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Studies on the innervation of the stapedius muscle of the cat.

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Studies on the Innervation of the Stapedius
Muscle of the Cat'
CHARLES E. BLEVINS2
Department of Biological Structure, University of Washington,
Seattle, Washington
ABSTRACT
A study of nerve-muscle relations and fine innervation patterns
is reported for the stapedius muscle of the cat. Gross relations of the muscle to its
associated nerves and other middle ear structures are shown by illustrations. Histologic features of innervation are studied with the Bodian silver method, the thiocholine technique, and iron hematoxylin and osmic acid stains.
The muscle is richly supplied by 3 to 4 facial nerve branches in which no axons
are found above 8 p and the majority are unimodally grouped at 2-4 /A. The auricular
nerve of the vagus does not penetrate the muscle. Three to four distinct zones of
fasciculi are found whose muscle fibers are 14-20 p in diameter. Each zone is supplied by primary nerves whose branches do not exhibit terminal overlap or multiple
innervation of muscle fibers. Short terminal axons lead to motor end plates from all
intramuscular nerve branches. Nerve branches and motor end plates are confined to
the basal third of the muscle. Sensory endings are not detected within the muscle
or its tendon. It is estimated that each motor neuron supplies no more than five
muscle fibers but more probably three or less.
In recent years the tympanic muscles
have been shown to exhibit remarkable
functional properties. Their response to
auditory stimuli is well known (Wever and
Lawrence, '54), but contraction may be
initiated by factors as variable as electrical
stimulation of the external auditory canal
(Klockhoff and Anderson, '59, '60) or air
jets applied to the orbital region (Klockhoff, '61). In addition, they may be conditioned to respond to light (Simmons et
al., '59). Their threshold is relatively
stable and their latency is shorter than
that of other muscles including the extrinsic ocular muscles. They do not summate subthreshold tones but respond additively to simultaneous stimulation by two
different tones of threshold or above
threshold intensity (Wersal, '58). The
development and maintenance of tension
closely parallel respectively, the initial discharge and final firing rates of cochlear
nerve fibers and cells of the cochlear
nuclei (Rose et al., '59). Fatigue is more
dependent on auditory frequency than
intramuscular phenomena (Kobrak et al.,
'41; WersaU, '58).
Although the tensor tympani and stapedius muscles are both acoustically active,
convincing data have been presented to
indicate that the stapedius is more domiANAT. REC., 149: 157-172.
nant in attenuating sound transmission
across the middle ear (Galambos and
Rupert, '59). The muscles have nearly
the same thresholds but the tension produced by the stapedius is greater than for
the tensor tympani and is developed more
rapidly (Wersiill, '58). The stapedius is
demonstrably more dynamic in producing
changes in cochlear microphonics (Simmons, '59). It is of interest to know if
such functional differences between the
tympanic muscles are reflected in their
neuromuscular substrate. The current
study was undertaken to compare the
pattern of innervation, nerve endings, and
innervation ratios of the stapedius muscle of the cat with those reported previously for the tensor tympani (Blevins, '63).
MATERIALS AND METHODS
Stapedius muscles were removed from
ten cats which had previously been perfused with isotonic saline and 10% neutral formalin. The facial nerve was sectioned approximately 1 cm proximal to
'Supported by the John C. and Edward Coleman
Memorial Fund, Department of Otolaryngology, Uniof California, San Francisco Medical Center
tate of Washington Initiative 171 Funds for
Research in Biology and Medicine.
zThe major po&on of this work was done in the
Department of Anatomy and .the . D ~ n s i o nof Otolarvnnolom, Universlty of Callforma, San Franclsco
Medical center.
%Pi
157
158
CHARLES E. BLEVINS
and 0.5 cm distal to the muscle. The
auricular branch of the vagus was cut
1 cm proximal to its fusion with the facial
nerve. By this means, it is possible to
obtain single, intact preparations of the
muscle and its associated nerves. To obtain cross sections for the study of innervation ratios, each specimen was straightened and rolled over as shown in figure 2.
Tissues were serially sectioned at 1030 Ifor study of nerve endings and at 5 II
for nerve fiber and muscle fiber counts.
