers.
These findings have been interpreted to
mean that the OCB may have inhibitory
influences on the afferent auditory system. It was discovered that the OCB also
could be stimulated and the same inhibitory effect measured if an acoustic signal
was presented to one ear and a noise was
presented to the opposite ear (Folsom &
Owsley, 1987). This has become known
as the “suppression effect,” which can be
measured using otoacoustic emissions or
evoked potentials in humans.
Activation of the OCB also has been
shown to enhance hearing in noise (Kawase & Liberman, 1993). If the OCB is
activated either electrically (as was done
in the Galambos’ study) or by contralateral noise when a listener is being asked
to detect a signal in noise, the detection
threshold will improve relative to the
conditions in which the OCB is not stimulated. A number of experimental studies
have shown similar effects, and this phenomenon is now considered to be one of
the mechanisms that allows hearing in
noisy situations to be enhanced (see Musiek & Baran, 2020; Sahley et al., 1997).
vasculaR suPPly foR
the auditoRy system
The Peripheral System
Functions of the peripheral and central
auditory systems are highly dependent
on blood supply. The external ear’s blood
supply comes from branches of the external carotid artery, whereas the middle ear
receives its blood supply from branches of
the internal carotid. The cochlea and AN’s
vascular supply comes from the vertebrobasilar system.
The pinna and external auditory
meatus are supplied mostly, but not exclusively, by the superficial temporal artery
and the posterior auricular artery. There is
some controversy regarding the tympanic
membrane’s blood supply, but playing
key roles are branches of the deep auricular and maxillary arteries. The middle ear
(soft tissue) receives vascular input from
branches of the internal carotid, maxillary
(tympanic branch), posterior auricular
(mastoid branch), and middle meningeal
(petrosal branch) arteries, as well as from
branches of the ascending pharyngeal and
pterygoid arteries (Anson & Donaldson,
1981; Clark & Ohlemiller, 2008; Musiek &
Baran, 2020).
The internal auditory or labyrinthine
artery (IAA), which is a main branch of
the basilar or anterior inferior cerebellar artery (AICA) (brainstem), is key for
blood supply of the cochlea and AN.
The IAA divides within the IAM into
cochlear and vestibular branches, which
supply the auditory and vestibular
nerves before proceeding externally to
the cochlea. The cochlear artery branches
into the spiral modiolar artery (SMA) and
the cochleovestibular artery (CVA). The
SMA spirals around the modiolus giving off branches en route, and the CVA
coils following the cochlea. These two
main vessels give off branches that make
up the network of radiating arterioles,
which supplies much of the cochlea
(Musiek & Baran, 2020; Smith, 1973). All
of these arteries have as their counterparts
veins and collecting venules in similar
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anatomic areas, which return the impure
blood back to the main veins and onto
the heart.
The Central System
The central auditory nervous system in
terms of its vascular supply can be segmented into the brainstem and cortex.
The brainstem auditory structures are
supplied by the vertebrobasilar system
and the cortex by the internal carotid
system. In the brainstem, the vertebral
arteries, which course rostrally on both
sides of the spine, join together to form
the basilar artery a few millimeters below
the pontomedullary junction. The basilar
artery is located on the ventral side of the
brainstem. The basilar artery gives off the
anterior inferior cerebellar artery (AICA),
which supplies blood to the cochlear
nucleus with circumferential branches.
Smaller branches off the basilar artery
called paramedial or pontine penetrating
arteries penetrate the pons to supply the
SOC and some of the LL deep in the pons
(Waddington, 1974).
Proceeding rostrally, another main
branch of the basilar artery is the superior
cerebellar artery, which indirectly supplies the LL and the IC. The MGB is most
likely supplied by the posterior thalamic
group arteries, which are a multivessel
complex arising from the posterior cerebral artery (Musiek & Baran, 2020; Waddington, 1974).
