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Adv. Oto-Rhino-Laryng., vol. 20, pp. 337-356 (Karger, Basel 1973)
Auditory Neurons of the Brain Stem l
D. K. MOREST
Department of Anatomy, Harvard Medical School and Eaton-Peabody
Laboratory of Auditory Physiology, Massachusetts Eye and Ear Infirmary,
Boston, Massachusetts
I. Introduction
One difficulty in understanding the role of the central auditory system
in hearing disorders is lack of information concerning the processes by
which auditory nerve cells in the brain normally represent acoustic information and transform it in the course of the auditory analysis. Electrophysiological observations have shown that auditory neurons respond to specific acoustic stimuli in predictable, even stereotyped patterns [KIANG et at., 1965a, b].
On the basis of this regularity, it would appear that the discharge patterns of
the neurons could constitute a kind of auditory code. However, it will not be
possible to explain how the auditory neurons are able to encode information
in specific electrical patterns until the structural basis for this kind of activity
has been clarified. Moreover, the morphological features of nerve cells that
are directly related to their analytical capacities must be recognized before
a functional neuropathology of the central auditory system can be developed at the cellular level. At the very least, one would like to know what
cytological features of nerve cells to study in pathological specimens.
Recent investigations delineated some of the morphological characteristics of nerve cells that may relate directly to the central hearing mechanisms
[MoREsT, 1964a, b, c, 1965a, b, 1968a, b, 1971]. The results suggest that, if
neurons are predictable and typical in their electrical discharge patterns,
they are equally regular and stereotyped in many of their morphological
attributes. Some of these structural regularities could be related to the functions of the auditory centers. Morphological features of nerve cells that
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1
Supported by US Public Health Service grant NS 06115, with the technical aid of
Mrs. A. B. GREENE.
MOREST
338
define their physiological capacities are the arrangement and geometrical
patterns of their processes, their axons and dendrites, and the sequences of
synaptic connections established by these processes.
Methods
The rapid Golgi technique, namely, silver chromate impregnation, is the method par
excellence for the microscopic demonstration of neurons and their processes. When this
technique is applied after perfusion-fixation of the brain [MOREST and MOREST, 1966], it
may be reliably correlated with observations by other methods, including silver degeneration methods [MOREST, 1965b] and electron microscopy [MOREST, 1971].
The relevant anatomical principles can be illustrated by reference to the
groups of auditory neurons in the trapezoid body, in particular the superior
olive and the medial nucleus of the trapezoid body. These cell groups are related to the ascending and descending auditory pathways (fig. 1). The ascending pathway begins with the secondary auditory neurons of the cochlear
nucleus, which receives the synaptic endings of the auditory nerve fibers
from the cochlea. The neurons of the cochlear nucleus send axons to the
superior olivary complex, including the medial nucleus of the trapezoid body,
and to the inferior colliculus. Neurons in the inferior colliculus project to
the medial geniculate body, which in turn projects upon the auditory cortex.
There are commissures of the cortex and the brain stem which provide for
bilateral representations of the cochlea. These are the main pathways of the
ascending auditory system. The superior olive and the inferior colliculus are
also important centers in the descending auditory system. The inferior colliculus receives corticofugal axons from the auditory cortex and in turn projects to peri-olivary cell groups of the superior olive. These parts of the superior olivary complex give rise to the olivo-cochlear bundle, which ends
upon hair cells in the cochlea.
