IOVS
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

(Investigative Ophthalmology and Visual Science. 2006;47:202-205.)
© 2006 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.05-0860
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow View responses
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (9)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by McClung, J. R.
Right arrow Articles by Goldberg, S. J.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by McClung, J. R.
Right arrow Articles by Goldberg, S. J.

Extraocular Connective Tissues: A Role in Human Eye Movements?

J. Ross McClung, Brian L. Allman, Diana M. Dimitrova, and Stephen J. Goldberg

From the Department of Anatomy and Neurobiology, Virginia Commonwealth University School of Medicine, Richmond, Virginia.


   Abstract
Top
 Abstract
Methods
 Results
 Discussion
 References
 
PURPOSE. This study presents a detailed anatomic analysis of the undisturbed connective tissues that surround the horizontal extraocular muscles (EOMs) of humans. Emphasis is placed on those EOM orbital side tissues that, in previous MRI studies, were assumed to couple the muscle to the pulley.

METHODS. Serial 5-µm sections were prepared from paraffin-embedded blocks of the lateral and medial rectus muscles and their surrounding connective tissues. The sections were treated with Masson’s trichrome stain for light microscopic examination of muscle fibers (red) and surrounding connective tissues (blue).

RESULTS. Rectus muscle sections demonstrated the orbital connective tissues to be a collagenous bridge between the distal third of the muscle and the orbital periosteum (i.e., check ligament [CL]). The CL attaches to the muscle by investing itself around orbital muscle fibers whereas, at the point of attachment, those fibers remain aligned with the remainder of the muscle. The CL on the orbital side and the reflected bulbar fascia on the global side of the muscle constitute a tubelike sheath. The posterior border of the sheath insinuates into the muscle belly and its anterior aspect blends into the sides of the portal through Tenon’s capsule.

CONCLUSIONS. All rectus EOM fibers participate in eye rotation. The CL is the band of tissue present on the MRI images, but was previously described as the orbital layer insertion for the active pulley hypothesis (APH). The APH should now be questioned. Alternate theories incorporating accepted neurophysiological, anatomic, and ophthalmological principles of EOM movement are discussed.

Recently, significant attention has been focused on the connective tissues found on the orbital surface (side near the bone) of the extraocular muscles (EOMs) in humans and other mammalian species.1 These connective tissues have been described in classic anatomic studies and books for over 70 years.2 Most anatomic textbooks3 contain descriptions and/or images of check ligaments (CLs) on the orbital side of the lateral and medial recti as well as the fascia behind the globe of the eye (i.e., Tenon’s capsule or the fascia bulbi). This fascia reflects back as a sleeve around the muscle and has portals (openings). The EOMs pass through these portals to insert on the sclera of the eye.

These long known connective tissue elements have apparently been renamed and a pulley function has been ascribed to them. As suggested by Miller,4 Demer et al.5 formally proposed that the EOMs are held in place and even deflected by pulleylike connective tissue near the portal through Tenon’s capsule. The pulley function was further elaborated by noting the presence of smooth muscle with parasympathetic innervation within these tissues. This gave the pulley a dynamic neural control component.6

When high-resolution magnetic resonance imaging (MRI) images of the pulley tissues were observed during levoversion and dextroversion of the eye, tissue projections from the orbital side of the EOMs were noted.7 These orbital projections were proposed to be the functional insertion of the orbital (peripheral, fine fiber) layer of the rectus muscles. It was further suggested that the orbital projection was a separate tendon of the orbital layer, with distinct neural control of the orbital layer muscle fibers capable of actively positioning the pulley throughout the full range of eye motion (active pulley hypothesis [APH]).1 7 This interpretation leaves the global layer muscle fibers alone to move the eye through its insertion on the sclera.

Because recent anatomic studies in monkeys, humans8 and rats9 have raised concerns regarding the pulleys, we performed a detailed anatomic analysis of the undisturbed connective tissues that surround the horizontal EOMs in humans, with emphasis on the anatomy of the orbital tissues that were shown in MRI images to couple the EOMs to the pulley.7 Thus, the purpose of our study was to determine whether the structure attached to the orbital side of the EOMs, observed in the MRI images,7 represents a true orbital layer insertion, and to observe whether the connective tissue in the orbit is consistent with pulleylike structures capable of mobility (i.e., adjusting position during EOM contraction) thereby providing a dynamic, anterior, functional origin of the EOMs. Some of this work has been published in abstract form (McNeer KW, et al. IOVS 2005;46:ARVO E-Abstract 5721).