The auditory cortex and associated
areas are supplied with blood primarily
by the middle cerebral artery (MCA). The
MCA has anterior, middle, and posterior
temporal arteries that branch to cover the
anterior, middle, and posterior temporal
lobe. The MCA also has a central sulcus
branch that supplies the parietal lobe. The
angular artery feeds the angular gyrus
and possibly the supramarginal gyrus.
The insula’s vascular supply is from the
fronto-opercular artery (from the anterior
MCA), which branches into a variety of
insular arteries. The corpus callosum’s
anterior four-fifths has as its vascular
supply the pericallosal artery, a branch
of the MCA, and its posterior one-fifth
is supplied by the posterior cerebral
artery (Musiek & Baran, 2020; Waddington, 1974).
the vestiBulaR system
Structure
The vestibular system serves to maintain
static and dynamic balance by initiating
a series of reflexes that are recruited when
the head is moved during daily activities
(e.g., walking or running). This system
also provides information regarding spatial orientation and a subjective sense of
movement. This elegant system consists
of five end organs for each ear that act
as integrating accelerometers, nerves
that carry electrical signals to the brainstem, and diffuse projections that are
routed throughout the central nervous
system. The peripheral vestibular system
is physically connected to the peripheral
auditory system at the vestibule, which
is located between the middle ear cavity
and the internal auditory meatus. Because
of this, it is not uncommon for disease to
affect both the auditory and vestibular
systems (e.g., Ménière’s disease). The
peripheral vestibular system is housed
in a series of hollow tunnels called the
bony labyrinth that are located within the
very hard petrous portion of the temporal bone. The membranous labyrinth is
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suspended within the bony labyrinth by
perilymph and supporting connective tissue. This membranous labyrinth is filled
with endolymphatic fluid, which has an
ionic composition that is higher in K+
and lower in Na+ than perilymph.
It is
believed that the “dark cells” of the cristae (i.e., semicircular canal system) and
maculae (utricle and saccule) are responsible for producing the endolymphatic
fluid found in the vestibular apparatus
(Kimura, 1969).
The peripheral vestibular system is
shown in Figure 2–38. The vestibular end
organs consist of the horizontal (or lat-
eral), anterior (or superior), and posterior
semicircular canals, along with the utricle
and the saccule. Together, there are a total
of five peripheral end organs, which act to
transduce acceleration of the head or the
head and body into an electrical code. The
three semicircular canals convert angular
acceleration and deceleration, and the latter two end organs (i.e., the utricle and
saccule) transduce linear acceleration and
deceleration. The anatomic structures of
the end organs are, at the same time, both
similar and different. Each consists of a
mass that sits atop stereocilia that project
from hair cells. It is the effect of inertia
Figure 2–38. The anatomy of the membranous labyrinth. Illustration by Mary Dersch from
Pender, 1992, with permission of Daniel Pender. This material is copyrighted and any further reproduction or distribution is prohibited without express permission of the author.
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on the mass that results in deflection of
the stereocilia and electrical transduction
at the base of the hair cells. Each of the
semicircular canals is oriented in a different plane making this system sensitive to angular acceleration in nearly all
directions. Specifically, the horizontal
canal is tilted upward approximately 30°
anteriorly in relationship to the horizontal plane and two vertical semicircular
canals are oriented at 45° in the sagittal plane. The sense organ located at the
ampullated end of each semicircular canal
is called the crista ampullaris. Each crista
consists of (1) a gelatinous mass that fills
the ampulla from the floor to the ceiling,
called the cupula; (2) hair cell stereocilia;
and (3) a single, taller, more rigid kinocilium that projects up from each hair cell
into the cupula (Figure 2–39). The stereocilia project from hair cells and both afferent and efferent nerve fibers approximate
the base of the hair cells. The cupula and
the endolymph have the same specific
gravity (i.e., 1.0; Money et al., 1971). When
the head is not in motion, the cupula is
suspended in the endolymph in a neutrally buoyant position and the primary
afferent nerve fibers fire at a tonic resting
rate. Acceleration of the head in the plane
of a canal results in the endolymph lagging behind the canal walls and results in
fluid motion. The cupula seals the ampullar cavity, and because the flow of endolymph is impeded, pressure is exerted on
the cupula, thus bending it in the direction
opposite that of the head movement. This
distortion of the cupula results in shearing
of the stereocilia and kinocilia that causes
a change in the electrical discharge rate at
the base of the hair cells. The end organs
are polarized so that head movement in
one direction results in depolarization of
the end organ and an increase in the firing
Figure 2–39. An illustration of the otolith system. The stimulation of the otolith organs is pro-
vided by the inertia of the otolith crystals.
