Extraocular muscles have fundamentally distinct properties that make them selectively vulnerable to certain disorders

Neuromuscular Disorders 15 (2005) 17–23 www.elsevier.com/locate/nmd

Extraocular muscles have fundamentally distinct properties that make them selectively vulnerable to certain disorders C.Y. Yu Wai Mana,b, P.F. Chinnerya,*, P.G. Griffithsb a

Department of Neurology, The Medical School, University of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH, UK b Department of Ophthalmology, Royal Victoria Infirmary, Newcastle upon Tyne, UK Received 14 July 2004; received in revised form 22 September 2004; accepted 1 October 2004

Abstract While skeletal muscles generally perform specific limited roles, extraocular muscles (EOMs) have to be responsive over a wider dynamic range. As a result, EOMs have fundamentally distinct structural, functional, biochemical and immunological properties compared to other skeletal muscles. While these properties enable high fatigue resistance and the rapid and precise control of extraocular motility, they might also explain why EOMs are selectively involved in certain disorders, such as chronic progressive external ophthalmoplegia (CPEO), myasthenia gravis and Graves’ ophthalmopathy. This review first gives an overview of the novel myofibre classification in EOMs and then focuses on those properties that might explain why ophthalmoplegia should be so prominent in these disorders. q 2004 Elsevier B.V. All rights reserved. Keywords: Extraocular muscles; CPEO; myasthenia gravis; Graves’ ophthalmopathy

1. Introduction

2. Types of muscle fibres

There are numerous differences between extraocular muscles (EOMs) and other skeletal muscles; so much so, that they can be classified into a distinct allotype separate from the limb/diaphragm and masticatory muscles [1]. EOMs also have disease susceptibilities that differ from other skeletal muscles. They are selectively spared in Duchenne muscular dystrophy and motor neuron disease but selectively targeted in chronic progressive external ophthalmoplegia (CPEO), myasthenia gravis and Graves’ ophthalmopathy. In this review, we focus on distinctions that exist between the extraocular muscle and limb muscle allotypes and speculate as to how these differences might account for the selective vulnerability of the extraocular muscle allotype in these disorders Table 1.

There are four types of skeletal muscle fibres: Type I (slow-twitch, fatigue resistant), Type IIA (fast twitch, fatigue resistant), Type IIB (fast twitch, fatigable) and Type IIX (fast twitch, fatigable). Some skeletal muscles contain predominantly one fibre type that determines their contractile properties and fatigue resistance (Type I in red muscles, Type IIA in intermediate muscles and Types IIB and IIX in white muscles) while others contain a mixture of different fibre types [2]. On the other hand, EOMs fibres have a novel classification scheme that is based on their color, location and innervation. Each EOM is divided into at least two main layers: a thin orbital layer adjacent to the bony walls of the orbit and an inner global layer immediately adjacent to the globe and optic nerve. Moreover, each of these two layers also consists of both singly and multiply innervated fibres (Section 3). To deal with terminology, EOMs fibres have thus been divided into six types: orbital singly innervated, orbital multiply innervated, global red singly innervated,

* Corresponding author. Tel.: C44 191 222 8334; fax: C44 191 222 8553. E-mail address: [email protected] (P.F. Chinnery). 0960-8966/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nmd.2004.10.002

18

C.Y. Yu Wai Man et al. / Neuromuscular Disorders 15 (2005) 17–23

Table 1 Differences in characteristics between the extraocular muscle and limb muscle allotypes Limb muscles

EOMs

Fibre classification [2]

Based on color (red, intermediate, white)

Innervation patterns [2,5] Mode of contraction [6,7] Motor unit size (fibres/motor neuron) [50] Contractile properties [15,50–55] Time to peak tension/ms

Single Twitch 100–2000

Novel classification into 6 fibre types based on color, location and innervation Single and multiple Twitch and tonic 13–20

One-half relaxation time/ms Twitch:tetanic tension ratio Maximum firing frequencies/Hz [13,50] Recruitment of motor units [2,14,15]

12.6 (extensor digitorum) 37.8 (soleus) 8.7 (extensor digitorum) 54.8 (soleus) 0.2 125 phasic 50 sustained Differentially recruited for specific subsets of fast and slow movements

global intermediate singly innervated, global pale singly innervated and global multiply innervated fibres [3,4].

