Accessing the Stapedius Muscle Via Novel Surgical Retrofacial Approach: A Cadaveric Feasibility Study.

HYPOTHESIS: Despite the complete embodiment of the stapedius muscle (SM) into the pyramidal eminence, it is possible to safely gain access to the SM belly via a retrofacial approach. This presents a novel approach to directly measure the electrically evoked stapedius reflex threshold (eSRT). BACKGROUND: Objective fitting of maximum comfortable loudness levels for cochlear implant users can improve the benefit introduced by the device. Sensing SM activity via direct surgical access represents a potential tool for objective eSRT fitting. METHODS: Eighteen human temporal bones (TBs) were used. Micro-computed tomography was performed for six TBs. Standard computed tomography for six TBs. Manual 3D-segmentation of the relevant middle and inner ear anatomy was performed on 12 TBs. Mastoidectomy and posterior tympanotomy allowed the access to middle ear of all 18 the TBs. Once identified the mastoidal segment of the facial nerve (FN), the retrofacial access to the SM was drilled. RESULTS: The total access rate was 72.2%. Only in the first three cases the posterior semi-circular canal was hit. The SM access was identified posterior to the FN at a 4 ±â€Š0.78 mm distance from the stapes' head, almost halfway to the chorda tympani's branching point along the FN direction. The drilling depth to access the SM posterior to the external surface of FN on average was 2 ±â€Š0.30 mm. The exposure took on average of 5 to 8 minutes. CONCLUSIONS: The retrofacial approach seems to offer a feasible and reproducible access to the SM belly opening an avenue to electromyographic sensing of eSRT.

Microsurgical anatomy of the stapedius muscle in adult cadavers.

This study aimed to establish an extended morphometric dataset regarding the stapedius muscle for anatomists and otologists. The tympanic cavity of ten cadavers (five females, five males) aged with 75.70 ± 13.75 years was bilaterally dissected. Morphometric properties of the stapedius muscle (i.e., its muscular belly and tendon) and its relationship with the neighborhood structures including the facial nerve was evaluated. The length of the entire stapedius muscle was found as 4.80 ± 1.13 mm. The depth between the entrance of the external auditory canal and stapedius muscle was measured as 18.23 ± 2.30 mm. The incudostapedial joint and stapedial tendon were found to be 1.66 ± 0.25 mm and 1.18 ± 0.19 mm away from the facial nerve, respectively. The stapedial tendon length was standardized as five types: Type 1, extremely short tendon (under 0.5 mm), 5% of cases; Type 2, short tendon (between 0.5 and 1 mm), 30% of cases; Type 3, normal tendon (between 1 and 2 mm), 55% of cases; Type 4, long tendon (between 2 and 2.5 mm), 10% of cases; and Type 5, extremely long tendon (above 2.5 mm), no cases. Our findings showed that the stapedius tendon size in adults was quite similar to fetuses. Therefore, probably entire muscle dimension does not alter after birth. Considering the concordance between direct anatomic (our findings) and radiologic measurements (literature findings) of the stapedius muscle, preoperative radiological evaluation may be important for otologists in terms of the choice of surgical tools such as diamond burr sizes. Due to the lack of standardization regarding the evaluation of the stapedius tendon size (i.e., extremely short or extremely long), we defined the tendons below 0.5 mm as extremely short (Type 1) and above 2.5 mm as extremely long (Type 5).

Microtomographic morphometry of the stapedius muscle and its tendon.