   Methods
Top
 Abstract
 Methods
Results
 Discussion
 References
 
Tissue was dissected from an orbital specimen taken during craniofacial reconstruction surgery (78-year-old man) and fixed in neutral buffered formalin immediately after removal. This research adhered to the tenets of the Declaration of Helsinki. Because this specimen contained the bones of the surrounding paranasal sinuses and anterior orbit, all the connective tissue elements and muscles around the eye were undisturbed. Serial 5-µm sections were prepared from paraffin-embedded blocks of the lateral and medial rectus muscles and their surrounding connective tissues. The sections were treated with Masson’s trichrome stain for light microscopic examination of muscle fibers (red) and surrounding connective tissues (blue). The horizontal rectus muscles were dissected separately from the intact specimen. The lateral rectus muscle was then cross-sectioned in the coronal plane, and the medial rectus muscle was sectioned longitudinally in the horizontal plane.


   Results
Top
 Abstract
 Methods
 Results
Discussion
 References
 
Rectus muscle cross sections with trichrome stain demonstrate the connective tissues on the orbital surface to be a discrete dense collagenous bridge between the distal third of the rectus muscle, proximal to where it passes through Tenon’s capsule and the periosteum of the orbital wall (Fig. 1A) . This is a classic description of the CL.2 No smooth or striated muscle was observed in the collagen bundles of the CL as they insinuate into the muscle belly (Fig. 1B) . Both orbital and global muscle fibers remain parallel within their connective tissue sheaths (perimysium) and are lined up with the muscle tendon (Fig. 1C) . The CL attaches onto the outer orbital muscle fiber layer by investing itself around individual muscle fibers (Fig. 1D) . The orbital muscle fiber layer remains aligned with the rest of the muscle at the point of CL attachment and does not extend outward into the ligament (Figs. 1C 1D) . Both the orbital and global portions of the muscle can be observed to course toward the single tendon of the muscle. There is no bifurcation of the muscle or its tendon. Tenon’s capsule was observed behind the sclera of the eye on the global side of the muscle (Fig. 1C) . This connective tissue layer, which in serial sections, was seen to wrap around the rectus muscles, also attaches onto the anterior third of the muscles by blending into the connective tissue of the muscle belly. A tubelike sheath is thus defined, with the CL on the orbital side and the reflected bulbar fascia on the global side. The posterior border of the sheath insinuates into the anterior third of the muscle belly and the anterior border of the sheath blends into the sides of the portal in Tenon’s capsule.



View larger version (115K):
[in this window]
[in a new window]
  		FIGURE 1. (A) A cross-section through the anterior third of the lateral rectus muscle approximately 3 mm posterior to the portal in Tenon’s capsule. The periosteum (P) and other connective tissues (blue) are visualized on the orbital side of the muscle (red). Dense bands of collagen are seen bridging the gap between the muscle and the orbit wall. These bands have been defined in the current study and other studies to be the CL. (B) A higher magnification view of the CL from the box in (A), demonstrating a lack of smooth or skeletal muscle (no red-stained fibers) within these bands of collagen. (C) A horizontal section through the superior aspect of the medial rectus muscle approximately 2.5 mm into the muscle belly. The CL can be seen coursing away from the orbital side of the muscle. The sclera of the eye and Tenon’s capsule (T) can be observe on the global side of the muscle. Note that no muscle fibers can be observed following the CL along its orbitally directed course. (D) A higher magnification view of the area in the box in (C), demonstrating the CL blending into the orbital side of the muscle by investing collagen filaments around the peripheral (orbital) muscle fibers. This insinuation of the connective tissue layer can also be seen on the global surface of the muscle.

 

   Discussion
Top
 Abstract
 Methods
 Results
 Discussion
References
 
We used histologic analysis to investigate the EOM fiber orientation from a human specimen with undisturbed connective tissues and to determine whether the orbital layer muscle fibers project to the pulley as suggested by the APH.1 7 Also, we examined anatomic structures that may be consistent with a pulley that could shift the functional insertion of the EOMs anteriorly (as is well known for the superior oblique muscle). Contrary to the APH, we present no evidence and found no indications that the orbital layer muscle fibers leave the belly of the muscle to insert on a pulley. Instead, our histologic findings show collagen bundles of connective tissue (usually described as the CL)2 that insinuate into the orbital layer of the rectus muscles (Figs. 1B 1D) . Based on an earlier electrical stimulation study,10 as well as a recent study from our laboratory,11 we believe the CL to be an apt description of these dense collagen bundles. After removal of the lateral orbital wall and connective tissues (i.e., any pulleylike structures), we found an increase in lateral and medial eye movement amplitude and velocity in response to electrical stimulation of the appropriate nerve or nucleus.11 This increased mobility (or release of connective tissue restraint) is consistent with the idea that the orbital collagen bundles are the CLs and have eye restraining (checking) actions.2 Thus, we suggest that the orbital projections in the MRI images (that formed the anatomic foundation for the APH7 ) are the collagen bundles of the CL, not a separate insertion for orbital layer muscle fibers. Future modeling of eye movements based on the APH may have to be adjusted accordingly.