Intramuscular nerve endings were studied
by the Bodian silver method (Bodian, ' 3 6 )
and the Gomori ('52) modification of the
thiocholine technique of Koelle. General
muscle architecture was observed on slides
stained with iron hematoxylin and aniline
blue. Cross sections of muscle fibers were
stained by the Bodian method, the Alzheimer Mann-Haggqvist method (Rexed,
'44), or with buffered osmic acid.
Nerve and muscle fiber counts were
made with a modification of the method
of Davenport and Barnes ( ' 3 5 ) . The diameter of nerve fibers was measured from
microphotographs at a magnification of
750 X (Haggqvist, '36; Rexed, '44).
OBSERVATIONS AND RESULTS
Gross nerve-muscle relations
The stapedius muscle of the cat lies in
a narrow cavity just posterior to the oval
and round windows (fig. 3). It is almost
entirely covered by bone except for its
tendon, which emerges from the cavity,
crosses ventral to the facial nerve and
inserts on the neck of the stapes. The
ventral limits of the muscle cavity are
outlined by a Y-shaped junction formed
by the anastomosis of the auricular branch
of the vagus nerve with the facial nerve.
To expose the muscle it is necessary to
dissect through relatively thick bone along
the facial canal and remove a thin lamina
of bone which covers the auricular branch
of the vagus.
Average muscle dimensions are 3 mm
in width at the base and 2-3 mm in length,
It is 1 mm thick at its border along the
facial nerve and tapers to a thin edge
within the deeper portions of its cavity.
The nerves to the stapedius muscle
emerge from the facial nerve near the
point where the latter is crossed by the
stapedial tendon. Three to four nerve
branches are commonly found leaving the
facial nerve, each from a separate peripheral fasciculus. They follow a course parallel to the tendon and enter the muscle
close to one another at slightly oblique
angles to the muscle fibers. A single nerve
branch is shown in figure 3 (insert).
Neuromuscular histology
Muscle fasciculi of the stapedius are
organized into three or four zones. The
zones are in the form of flexible cones
which are compressed in the bilateral axis
of the muscle. The bases of the cones fold
over one another but are separated by connective tissue and fat. Muscle fasciculi
close to the plane of the tendon are shorter
than those farther removed. The amount
of interfascicular connective tissue is
progressively reduced toward the tendon.
Muscle fibers, whose diameters range from
14-20 u, originate on the periosteum of
the stapedial cavity and insert tangentially upon the tendon. Near the base of the
muscle, the epimysium is continuous with
the periosteum as well as the epineurium
of the facial nerve and the auricular
branch of the vagus. The muscle and its
associated nerves are thus enclosed in a
common connective tissue sheath.
Primary nerves originate from peripheral fasciculi of the facial nerve at different levels (figs. 4-6). They are not confined in the same region of the parent
nerve trunk prior to their origin as separate nerves. They pass parallel to or at
slightly oblique angles to the muscle,
gradually arch toward it, and finally penetrate their respective muscle zones at
right angles to the muscle fibers. In most
instances each primary nerve supplies
muscle fasciculi of a single zone. Within
the muscle, secondary nerve branches
are formed which wind around the fasciculi. These in turn give rise to smaller
branches which form an increasing complex network about individual muscle fibers. Terminal axons branch from all
nerve branches except the primary ones,
but in greater numbers as smaller nerves
appear. They run for very short distances
before terminating on motor end plates.
INNERVATION OF STAPEDIUS
159
range except for the smallest intramuscular nerve branch (fig. 1,D).
Despite elaborate branching, all nerves are
restricted to a narrow region within the
basal third of the muscle.
Nerve endings
The stapedius muscle of the cat is
generously endowed with motor endings.
Numerous motor end plates with a variety
of form are observed in both silver and
histochemical preparations. Sole plate
dimensions vary from 14-30
in length.
Terminal arborizations and sole plate nuclei are clearly evident in silver preparations (fig. 7). Cholinesterase preparations
consistently demonstrate the synaptic gutters of extrafusal motor end plate formations (figs. 8, 9).
Caliber spectra of nerves
Although it is difficult to obtain cross
sections of all nerves throughout their
entire course, it is possible to count nerve
fibers and measure their diameters at specific points, before axon branching occurs.