All of these pathways are tonotopically organized. First of all, in the
cochlear nuclei of the cat the auditory nerve fibers establish a sequential
correspondence between points in the cochlea and sectors in the cochlear
nucleus (fig. I), such that the basal coil is represented most dorsomedially
in each cochlear nucleus and the more apical turns successively more ventrolaterally in the cochlear nuclei [LORENTE DE No, 1933; RASMUSSEN et al.,
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Results
Auditory Neurons of the Brain Stem
339
Base
Cochlea
Nuclei
Apex
Corp. callosum
~a
cortex~
Radiation
Thalamus
Midbrain
Pons
Medulla
Descending
Fig. 1. Top: Scheme of the tonotopic organization of the dorsal and ventral cochlear
nuclei as a frequency map of the cochlea. The spatial arrangement of the axonal branches
of the auditory nerve in the cochlear nucleus establishes an orderly correspondence of
successively more apical regions of the cochlea with progressively more ventrolateral sectors of the cochlear nucleus. Bottom: The principal pathways of the ascending and descending auditory systems. RF = reticular formation; ML = medial lemniscus.
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Ascending
MOREST
340
Fig. 2. Photomicrograph of a rapid Golgi impregnation of the trapezoid body in a
transverse section from a I-day-old cat. A = fibers of the trapezoid body; B = medial
trapezoid nucleus; C = medial superior olive; D = dorsomedial peri-olivary nucleus;
E = ventral trapezoid nucleus; F, G, and K = peri-olivary cell groups; H = lateral superior olive; J = lateral trapezoid nucleus. Thin arrows (left) show the direction of unimpregnated abducens nerve rootlets. Scale = 100 flm.
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1960]. This amounts to a frequency map of the cochlea. This tonotopic map
is maintained in all subsequent parts of the auditory system [WARR, 1966;
MOREST, 1968a; ROSE et al., 1963; MOREST, 1965a; WOOLSEY and WALZL,
1942]. As it will be seen, this circumstance permits us to interpret several
prominent features of the auditory neurons.
The superior olive in the cat receives bilateral projections ofaxons from
the cochlear nuclei (fig. 1). Within the superior olivary complex these afferent
341
axons end in distinct groups of neurons (fig. 2). These may be distinguished
by the different patterns in which the afferent axons are arranged. In the
medial superior olive the ends of the axons line up along the dorsoventral
axis of the nucleus. A linear arrangement of the different axons also occurs in
the lateral superior olive, but this nucleus has an S-shaped configuration.
Nests of axonal endings from the cochlear nucleus occur in several groups of
neurons located on the perimeter of the medial and lateral superior olivary
nuclei. These nests define the peri-olivary nuclei, which are thought to give
rise to the crossed and uncrossed olivo-cochlear bundles. The dorsomedial
peri-olivary nucleus in particular has been indicated by RASMUSSEN [1946] as
one source of the crossed olivo-cochlear bundle. Medial to the medial superior olive is the medial trapezoid nucleus, in which many of the axons
from the contralateral ventral cochlear nucleus form very large endings, the
calyces of Held.
The medial superior olive receives projections from both cochlear nuclei.
It is one of several places in the brain where acoustic information from both
sides of the head can be integrated, and binaural interactions may occur.
The afferent axonal endings from the cochlear nuclei line up in an orderly
dorsal-to-ventral sequence. This sequence corresponds to the tonotopic organization of the cochlear nucleus and cochlea, such that the more basal segments of the cochlea are represented more ventrally [WARR, 1966]. The axons
from the contralateral cochlear nucleus end in the medial half of the nucleus;
those from the ipsilateral cochlear nucleus end in the lateral half [STOTLER,
1953]. The end of each axon branches predominantly within a horizontal
layer of the nucleus. At the same time the neurons of the medial superior
olive have a layered arrangement (fig. 3). The dendrites of these neurons
occupy elongated, oval fields, flattened in the horizontal plane. This arrangement of afferent axons and the dendrites would provide the optimum
structural basis for preserving the tonotopic map of the cochlear nucleus.
The dendritic field of each neuron would intercept a narrow sector of the
spectrum of afferent fibers, each of which branches so as to maximize the
synaptic contacts within a narrow sector of the nucleus. Since the medial
dendrites would receive synaptic endings from the contralateral cochlear
nucleus, binaural interactions could occur in the individual auditory neurons.