The aim of this study was to evaluate the morphology of the stapedius muscle and its tendon with the use of microCT and to describe their anatomic relationship with facial nerve and incudostapedial joint. The study was performed on 16 fresh cadaveric temporal bones scanned in microtomography (microCT). Stapedius muscle and its tendon were identified in each set of images. The length of the medial and lateral border of the stapedius tendon (STL-med, STL-lat), width at the insertion to stapes (STW-s), at the point it emerges from the pyramidal eminence (STW-p) and in the half way from the pyramidal eminence to stapes (STW-m), and the length and the width of the belly of stapedius muscle (BSML and BSMW) were measured in modified axial plane. The shortest distance between the facial canal and incudostapedial joint (FN-isj), and between the facial canal and stapedius tendon (FN-st) were measured in the Pöschl plane. The average values of all distances measured were: STL-lat 1.29 ± 0.50 mm, STL-med 1.27 ± 0.44 mm, BSML 2.98 ± 0.51 mm, STW-s 0.47 ± 0.10 mm, STW-p 0.46 ± 0.12 mm, STW-m 0.35 ± 0.12 mm, BSMW 1.26 ± 0.29 mm, FN-isj 1.72 ± 0.33 mm, FN-st 1.35 ± 0.30 mm. The stapedius muscle complex consists of the tendon and the belly, and the border between them in microCT scans is not always evident. The distance between the facial nerve and the incudostapedial joint is greater than the distance between the facial nerve and the stapedius muscle tendon.

The Endoscopic Relationship of the Stapedius Muscle to the Facial Nerve: Implications for Retrotympanic Surgery.

OBJECTIVES: The stapedius muscle, tendon, and pyramidal eminence are structures within the retrotympanum. In cholesteatoma surgery, the retrotympanum is a common site of residual disease. The removal of the pyramidal eminence during surgery is sometimes necessary to obtain better visualization of the superior retrotympanum during surgery. Understanding the relational anatomy of structures in the region to the facial nerve allows the surgeon to safely access regional disease. This study aims to better understand the anatomical relationship between the mastoid portion of the facial nerve, the pyramidal eminence, and the stapedius muscle. A secondary aim is to demonstrate that removal of the stapedius muscle in the cadaver model, can increase exposure to the retrotympanic space. STUDY DESIGN: Anatomical cadaveric observation study. METHODS: Endoscopic dissection of cadaveric heads was undertaken. Classification of the superior and inferior retrotympanic area was performed. The anatomy of the stapedius muscle was described including relationships, depth, course, and angle with respect to the facial nerve. The pyramidal eminence and stapedius muscle were removed in all specimens and the exposure of the retrotympanum re-evaluated to determine if exposure of the region was increased. RESULTS: In all cases (11 ears), the stapedius muscle was located medial and anterior to the mastoid portion of the facial nerve, with the second genu superior. The mean antero-posterior distance from the apex of the pyramidal eminence, which the stapedius tendon enters, to the stapes itself was 4.10 mm (range, 2.92-5.73 mm; standard deviation [SD] 0.90 mm). In all cases, irrespective of sinus tympani conformation, removal of the pyramidal eminence and stapedial bony crest in proximity to the facial nerve allowed exposure of the whole retrotympanic region, using a 0-degree endoscope. CONCLUSIONS: The pyramidal eminence and stapedius muscle have a relatively constant relationship to the facial nerve. Removal of the stapedius muscle in the human cadaver model increases the exposure of the sinus tympani and subpyramidal space. Increased visualization in this region, may reduce risk of residual cholesteatoma in patients.

Three-dimensional temporal bone reconstruction from histological sections.

OBJECTIVE: To produce a high-resolution, three-dimensional temporal bone model from serial sections, using a personal computer. METHOD: Digital images were acquired from histological sections of the temporal bone. Image registration, segmentation and three-dimensional volumetric reconstruction were performed using a personal computer. The model was assessed for anatomical accuracy and interactivity by otologists. RESULTS: An accurate, high-resolution, three-dimensional model of the temporal bone was produced, containing structures relevant to otological surgery. The facial nerve, labyrinth, internal carotid artery, jugular bulb and all of the ossicles were seen (including the stapes footplate), together with the internal and external auditory meati. Some projections also showed the chorda tympani nerve. CONCLUSION: A high-resolution, three-dimensional computer model of the complete temporal bone was produced using a personal computer. Because of the increasing difficulty in procuring cadaveric bones, this model could be a useful adjunct for training.

Realistic 3D computer model of the gerbil middle ear, featuring accurate morphology of bone and soft tissue structures.