Muscle fibers connect to each other by connective tissue sleeves (endomysium and perimysium), which condense into the dense regular connective tissue structure called the muscle tendon. Thus, EOM fibers would ultimately act through their lateral and terminal connective tissues to move the eye. We found that all the muscle fibers (orbital and global) of the EOMs were aligned with the single muscle tendon that inserts on the eye. A recent anatomic study of 21 human cadavers shows both the orbital and global sides of the anterior third of the lateral and medial rectus muscles inserting on the sclera of the eye.12 Similarly, Felder et al.9 clearly showed (in the rat inferior rectus muscle) that the orbital layer continued well anterior to the attachment of the orbital connective tissue band, which we believe is the CL. Therefore, the orbital layer must insert on the muscle tendon that attaches to the sclera. Indeed, physiological studies have shown that major surgical insults to the tendon13 or muscle14 belly do not significantly effect electrically evoked conjugate eye movements13 or muscle force.14 These latter studies confirm that muscle fiber force is transferred laterally within the muscle and ultimately acts on the eye itself.15 16

In contrast with our present study, Ruskell et al.8 noted that by following muscle fibers through serial sections, "single muscle fibers, or sometimes two or three, continued for a short distance" ({approx}1.0 mm) and could be observed to enter the orbital connective tissues (the CL). However, a small number of fibers that have a tight relationship with the orbital connective tissues (CL) in no way constitutes a "double insertion"8 or a separate orbital layer insertion.1 Finally, we showed in the present study that collagen fibrils from the global connective tissues also insinuate around the peripheral muscle fibers on the global surface of the muscle. This observation supports that of both Ruskell et al.8 and Felder et al.9 Therefore, if the orbital layer inserts on a pulley, then the global layer does, as well. This observation, now by three separate laboratories, confounds the idea of an independent pulley action for the orbital layer of the EOMs.

In relation to this exclusive pulley role for the orbital layer of the EOMs, a recent comprehensive review1 asserted that fundamentally different neural commands would be needed for ocular rotation by the global layer and pulley translation by the orbital layer. We agree with this logical assertion. One would expect, therefore, distinct motoneuron firing patterns by these two pools of motoneurons (orbital and global) recorded in alert animals during eye movements. However, this does not appear to be the case. All motoneurons have consistently been shown to be involved in every eye movement, and distinct populations of motoneurons with specific firing patterns have not been found.17 Moreover, the suggestion that the orbital layer serves to control pulley position, but not to rotate the eyeball, conflicts with the finding that feline lateral rectus muscle motor units with muscle fibers located in the orbital layer transmit force to the muscle insertion on the eye, similar to global layer motor units.18 In addition, a significant portion of feline abducens nucleus single motoneurons innervate muscle fibers in both the orbital and global layers.18 That is, single lateral rectus motor units are not necessarily confined to a single EOM layer. In humans, horizontal rectus muscles have been shown to produce similar force levels when attached to the globe or disinserted from it (Lennerstrand G, et al. IOVS 2003;44:ARVO E-Abstract 2735). These physiological findings do not support the APH but they are consonant with the usual single insertion for EOMs on the eye’s sclera.

Of interest, even though we strongly question the separate orbital insertion idea, our findings do not challenge the idea that there are pulleylike restraints on the dynamics of eye movement.2 4 5 11 We assume the pulley to be the sling of tissue encircling the portal for the muscle as it passes through the fascia bulbi. This fascia adheres to the bones of the anterior orbit, providing a firm anterior attachment for the EOMs. Therefore, our images showing the rectus muscles having a sleeve of connective tissue firmly anchored into the muscle belly as well as into this portal define a mobile pulley. The movement of the pulley would be accomplished by the shortening of the entire muscle (both orbital and global portions) during muscle contractions that rotate the eye. No differential action of the orbital layer is necessary, and yet the pulley is moved along with the muscle to maintain the distance between the pulley and the scleral insertion of the tendon. As has been stated in the APH, "this coordination of pulley position is proposed to underlie Listing’s law of ocular torsion" (Vijayaraghavan A, et al. IOVS 2005;46:ARVO E-Abstract 4675).