The results of such enumeration are shown
in figure 1. The level of nerve section is
indicated by letters A-D. All nerve fibers
are less than 7.9 I.I in diameter and a
unimodal spectrum is evident. The peak
of fiber population lies within the 3-3.9 LI
DIAGRAM OF THE CALIBER SPECTRUM
OF THE NERVES TO THE STAPEDIUS
MUSCLE OF THE CAT
60
i
k
=
30
zo
10
k
x o
A.
1-13 2-293-394-4.9
5-596-6.97-29
1-1.9 2-ZB3-394-4.9 5-5.96-69
D I A M E T E R I N MICRONS
FIBER DIAMETER OF I NERVE
WITHIN THE P€RIMYSIUM
8o
b4%
8.
FIBER DIAMETER OFA
SECOND NERVE WITHIN
THE PERIMYSIUM
-
r
76%
43%
20
n
c."
1-1.92-2.93-394-495-596-6.9 7-7.9
DIAMETER IN
Fl6ER OlAMtTER OF A
SMALL INTRAMUSCULAR
NERVL
Fig. 1
c
r
1-19 2-2.9 3-394-4.95-596-69
ICRONS
FleER DIAMETER OF
A S€CONO SINGLE INTRAMUSCULAR NERVE
Caliber spectrum of nerve fibers in the nerve to the stapedius muscle of the cat.
160
CHARLES E. BLEVINS
In silver preparations, motor end plates
appear to be grouped closely together and
are not spread throughout the muscle.
Single axons leading to them are characteristically short, often passing over no
more than two or three muscle fibers
before forming terminal arborizations.
Terminal axons do not supply more than
one end plate and, in addition, muscle
fibers are not observed with more than
one motor end plate.
The short length of terminal axons and
the restriction of nerve branches to a narrow intramuscular region strongly suggested that the area of motor innervation
lies within the same limited area. This
impression is confirmed by cholinesterase
preparations of both whole and longitudinally sectioned specimens. Figure 10
shows a whole muscle preparation in
which the sites of cholinesterase activity
stand out clearly within the basal third of
the muscle. The greatest concentration of
cholinesterase positive endings occurs in
the thickest portion of the muscle where
muscle fibers are more obliquely oriented
to the tendon. In longitudinal section
(fig. l l ) , the end plates are clearly outlined in orderly fashion following the contour of the muscle. These are also restricted to the basal third of the muscle
where they constitute a band of innervation.
Although elaborate motor innervation
of the stapedius muscle may be detected
by these means, it is difficult to determine
the presence and/or nature of sensory
innervation. Neither muscle spindles nor
free sensory endings were detected in the
current study. Earlier workers have described encapsulated endings in the stapedius of the cow and horse (Krebs, '05),
spindles in the human stapedius (Steinitz,
' 0 7 ) , and sensory corpuscles in the rat
stapedius (Muller, '42). More recently,
Malmfors and Wersiill ('60a) observed
free endings in the rabbit stapedius which
they presumed to be sensory. In the current study, structures were observed which
resembled sensory formations, but they
lacked sufficient histologic clarity to justify interpretation as such. Figure 12
shows a fine nerve fiber which appears to
end blindly on the muscle fiber. However,
closer observation at a different micro-
scopic level reveals fine terminal arborizations and faintly stained sole plate nuclei
(fig. 13). Although the nerve fiber resembles the formations of Malmfors and
Wersdl, they are always found to tenninate in the vicinity of nuclei. In this
paper, the interpretation is made that such
free endings are in reality terminal axons
whose end plates have capriciously escaped the deposit of silver or are not in
the plane of section. Other argyrophyllic
structures can be observed which are suggestive of sensory endings but they are
identified as reticular and collagenous
connective tissue fibers (fig. 14).
Innervation ratios
Data for the study of innervation ratios
are shown in table 1. Nerve fibers were
counted in all the nerve branches to each
muscle as close as possible to their point
of origin. The total number for each specimen is tabulated on the left side of the
second column and the number of muscle
fibers is shown in the third column. From
these figures, the innervation ratio (ratio
of nerve fibers to muscle fibers) is calculated. Although variation appears among
individual nerve and muscle fiber counts,
the innervation ratio for each specimen
is remarkably close to the mean value of
1 :2.4. Such values are interpreted to mean
that each motor neuron supplies three or
less muscle fibers and does not make
allowances for sensory fiber population.