It is not yet certain whether these responses are involved in localizing sounds
in space, in the lateralization of acoustic reflexes, or in some other binaural
phenomenon.
Thus it appears that in the medial superior olive of the cat the geometrical
pattern of the axonal endings and of the dendritic processes contacting
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Auditory Neurons of the Brain Stem
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Fig. 3. Scheme of the trapezoid body as reconstructed by serial transverse sections
from a rapid Golgi impregnation of a 14-day-old cat. A, B, C, and D = axonal calyces,
principal neuron, stellate neuron, and elongate neurons, respectively, of the medial trapezoid nucleus; E and F = radiate and elongate neurons, respectively, of the dorsomedial
peri-olivary nucleus; G = large neurons of the nucleus pont is centralis caudalis; H =
small neuron of the same nucleus: the axon forms collateral branches to the left of the perikaryon; J = neuron in the dorsal peri-olivary group; K = collateral of an axon crossing
in the trapezoid body, which ascends in the root of the abducens nerve (6). LSO = medial
lobe of the lateral superior olivary nucleus; MSO = medial superior olivary nucleus;
TV = ventral trapezoid nucleus; TVM = a medial extension of the preceding, associated
with crossing trapezoid fibers. a = Axonal collateral of principal neuron projecting to the
dorsomedial peri-olivary nucleus; b = collaterals of calyx fibers projecting to the dorsomedial peri-olivary nucleus; c = ventromedial peri-olivary group. Modified from figure 4
in MOREST [1968a].
~
~
I
/-/F~~
~m
~,
Fig. 4. Calyces of Held and axonal collaterals in a transverse plane of the medial trapezoid nucleus of a newborn cat. The neurons at the top actually appear in an adjacent section. A = collaterals of the principal neuron that leave the section in a ventrolateral direction; B = collateral of the preceding, arborizing in the dorsomedial peri-olivary nucleus;
C = afferent axon of the medial trapezoid body, crossing the myelin sheaths of the abducens
rootlet (6) and the dorsomedial peri-olivary nucleus to form the type of terminal arborization consistently associated with the dendrites of principal neurons; D = collateral of a
calyciferous axon in the medial trapezoid nucleus, which ramifies in the dorsomedial periolivary nucleus; E = afferent axon, arborizing in this nucleus; F = collaterals of calyciferous
axons, ending in the medial trapezoid nucleus; G = large solitary process of a calyx; H =
short collaterals of a calyx; J = collateral growth cones. * = Axon of dorsomedial periolivary neuron. Rapid Golgi method. Modified from figure 9 in MOREsT [J968aJ.
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30
344
them would correspond to a frequency map on the dorsoventral axis of the
nucleus and to a timing mechanism on the mediolateral axis. Evidently slight
differences in the times at which signals arrive from the two sides can be
critical in determining if the activity of the individual neurons is enhanced or
suppressed [GALAMBOS et al., 1959].
The medial nucleus of the trapezoid body contains three varieties of
neurons in the cat (fig. 3 B, C, D). A detailed description appears elsewhere
[MOREST, 1968a]. The principal neurons are the most numerous, and perhaps the most interesting, since they receive the calyces of Held, the large
synaptic endings ofaxons from the contralateral ventral cochlear nucleus
that establish a point-to-point correspondence with the neurons of the medial trapezoid nucleus (fig. 3 A). The principal neurons and the calyces of
the medial trapezoid nucleus maintain a special relationship with the neurons of the crossed olivo-cochlear bundle in the dorsomedial peri-olivary
nucleus (fig. 3 E, F). The peri-olivary neurons receive endings of axonal
branches from the principal neurons of the medial trapezoid nucleus. The
calyciferous axons, contacting the principal neurons, also send side-branches
to the dorsomedial olivo-cochlear neurons. In order to form a useful notion
of the possible significance of this relationship, it is necessary to have a clear
idea of the structure and function of the calyciform synapse.