3. Innervation patterns Skeletal muscles fibres have a single nerve axon connected to an en-plaque endplate at the mid-belly of each fibre. In contrast, only four of the six types of EOM fibres are singly innervated fibres (SIFs) [2]. The other two types are multiply innervated fibres (MIFs) with multiple neuromuscular junctions along the length of each fibre and they possess an additional type of endplate, the en-grappe endplate, that arises from one or more nerve fibres [5]. Firing at the synapses of limb muscle fibres and EOM SIFs leads to an action potential that creates an all-or-none twitch mode of contraction. On the other hand, EOM MIFs have a tonic mode of contraction that is activated focally at each synapse without generation or propagation of action potentials [6]. These tonic contractions are slow and graded and only slightly and slowly increase muscle tension. While global MIFs only have a tonic mode of contraction, orbital MIFs have a twitch mode of contraction at mid-belly and a tonic mode of contraction at their proximal and distal ends [7] (Fig. 1). In limb muscles, the endplate potential amplitude is larger than the minimum depolarisation needed to trigger

Fig. 1. Schematic representation of the modes of contraction in the different types of EOMs fibres (SIF, singly innervated fibre; MIF, multiply innervated fibre) [2,6,7].

4.4 4.8 0.13 O600 phasic O200 sustained Wide functional repertoire: almost every motor unit can participate in saccades, tracking and vergeance movements

a propagated action potential. This difference is called the safety factor. EOM twitch fibres have a lower safety factor as they have less prominent synaptic folds and therefore one would predict, fewer acetylcholine receptors on the postsynaptic membrane [8–11]. The lower safety factor might thus make EOM twitch fibres more vulnerable to the reduction in synaptic depolarisation that occurs in myasthenia. On the other hand, EOM tonic fibres have no safety factor and the force generation is directly proportional to the membrane depolarisation [10]. Any reduction in synaptic depolarisation would thus lead to symptomatic muscle weakness in EOM tonic fibres. Moreover, destruction of the neuromuscular junction by myasthenia is complement mediated. Porter et al. found that EOMs express low levels of decay accelerating factor (Daf) [12] (Section 8.3). As Daf is an inhibitor of complement deposition at the neuromuscular junction, this might allow the complement mediated response to affect EOMs more severely in myasthenia [11].

4. Motor units and contractile properties Ocular motor units are an order of magnitude smaller than limb muscles motor units which is consistent with the capacity of EOMs to vary their contractile forces by small increments [2,10]. Moreover, the maximum firing frequencies of ocular motor units in the phasic and sustained phases are about four times greater than those of limb muscles motor units [13]. The higher firing frequencies might make EOMs more prone to the neuromuscular transmission failure in myasthenia [10]. To allow them to operate at the higher firing frequencies, EOMs also have faster contractile properties with their time to peak tension and their one-half relaxation time being at least half those in limb muscles [2]. Limb movements are executed by the differential recruitment of motor units for specific subsets of fast and slow movements [14], and by the frequency modulation of already active motor units [2]. In contrast, almost every

C.Y. Yu Wai Man et al. / Neuromuscular Disorders 15 (2005) 17–23

ocular motor unit is able to participate in saccades, tracking and vergeance movements [2]. At the primary position of gaze, 70% of ocular motor units are already active and frequency modulation is the major means of executing eye movements [15]. EOMs also have a smaller twitch: tetanic tension ratio than limb muscles which is consistent with their high capacity to frequency modulate their force output [15]. The higher firing frequencies, the faster contractile properties and the higher percentage of recruitment of ocular motor units in almost every eye movement all contribute to make the properties of EOMs more energy demanding than those of limb muscles.