In order to improve realism in middle ear (ME) finite-element modeling (FEM), comprehensive and precise morphological data are needed. To date, micro-scale X-ray computed tomography (µCT) recordings have been used as geometric input data for FEM models of the ME ossicles. Previously, attempts were made to obtain these data on ME soft tissue structures as well. However, due to low X-ray absorption of soft tissue, quality of these images is limited. Another popular approach is using histological sections as data for 3D models, delivering high in-plane resolution for the sections, but the technique is destructive in nature and registration of the sections is difficult. We combine data from high-resolution µCT recordings with data from high-resolution orthogonal-plane fluorescence optical-sectioning microscopy (OPFOS), both obtained on the same gerbil specimen. State-of-the-art µCT delivers high-resolution data on the 3D shape of ossicles and other ME bony structures, while the OPFOS setup generates data of unprecedented quality both on bone and soft tissue ME structures. Each of these techniques is tomographic and non-destructive and delivers sets of automatically aligned virtual sections. The datasets coming from different techniques need to be registered with respect to each other. By combining both datasets, we obtain a complete high-resolution morphological model of all functional components in the gerbil ME. The resulting 3D model can be readily imported in FEM software and is made freely available to the research community. In this paper, we discuss the methods used, present the resulting merged model, and discuss the morphological properties of the soft tissue structures, such as muscles and ligaments.

Controlled round-window stimulation in human temporal bones yielding reproducible and functionally relevant stapedial responses.

Stimulation of the round window (RW) has gained increasing clinical importance. Clinical, as well as human temporal bone and in-vivo animal studies show considerable variability. The influence of RW stimulation on the cochlea remains unclear. We designed a human temporal-bone study with controlled direct mechanical stimulation of the RW membrane to identify conditions for successful RW stimulation. Eight human temporal bones were stimulated on the RW by piezoelectric stack actuators with cylindrical aluminium rods of diameter 0.5 mm and with either flat or 30° inclined top surface. Using a dedicated two-stage positioning protocol for the actuator, we achieved highly reproducible measurements of the stimulus vibration at the RW and of the resultant vibration of the stapes footplate. The reverse transmission, characterized by the displacement ratio of the stapes-footplate relative to the actuator tip on the RW membrane, yielded an average displacement ratio of 0.089 up to 1.2 kHz when the actuator was coupled without angular misalignment to the RW membrane. The results suggest that 90-µm pretension of the RW membrane is essential for optimum and reproducible RW stimulation. The displacements are shown to be roughly consistent with the equal-volume displacement hypothesis under specific assumptions about the displacement mode of the RW membrane. It is further suggested that the large inter-patient variability in the effectiveness of RW stimulation might be due primarily to the success of coupling, rather than to the variability of functionally relevant anatomical parameters.

Development of the stapedius muscle canal and its possible clinical consequences.

OBJECTIVE: To study the development of the stapedius muscle canal in human embryos and foetuses. MATERIALS AND METHODS: 46 temporal bones with ages between 9mm and new-borns were studied. The preparations were dyed using Martins' trichrome technique. RESULTS: Two areas of different embryological origin form the stapedius muscle canal, which contains this muscle and the facial nerve. On the otic capsule, at 11 weeks an extension starts to grow from its caudal part, which moves outwards and near to Reichert's cartilage, forming the footplate and internal wall. The pyramidal eminence comes from the mesenchyme that surrounds the muscle, forming a partition to separate it from the laterohyale portion of Reichert's cartilage. Extensive connections are observed in its development between bone marrow and mesenchyme. At 35 weeks the muscle and nerve start to separate by means of a bony partition. If this partition does not form, there is going to be a dehiscence that could cause peripheral nerve pathology due to the repeated contraction of the muscle, or the dissemination of infections from middle ear. CONCLUSION: During the development of the stapedius muscle canal the presence of dehiscences between the facial nerve and the muscle may have clinical repercussions.