The calyces of Held are large axosomatic endings (fig. 4). They are probably the largest synaptic endings in the brain [see MOREST, 1968a]. The calyx
is formed by thin, broad, petal-like expansions of the terminal axon. These
terminal processes, much like the petals of a tulip, enclose a large portion of
the cell body upon which they synapse. Normally each axon forms one, and
only one, calyx ending on one, and only one, principal neuron. Each principal neuron receives only one calyx. Numerous, thin, local collaterals
emanate from the calyces. These local collaterals form many small endings
in association with the cell bodies of surrounding principal neurons, which
also receive calyx endings. The principal neuron is a highly specialized and
stereotyped variety of neuron, clearly distinguished from other types of
neurons in the region by its typical cell body in Nissl-stained preparations
and by its characteristic dendritic branching pattern in Golgi impregnations
(fig. 4). The dendrites receive the endings ofaxons from the cochlear nucleus,
other than the calyciferous fibers (fig. 4 C). But no other type of neuron
receives calyces. This highly specific synaptic arrangement establishes a pointto-point, cell-to-cell correspondence between one portion of the ventral
cochlear nucleus and the medial trapezoid nucleus.
In electron micrographs it is possible to identify the synaptic contacts of
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MOREST
345
Fig. 5. Electron micrograph of a portion of a principal cell from the medial trapezoid
nucleus of a cat. NU = nucleolus; N = nucleus; NS = Nissl substance; M = mitochondria; L = lysosome; S = synaptic endings of afferent axons; * = synaptic complexes;
A = myelinated axon; G = glial processes, Scale = 0.5,um.
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Auditory Neurons of the Brain Stem
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Fig. 6. Part of a calyx (C) ending on the cell body of a principal neuron (P). F =
myelinated parts of calyx fibers; FM = central core of filaments and mitochondria in
fiber endings; C' = part of another ending. Scale = 0.5 11m.
.---.',
, ';t~
'
.
lj)
,
•
'/<
~~
II
.
'
Fig. 7. Cross-section of the central stem of a calyx ending (F) in synaptic contact with
a principal cell body (P) and also with an unidentified dendritic profile (D). F= bundles
of calyx filaments; * = synaptic complexes. Scale = 0.5 fl-m.
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...
I ,
.,
348
Fig. 8. Part of a calyx process (C) in contact with a principal cell body (P). In the thin
section this calyx process extends for a length of 10 .urn. The external membrane of such
an ending is separated from the adjacent cell membranes by intercellular spaces. This holds
for the synaptic complexes (*) as well as the surface invaginations ( +). Although the pre-
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MOREST
Auditory Neurons of the Brain Stem
349
Fig. 9. Synaptic complex (*) of a principal cell body (P) with a calyx ending (C),
which contains many clear vesicles and part of a mitochondrion (M). D = large densecored vesicles, associated with a multivesicular body; arrows = cisternae of endoplasmic
reticulum. Scale = 0.5 ,um.
and post-synaptic membranes may be sectioned tangentially (lower edge of the synaptic
complex), so far, close junctions between them have not been observed. The surface invaginations often contain exceedingly thin processes (arrow) or their remnants, some of
which probably derive from the filamentous astrocytic processes (G) encasing the synaptic
endings. Evidently an elaborate network of these delicate processes extends over the surface of the principal cell between and beneath the calyx processes. This network often
appears in rapid Golgi impregnations of these cells and other cells; it probably corresponds
to the pericellular net of GOLGI [1903], observed, illustrated, and variously interpreted by
BETHE [1900], HELD [1902], and others [see RAM6N Y CAJAL, 1934]. Scale = 0.5,um.