5. Myosin expression The differences in contractile properties between EOMs and limb muscles are dictated by their differences in myosin expression (Table 2). During development, limb muscles express both of the developmental myosin heavy chain (MyHC) isoforms, MyHCemb and MyHCneonatal. Thereafter, limb muscle fibres express only one of the adult MyHC isoforms I, IIa, IIb and IIx and it is the type of MyHC isoform expressed that determines the contractile velocity, the contractile force and the ATP consumption of the muscle. In contrast, EOMs fibres express almost all known MyHC isoforms and the expression of more than one MyHC isoform in single muscle fibres is characteristic of EOMs [7,16–19]. Firstly, embryonic and neonatal MyHC isoforms persist in adult EOMs [7,16,18]. Secondly, extraocular MIFs express the MyHCa-cardiac isoform that is only also seen in heart and masticatory muscles [20]. Thirdly, EOMs express the MyHCeom isoform that is otherwise only seen in laryngeal muscles [21,22]. Briggs et al. also found that EOMs have longitudinal variation in MyHC expression along single muscle fibres with the fast MyHC isoforms being more prominent in the central innervation zone [22]. The high expression of the fast MyHC isoforms in the central innervation region might thus be the reason why all

19

EOMs fibres are able to exhibit such fast contractile properties despite the fact that they have kinetically slower MyHC isoforms in other fibre sections [22].

6. Structural and metabolic adaptations EOMs need a higher fatigue resistance to fulfil their more energy demanding properties [23]. Fuchs et al. demonstrated that even after continuous strenuous saccadic movements, the peak saccadic velocities in EOMs only decreased on average by less than 10% and even then, the authors argued that this small decrease could largely be attributed to inattention and lack of motivation of the subjects [23]. To enjoy such a high fatigue resistance, EOMs possess a highly developed microvascular bed [4], a higher blood flow [24], a higher mitochondrial content [25] and a higher metabolic rate [26]. 6.1. Higher mitochondrial content EOMs have a higher mitochondrial content than skeletal muscles [25]. Carry et al. also found that the fibres in the orbital layer have a relatively higher mitochondrial content than the corresponding fibres in the global layer [25]. Demer et al. proposed that the differences in mitochondrial content between the orbital and global layers might be due to their functional specialisation [27]. In rectus EOMs, the global layer inserts onto the sclera to mainly rotate the globe while the orbital layer inserts onto fibrous pulleys in the orbit to adjust the position of the fibres in the global layer [27]. In order to support the continuous elastic loading of their fibrous pulley insertions, the orbital layer needs a higher fatigue resistance and thus has a higher mitochondrial content. The high dependence of EOMs on oxidative phosphorylation for their normal functioning is consistent with their selective vulnerability to the respiratory chain dysfunction that occurs in mitochondrial disorders such as CPEO [26]. 6.2. Higher metabolic rate

Table 2 Differences in myosin expression between EOMs and limb muscles [7,16–22]

Fast MyHC isoforms

Slow MyHC isoforms

Developmental MyHC isoforms

MyHCIIa MyHCIIb MyHCIIx MyHCeom MyHCI MyHCacardiac MyHCsto MyHCemb MyHCneonatal

EOMs

Limb muscles

O O O O O O

O O O X O X

O O O

X Only during development

Other muscles

Laryngeal Heart, masticatory

Chang et al. found that mitochondrial DNA (mtDNA) mutations in mitochondrial disorders seem to be preferentially distributed in tissues with high oxidative metabolisms such as EOMs [26]. The hypothesis is that the higher metabolic rate of EOMs, which is needed to fulfil their more energy demanding properties, might in turn make them more prone to free radical-mediated enzyme and mtDNA damage. This theory is consistent with the faster age-related decline in respiratory chain function that occurs in EOMs compared to other skeletal muscles [28]. Muller-Hocker et al. analysed histochemically the age-related increase in the biochemical defect in cytochrome c oxidase (COX) in normal EOMs and compared it to those in normal limb muscles and diaphragms [29,30]. They found that

20

C.Y. Yu Wai Man et al. / Neuromuscular Disorders 15 (2005) 17–23

COX-deficient fibres were already present from the second decade and that they increased with age in all three types of muscles. However, the rate of increase was found to be in overall about six times higher in EOMs compared to the other muscles [29]. As COX is part of the energy-producing mechanism that is frequently deficient in mitochondrial disorders, Muller-Hocker proposed that it might be the higher age-related progression in COX defect density in EOMs that selectively predisposes the latter to CPEO [29,31]. It has been argued that the age-related decline in respiratory chain function in limb muscles might simply reflect physical inactivity in older people [32]. However, this is unlikely to be the case in EOMs as eye movements continue even in the most sedentary patients.