Human oscicular chain articulations: asymmetric sound transmission / Articulaciones de la cadena oscicular humana: transmisión asimétrica del sonido

The mechanism for conducting acoustic energy via middle ear oscicles is still a controversial topic and will remain so until consensus is reached regarding whether it amplifies or reduces sound transmission. 22 paired human temporal bone blocks and 1 left block were studied. Digital measurements were taken of the tympanic membrane, oscicular articulations and the oval window; the results were then correlated. A relationship of non-lineal areas was found amongst the structures being studied, suggesting a sound transmission relationship combining both sound reduction and amplification. A complex relationship of levers could be observed originating in an oscicular and articular asymmetric relationship suggesting an amplifying function in initial sound transmission and also a final reducer destination for such conduction.

El mecanismo para conducir la energía acústica a través de oscículos del oído medio es todavía un tema controvertido y lo seguirá siendo hasta alcanzar un consenso con respecto a si se amplía o reduce la transmisión del sonido. Fueron estudiados 22 pares de bloques de hueso temporal izquierdo y uno derecho hmanos. Mediciones digitales fueron tomadas de la membrana timpánica, articulaciones osciculares y de la ventana oval, siendo estos resultados correlacionados. Una relación de las áreas no-lineal se encontró entre las estructuras, lo que sugiere una relación de transmisión de sonido que incluye tanto la reducción de sonido y amplificación. Una compleja relación de las palancas se pudo observar originando una relación oscicular y articular asimétrica, lo que sugiere una función de amplificación de la transmisión del sonido inicial y también un destino reductor final para su conducción.

[Stapedial artery in rat: an anatomical study]. / Tetnica strzemiaczkowa u szczura: morfologia i zakres unaczynienia.

OBJECTIVE: The thorough knowledge about anatomy and morphology of the stapedial artery is of such importance to the laryngologist. In rat this artery persists throughout life. The following study was performed to analyze the morphology and course of stapedial artery in rat. METHODS: 30 Wistar rats weighing 300-400 g were used to analyze the stapedial artery. After the anesthetic induction with ether, the lethal doses of thiopental were administered. The stapedial arteries were dissected after latex injection and an immersion and preservation in 9% formalin solution. RESULTS: The stapedial artery branches off internal carotid artery and course through the stapes. After that it gives middle meningeal artery and continues in a bony canal laterally to the tegmen tympani. In the orbit stapedial artery gives off ophtalmic artery to supply mainly the orbit structures (muscles, lacrimal gland and eyeball) and the infraorbital artery with palatine artery. Additionally, the ophtalmic artery gives off the central retinal artery. CONCLUSION: Our study reveals that the stapedial artery and its distal branches are the only vessels supplying all tissues of the orbit, including the eyeball in rats.

Middle ear structure and bone conduction in Spalax, Eospalax, and Tachyoryctes mole-rats (Rodentia: Spalacidae).

There is evidence that spalacine, tachyoryctine, and myospalacine mole-rats all communicate with conspecifics through a form of seismic signaling, but the route for the detection of these signals is disputed. It has been proposed that two unusual anatomical adaptations in Spalax allow jaw vibrations to pass to the inner ear via the incus and stapes: a pseudoglenoid (=postglenoid) fossa which accomodates the condylar process of the mandible, and a bony cup, supported by a periotic lamina, through which the incus articulates with the skull. In this study, a combination of dissection and computed tomography was used to examine the ear region in more detail in both Spalax and its subterranean relatives Tachyoryctes and Eospalax, about which much less is known. Tachyoryctes was found to lack a pseudoglenoid fossa, while Eospalax lacks a periotic lamina and bony cup. This shows that these structures need not simultaneously be present for the detection of ground vibrations in mole-rats. Based on the observed anatomy, three hypothetical modes of bone conduction are argued to represent more likely mechanisms through which mole-rats can detect ground vibrations: ossicular inertial bone conduction, a pathway involving sound radiation into the external auditory meatus, and a newly-described fluid pathway between pseudoglenoid fossa and cranial cavity. The caudolateral extension of the tympanic cavity and the presence of a bony cup might represent synapomorphies uniting Spalax and Tachyoryctes, while the loss of the tensor tympani muscle in Spalax and Eospalax may be convergently derived.