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the calyces in the cat by limiting the observations to cell bodies with the typical features of principal neurons [see MOREST, 1968a]. When this is done,
the typical long, thin, petal-like profiles of the calyx endings are readily recognized (fig. 5-8). Many of the smaller synaptic profiles probably represent
partial sections of calyx processes and the endings of local collaterals from
neighboring calyces. The surface of the cell body is extensively covered with
synaptic endings. In every profile of the principal cells so far observed, perhaps half of the surface is covered with synaptic endings. Here and there,
thin, thready processes weave between the calyx and the perikaryal surface;
sometimes such processes gather in strands or patches. Some of these processes are seen in continuity with astrocytic branches (fig. 8); perhaps others
represent appendages of the calyces or the principal cell body. The large
350
central core of the calyx contains aggregates of neurofilaments (fig. 6 and 7).
The synaptic complexes consist of numerous clear vesicles, thickened preand post-synaptic membranes, a prominent sub-synaptic density in the principal cell, and an intermediate density in the slightly-widened synaptic cleft
(fig. 9). A number of small synaptic complexes may be found in each calyx
profile (fig. 7 and 8). Since these complexes appear to be relatively uniform
in size, it seems unlikely that many of them are connected to each other.
Thus a single calyx ending apparently may form many synaptic contacts with
the principal cell soma. This observation, together with the large surface
area occupied by synaptic endings, suggests that the synaptic activity of the
calyx would dominate the post-synaptic membrane. These are cytological
features typically associated with chemical synapses in the nervous system.
Similar findings have been reported in the rat [LENN and REESE, 1966].
Close junctions at the calyces of Held have not yet been demonstrated. Thus
there is neither morphological nor electrophysiological evidence for electrical
synapsis at the calyx of Held. However, intracellular recordings from the
principal neurons remain to be made. The identity of the chemical transmitter
at the calyx of Held is not known. Evidently acetylcholinesterase activity has
not been localized in the calyces, as it has in the olivo-cochlear bundle and
in the cell bodies of the dorsomedial and other peri-olivary neurons forming
that bundle [RASMUSSEN, 1964; OSEN and ROTH, 1969].
In collaborative studies with GUINAN [1968 and unpublished], we have
identified the principal neuron with a particular type of unit activity recorded
with microelectrodes in the medial trapezoid nucleus. Not surprisingly this
unit has a narrow tuning curve. A tonotopic arrangement in the nucleus is
suggested by the sequences of these units' best frequencies [GUINAN et al.,
1969]. Possibly this sequence in the adult cat corresponds to the dorsal-toventral, or perhaps, more accurately, dorsolateral-to-ventromedial, sequence,
deduced from the laminar arrangement of the peri-dendritic plexus in the
newborn kitten (fig. 10) [MOREST, 1968a]. The combined anatomical and
electrophysiological data [MOREST, 1968 a; GUINAN, 1968; GUINAN et al.,
1969; LI and GUINAN, 1971] suggest that the principal neurons and their
calyces exhibit spontaneous activity and, in response to simple acoustic stimuli (tone bursts or clicks), produce characteristic temporal response patterns
with very short latencies, as shown by extracellular micro electrode recordings. In the above respects, these units closely resemble the auditory nerve
fibers [KIANG et al., 1965b]. However, the potentials of the calyx units are
associated with a complex wave form. The complex wave consists of an
early positive potential, which is apparently generated by the calyx itself,
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MOREST
Auditory Neurons of the Brain Stem
1 100 iJ
351
m
~
y
Fig. 10. The tonotopic correspondence between the medial superior olivary, medial
trapezoid, and dorsomedial peri-olivary nuclei, as seen in the transverse plane from a
newborn cat. X, Y, and Z = axons of the medial trapezoid body, projecting to the medial
region of the medial superior olivary nucleus (MSO) in a dorsoventral tonotopic sequence,
contribute collaterals to the peri-dendritic plexus of the medial trapezoid nucleus, also in a
nearly dorsoventral sequence. The basal tum of the cochlea is represented most ventrally.