7. Continuous remodelling Mature mammalian skeletal myofibres are post-mitotic in nature but can regenerate in injury by activation of normally quiescent satellite cells and upregulation of myogenic regulatory factors such as MyoD. In contrast, McLoon et al. found that uninjured EOMs contained satellite cells that are continually dividing [33,34] and thus proposed that the continuous remodelling in EOMs might make them selectively vulnerable to certain muscle disorders such as CPEO [33,35]. The hypothesis is that the repeated cycling of satellite cells might cause a gradual accumulation of damaged DNA in the mitochondria and nuclei of EOMs [35]. However, McLoon’s recent findings have not yet been confirmed in other studies. Besides, although uninjured EOMs contained about twice the percentage of MyoDpositive satellite cells compared to uninjured skeletal muscles, the frequency of MyoD-positive nuclei found in uninjured EOMs was rather low with an overall figure of only about 5 per 100 myofibres [33]. The continuous remodelling theory of McLoon et al. is also unlikely to be true unless it can be reconciled with the work of Clark et al. Using human quadriceps, the latter showed that satellite cells have low levels of mtDNA mutations and that the biochemical defect in COX could be reversed by inducing muscle degeneration with the injection of the local anaesthetic bupivacaine [36]. According to Clark’s findings, a higher percentage of activated satellite cells in EOMs would in fact be a protective mechanism rather than a predisposing factor in CPEO.

fibroblasts as being the primary targets [39–41]. The proposed pathogenesis is that T-cells recognise the same antigen on orbital fibroblast as on thyroid follicular cells and this causes a release of cytokines, e.g. IFN-g and TNF-a. The latter enhance the expression of immunomodulatory proteins, e.g. ILA-DR, ICAM-1 and HSP-72, and the end result is an increased production of glycosaminoglycans by orbital fibroblasts [39]. As EOMs are located in a restricted orbital space, the increase in the orbital connective tissue volume causes compression of EOMs and fibrotic restriction of their movements and thus leads to the clinical manifestations of ophthalmopathy [39]. 8.2. Acetylcholine receptor (AChR) isoforms The principal antigenic target in myasthenia gravis is the nicotinic acetylcholine receptor (AChR) [42]. The latter is a pentameric protein that exists in two isoforms in mammalian muscles. The structure of the fetal isoform is a2bdg while that of the adult isoform is a2bd3. As adult EOMs coexpress both the fetal and adult AChR isoforms [10,43–45], this has led to the hypothesis that it might be the g subunit in the fetal AChR isoform that is selectively targeted in myasthenia. However, this theory is unlikely to be true unless it can be reconciled why the levator palpebrae superioris, which is frequently affected in myasthenia, does not express the fetal AChR isoform [46]. 8.3. Complement-mediated immune response In the classical pathway, Porter et al. found that the decay accelerating factor (Daf), which is an inhibitor of the central C3 amplification convertases, is downregulated in EOMs compared to other skeletal muscles [12] (Table 3). On the other hand, Cd59a, which is an inhibitor of complement deposition on the cell surface, was found to be upregulated in EOMs. In the alternative pathway, Porter et al. also found that negative regulators such as complement factor H-related protein and complement component factor h (Cfh), were upregulated in EOMs compared to other Table 3 Differences in complement mediated immune response in EOMs v/s leg and jaw muscles [12] Genetic expression in EOMs v/s leg and jaw muscles Classical pathway

8. Immunological properties 8.1. Orbital components and space Cell-mediated cytotoxicity against EOMs fibres has been reported in Graves’ ophthalmopathy [37,38] but the compelling evidence in the literature points towards orbital

Alternative pathway

Decay accelerating factor 1 (Daf1)

Downregulated

Decay accelerating factor 2 (Daf2) CD59 antigen (Cd59a) Complement factor H-related protein Complement component factor h (Cfh)

Downregulateda Upregulateda Upregulated Upregulateda

a Indicates comparison did not meet criteria for significance between EOMs and leg muscles.