However, we still must answer a very important question. Are these fascia bulbi tissues strong enough to bend the rectus muscle tendons while force is being applied? The strength of the tissues has yet to be proven (Jampel RS, et al. IOVS 2005;46:ARVO E-Abstract 4677).8 These sleeves of connective tissue around the EOMs may act as a pulley or they may not. But certainly, the bands of collagen that attach to the orbital wall and help to restrain EOM movement2 11 should still be referred to as CLs. We hope this study will give the scientific and clinical ophthalmology communities a thorough anatomic description of the connective tissues around the pulleys and help with this important modeling of eye movements.


   Acknowledgements
 
The authors thank Robert E. Revels, Jr., for expert assistance with the histology; Keith W. McNeer, MD, and Mary G. Tucker, MD, for supplying the surgical specimen and encouraging us to use histopathology as a means of directly assessing the site of insertion of the orbital myofibers in human EOMs; their ideas and involvement are demonstrated in an earlier version of this work, published in abstract form (McNeer KW, et al. IOVS 2005;46:ARVO E-Abstract 5721).


   Footnotes
 
Supported by National Eye Institute Grant EY011249 (SJG), Jeffress Memorial Trust Grant J-745 (JRM), and the Smith-Kettlewell Eye Research Institute.

Submitted for publication July 6, 2005; revised August 9, 2005; accepted November 16, 2005.

Disclosure: J.R. McClung, None; B.L. Allman, None; D.M. Dimitrova, None; S.J. Goldberg, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Corresponding author: Stephen J. Goldberg, Department of Anatomy and Neurobiology, PO Box 980709, Virginia Commonwealth University, 1101 E. Marshall Street, Richmond, VA 23298-0709; sgoldber{at}hsc.vcu.edu.


   References
Top
 Abstract
 Methods
 Results
 Discussion
 References
 

  1. Demer JL. Pivotal role of orbital connective tissues in binocular alignment and strabismus: The Friedenwald Lecture. Invest Ophthalmol Vis Sci. 2004;45:729–738.[Free Full Text]
  2. Whitnall SE. The Anatomy of the Human Orbit and Accessory Organs of Vision. 1932; 2nd ed. Oxford University Press Oxford, UK. taken from 2nd ed. Huntington, NY: Robert E. Krieger; 1979
  3. Drake RL, Vogl W, Mitchell AWM. Gray’s Anatomy for Students. 2005;837–839. Elsevier Philadelphia.
  4. Miller JM. Functional anatomy of normal human rectus muscles. Vision Res. 1989;29:223–240.[CrossRef][ISI][Medline][Order article via Infotrieve]
  5. Demer JL, Miller JM, Poukens V, Vinters HV, Glasgow BJ. Evidence for fibromuscular pulleys of the recti extraocular muscles. Invest Ophthalmol Visual Sci. 1995;36:1125–1136.[Abstract/Free Full Text]
  6. Demer JL, Poukens V, Miller JM, Micevych P. Innervation of extraocular pulley smooth muscle in monkeys and humans. Invest Ophthalmol Visual Sci. 1997;38:1774–1785.[Abstract/Free Full Text]
  7. Demer JL, Oh SY, Poukens V. Evidence for active control of rectus extraocular muscle pulleys. Invest Ophthalmol Visual Sci. 2000;41:1280–1290.[Abstract/Free Full Text]
  8. Ruskell GL, Kjellevold Haugen I-B, Bruenech JR, van der Werf F. Double insertions of extraocular rectus muscles in humans and the pulley theory. J Anat. 2005;206:295–306.[CrossRef][ISI][Medline][Order article via Infotrieve]
  9. Felder E., Bogdanovich S., Rubinstein NA., Khurana TS. Structural details of rat extraocular muscles and three-dimensional reconstruction of the rat inferior rectus muscle and muscle-pulley interface. Vision Res. 2005;45:1945–1955.10.[CrossRef][ISI][Medline][Order article via Infotrieve]
  10. Jampel RS, Bloomgarden CI. Individual extraocular muscle function from faradic stimulation of the oculomotor and trochlear nerves of the macaque. Invest Ophthalmol. 1963;2:265–271.[Abstract/Free Full Text]
  11. Dimitrova DD, Shall MS, Goldberg SJ. Stimulation-evoked eye movements with and without the lateral rectus muscle pulley. J Neurophysiol. 2003;90:3809–3813.[Abstract/Free Full Text]
  12. Jaggi GP, Laeng HR, Müntener M, Killer HE. The anatomy of the muscle insertion (scleromuscular junction) of the lateral and medial rectus muscle in humans. Invest Ophthalmol Vis Sci. 2005;46:2258–2263.[Abstract/Free Full Text]
  13. Jampel RS. Multiple motor systems in the extraocular muscles of man. Invest Ophthalmol. 1967;6:288–293.[Abstract/Free Full Text]
  14. Goldberg SJ, Wilson KE, Shall MS. Summation of extraocular motor unit tensions in the lateral rectus muscle of the cat. Muscle Nerve. 1997;20:1229–1235.[CrossRef][ISI][Medline][Order article via Infotrieve]
  15. Monti RJ, Roy RR, Hodgson JA, Edgerton VR. Role of motor unit structure in defining function. Muscle Nerve. 2001;24:848–866.[CrossRef][ISI][Medline][Order article via Infotrieve]
  16. Shall MS, Dimitrova DM, Goldberg SJ. Extraocular motor unit and whole muscle contractile properties in the squirrel monkey: summation of forces and fiber morphology. Exp Brain Res. 2003;151:338–345.[CrossRef][ISI][Medline][Order article via Infotrieve]
  17. Scudder CA, Kaneko CRS, Fuchs AF. The brainstem burst generator for saccadic eye movements: a modern synthesis. Exp Brain Res. 2002;142:439–462.[CrossRef][ISI][Medline][Order article via Infotrieve]
  18. Shall MS, Goldberg SJ. Lateral rectus EMG and contractile responses elicited by cat abducens motoneurons. Muscle Nerve. 1995;18:948–955.[CrossRef][ISI][Medline][Order article via Infotrieve]