A, B, and C = the main axons of principal neurons in the medial trapezoid nucleus, before
leaving the section laterally, send collaterals, a, b, and c, to the dorsomedial peri-olivary
nucleus, so that the dorsal-to-ventral extent of the first nucleus is projected across the lateral-to-medial axis of the second. The large calyx fibers ending next to the principal neurons and their long collaterals to the dorsomedial peri-olivary nucleus respect the topographical correspondence as indicated. The cells corresponding to B, b, and C, c were taken from adjacent sections but retain their relative positions in the drawing. The scale at
the upper left is parallel to the median raphe. P = pyramid. Rapid Goigi method. Modified from figure 10 in MOREST [1968a).
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z
352
whereas a later, negative potential corresponds to a post-synaptic event,
perhaps in the initial segment of the principal neuron. Since both components of the wave nearly always occur together, at least with simple acoustic
stimuli, it is probable that nearly every signal invading a calyx will cause the
post-synaptic principal neuron to fire and that the synapse is excitatory conclusions in harmony with the cytological features of the calyciform
synapse.
The effects of the calycine collaterals on surrounding cells are not
known. Even if the calyx forms an excitatory synapse, the collaterals could
elicit either excitatory or inhibitory post-synaptic responses, depending on
the properties of the post-synaptic membranes. If inhibitory, the local collaterals of the calyx could produce an inhibitory surround, which might influence the shapes of the units' tuning curves or the temporal patterns of
their discharge.
The dorsomedial peri-olivary nucleus receives the long collaterals of the
calyx fibers and of the principal meurons and other afferents, coming directly
from the cochlear nuclei. These afferent axonal endings are arranged in more
or less vertical layers, in parallel with elongate neurons - a pattern that we
have learned to associate with a topographic organization. The axonal collaterals of both the calyx fibers and of the principal neurons project into these
layers in an orderly sequence (fig. 4). The result is that a rigid correspondence obtains between sectors of the medial trapezoid nucleus and the layers
of the dorsomedial peri-olivary nucleus. Insofar as this correspondence correlates with the frequency organization of the auditory system in this region,
we may deduce the probable tonotopic sequence as follows:
The dendrites of the principal neurons engage an elaborate plexus of
axonal endings (fig. 4). These afferent axons arborize in a pattern that recapitulates that of the dendritic branches on which they end. The arbors of
the peri-dendritic plexus are flattened in nearly horizontal planes, and their
arrangement forms a lamination pattern in the medial trapezoid nucleus
(fig. 10). This laminar pattern probably corresponds to the tonotopic organization of the nucleus. Much of the peri-dendritic plexus derives from collaterals ofaxons passing through the medial trapezoid nucleus en route from
the cochlear nucleus to the medial superior olive. Since we know that the
afferent axons of the medial superior olive are tonotopically arranged in a
dorso-ventral sequence, the layers of the peri-dendritic plexus in the medial
trapezoid nucleus must also maintain a corresponding dorsolateral-to-ventromedial, low-to-high frequency, tonotopic sequence. It seems likely that the
tonotopic arrangement of the calyces would also follow this pattern. The
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MOREST
353
more medial and dorsal principal neurons send long axonal collaterals to the
more medial and ventral layers of the dorsomedial peri-olivary nucleus, and
the long collaterals of the calyces also conform to this pattern. From this we
may infer that a tonotopic sequence occurs across the mediolateral axis of
this nucleus, such that the lower frequencies are represented more medially.
It might be expected that this frequency organization would correspond to
the range of tuning curves already shown among units in the crossed olivocochlear bundle [FEx, 1962]. A systematic exploration of unit electrical activity in the peri-olivary origins of the olivo-cochlear bundle remains to be
done.
