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Figure 7–1. Cross-section of the human organ of Corti. A. Illustration with modiolus on right. (From http://image.slide
sharecdn.com/anatomyonnerearhk-141128002230-conversion-gate02/95/anatomy-of-inner-ear-hk-32-638.jpg?cb=142
7586161.) B. Histology section with modiolus on left. (From https://histologydrawings.blogspot.com/2016/03/ear.html.)
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1177. The Cochlea, Vestibule, and Central Connections
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Figure 7-2A. Section of semicircular canal crista and otolithic macula. (From David King, PhD, SIU Anatomy: http://www
.siumed.edu/~dking2/idex.htm, used with permission.)
The structure of the macula located in the utricle
and the saccule differs from the crista in that the macula is flattened like a patch. The center of the macula
has a narrow curved zone extending through its middle called the striola. Type 1 hair cells predominate on
the striola while type 2 hair cells are located away from
it. The gelatinous matrix covering the macular surface
contains small calcium carbonate crystals and is called
the otoconial (or otolithic) membrane.
The unique geometrical arrangement of the cristae to the semicircular canals and the maculae to the
utricle and saccule allow the semicircular canals to respond to angular acceleration and the otoconial organs
to respond to gravitational pull.
CLINICAL CAVEAT: Benign paroxysmal positional
vertigo (BPPV) is caused by dislodgement of otoconia from the macule. When the head is placed into
the provoking position, the free otoconia fall either
inferiorly onto the posterior semicircular canal crista
(most commonly) or horizontally onto the lateral
se micircular canal crista. Displacement of the cupula by the weight of the calcium carbonate crystals
stimulates that angular acceleration receptor organ,
giving the patient a false sense of spinning. Using a
canalith repositioning technique moves the crystals
from the ampullary (active) end of the canal to the
nonampullated end, and often resolves the problem.
Figure 7-2B. Magnied image of semicircular canal crista showing hair cells and cupula. The more common spelling is ‘cu-
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pula’, but ‘cupola’ is acceptable. (From David King, PhD, SIU Anatomy: http://www.siumed.edu/~dking2/idex.htm, used with
permission.)
Figure 7-2C. Magnied image of otolithic macula showing hair cells and otoconia (otoliths). (From David King, PhD, SIU
Anatomy: http://www.siumed.edu/~dking2/idex.htm, used with permission.)
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7. The Cochlea, Vestibule, and Central Connections 119
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AFFERENT PATHWAYS
1. Afferent Auditory Pathways
These consist of dendrites of neurons whose cell bodies comprise the spiral ganglion in Rosenthal’s canal in
the modiolus of the cochlea. The total neuronal population of the human cochlea is approximately 35,000.
There are two types of neurons. The type 1 neurons
are large and bipolar and connect exclusively with inner hair cells. Type 1 neurons constitute about 95% of
the neuronal population. Type 2 cochlear neurons have
very small fibers, connect exclusively to outer hair cells,
and constitute the other 5% of the total neuronal population. (See Figure 2–11.) Each inner hair cell is innervated by 10 to 20 type 1 neurons. Conversely, each type 2
neuron innervates several outer hair cells. Modiolar fi bers from the cochlear neurons cross the tunnel of Corti
to reach the bases of outer hair cells.
The orderly spatial arrangement of the cochlear
neurons is maintained in the cochlear nerve trunk and
continues into the cochlear nuclei. The nerve fibers from
the basal turn are peripherally located, and those from
the apical turn are in the central region of the nerve trunk.
On entering the brainstem, the fibers end in the cochlear
nucleus, the major portion going to the ventral cochlear
nucleus. Ascending fibers from the dorsal nucleus cross
the midline to enter the contralateral lateral lemniscus.
From there, fibers ascend to the inferior colliculus. From
the ventral cochlear nucleus fibers go to both the contralateral and ipsilateral superior olive (Figure 7–3).
The distant projections from the inferior colliculus are mainly to the medial geniculate body. There are
crossover connections between the two inferior colliculi—but no such connections connect the two medial geniculate bodies. Each cochlea has nearly equal
bilat eral neuronal connections to the level of medial genic ulate bodies and thus to the auditory cortices.