C.Y. Yu Wai Man et al. / Neuromuscular Disorders 15 (2005) 17–23

21

Fig. 2. Schematic representation of the properties of EOMs that might lead to a selective vulnerability to certain disorders.

skeletal muscles [12]. As complement activation by both the classical and alternative pathways has been implicated in muscle disorders [47–49], these specific patterns of complement-mediated immune response might contribute towards the selective vulnerability of EOMs in autoimmune disorders such as myasthenia gravis and Graves’ ophthalmopathy.

9. Conclusions The selective vulnerability of EOMs to certain disorders can, in part, be explained by their fundamentally distinct structural, functional, biochemical and immunological properties compared to other skeletal muscles (Fig. 2). The importance of determining the characteristics of EOMs and the mechanisms that make them selectively vulnerable to certain disorders is that it will extend our knowledge of muscle biology but more importantly, it might lead to new treatment regimes. Unfortunately, the relative inaccessibility of EOMs compared to skeletal muscles and the relatively smaller volume of tissue available have hampered research in this area. Recent genome profiling studies carried out on EOMs lend substantial support to the notion that they have a unique pattern of gene expression so that

advances in the field of molecular genetics hold the potential to reveal yet more differences between EOMs and other skeletal muscles.

References [1] Hoh JFY, Hughes S, Hugh G, Pozgaj I. Three hierarchies in skeletal muscle fibre classification: allotype, isotype and phenotype. In: Kedes LH, Stockdale FE, editors. Cellular and molecular biology of muscle development. New York: Liss; 1989. p. 15–26. [2] Porter JD, Baker RS. Muscles of a different ‘color’: the unusual properties of the extraocular muscles may predispose or protect them in neurogenic and myogenic disease. Neurology 1996;46(1): 30–7. [3] Mayr R. Structure and distribution of fiber types in the external eye muscles of the rat. Tissue Cell 1971;3:433–62. [4] Spencer RF, Porter JD. Structural organization of the extraocular muscles. In: Buttner-Enever JA, editor. Reviews in oculomotor research. New York: Elsevier; 1988. [5] Khanna S, Richmonds CR, Kaminski HJ, Porter JD. Molecular organization of the extraocular muscle neuromuscular junction: partial conservation of and divergence from the skeletal muscle prototype. Invest Ophthalmol Vis Sci 2003;44(5):1918–26. [6] Chiarandini DJ, Stefani E. Electrophysiological identification of two types of fibres in rat extraocular muscles. J Physiol 1979;290(2): 453–65.

