Neuroanatomy – Auditory System Lecture -
Dr. J. Art
Introduction: The three primary functions of the auditory system are the detection, localization and
comprehension of acoustic stimuli. To fulfill all three tasks requires the transduction of mechanical to electrical
energy in the inner ear, the transmission and comp arison of information carried bilaterally by primary afferents in
the VIIIth cranial nerves to the brainstem, and the further processing of auditory inform ation at all levels of neural
axis up to and including the prim ary and association co rtices of the cerebral hemispheres.
To understand each phase in the process there are five general principles that sho uld be borne in mind. First,
many of the areas of the auditory system are arranged tonotopically, with an epithelium, nucleus or cortex arranged
with a smooth and continuous gradient of cells that respond best to low frequencies at one end and high frequencies
at the other of the structure. Second, the detection of a stimulus is under dynamic control at all levels of the nervous
system, and in general all signals ascending th e neural axis are modified by active feedback from higher levels.
Th ird, the lo calization of stimuli in three d imen sions is a com plex comparison of signals from the two ears
dependen t on the anatomy of external ear, and differences in b oth the intensity and timing of the mech anical stimuli
reaching the inner ear. Fourth, after the primary afferents synapse onto the seco nd-order neurons in the brainstem , all
subsequent aud itory information is carried bilaterally in the CNS. Fifth, during development locations in acoustic
space are m apped onto th e equivalent points in visual and som atic sp ace.
The Auditory P eriphery:
The external ear, the pinna and external auditory meatus, collects sound (acoustic waves) from the environm ent.
Th e length of the external ear canal influences perception. W ith a length of 2.5 to 3 cm, the ear canal will resonate at
roughly 3700 Hz and 13 kHz, so human hearing is naturally most sensitive to sounds in frequencies near the lower
resonance (2k – 5kHz). In addition, any complex sound containing many frequencies that strikes the pinna from
different locations in the vertical plane will interact with the rough and intricate surface of the external ear to
produ ce position-dependent acoustic spectra at the eardrum. Th ese spectral differences are used more centrally to
localize a sound source in the vertical plane. In hum ans the ability to move the pinn a, an obvious feature in a variety
of other mammalian species, is rather modest and is thought to have little impact on either signal detection or
localization.
To a first approximation the middle ear acts as a transformer between the mechanical vibrations in the lowdensity gas that surrounds us, and the pressure waves in the fluid-filled scalae of the cochlea in the inner ear. The
three o ssicles within the middle ear are the malleu s, the incus, and the stapes. T he m alleus is th e mo st lateral
element, and it attaches to the tympanic membrane via its manubrium and lateral process. Embryologically the head
and neck of the malleus come from the mandibular (first) arch, and the manubrium of the malleus comes from the
hyoid (second) arch. The tip of the manubrium is on the umbo of the eardrum, and onto its medial side is attached
the tendon of the tensor tympani muscle. Th e incus is the medial elem ent of the trio, with a bod y and short process
in the epitympanic region. The articular facet forms the joint with the head of the malleus, while the lenticular
process forms th at with th e head of the stapes. Embryologically the body and short process are from the mandibu lar
arch, and the long and lenticular processes are from the hyoid arch. The third ossicle is the stapes, which consists of
a body, two crura and a footplate. The head articulates with the incus, and the footplate fits into the oval window of
the scala vestibuli. The tendo n of the stapediu s muscle inserts into the po sterior part of the head of the stapes.
Middle-ear reflexes cau se contraction of either or both the ten sor tympani and the stapedius mu scles, an d m odify
both th e intensity and the frequency response of the m echanical energy transmitted through the mid dle ear.
To permit energy to flow from the low-density fluid of air into the high-density fluid of the inner ear requires two
tricks of impedance matching across the middle ear. The first is the pressure gain brou ght about by the large
difference in the size of the tympanic memb rane and the footplate of the stapes (15X). The second is a mechanical
advan tage brought ab out by the lever action of the three ossicles resulting in a m echanical advantage of about 1.6X.
The auditory inner ear, the cochlea, is the region that transduces mechanical energy into neural activity encoded
on the au ditory part o f the vestibulocochlear nerve, CN VIII.
The cochlea is divided into three scala. The scala vestibuli and scala tympani are perilymphatic spaces filled
with fluid (mu ch like CSF) high in Na and Ca and low in K. Th e scala media, between the other two, is an
endolymphatic space with a very unusual fluid high in K and low in Na. The endolymph is produced by cells in the
stria vascularis along the lateral wall of the cochlea. Separating th e scala vestib uli and the scala media is Reissner’s
membrane, a thin squamous epithelium . The sep aration between the scala tympani and the scala media is
anatomically more complex since it occurs at the apex of the primary sensory cells and along the other tight
junctions of the ep itheliu m lining the scala media.