Microscopic guide to the middle ear anatomy in guinea pigs.

OBJECTIVES: To reveal microscopic surgical anatomy of the middle ear in guinea pigs (GPs). MATERIALS AND METHODS: Twenty ears of GPs were dissected under dissection microscopy via inferior approach. By using digital equipment, the most suitable photograph was taken for each step. Differences and similarities between the ears of GPs and those of human beings are discussed to show the advantages and disadvantages of the GP ear for experimental studies. RESULTS: Tympanic membrane of the GP merely consists of pars tensa and above this there is a bony segment called as supratympanic crest. There were two spaces called bulla and hypotympanium in the GP,s middle ear. Cochlea, which is normally found in the inner ear in humans, separates these two spaces. Upper part of this space which is called epitympanium is a slit like cavity having a bony complex called malleoincudal complex. Bulla, the largest cavity in the middle ear, is a hemispherical cavity having a smooth surface. It corresponds to hypotympanum and mesotympanum of human middle ear. Cochlea has 3.5 turns from basal turn to apical turn. While the oval window is placed vertically, the round window is placed horizontally. While the stapes is almost identical to that of the humans, there is a bony bridge between the crura of the stapes called crista stapedius. Stapes can not removed unless this bony bridge is taken. CONCLUSION: The middle ear in GP differs from the human middle ear in many aspects. Researchers who are planning to the study with this animal model should be aware of all these differences.

The incudostapedial articulation: new concepts.

HYPOTHESIS: To study the detailed anatomy of the incudostapedial joint (ISJ). BACKGROUND: Detailed study of the anatomy of the ISJ has been surprisingly neglected and continues to be controversial. METHODS: We studied the joint in 86 temporal bones from 51 subjects, aged 12 to 104 years, by light microscopy and three-dimensional images of computer-based reconstructions. RESULTS: In the course of this study, we found that the lenticular process is extremely delicate, consisting of thin trabeculae and spaces filled with areolar tissue. The ISJ is at least partially a bichambered diarthrodial joint divided by a fibrous articular disk into unequal joint spaces. This disk could be identified in all 86 specimens. In addition, we identified for the first time a concavity on the anterior face of the inferior end of the long process of the incus. The asymmetric joint capsule of the ISJ, which completely envelops the lenticular process, extends to this incudal concavity, thereby incorporating part of the long process into the joint complex. We also found that the attachment of the tendon of the stapedius muscle to be more complex than previously described with contributions to the posterior and probably the inferior joint capsule. CONCLUSION: On the basis of its finely cohesive anatomic structure, we believe that the incudostapedial articulation should be considered to be a single complex and to be a single entity physiologically.

[Functional model of the middle ear ossicles].

In students' dissection practice, it is very difficult to teach students the structures and functions of the middle ear ossicles. The middle ear ossicles are too small to explain their structures and functions. Models are useful in explaining these points, but there have been no models that accurately explain the movements of the middle ear ossicles and the functions of the muscles in the middle ear. This time, we have made a model of middle ear ossicles. Our ear ossicles are made of paper-mache with metal in it. The incudomalleolar and incudostapedial articulations are made of rubber. The tensor tympani and the stapedius muscles are made of wire and the two wires can be fixed by cord stoppers. Our model explains clearly the following mechanisms of the middle ear ossicles. 1. The mechanism of sound conduction system. When the sound vibrates the tympanic membrane, malleus and incus rotate together. The long process of the incus pushes the head of the stapes. The sound is amplified by leverage. 2. Attenuation of sound by contractions of tensor tympani and stapedius muscles. When a loud sound is transmitted through the ossicular system, the tensor tympani muscle pulls the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and increase rigidity of the ossicular system, thus reducing the ossicular conduction. 3. The mechanism of how paralysis of stapedius muscle, caused by an injury to the facial nerve, results in hyperacusis. 4. This model also suggests a possible reason why the pars lucida of the tympanic membrane exists.