This would seem a justifiable interpretation since definitive sensory nerve endings
were not observed in this species. If such
endings are present but remain undetected, they are apparently not numerous
and their presence would not change the
innervation ratio to any great degree. To
further amplify this point and to allow
comparison of tympanic muscle innervation ratios with those of skeletal muscles
supplied by spinal nerves of known afferent composition, the traditional allowances
of one-half to one-third were made for
sensory fibers according to the methods
of Sherrington (1894) and Clark ('31).
The results are shown in the right side
of the second column and in the fifth column. For each muscle the uDper figure of
column 2 indicates one-half of the total
nerve fibers and the lower figure two-
161
INNERVATION OF STAPEDIUS
TABLE 1
Innervation ratios and size of motor units for the stapedius muscle of the cat
Specimen
Nerve
fiber
count
Muscle
fiber
count
N/M ratio
(innervation
ratio)
663/332
\444
1848
1:2.8
63d315
\442
1711
Motor
unit
size
~
1 :5.5
1
2
1:4.1
1 :5.5
1:2.7
1:4.1
1 :5.6
1324
3
1:2.8
1:4.1
1:4.6
2089
4
5
6
\607
,431
862/
\574
,375
74d
\566
Mean
1:2.2
1:3.5
1:4.0
1715
1:2.0
1:3.0
1:4.5
1694
1 :2.3
1:3.0
1:4.8
1730
1:2.4
1:3.6
thirds. If these values are divided into the
number of muscle fibers, a maximum and
a minimum value are obtained for the
size of the motor unit (column 5). Again,
individual variations are close to the mean.
The interpretation is made that if sensory
innervation is present in Sherringtonian
proportions, each nerve fiber supplies five
or less muscle fibers. Regardless of which
value is accepted, both ratios are strikingly small and the motor unit ratio is
only slightly larger than the innervation
ratio.
The number of motor units in the stapedius is slightly less than those observed
in the tensor tympani. Division of the
total number of muscle fibers by the number of muscle fibers in each unit yields a
value of 346-432 units in contrast to 564745 in the tensor tympani (Blevins, '63).
If one uses the numbers obtained from
calculating innervation ratios, since there
are approximately three muscle fibers per
motor neuron, there would be 577 groups
of muscle fibers, each of which is supplied
by a single motor neuron. This value is
approximately half that observed in the
tensor tympani.
DISCUSSION
The rich nerve supply of the stapedius
muscle of the cat begins with a variable
number of short myelinated branches of
facial nerve origin. They emerge from
separate peripheral fasciculi instead of a
common one. The number and size of
myelinated axons found in this study agree
closely with the observations of others for
tympanic muscle nerves. Foley and DuBois ('43) found several nerve filaments
to the cat stapedius, the majority of whose
axons were 2.5-4.5 v, with a caliber range
of 1.5-11.5 CI.Malmfors and Wersi-ill ('60a)
found no fibers larger than 7~ in the
stapedial nerve of rabbits and 70.9% of
the fibers were between 2-4 M. Fiber size
for the stapedius is almost identical to
that observed in the tensor tympani nerve
of the cat (Blevins, '63) and the rabbit
(Malmfors and Wersiill, '60b).
In contrast to the work of Breusch ('44),
nerve filaments from the auricular branch
of the vagus were not seen in the muscle.
Despite the proximity of this nerve to the
muscle, its anatomical integrity was maintained until its union with the facial nerve.
Muscle fibers of the stapedius are essentially similar to those of other skeletal
162
CHARLES E. BLEVINS
muscles except for their shortness and
small diameter. The larger ones predominate in the larger muscle fasciculi which
are near the facial nerve in the more spacious portion of the stapedial cavity. Those
more deeply located are smaller in diameter, a possible reflection of pressures
exerted by concomitant development of
surrounding bone. Spiral-shaped myofibrils or "Ringbinden" found in tympanic
muscles by others (Malan, '34a, b; Kobayashi, '56) were not detected in any of
the preparations in this study.