The possible significance of the long collaterals projecting to the dorsomedial olivo-cochlear neurons is now open for exploration. One consequence
of the arrangement of the collaterals is that the neurons of the olivo-cochlear
bundle would be in a position to sample both the input and output of the
principal neurons. If the olivo-cochlear bundle forms part of a feedback loop
[FEx, 1962], it could function in a physiological automatic gain control of
the auditory signals relayed by the cochlear nucleus. In that case the collaterals of the calyx fibers could negotiate an increase in the activity of the
crossed olivo-cochlear neurons, in response to increased sound levels and the
accompanying increased afferent fiber activity. In the case of a physiological
control system it may be just as important to monitor the output of central
neurons in response to input alterations as it is to monitor the change of
input itself. In this context the specific pattern of the collaterals just described may possibly have some significance, since these collaterals should permit
the dorsomedial olivo-cochlear neurons to sample the input to the principal
neurons by way of the calyx collaterals and the output of the principal neurons by way of the principal axonal collaterals. But, since the probability
of synaptic transmission is so high at the calyciform synapse, it is difficult to
see what new information could be transmitted in this way. Nevertheless,
the input from the cochlear nucleus by way of the peri-dendritic plexus could
playa significant role in the output of the principal neurons. When the level
of activity in the calyx fibers is high, it seems unlikely that the post-synaptic
potentials initiated by the peri-dendritic plexus could influence the firing
pattern of the principal neurons very much. However, when the calyx activity is slow enough, as with low sound levels perhaps, then the synaptic activity of the peri-dendritic plexus might well influence the time-pattern of
response greatly enough to be reflected, by way of the collaterals, as a change
in the level of excitation of the olivo-cochlear neurons. In other words, it
seems possible that the special anatomical constellation defined here could
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Auditory Neurons of the Brain Stem
MOREST
354
provide for a differential pattern of response in the olivo-cochlear bundle,
depending both on the stimulus level and on the relative activity in different
parts of the cochlear nucleus and the auditory system.
Discussion
It is not possible to explain the function of any of the central auditory
neurons at present in the cat, not to mention the human. Nevertheless, two
lessons might be drawn from the present analysis for the purpose of this
symposium.
First, it should be evident that certain morphological attributes of the
central auditory neurons are essential to a definitive explication of central
auditory function. Electrophysiological studies alone cannot explain function. Morphological factors that seem to hold great promise for elucidating
the physiology of the auditory system include the geometrical arrangements
of the axons and dendrites making synaptic contacts and the sequential
patterns of the synaptic connections.
The second point concerns the lack of information on the functions of
auditory neurons in the human. Since it is unlikely that the necessary electrophysiological correlations with the morphology can be made directly in
humans, as in cats, we may have to rely primarily on microscopical observations of the homologous structures in autopsy specimens to clarify the
normal and pathological processes in the auditory system of man. The relevant anatomical studies in the human remain to be done. The current
analyses in the cat hopefully provide a basis for the first steps in that direction.
Morphological features of auditory neurons that define their physiological capacities
are the spatial arrangement of the axons and dendrites and the sequences of the synaptic
connections of these processes. In the medial superior olive the spatial arrangement of
dendrites and afferent axons would allow for a tonotopic arrangement in one axis of the
nucleus, while in the other axis the sequence of connections would permit binaural interactions. In the medial trapezoid nucleus and the dorsomedial peri-olivary nucleus the
spatial arrangement of the axonal calyces of Held and the peri-dendritic axonal plexus,
together with their collaterals, may establish a tonotopic correspondence between these
nuclei, the cochlear nucleus and the medial superior olive, and the crossed olivo-cochlear
bundle. The sequences of synaptic connections elaborated by these axonal coIIaterals may
permit the olivo-cochlear bundle to adjust its level of activity as a function of changing
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Summary
Auditory Neurons of the Brain Stem
355
inputs and outputs of the medial trapezoid neurons. The approach developed in this study
holds promise of fruitful application to the human auditory system.
BETHE, A.: Uber die Neurofibrillen in den Ganglienzellen von Wirbelthieren und ihre Beziehungen zu den Golginetzen. Arch. mikr. Anat. 55: 513-558 (1900).
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