The organ of corti is the sensory structure of the auditory system. It rests upon a flexible basilar membrane
stretching between the osseous spiral lamina housing the spiral ganglion (the auditory primary afferents) and the
lateral wall of the coch lea. With acoustic stimulation , vibrations of the footplate of the stapes prod uce pressure
Neuroanatomy – Auditory System Lecture -
Dr. J. Art
differences between the scala vestibu li, scala med ia and scala tympani causing displacement of the basilar
membrane. This displacement in turn produces a relative motion between the sensory epithelium of the organ of
corti and an overlying gelatinous tectorial membrane. A shearing force on the microvilli, the stereocilia, at the apex
of each hair cell results in the flow of transduction current into the cell. The resulting depolarization of the cells
opens voltage- and ion-sensitive channels along the basolateral surface of the hair cells with a resulting influx of
calcium. Elevation of calcium near the afferent synapse causes fusion of synaptic vesicles with the basolateral wall
of the hair cell, and the release of an excitatory amino acid, most likely glutam ate, onto the prim ary afferent fibers.
The sensory epithelium within the organ of corti can be further differentiated, and consists of a single row of
inner hair cells, IHCs, near the spiral ganglion, that synapse with 95 % of the primary afferents (type I primary
afferents). Efferent fibers from the CNS synapse directly onto these primary afferen ts to m odulate their sensitivity,
but, in general, do not contact the IHCs. In stark contrast, there are three rows of outer hair cells, OHCs, that receive
the majority of the input from the efferent fibers originating in the crossed olivary-cochlear bundle, COCB. The
action of these efferent fibers can modify the mechanical properties of the outer hair cells, and consequently, the
tuning o f the basilar membrane. T he afferent innervation of OHC s is rather modest, with a large num ber of cells
converging onto a single type II auditory afferent. The organ of corti is arranged tonotopically with cells sensitive to
high frequencies closest to the oval window and the footplate of the stapes, and cells sensitive to low frequencies
furthest from the oval window at the apex of the cochlea, near the helicotrem a, a connection between the scala
vestibuli and tymp ani.
Loss of F unction: Deafness may be a ‘conductive loss’ involving either the external or middle ears, or it may be
a ‘sensory-neural loss’ either at the level of hair cell, involving processes related to mechano-electrical transduction,
or downstream at the level of the primary afferents or the cochlear nuclei. A tuning fork test is used to differentiate
between these two types of deafness. To test for conductive losses, the handle of a tuning fork is applied directly to
the skull, and mechanical energy via bone conduction will bypass the external and middle ears and stimulate the
cochlea directly. Percep tion of such a stimulus suggests that deafness is due to a loss in conduction of mechanical
energy to the inner ear. If such a stimulus were not perceived, it would suggest that deafness resulted from a
sensory-neural hearing loss, perhaps at the level of the inner ear or primary afferents.
The Auditory Pathway:
The CN V III consists of both afferent fibers traveling centripetally, toward the CNS, and efferent fibers traveling
centrifugally toward the cochlea. The afferent fibers: Type I afferents contact a single inner hair cell (IHC); each
IHC receives ~20 afferent endings. Type II afferents contact 15 - 20 outer hair cells (OHCs). Each OHC receives 6
- 10 afferent endings. The efferent fibers: Thick mylinated fibers innervate OHCs, and are mostly crossed, coming
from the con tralateral olivary complex . Thin fibers are unmylinated fibers that are uncrossed and do no t synapse
onto th e IHC, but directly onto the type I afferents.
The CN VIII enters at the angle between the po ns and the medulla at the cerebellop ontine angle. All primary
afferents enter and bifurcate:
1). An ascending branch goes to anterior ven tral coch lear nucleus (AVCN).
2). A descending branch goes first to the posterior ven tral coch lear nucleus (PVC N), & then to the dorsal
cochlear nucleus (DC N).
The nerve also has a cochleatopic or tono topic map: Low characteristic frequency (CF) fibers from the apex of
the cochlea bifurcate at the moment they enter the CN. The ascending branch stays ventrolateral in the AVCN, and
the descen ding branch stays ventrolateral in PVCN and DCN. High CF fibers go do rsomedial in VCN before
bifurcating. The ascending branch goes dorsomedial in AVCN & descending branches go to the dorsom edial
portion s of the PVCN and DCN. Therefore low CF information is represented ventro-laterally, and high frequency
information is represented dorso-medially. This organization (discovered in cats and primates) is maintained
throughout the pathway, and is presumably found also in man.