Certain features of intramuscular nerve
branching in the stapedius differ from
those observed in extrinsic ocular muscles
(Feindel et al., '52). Nerve branches are
confined to a narrow region in the basal
third of the muscle instead of spreading
throughout. A similar confinement is
found in the tensor tympani of the cat
but it is restricted to the middle third of
the muscle fibers (Blevins, '63). Single
terminal axons are short, usually passing
over no more than a few muscle fibers
before termination. Moreover, multiple
end plates are not observed on a single
muscle fiber. Degeneration experiments
were not performed to see if primary nerve
fibers supply muscle fibers in more than
one zone. In one specimen a small nerve
branch originating in one muscle zone
was found to continue to the edge of the
next zone. If anatomical overlap of motor
innervation occurs in the stapedius as in
other muscles (Feindel, '54), i t would
seem, in most instances, to be restricted
to the muscle fasciculi of each of the zones
described.
The rich motor innervation of the stapedius muscle in the cat is in keeping with
that of the tensor tympani (Blevins, '63)
and with both tympanic muscles of the
rabbit (Malmfors and Wersall, '60a, b).
A great variety of motor end plates are
present which are supplied mainly by
axons 2-4
in diameter. Their morphology is similar to those in other skeletal
muscles. In terms of spatial distribution,
it is apparent that the greatest numbers
are located in the thickest portion of the
muscle whose muscle fibers are the most
obliquely arranged. The greatest force of
contraction is, therefore, apparently directed
at
angles to the tendon. a
___
.... obliaue
_
.~
_
v
factor which presumably dampens stresses
upon the ossicular chain. The consistent
location of motor endings in the basal
third of the muscle instead of the middle
third offers morphological contrast but
would seem insufficient to contribute to
specific functional differences between the
stapedius and tensor tympani.
The usual type of muscle spindles and
tendon organs are either not present in
the tympanic muscles or are not detectable
by silver or thiocholine techniques. Additional support for the absence of such
endings can be drawn from the nerve caliber data of others. Barker ('48) found
that nerve fibers entering muscle spindles
were between 6-12 u i n diameter while
those innervating tendon organs ranged
between 8-12 v. Hagbarth and Wohlfart
('52) found nerve fibers to muscle spindles
whose diameters varied from 8-15 p. If
one applies these criteria to nerve fibers
of the cat stapedius muscle, such sensory
endings are not present, for nerve diameters were not observed above 5 v in the
small nerve branches within the muscle.
It is evident that the number of stapedial muscle fibers subserved by each motor
neuron is five or less but more probably
three or less. The consistent absence of
sensory endings indicates that the smaller
number is more representative of the true
state of nerve-muscle relations. The
larger value of five is smaller than that
reported for the rabbit stapedius. Malmfors and Wersall ('60a) found each motor
unit contained between 14-20 muscle fibers while Berlendis and DeCaro ('55)
observed a mean value of 27 muscle fibers
per unit. These differences may reflect
the difficulties in estimating the number
of muscle fibers, but such values point to
a fine degree of motor innervation compared to other muscles. The number of
stapedial muscle fibers per motor neuron
in the cat is only one to two less than that
observed in the tensor tympani (Blevins,
'63). It is unlikely that such small differences could account for the different physiological characteristics of the two muscles
or their respective roles in audition, but an
interesting anatomical corollary is offered
by the smaller number of muscle fibers
per motor neuron in the stapedius muscle.
INNERVATION O F STAPEDIUS
LITERATURE CITED
Barker, D. 1948 The innervation of muscle
spindles. Quart. J. Microscop. Sci., 89: 143186.
Berlendis, P., and L. G. DeCaro 1955 L’unita
motoria del muscolo stapedio. Boll. SOC.Med.chir. Pavia, 69: 33-36.
Blevins, C. E. 1963 Innervation of the tensor
tympani muscle of the cat. Am. J. Anat.,
113: 287401.
Bodian, D. 1936 A new method for staining
nerve fibers and nerve endings in mounted
paraffin sections. Anat. Record, 65: 89-97.
Breusch, S. R. 1944 The distribution of myelinated afferent fibers in the branches of the
cat’s facial nerve. J. Comp. Neur., 81: 169191.