The inferior division lying on the inferior vestibular
nerve supplies the posterior semicircular canal crista
and the saccular macula. Medial to the vestibular ganglion, the two nerves merge into a single trunk which
enters the brainstem. The average number of vestibular fibers in each ear is approximately 18,000.
Central projections: The vestibular fibers end in
four major vestibular nuclei termed superior, medial,
lateral, and descending. They are all in a single compact oval mass and located medial to the cochlear nucleus in the brainstem. Fibers from the two maculae
reach the medial and lateral vestibular nuclei; fibers
from the semicircular canals reach three nuclei—superior, medial, and lateral. From the four nuclei two
connections are made—one vestibulo-ocular and the
other vestibulo-spinal. Vestibulo-ocular fibers arise
from the superior, medial, and lateral nuclei, pass via
the medial longitudinal bundle, and make connections
with the third, fourth, and sixth cranial nerve nuclei
(Fig ure 7–4).
Vestibulo-spinal fibers arise from the lateral, medial, and descending nuclei, reach the anterior horns
of the spinal cord, and mediate trunk and limb muscle
reflexes (Figure 7–5).
CLINICAL CAVEAT: An unusual but reported cause
of persistent vestibulopathy following labyrinthectomy is development of a neuroma in Scarpa’s ganglion in the IAC.
EFFERENT PATHWAYS
2. Afferent Vestibular Pathways
The vestibular ganglion, also known as Scarpa’s ganglion, is made up of bipolar neurons lying in two sets
of cell masses attached to the two vestibular nerves
(superior and inferior) as they lie in the internal auditory canal. The superior division lying on the superior
vestibular nerve supplies the crista of the superior and
lateral semicircular canals and the utricular macula.
1. Efferent Cochlear Pathways
There exist many efferent neurons that descend paralleling the ascending pathways and link the auditory
cortex with lower auditory centers and the organ of
Corti. The efferent bundle that originates in the superior olivary complex and termed the olivocochlear bundle (of Rasmussen) is composed of 500 to 600 crossed
and uncrossed fibers. This efferent nerve bundle sends
collaterals to the ventral cochlear nucleus and travels
in the vestibular root to emerge from the brainstem in
the inferior vestibular nerve. The bundle then joins the
Figure 7–3. Afferent auditory pathways at brainstem. (From Henry Vandyke Carter—Henry Gray (1918), Anatomy of the Human
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Body. Bartleby.com: Gray’sAnat omy, Plate 691, public domain, https://commons.wikimedia.org/w/index.php?curid=541502.)
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Figure 7–4. Vestibular nuclei and pathways to cortices through the thalamus. ATD - ascending tract of Deiters; ASS - anterior
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suprasylvian cortex; DVN - descending vestibular nuclei; LG - lateral geniculate; LVN - lateral vestibular nuclei; MLF - medial
longitudinal fasciculas; MG - medial geniculate nucleus; MVN - medial vestibular nuclei; PIVC - parieto insular vestibular cortex (in primates); Post Priet C - posterior parietal cortex; Pulv - pulvinar; SG - suprageniculate nucleus; SCP- superior cerebellar
pedunculus; SVN - superior vestibular nuclei; Temporal - temporal cortex; Vim - nucleus ventralis intermedius; VPI - ventral
posterior inferior nucleus; VPL - ventral posterior lateral nucleus; VPM - ventral posterior median nucleus; VPP - nucleus ventralis posterior pars posterior; VPS - ventral posterior superior nucleus. Numbers 5 and 3b refer to Brodmann areas. (Illustration
from Hitier M, Bes nardS, Smith PF. Front Integ Neurosci 2014, 8:59. http://journal.frontiersin.org/article/10.3389/fnint.2014
.00059/full [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons.)
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Figure 7–5. The VOR and VSR reex arcs. S, L, M, and D indicate the superior, lateral, medial, and descending vestibular nu-
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clei, respectively. The lateral vestibulospinal and medial vestibulospinal tracts are shown as heavy and light lines, beginning in
the lateral and medial vestibular nucleus, respectively. (FromBrodal, A., Neurological anatomy in relation to clinical medicine.
Third edition. New York, Oxford University Press, 1981). Reproduced with permission from Hain TC, http://www.dizziness-and
-balance.com/anatomy/physiology/compensation.htm. Aug 1, 2016.)