The anterior ventral cochlear nucleus: Afferent fibers are from CN VIII. The nucleus can be divided by the
types of cells found histologically in different regions. Anteriorly are the spherical bushy cells. Posterior are the
glob ular bushy cells. The afferent fibers make large calyceal endings, end bulbs o f Held, on the bushy cells.
Th erefore the firing properties of the bushy cells are ‘primary like’.
Neuroanatomy – Auditory System Lecture -
Dr. J. Art
The posterior ventral cochlear nucleus: Afferent fibers from CN VIII. Principle cell in this nucleus is the
octopus cell. Dendrites of these cells are at right angles to the type I and type II primary afferents, and therefore the
cells are more sensitive to bands of frequencies, rather than pure tones. The axons of the PVCN leave the nucleus
dorsal to the restiform body and form the intermediate acoustic stria (stria of Held), which enters the pontine
tegmentum.
The dorsal co chlear nucleus: Afferent fibers from CN VIII, and the response to pure tones is complex. The
DCN has three layers: molecular, fusiform , and central region. The last contains stellate cells. DCN receives
abund ant input from other somatosensory regions of the brain. The axons of the stellate cells leave in the dorsal
acoustic stria (stria of Manakow), which is dorsal to the restiform body, and they too enter the pontine tegmentum.
Superior Olivary Complex, SOC: Is composed of the lateral and medial superior olives and the medial nucleus
of the trapezoid body. Most of the synaptic input is from the AVCN via the ventral acoustic stria. Sizes: The M SO
is largest, the LSO is quite small, and the MNT B is even smaller (if it exists at all in man).
The Medial Superior Olive, MSO: is large in humans and small in sp ecies that lack lo w frequency hearin g.
Principle neurons of the MSO are multipo lar cells. On a given side the inp ut is from axons of spherical bu shy cells
of both left and right AVCN. Th erefore the MSO is the first place we have binau ral processing of auditory
information. In the MSO we can localize low frequency sounds in the horizontal plane. For a low frequency
acoustic stimulus that is off axis there is little intensity difference between the two ears because sound is diffracted
around the head, not reflected. Ho wever there is a timing difference, measured in µs, betw een informatio n that
reaches the two ears. Therefore there is a phase difference between the signal in the two ears, and the differences in
time of arrival of the signal determines how MSO cells will respond and encode the horizontal position.
Medial Nucleus of the Trapezoid Body, MNTB : Contain s large mu ltipolar cells. In this nucleus there are
calyceal endings, this time from the con tralateral globular bushy cells. T herefore there is m ore high frequency than
low frequency informatio n in th e MNT B. Axons from the M NT B term inate on the ipsilateral neurons of the lateral
superior olive (LSO).
Lateral Superior Olive, LSO: Has small multipolar cells. Each cell receives bouton-like endings from the
ipsilateral spherical bushy cells of the AVCN, and the ipsilateral principle cells of the MNTB. Like the MNTB
neurons, the LSO receives more high than low frequency information. The LSO codes for horizontal position using
high frequency cues. High frequency sounds are reflected by the head rather than diffracted, and thus there are
intensity differences between the high frequency sounds reaching the right and left ears. Therefore the activity
of the LSO neurons depend on the intensity differences. Axons of the LSO contribute bilaterally to the lateral
lemniscus on both sides.
Periolivary Nuclei and Olivocochlear B undle, OCB: The multipolar cells of the OCB form two separate
groups. Medial group: Most axons extend contralaterally and leave the brain as part of the vestibular division of the
eighth nerve. They have large axons. These fibers leave the vestibular part of the nerve at the base of the cochlea
and join the cochlear division as the anastomosis of Oort. The fibers travel radially across the organ of corti and end
on outer hair cells. Lateral group: Smaller cells that send there axon s ipsilaterally, join with crossed fibers to reach
the cochlea. Once through the habenula perforata, they synapse on the type I primary afferents.
Lateral Lem niscus: This is the major auditory tract from the pontine tegmentum to the inferior colliculus. It
forms lateral to the SO C and is composed of axons from : 1) Contralateral multipolar neurons o f VCN via the ventral
acoustic stria & trapezoid bo dy; 2) Contralateral stellate cells of DCN via dorsal acoustic stria; 3) Ipsilateral
Neuroanatomy – Auditory System Lecture -
Dr. J. Art
principle cells of MSO; 4) Both ipsilateral and contralateral principle neuron s of LSO; 5) Contralateral octopus cells
of PVC N via intermediate acou stic stria.
All but #5 go all the way to the inferior colliculus (IC). The axons from the PVCN end in the ventral nucleus
of the lateral lemniscus (VNLL). Axons from VNLL run in the LL to the IC. Therefore the L L has 2 nd order cells
from the VC N and DCN ; 3 rd order cells from the MSO and VNLL and 4 th order axons from the LSO. Most axons
of the LL end on neurons of the ipsilateral central nucleus of the IC. But the LL also contains interstitial nuclei, the
ventral, intermediate, and dorsal nuclei of the lateral lemniscu s. Most of the projections from these nuclei go on to
the ipsilateral IC, but some from the dorsal coch lear nucleus cross the midline as the com misure of Probst to synapse
on the contralateral central nucleus of the inferior colliculus.