Clark, D. A. 1931 Muscle counts of motor
units. A study in innervation ratio. Am. J.
Physiol., 96: 296-304.
Davenport, H. A., and J. R. Barnes 1935 The
strip method for counting nerve fibers and/or
other microscopic units. Stain Technol., 10:
139.
Feindel, W. 1954 Anatomical overlap of motor
units. J. Comp. Neur., 101: 1-13.
Feindel, W., J. R. Hinshaw and G. Weddell 1952
The pattern of motor innervation in mammalian striated muscle. J. Anat., 86: 35-48.
Galambos, R., and A. Rupert 1959 Action of
middle ear muscles in normal cats. J. Acc.
SOC.Am., 31: 349-355.
Gomori, G. 1952 Microscopic Histochemistry.
Principles and Practices. Chicago, University
of Chicago Press, p. 210.
Foley, J. O., and F. DuBois 1943 A n experimental study of the facial nerves. J. Comp.
Neur., 79: 70-101.
Hagbarth, L. E., and G. Wohlfart 1952 The
number of muscle spindles in certain muscles
in the cat in relation to the composition of
muscle nerves. Acta Anat., 15: 85-105.
Haggqvist, G. 1936 Analyse der Faserveteilung
in einem Riickenmarkquerschnitt. Z. Mikroskop. Anat. Forsch., 39: 1.
Klockhoff, I. 1961 Middle ear muscle reflexes
in man. Acta Oto-Laryngol. Suppl., 164: 1-92.
Klockhoff, I., and H. Anderson 1959 Recording
of the stapedius reflex elicited by cutaneous
stimulation. Acta Oto-Laryngol., 50: 541-554.
1960 Reflex activity in the tensor tymoani muscle recorded in man. Acta Oto-Laryngol., 51: 184-188.
Kobavashi, M. 1956 The comparative anatomical study of the stapedial muscles of the
I
~~
163
various kinds of mammalian animals. Hiroshima J. Med. Sci., 5: 63-84.
Kobrak, H. G., J. R. Lindsay and H. B. Perlman
1941 Experimental observations on the question of auditory fatigue. Laryngoscope, 51:
1-12.
Krebs, P. 1905 Nervenendigungen i m Musculus
stapedius. Arch. Mikroskop. Anat., 65: 704727.
Malan, E. 1934a Morphologica comparata de
muscolo “tensor tympani.” Z. Anat., 103:
407-437.
1934b Etude d’histologie comparbe sur
quelques modifications particulieres des fibres
du Tensor Tympani dues a la senescence.
Arch. Biol., 45: 355-361.
Malmfors, T., and J. Wersall 1960a Innervation
of the middle ear muscles in the rabbit with
special reference to nerve calibres and motor
units. 11. Musculus Stapedius. Acta Morphol.
Neer1.-Scand., 3: 163-169.
1960b Innervation of the middle ear
muscles in the rabbit with special reference to
nerve calibers and motor units. I. Musculus
Tensor Tympani. Acta Morphol. Need-Scand.,
3: 157-162.
Muller, A. E. 1942 Etude de l’innervation
totale d’un muscle, le M. stapedius du Rat.
Compt. Rend. de SBances de la Socibtb de
Physique et d‘Histoire Naturelle de Geneve,
59:
.. 174-176.
- . - - .- .
Rexed, B. 1944 Contributions to the knowledge
of postnatal development of the peripheral
nervous system in man. Acta Psychiat. Neurol. Scand. Suppl., 33: 1-205.
Rose, J. E., R. Galambos and J. R. Hughes 1959
Microelectrode studies of the cochlear nucleus
of the cat. Bull. Johns Hopkins Hosp.. 104:
211-251.
Sherrington, C. S. 1894 On the anatomical
constitution of nerves of skeletal muscles. J.
Physiol., 17: 211-258.
Simmons, F. B. 1959 Middle ear muscle activity at moderate sound levels. Ann. Otol. Rhinol.
and Laryngol., 68: 1126-1143.
Simmons, F. B., R. Galambos and A. Rupert
1959 Conditioned responses of middle ear
muscles. Am. J. Physiol., 197: 537-538.