Development of the stapedius muscle and pyramidal eminence in humans.

The aim of the study was to systematize the key developmental phases of the stapedius muscle and the pyramidal eminence to clarify their formation, as well as to understand the variations and anomalies that can affect these structures. Sixty human embryos and fetuses between 38 days and 17 weeks of development were studied. The stapedius muscle is formed by two anlagen, one for the tendon, which derives from the internal segment of the interhyale, and another for the belly, located in the second pharyngeal arch medial to the facial nerve and near the interhyale but forming a completely independent anlage. In the interhyale, two segments were differentiated, these forming an angle; at the vertex, the belly of the stapedius muscle is attached. The internal segment is located from the attachment of the belly of the stapedius muscle to the anlage of the stapes, forming the anlage of the tendon of the stapedius muscle. The external segment completely disappears at the beginning of the fetal period. The pyramidal eminence is formed by an anlage independent of Reichert's cartilage, from the mesenchymal tissue of the tympanic cavity, which condenses around the belly of the stapedius muscle from 12 weeks of post-conception development. The length of the tendon of the stapedius muscle in adults varies, depending on the attachment site of the belly of the stapedius muscle in the interhyale, which would determine the length of the internal segment (anlage of the tendon) and consequently the tendon length. This variation depends on the greater or lesser persistence of the angulation observed during development, between the tendon and the belly of the stapedius muscle.

[The stapedius muscle--the present opinions about anatomy and physiology]. / Miesien strzemiaczkowy - aktualne poglady na temat anatomii i fizjologii.

The authors present current opinions about anatomy and physiology of the stapedius muscle and its role of the hearing process. The stapedius muscle is the smallest striped muscle of the human body and contracts reflexive in response to acoustic stimulation. The stapedius muscle puls the neck of the stapes in the direction of the stapedius tendon. This movement causes stiffening of the incus and the malleus and also changes the pressure of the perilymph in the inner ear. This is the protective inner ear action of the stapedius reflex against hearing damage by noise. The stapedius reflex shows bilateral interactions and its center is located in the brainstem. The binaural interaction of the stapedius reflex plays an important role in the maintaining of the sound direction. The stapedius tendon also plays role in the vascularization of the long process of the incus.

Topographical anatomy of the guinea pig temporal bone.

Systematic anatomical description of the various structures of the temporal bone have been performed based on dissection of 16 guinea pigs (32 temporal bones). It has been found that besides two main air spaces in the middle ear, the tympanic bulla and dorsal bulla described in literature, there are also additional air cells in the mastoid process and facial nerve region in the temporal bone of a guinea pig. Moreover recesses were found in the walls of the tympanic bulla that formed almost completely separated partitions of tympanic cavity. The malleus head, the body of the incus and the superior and lateral semicircular canals as well as the facial nerve are easily accessible from the dorsal bulla. From the ventral tympanic bulla, one can access both windows and the cochlea. The semicircular canals are relatively large, the lateral canal is largest and the posterior the smallest. The cochlea has thin bony wall, and is composed of 3.5-3.75 turns.

Stapedius muscle fibre composition in the rat.