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7. The Cochlea, Vestibule, and Central Connections 123
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cochlear nerve via the vestibulocochlear anastomosis
(of Oort). It is characteristic for the efferent fibers to
ramify numerously at every level before terminating.
The fibers enter Rosenthal’s canal in the modiolus.
Branched fibers end on the nerve chalice of inner hair
cells and also pass through the tunnel of Corti to end
on cell bodies of outer hair cells. The total number of
efferent cochlear fibers is approximately 40,000.
2. Efferent Vestibular Pathways
It has been shown that efferent vestibular pathways do
exist. They travel along with cochlear efferents until
meeting up with the inferior vestibular nerve. Passing
further, the fibers disperse as scattered fibers to the rami
supplying the maculae of the otolithic organs and cristae of the semicircular canals. The total number of efferent vestibular fibers is between 200 and 300.
REFERENCES
1. Belal A, Ylikoski J. Pathology as it relates to ear surgery II.
Labyrinthectomy. J Laryngol Otol. 1983 Jan;97(1):1–10.
2. Bhattacharyya N, Baugh RF, Orvidas L, et al. Clinical prac-
tice guideline: benign paroxysmal positional vertigo. Oto-
laryngol Head Neck Surg. 2008 Nov;139(5 Suppl 4):S47–81.
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CHAPTER 8
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Vertical Temporal Bone Sections with
Corresponding Radiographic Images
Hosakere K. Chandrasekhar and Sujana S. Chandrasekhar
Over the following vertical temporal bone histology
sections, the reader will be able to see detailed anatomy and follow structures in the same plane as is seen
during the lateral approach to the mastoid in surgery.
They are paired with sagittal CT and MR images. Normally, on radiographic images, the temporal bone is
viewed in the axial and coronal planes, with sagittal
views reserved for particular conditions such as superior semicircular canal dehiscence syndrome (SSCD or
SCDS, Minor’s syndrome)1 or as an adjunct view. Temporal bone histology sections are never prepared in the
coronal plane; therefore, the clinician must extrapolate
their knowledge of horizontal and vertical laboratory
slides in order to appreciate the structures seen on coronal radiologic images. The reader is again asked to
look at the unmarked pictures first and try to identify
the structures on both histology and radiograph, and
then turn the eye to the marked pictures as a form of
self-guided learning and assessment. Along the way,
the reader will find some clinical caveats of interest to
the ear surgeon as well as the radiologist interpreting
these studies for the otolaryngologist.
Please bear in mind that the histology sections are
20 µm (0.02 mm) thick while the CT images are 1 mm
thick and the MR images are 0.3 mm thick. The CT
images have been reformatted in this plane and are
presented in bone windows as these are the most effective for temporal bone assessment. MR images are
acquired in this plane but do not offer the type of osseous definition seen in CT; they are useful for soft tissue
delineation.
For the vertical sections, the images will begin laterally at the external auditory canal (EAC) and mastoid
antrum and go medially. The hisotologic sections are
20 µm thick and every 10th section is stained, so that each
successive figure is 0.2 mm from the previous one. The
orientation of the images is as if for a left sagittal CT
scan, or as if the patient is sitting upright with the left
ear toward the examiner. The anterior structures such
as the EAC are to the reader’s left, the posterior structures such as the mastoid are to the right; the middle
fossa dura is superior, and the mastoid tip is inferior.
For a more surgical orientation, the reader should rotate the image 90 degrees to the right, so that the EAC
is superior. A representative surgical photograph in the
proper surgical orientation is shown in Figure 8–5 on
page 136.
VERTICAL SECTION 1
These lateral-most vertical sections and sagittal CT
scans demonstrate the relationship of the EAC, mastoid antrum, and tegmen. The vertical nerve seen on
the histologic image is the chorda tympani nerve, but
it indicates how lateral the vertical (mastoid) facial
nerve is. The facial nerve’s vertical lie from the second
surgical genu to the stylomastoid foramen moves from
medial to lateral. In a study of 25 cadaveric temporal
bones, the mean depth of the second genu from the
outer cortex was 21.6 mm ± 2.62 (range 18 mm to
26 mm), while the mean depth of the facial nerve from
the cortex at the stylomastoid foramen was 12.8 mm
± 2.42 (range 9 mm to 17 mm).
stylomastoid foramen marking the exit of the facial
nerve from the temporal bone is seen.
1
In the CT image, the
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