Steinitz, W. 1907 Beitrage zur anatomia musculus stapedius. Arch. Ohrenheilk., 70: 45-50.
Wersall, R. 1958 The tympanic muscles and
their reflexes. Acta 0to.-Laryngol., Suppl.,
139: 1-106.
Wever, E. G., and M. Lawrence 1954 Physiological Acoustics. Princeton, Princeton University Press.
~
PLATE 1
EXPLANATION O F FIGURES
2 Relations of the stapedius muscle to the facial nerve (VII) and the
auricular branch of the vagus nerve. Note the muscle lying in the
Y-shaped junction of the two nerves. By rolling the muscle toward
the proximal end of the facial nerve and straightening the distal end
of the facial nerve below the anastomosis, it is possible to align the
muscle and its nerves for sectioning. Serial sections in the plane
AA' are utilized for the enumeration of nerve and muscle fibers.
(Drawings by Mrs. Lynn H. Young.)
3
164
Ventral aspect of the posterior portion of the middle ear of the cat.
The bony covering of the lateral tympanic chamber, the annulus,
and the tympanic membrane have been removed. The left of the
field is anterior and the lower margin is lateral. Note the position of
the stapedius muscle in the larger view and the details of nervemuscle relations in the inset. Only one nerve is shown. For further
details of the anterior part of the middle ear see Blevins ('63).
(Drawings by Mrs. Lynn H. Young.)
INNERVATION OF STAPEDIUS
Charles E. Blevins
PLATE 1
165
PLATE 2
EXPLANATION OF FIGURES
166
4
Diagram of the facial nerve branches to the stapedius muscle. Three
nerves (1-3) are shown, each supplying a distinct zone or group of
muscle fasciculi (a-c). In this specimen, one small nerve branch
( 2 ) from zone b was found to supply a few muscle fibers in the
periphery of zone a. (Drawing by Mrs. Lynn H. Young.)
5
Origin of one facial nerve branch to the stapedius muscle. The
nerve branch is arched toward the muscle fibers shown on the left,
which it supplied in a subsequent section. The course followed by
this nerve is represented by nerve branch no. 1 in figure 4. (VII,
facial nerve; 1, stapedial nerve 1; a, muscle fibers, zone a; Aur. Br.
X, auricular branch of vagus nerve.)
6
Origin of a second nerve branch to the stapedius muscle. This nerve
( 2 ) supplied muscle fibers in the zone of fasciculi in the upper portion of the muscle in this section. (b, muscle fibers, zone b; other
abbreviations as in fig. 5 . )
INNERVATION OF STAPEDIUS
Charles E. Blevins
PLATE 2
167
PLATE 3
EXPLANATION O F FIGURES
7
Motor end plate. Note the terminal axon and large sole plate nuclei.
(Bodian stain.)
8
Motor end plate. The subneural apparatus is heavily stained. (Cholinesterase stain.)
9
Cholinesterase preparation of motor end plates showing the subneural apparatus. (Phase contrast.)
10 Diagram of a whole muscle preparation of the stapedius muscle and
the facial nerve. Note the sites of cholinesterase activity (black
spots) which are more heavily concentrated i n the thick portion of
the muscle near the facial nerve. (Drawing by Jessie W. Phillips.)
168
INNERVATION O F STAPEDIUS
Charles E. Blevins
PLATE 3
169
PLATE 4
E X P L A N A T I O N O F FIGURES
11
LongitudinaI section of a cholinesterase preparation of the stapedius.
Note the band of cholinesterase activity following the muscle contour
i n the basal third of the muscle fibers. (T, tendon; MF, muscle
fibers, IB, innervation band.)
12
Terminal axon ending on a muscle fiber. The axon appears to end
blindly on the muscle fiber surface. A network of small, faintly
stained terminal arborization filaments can be seen. Compare with
figure 13. (Bodian stain.)
13 This photograph was taken at a different microscopic level than
figure 12. Note that the terminal axon is out of the plane of focus
but large, faintly stained sole plate nuclei are visible. (Bodian stain.)
14
170
Collagenous and reticular connective tissue fibers associated with
muscle fibers. (Bodian stain, counterstained with aniline blue.)
INNERVATION OF STAPEDIUS
Charles E. Blevins
PLATE 4
171
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