The stapedius muscle (SM) is supposed to prevent cochlear damage by noise. Consequently functional demands are the ability of fast contraction with long endurance. This implies the presence of a large fraction of myosin type II fibres with an appreciable oxidative capacity. We determined the myosin composition of SM fibres using consecutive complete SM cross-sections (6 week old rats) which were processed by enzyme histochemistry (EHC) to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) or by immunohistochemistry (IHC) using myosin heavy chain (MyHC) antibodies. Method accuracy was determined in co-processed extensor digitorum longus (EDL). Four hundred SM and 200 EDL fibres were assigned to mATPase type I, IIA, IIB, IIX or 'miscellaneous' ('Misc') categories. Per mATPase category the fibres were attributed to groups with specific MyHC composition. In the EDL, mATPase type I and IIB fibres expressed only MyHC I and IIB respectively, whereas about 10% of the type IIA and 40% of the type IIX fibres expressed more than one MyHC. Thus IHC detects amounts of myosin isoforms which are not detected by EHC. The mATPase IIX category criterion leaves the possibility that this category contains fibres with myosin type IIA and/or IIB in larger amounts. The criteria of the mATPase categories type I, IIA or IIB preclude assignment to these categories of fibres which also contain other myosin isoforms in larger amounts. Such fibres were classified in one of the mATPase 'Misc' categories. Thus in the EDL the capability of the EHC criteria to select 'pure' fibres in terms of myosin differs per mATPase category. None of the SM fibres were assigned to the mATPase type I or IIB categories, about 25% to the type IIA, 60% to type IIX and 15% (including most fibres which expressed MyHC I) to a 'Misc' category. All SM fibres expressed two or more MyHC isoforms, MyHC IIB occurring in all fibres and substantial amounts of MyHC IIA and/or IIX in most. These findings confirm the hypothesis that such fibres have the capacity to contract fast and have the better fatigue resistance.

Observations on the number, distribution and morphological peculiarities of muscle spindles in the tensor tympani and stapedius muscle of man.

Although the middle ear muscles have been described for the first time more than four hundred years ago their role in modulation and transmission of sound is not yet fully understood. Surprisingly very little is known about proprioceptors in these muscles, especially in man, although this seems to be the key to the understanding of their various functions. Therefore, the question for proprioceptive sensory organs in these muscles is still relevant. The tensor tympani and stapedius muscles of four women who had donated their bodies to our institute were taken. Complete serial sections of these muscles were made which were either impregnated with silver, stained with ferric oxide for acidic polysaccharides or incubated with antibodies against S-100 protein. Thereby four to eight (mean five) muscle spindles distributed along the whole muscle could be detected in the tensor tympani muscles. These spindles contain one to three intrafusal muscle fibres and their length ranges from 140 to 4270 microm (mean 1492.8 microm). Furthermore, in three stapedius muscles one to two (mean 1.7) muscle spindles were found. They were from 350 to 500 microm (mean 482 microm) long and contained only one intrafusal muscle fiber. Regarding the diameter of intrafusal muscle fibers in both, the tensor tympani as well as the stapedius muscle, no difference to extrafusal muscle fibers of these muscles could be detected. The structure of these spindles differs considerably from those found in skeletal muscles. The morphological findings presented strongly suggest that muscle spindles occur regularly in both middle ear muscles. The results presented herein are consistent with clinical findings obtained from electromyographic studies and may help to elucidate all functions the middle ear muscles might serve in man.

Motoneuron axon distribution in the cat stapedius muscle.

Stapedius-motoneuron cell bodies in the brainstem are spatially organized according to their acoustic response laterality, as demonstrated by intracellular labeling of physiologically identified motoneurons [Vacher et al., 1989. J. Comp. Neurol. 289, 401-415]. To determine whether a similar functional spatial segregation is present in the muscle, we traced physiologically identified, labeled axons into the stapedius muscle. Ten labeled axons were visible in the facial nerve and five could be traced to endplates within the muscle. These five axons had 39 observed branches (others may have been missed). This indicates an average innervation ratio (> or = 7.8) which is much higher than that obtained from previous estimates of the numbers of stapedius motoneurons and muscle fibers in the cat. One well-labeled stapedius motor axon innervated only a single muscle fiber. In contrast, two labeled axons had over 10 endings and innervated muscle fibers spread over wide areas in the muscle. Two of the axons branched and coursed through two primary stapedius fascicles, indicating that the muscle zones innervated by different primary fascicles are not functionally segregated. In another series of experiments, retrograde tracers were deposited in individual primary nerve fascicles. In every case, labeled stapedius-motoneuron cell bodies were found in each of the physiologically identified stapedius-motoneuron regions in the brainstem. These observations suggest there is little, if any, functional spatial segregation based on separate muscle compartments in the stapedius muscle, despite there being functional spatial segregation in the stapedius-motoneuron pool centrally.