22

C.Y. Yu Wai Man et al. / Neuromuscular Disorders 15 (2005) 17–23

[7] Jacoby J, Ko K, Weiss C, Rushbrook JI. Systematic variation in myosin expression along extraocular muscle fibres of the adult rat. J Muscle Res Cell Motility 1990;11(1):25–40. [8] Salpeter MM. Vertebrate neuromuscular junctions: general morphology, molecular organisation, and functional consequences. In: Salpeter MM, editor. The vertebrate neuromuscular junction. New York: Alan R. Liss; 1987. p. 1–54. [9] kim Y, Zahm D, Liu H, Johns T. Safety margin of neuromuscular transmission in rat extraocular muscle. Noc Neurosci 1982;8:616. [10] Kaminski HJ, Maas E, Spiegel P, Ruff RL. Why are eye muscles frequently involved in myasthenia gravis? Neurology 1990;40(11): 1663–9. [11] Kaminski HJ, Richmonds CR, Kusner LL, Mitsumoto H. Differential susceptibility of the ocular motor system to disease. Ann N Y Acad Sci 2002;956:42–54. [12] Porter JD, Khanna S, Kaminski HJ, et al. Extraocular muscle is defined by a fundamentally distinct gene expression profile. Proc Natl Acad Sci USA 2001;98(21):12062–7. [13] Robinson DA. Oculomotor unit behavior in the monkey. J Neurophysiol 1970;33(3):393–403. [14] Burke RE. The structure and function of motor units. In: Karpati G, Hilton-Jones D, Griggs RC, editors. Disorders of voluntary muscle. Cambridge: Cambridge University Press; 2001. p. 3–25. [15] Nelson JS, Goldberg SJ, McClung JR. Motoneuron electrophysiological and muscle contractile properties of superior oblique motor units in cat. J Neurophysiol 1986;55(4):715–26. [16] Brueckner JK, Itkis O, Porter JD. Spatial and temporal patterns of myosin heavy chain expression in developing rat extraocular muscle. J Muscle Res Cell Motility 1996;17(3):297–312. [17] Rubinstein NA, Hoh JF. The distribution of myosin heavy chain isoforms among rat extraocular muscle fiber types. Invest Ophthalmol Vis Sci 2000;41(11):3391–8. [18] Wieczorek DF, Periasamy M, Butler-Browne GS, Whalen RG, NadalGinard B. Co-expression of multiple myosin heavy chain genes, in addition to a tissue-specific one, in extraocular musculature. J Cell Biol 1985;101(2):618–29. [19] McLoon LK, Rios L, Wirtschafter JD. Complex three-dimensional patterns of myosin isoform expression: differences between and within specific extraocular muscles. J Muscle Res Cell Motility 1999; 20(8):771–83. [20] Rushbrook JI, Weiss C, Ko K, et al. Identification of alpha-cardiac myosin heavy chain mRNA and protein in extraocular muscle of the adult rabbit. J Muscle Res Cell Motility 1994;15(5):505–15. [21] Briggs MM, Schachat F. Early specialization of the superfast myosin in extraocular and laryngeal muscles. J Exp Biol 2000;203(Pt 16): 2485–94. [22] Briggs MM, Schachat F. The superfast extraocular myosin (MYH13) is localized to the innervation zone in both the global and orbital layers of rabbit extraocular muscle. J Exp Biol 2002;205(Pt 20):3133–42. [23] Fuchs AF, Binder MD. Fatigue resistance of human extraocular muscles. J Neurophysiol 1983;49(1):28–34. [24] Wooten GF, Reis DJ. Blood flow in extraocular muscle of cat. Arch Neurol 1972;26(4):350–2. [25] Carry MR, Ringel SP, Starcevich JM. Mitochondrial morphometrics of histochemically identified human extraocular-muscle fibers. Anatomical Record 1986;214(1):8–16. [26] Chang TS, Johns DR, Walker D, de la Cruz Z, Maumence IH, Green WR. Ocular clinicopathologic study of the mitochondrial encephalomyopathy overlap syndromes. Arch Ophthalmology 1993; 111(9):1254–62. [27] Demer JL, Oh SY, Poukens V. Evidence for active control of rectus extraocular muscle pulleys. Invest Ophthalmol Vis Sci 2000;41(6): 1280–90. [28] Mullerhocker J, Seibel P, Schneiderbanger K, Kadenbach B. Different insitu hybridization patterns of mitochondrial-DNA in

[29]

[30]

[31] [32]