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rate, whereas movement in the opposite
direction results in hyperpolarization and
a decrease in the discharge rate.
The saccule and the utricle are located
in the vestibule and each has a sensory
neuroepithelium called a macula. The
macula of the saccule is oriented primarily
in the vertical plane, and the macula of the
utricle is oriented in the horizontal plane.
The mass in the macular organs (see Figure 2–39) are calcium carbonate crystals
(i.e., 5–7 µm), referred to as “otoliths” or
“otoconia” (Lindeman, 1969). These crystals reside on a sticky, reticulated “net”
called the otolith membrane. The stereocilia and kinocilia project from hair cells
up into the otolith membrane; however,
the polarization pattern is more complex
than in the cristae. Each macular organ
has an anatomic line running through it
called a striola. The hair cells are oriented
in opposing directions around the striola.
In the utricle, the hair cells are oriented
toward the striola; whereas in the saccule,
the hair cells are oriented away from the
striola. This unique organization of hair
cells allows the otolith organs to transduce linear head motion in nearly any
direction. Here again, both afferent and
efferent primary nerve fibers approach
the base of the hair cells.
Electrical activity from the vestibular
end organs is routed through either the
superior or inferior division of the vestibular portion of the eighth cranial nerve.
The superior vestibular nerve contains
afferent fibers from the horizontal and
anterior semicircular canals, the utricle,
and part of the saccule, which provide
electrical impulses to this pathway. The
inferior vestibular nerve contains afferent fibers from the posterior semicircular
canal and the majority of the saccule. In
total, the vestibular branches of the eighth
nerve consist of 18,000 single nerve fibers
that discharge at a rate that varies between
10 and 100 spikes per sec. This means that,
at any moment, there can be well over
1 million spikes per sec flowing from each
of the two peripheral vestibular systems
to the central vestibular system.
The origin of the second-order neuron in the vestibular system is the vestibular nuclei (i.e., the vestibular equivalent of
the cochlear nuclei). There are four vestibular nuclei in the pons: the superior,
medial, lateral, and inferior vestibular
nuclei. The vestibular nuclei act as gating
centers for electrical activity generated
from the peripheral vestibular system
(see Figure 2–40). For example, electrical activity from the vestibular nuclei is
routed to the eye movement pathways so
that we can view a stationary object while
walking or running in such a way that the
target remains still on the center of clear
vision of the retina. The cerebellum regulates vestibular responses. Specifically, the
vestibulocerebellum (i.e., flocculus, nodulus, uvula, and vermis) receives input
from the primary vestibular afferents and
sends projections to the vestibular nuclei
(see Figure 2–40). In this regard, inhibitory connections between the flocculus
and the vestibular nuclei make it possible
for patients to centrally compensate (i.e.,
improve the dynamic properties of the
vestibulo-ocular reflex) after the loss of all
or part of a single end organ system (Ito,
1993). Connections between the vestibular
nuclei, the cerebellum, and the descending spinal cord pathways (e.g., the lateral
and medial vestibulospinal pathways and
the reticulospinal pathway) also enable
us to correct postural deviations so we
do not fall (Figure 2–40). Connections between the vestibular nuclei, the reticular
activating system, and the autonomic nervous system are responsible for the generation of secondary reactions such as pal-
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