Inferior Colliculus (IC): The major m idbrain audito ry structure & largest audito ry structure in the brainstem.
The largest nucleus is the central nucleus. Smaller surrounding nuclei are parts of the descending rather than the
ascending pathways. Central nucleus of the IC (CNIC) has tonotopic organization. Commisure of the IC (CIC)
interconn ects the right and left IC. Minority of the fibers bypass the contralateral IC and join the contralateral
brachium of IC. Most axons of the CIC stay ipsilateral and form the brachium of the IC. Brachium lies on
dorso lateral surface of midbrain and leaves rostrally to enter the thalamus. Fibers end massively on the medial
geniculate body, the principle auditory mucleus of the thalamus.
Medial Geniculate Body of the thalamus (M GB): It has three divisions: ventral, dorsal and medial.
Ventral division of the MGB has bitufted neurons in organized, parallel, tonotopically organ ized laminae.
Most fibers of the brachium of the IC synapse on ventral division of MGB. Axons from here project via the
sublenticular portion of the internal capsule to the primary auditory cortex. The primary and first association cortex
project axons back to the ven tral division of the MGB. Function of the ventral division is to transmit specific
auditory discriminative information to the cortex.
Dorsal division of the MGB is complex(!), with as many as 10 divisions and 8 different neuronal types. Input
is from the CNIC, peripheral IC, brainstem reticular neurons, ventral MGB and other thalamic nuclei. Dorsal axons
project to the auditory association cortex. It is proposed that the do rsal division of the MGB functions in
maintaining and directing auditory attention.
Medial division of the MG B had axons from the vestibular nuclei, spinal cord, superior colliculus, and a small
input from near the SOC. There is sparse projection from Medial MGB to the auditory cortex, the putamen,
amygdala, and other cortices. There is a projection to the Medial MGB from all auditory cortical regions, & some
non-auditory cortex via the internal capsu le. Function: Multisenso ry arousal system?!?!?
Auditory cortex: In the temporal lobe, it is the ‘transverse gyri of Heschl.’ On the superior surface of the
temp oral lobe it is continuou s with, but not part of, superior temporal gyrus. There m ay be one to three, rarely four,
transverse gyri. The smoother portion of the superior surface of temporal lobe caudal to transverse gyri is called the
‘planum tem porale’, PT. PT is generally larger on the left than right (65% :35%).
There is confusion in the neuroanatomy texts concerning primary auditory cortex & its location within the
temp oral lobe. Generally what is called the primary cortex is actually part of the superior temporal gyrus, rather than
the gyrus o f Heschl which is no t visible on th e surface until one opens the lateral fissure. The human primary
cortex, koniocortex, is variably called area 41, A1, TC, KAm or KAt. Cytoarchitectonic studies looking for
koniocortex find it within the first transverse gyrus of Heschl. Parakoniocortex (42, TB, PA) has different
architecture. It surrounds the ko niocortex and extend s a variable distance beyond the transverse gyri of Heschl onto
the planum temporale. Studied extensively in other animals, it as been found that in cats and nonhum an primates
Neuroanatomy – Auditory System Lecture -
Dr. J. Art
there is a precise tonotopic organization of the koniocortex. In cats along the iso-frequency bands there are sub
bands with 1) excitatory response from either ear, and 2) excitatory from the contralateral and inhibitory from the
ipsilateral side.
Planum temporale & superior temporal gyrus: PT behind the parakoniocortex & posterior part of superior
temporal gyrus. Roughly behind the vein of Labbe. Area 22 also extends onto the parietal operculum and the
inferior parietal lobe. In 90% of humans area 22 is ‘Wernicke’s area’ - a speech receptive area. (Some authors also
include the angular gyrus (area 39) and the sup ra marginal gyrus (area 40). In humans PT is larger on the left in
65% of individuals, and larger on the right in 11% . It is of equal size in 24%. PT is also larger on left side in
musicians with perfect pitch .
Language related cortices: The arcuate fasciculus interconnects the inferior parietal area 22 and the triangularus
of the inferior frontal gyrus. Supramarginal gyrus (40) and (39) make up the inferior parietal lobule. T his area is
meant to integrate the visual, auditory and somesthetic information. The inferior frontal gyrus - the area triangularus
(areas 44 & 45) are Broca’s area, an area impo rtan t in expressive speech and language. As with the PT it is also left
dom inant.