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

cytochrome-c oxidase-deficient extraocular-muscle fibers in the elderly. Virchows Arch a-Pathol Anat Histopathol 1993;422(1): 7–15. Mullerhocker J, Schneiderbanger K, Stefani FH, Kadenbach B. Progressive loss of cytochrome-c-oxidase in the human extraocularmuscles in aging—a cytochemical-immunohistochemical study. Mutation Res 1992;275(3–6):115–24. Muller-Hocker J. Cytochrome c oxidase deficient fibres in the limb muscle and diaphragm of man without muscular disease: an agerelated alteration. J Neurological Sci 1990;100(1–2):14–21. Wallace DC. Mitochondrial DNA mutations in diseases of energy metabolism. J Bioenerg Biomembranes 1994;26(3):241–50. Brierley EJ, Johnson MA, James OF, Turnbull DM. Effects of physical activity and age on mitochondrial function. Q J Med 1996; 89(4):251–8. McLoon LK, Wirtschafter JD. Continuous myonuclear addition to single extraocular myofibers in uninjured adult rabbits. Muscle Nerve 2002;25(3):348–58. McLoon LK, Wirtschafter J. Activated satellite cells in extraocular muscles of normal adult monkeys and humans. Invest Ophthalmol Vis Sci 2003;44(5):1927–32. McLoon LK, Wirtschafter J. Activated satellite cells are present in uninjured extraocular muscles of mature mice. Trans Am Ophthalmological Soc 2002;100:119–23 (discussion 123–124). Clark KM, Bindoff LA, Lightowlers RN, et al. Reversal of a mitochondrial DNA defect in human skeletal muscle. Nature Genet 1997;16(3):222–4. Blau HM, Kaplan I, Tao TW, Kriss JP. Thyroglobulin-independent, cell-mediated cytotoxicity of human eye muscle cells in tissue culture by lymphocytes of a patient with Graves’ ophthalmopathy. Life Sci 1983;32(1–2):45–53. Wang PW, Hiromatsu Y, Laryea E, Wosu L, How J, Wall JR. Immunologically mediated cytotoxicity against human eye muscle cells in Graves’ ophthalmopathy. J Clin Endocrinology Metabolism 1986;63(2):316–22. Bahn RS, Heufelder AE. Pathogenesis of Graves’ ophthalmopathy. New England J Med 1993;329(20):1468–75. Hufnagel TJ, Hickey WF, Cobbs WH, Jakobiec FA, Iwamoto T, Eagle RC. Immunohistochemical and ultrastructural studies on the exenterated orbital tissues of a patient with Graves’ disease. Ophthalmology 1984;91(11):1411–9. Tallstedt L, Norberg R. Immunohistochemical staining of normal and Graves’ extraocular muscle. Invest Ophthalmol Vis Sci 1988;29(2): 175–84. Newsom-Davis J, Vincent A, Willcox HN. Autoimmune disorders of the neuromuscular junction. In: Lachmann PJ, Peters DK, Rosen FS, Walport NJ, editors. Clinical aspects of immunology, 5th ed. Oxford: Blackwell Scientific Publications; 1993. p. 2091–111. Horton RM, Manfredi AA, Conti-Tronconi BM. The ‘embryonic’ gamma subunit of the nicotinic acetylcholine receptor is expressed in adult extraocular muscle. [comment]. Neurology 1993;43(5): 983–6. Kaminski HJ, Kusner LL, Block CH. Expression of acetylcholine receptor isoforms at extraocular muscle endplates. Invest Ophthalmol Vis Sci 1996;37(2):345–51. Missias AC, Chu GC, Klocke BJ, Sanes JR, Merlie JP. Maturation of the acetylcholine receptor in skeletal muscle: regulation of the AChR gamma-to-epsilon switch. Dev Biol 1996;179(1):223–38. Kaminski HJ, Kusner LL, Nash KV, Ruff RL. The gamma-subunit of the acetylcholine receptor is not expressed in the levator palpebrae superioris. Neurology 1995;45(3 Pt 1):516–8. Gasque P, Morgan BP, Legoedec J, Chan P, Fontaine M. Human skeletal myoblasts spontaneously activate allogeneic complement but are resistant to killing. J Immunol 1996;156(9):3402–11. Lang TJ, Shin ML. Activation of the alternative complement pathway and production of Factor-H by skeletal myotubes. J Neurophysiol 1993;44(2):185–92.

C.Y. Yu Wai Man et al. / Neuromuscular Disorders 15 (2005) 17–23 [49] Navenot JM, Villanova M, LucasHeron B, Malandrini A, Blanchard D, Louboutin JP. Expression of CD59, a regulator of the membrane attack complex of complement, on human skeletal muscle fibers. Muscle Nerve 1997;20(1):92–6. [50] Burke RE. Motor units: anatomy, physiology, and functional organization. In: Brooks VB, editor. Handbook of physiology, section 1, the nervous system. Bethesda: American Physiological Society; 1981. p. 345–422. [51] Barmack NH. Recruitment and suprathreshold frequency modulation of single extraocular muscle fibers in the rabbit. J Neurophysiol 1977; 40(4):779–90.

23

[52] Hanson J, Lennerstrand G. Contractile and histochemical properties of the inferior oblique muscle in the rat and in the cat. Acta Ophthalmologica 1977;55(1):88–102. [53] Gurahian SM, Goldberg SJ. Fatigue of lateral rectus and retractor bulbi motor units in cat. Brain Res 1987;415(2): 281–92. [54] Meredith MA, Goldberg SJ. Contractile differences between muscle units in the medial rectus and lateral rectus muscles in the cat. J Neurophysiol 1986;56(1):50–62. [55] Close RI, Luff AR. Dynamic properties of inferior rectus muscle of the rat. J Physiol (Lond) 1974;236:259–70.