This article has been cited by other articles:


Home page
 IOVSHome page
M. Fesharaki, P. Karagiannis, D. Tweed, J. A. Sharpe, and A. M. F. Wong
Adaptive Neural Mechanism for Listing's Law Revealed in Patients with Skew Deviation Caused by Brainstem or Cerebellar Lesion
Invest. Ophthalmol. Vis. Sci., January 1, 2008; 49(1): 204 - 214.
[Abstract] [Full Text] [PDF]

Home page
 IOVSHome page
K. H. Lim, V. Poukens, and J. L. Demer
Fascicular Specialization in Human and Monkey Rectus Muscles: Evidence for Anatomic Independence of Global and Orbital Layers
Invest. Ophthalmol. Vis. Sci., July 1, 2007; 48(7): 3089 - 3097.
[Abstract] [Full Text] [PDF]

Home page
 IOVSHome page
R. Hoerantner, T. Kaltofen, S. Priglinger, C.-M. Fock, M. Buchberger, and T. Haslwanter
Model-Based Improvements in the Treatment of Patients with Strabismus and Axial High Myopia
Invest. Ophthalmol. Vis. Sci., March 1, 2007; 48(3): 1133 - 1138.
[Abstract] [Full Text] [PDF]

Home page
 IOVSHome page
I. Schoemaker, P. P. W. Hoefnagel, T. J. Mastenbroek, C. F. Kolff, S. Schutte, F. C. T. van der Helm, S. J. Picken, A. F. C. Gerritsen, P. A. Wielopolski, H. Spekreijse, et al.
Elasticity, Viscosity, and Deformation of Orbital Fat
Invest. Ophthalmol. Vis. Sci., November 1, 2006; 47(11): 4819 - 4826.
[Abstract] [Full Text] [PDF]

Home page
 J. Neurosci.Home page
E. M. Klier, H. Meng, and D. E. Angelaki
Three-dimensional kinematics at the level of the oculomotor plant.
J. Neurosci., March 8, 2006; 26(10): 2732 - 2737.
[Abstract] [Full Text] [PDF]

eLetters:

Read all eLetters

Active Pulley Hypothesis Withstands Challenge of Exenteration
Joseph Demer
IOVS Online, 28 Apr 2006 [Full text]
Last Gasp of Oculomotor Fundamentalism?
Joel Miller
IOVS Online, 28 Apr 2006 [Full text]
Classical Histology and Physiology Leave APH Proponents Gasping for a Coherent Explanation
Stephen J. Goldberg
IOVS Online, 28 Apr 2006 [Full text]
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow View responses
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (9)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by McClung, J. R.
Right arrow Articles by Goldberg, S. J.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by McClung, J. R.
Right arrow Articles by Goldberg, S. J.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS