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Korb A, Tajbakhsh S, Comai GE. Functional specialisation and coordination of myonuclei. Biol Rev Camb Philos Soc 2024. [PMID: 38477382 DOI: 10.1111/brv.13063] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2023] [Revised: 01/30/2024] [Accepted: 02/02/2024] [Indexed: 03/14/2024]
Abstract
Myofibres serve as the functional unit for locomotion, with the sarcomere as fundamental subunit. Running the entire length of this structure are hundreds of myonuclei, located at the periphery of the myofibre, juxtaposed to the plasma membrane. Myonuclear specialisation and clustering at the centre and ends of the fibre are known to be essential for muscle contraction, yet the molecular basis of this regionalisation has remained unclear. While the 'myonuclear domain hypothesis' helped explain how myonuclei can independently govern large cytoplasmic territories, novel technologies have provided granularity on the diverse transcriptional programs running simultaneously within the syncytia and added a new perspective on how myonuclei communicate. Building upon this, we explore the critical cellular and molecular sources of transcriptional and functional heterogeneity within myofibres, discussing the impact of intrinsic and extrinsic factors on myonuclear programs. This knowledge provides new insights for understanding muscle development, repair, and disease, but also opens avenues for the development of novel and precise therapeutic approaches.
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Affiliation(s)
- Amaury Korb
- Institut Pasteur, Université Paris Cité, CNRS UMR 3738, Stem Cells & Development Unit, 25 rue du Dr. Roux, Institut Pasteur, Paris, F-75015, France
| | - Shahragim Tajbakhsh
- Institut Pasteur, Université Paris Cité, CNRS UMR 3738, Stem Cells & Development Unit, 25 rue du Dr. Roux, Institut Pasteur, Paris, F-75015, France
| | - Glenda E Comai
- Institut Pasteur, Université Paris Cité, CNRS UMR 3738, Stem Cells & Development Unit, 25 rue du Dr. Roux, Institut Pasteur, Paris, F-75015, France
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2
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Hernández-Morato I, Yu VX, Pitman MJ. A review of the peripheral proprioceptive apparatus in the larynx. Front Neuroanat 2023; 17:1114817. [PMID: 36910514 PMCID: PMC9998684 DOI: 10.3389/fnana.2023.1114817] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2022] [Accepted: 01/19/2023] [Indexed: 03/14/2023] Open
Abstract
The larynx is an organ of the upper airway that participates in breathing, glutition, voice production, and airway protection. These complex functions depend on vocal fold (VF) movement, facilitated in turn by the action of the intrinsic laryngeal muscles (ILM). The necessary precise and near-instantaneous modulation of each ILM contraction relies on proprioceptive innervation of the larynx. Dysfunctional laryngeal proprioception likely contributes to disorders such as laryngeal dystonia, dysphagia, vocal fold paresis, and paralysis. While the proprioceptive system in skeletal muscle derived from somites is well described, the proprioceptive circuitry that governs head and neck structures such as VF has not been so well characterized. For over two centuries, researchers have investigated the question of whether canonical proprioceptive organs, muscle spindles, and Golgi tendon organs, exist in the ILM, with variable findings. The present work is a state-of-the-art review of the peripheral component of laryngeal proprioception, including current knowledge of canonical and possible alternative proprioceptive circuitry elements in the larynx.
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Affiliation(s)
- Ignacio Hernández-Morato
- Department of Otolaryngology-Head and Neck Surgery, Columbia University Irving Medical Center, New York, NY, United States
| | - Victoria X Yu
- Department of Otolaryngology-Head and Neck Surgery, Columbia University Irving Medical Center, New York, NY, United States
| | - Michael J Pitman
- Department of Otolaryngology-Head and Neck Surgery, Columbia University Irving Medical Center, New York, NY, United States
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Carrero-Rojas G, Hernández RG, Blumer R, de la Cruz RR, Pastor AM. MIF versus SIF Motoneurons, What Are Their Respective Contribution in the Oculomotor Medial Rectus Pool? J Neurosci 2021; 41:9782-9793. [PMID: 34675089 PMCID: PMC8612643 DOI: 10.1523/jneurosci.1480-21.2021] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2021] [Revised: 08/18/2021] [Accepted: 10/11/2021] [Indexed: 12/18/2022] Open
Abstract
Multiply-innervated muscle fibers (MIFs) are peculiar to the extraocular muscles as they are non-twitch but produce a slow build up in tension on repetitive stimulation. The motoneurons innervating MIFs establish en grappe terminals along the entire length of the fiber, instead of the typical en plaque terminals that singly-innervated muscle fibers (SIFs) motoneurons establish around the muscle belly. MIF motoneurons have been proposed to participate only in gaze holding and slow eye movements. We aimed to discern the function of MIF motoneurons by recording medial rectus motoneurons of the oculomotor nucleus. Single-unit recordings in awake cats demonstrated that electrophysiologically-identified medial rectus MIF motoneurons participated in different types of eye movements, including fixations, rapid eye movements or saccades, convergences, and the slow and fast phases of the vestibulo-ocular nystagmus, the same as SIF motoneurons did. However, MIF medial rectus motoneurons presented lower firing frequencies, were recruited earlier and showed lower eye position (EP) and eye velocity (EV) sensitivities than SIF motoneurons. MIF medial rectus motoneurons were also smaller, had longer antidromic latencies and a lower synaptic coverage than SIF motoneurons. Peristimulus time histograms (PSTHs) revealed that electrical stimulation to the myotendinous junction, where palisade endings are located, did not recurrently affect the firing probability of medial rectus motoneurons. Therefore, we conclude there is no division of labor between MIF and SIF motoneurons based on the type of eye movement they subserve.SIGNIFICANCE STATEMENT In addition to the common singly-innervated muscle fiber (SIF), extraocular muscles also contain multiply-innervated muscle fibers (MIFs), which are non-twitch and slow in contraction. MIF motoneurons have been proposed to participate only in gaze holding and slow eye movements. In the present work, by single-unit extracellular recordings in awake cats, we demonstrate, however, that both SIF and MIF motoneurons, electrophysiologically-identified, participate in the different types of eye movements. However, MIF motoneurons showed lower firing rates (FRs), recruitment thresholds, and eye-related sensitivities, and could thus contribute to the fine adjustment of eye movements. Electrical stimulation of the myotendinous junction activates antidromically MIF motoneurons but neither MIF nor SIF motoneurons receive a synaptic reafferentation that modifies their discharge probability.
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Affiliation(s)
- Génova Carrero-Rojas
- Departamento de Fisiología, Facultad de Biología, Universidad de Sevilla, Seville 41012, Spain
- Center of Anatomy and Cell Biology, Medical Imaging Cluster, Medical University Vienna, Vienna 1090, Austria
| | - Rosendo G Hernández
- Departamento de Fisiología, Facultad de Biología, Universidad de Sevilla, Seville 41012, Spain
| | - Roland Blumer
- Center of Anatomy and Cell Biology, Medical Imaging Cluster, Medical University Vienna, Vienna 1090, Austria
| | - Rosa R de la Cruz
- Departamento de Fisiología, Facultad de Biología, Universidad de Sevilla, Seville 41012, Spain
| | - Angel M Pastor
- Departamento de Fisiología, Facultad de Biología, Universidad de Sevilla, Seville 41012, Spain
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Abstract
Eye movements are indispensable for visual image stabilization during self-generated and passive head and body motion and for visual orientation. Eye muscles and neuronal control elements are evolutionarily conserved, with novel behavioral repertoires emerging during the evolution of frontal eyes and foveae. The precise execution of eye movements with different dynamics is ensured by morphologically diverse yet complementary sets of extraocular muscle fibers and associated motoneurons. Singly and multiply innervated muscle fibers are controlled by motoneuronal subpopulations with largely selective premotor inputs from task-specific ocular motor control centers. The morphological duality of the neuromuscular interface is matched by complementary biochemical and molecular features that collectively assign different physiological properties to the motor entities. In contrast, the functionality represents a continuum where most motor elements contribute to any type of eye movement, although within preferential dynamic ranges, suggesting that signal transmission and muscle contractions occur within bands of frequency-selective pathways.
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Affiliation(s)
- Anja K E Horn
- Institute of Anatomy and Cell Biology I, Ludwig-Maximilians-University Munich, 80336 Munich, Germany;
| | - Hans Straka
- Department Biology II, Ludwig-Maximilians-University Munich, 82152 Planegg, Germany
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Mayadali ÜS, Fleuriet J, Mustari M, Straka H, Horn AKE. Transmitter and ion channel profiles of neurons in the primate abducens and trochlear nuclei. Brain Struct Funct 2021; 226:2125-2151. [PMID: 34181058 PMCID: PMC8354957 DOI: 10.1007/s00429-021-02315-7] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2021] [Accepted: 06/04/2021] [Indexed: 01/28/2023]
Abstract
Extraocular motoneurons initiate dynamically different eye movements, including saccades, smooth pursuit and vestibulo-ocular reflexes. These motoneurons subdivide into two main types based on the structure of the neuro-muscular interface: motoneurons of singly-innervated (SIF), and motoneurons of multiply-innervated muscle fibers (MIF). SIF motoneurons are thought to provoke strong and brief/fast muscle contractions, whereas MIF motoneurons initiate prolonged, slow contractions. While relevant for adequate functionality, transmitter and ion channel profiles associated with the morpho-physiological differences between these motoneuron types, have not been elucidated so far. This prompted us to investigate the expression of voltage-gated potassium, sodium and calcium ion channels (Kv1.1, Kv3.1b, Nav1.6, Cav3.1-3.3, KCC2), the transmitter profiles of their presynaptic terminals (vGlut1 and 2, GlyT2 and GAD) and transmitter receptors (GluR2/3, NMDAR1, GlyR1α) using immunohistochemical analyses of abducens and trochlear motoneurons and of abducens internuclear neurons (INTs) in macaque monkeys. The main findings were: (1) MIF and SIF motoneurons express unique voltage-gated ion channel profiles, respectively, likely accounting for differences in intrinsic membrane properties. (2) Presynaptic glutamatergic synapses utilize vGlut2, but not vGlut1. (3) Trochlear motoneurons receive GABAergic inputs, abducens neurons receive both GABAergic and glycinergic inputs. (4) Synaptic densities differ between MIF and SIF motoneurons, with MIF motoneurons receiving fewer terminals. (5) Glutamatergic receptor subtypes differ between MIF and SIF motoneurons. While NMDAR1 is intensely expressed in INTs, MIF motoneurons lack this receptor subtype entirely. The obtained cell-type-specific transmitter and conductance profiles illuminate the structural substrates responsible for differential contributions of neurons in the abducens and trochlear nuclei to eye movements.
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Affiliation(s)
- Ümit Suat Mayadali
- Institute of Anatomy and Cell Biology, Dept. I, Ludwig-Maximilians-University Munich, Pettenkoferstrasse 11, 80336, Munich, Germany
- Graduate School of Systemic Neurosciences, Ludwig-Maximilians-University Munich, Planegg-Martinsried, Germany
| | - Jérome Fleuriet
- Washington National Primate Research Center, Department of Ophthalmology, University of Washington Seattle, Seattle, WA, USA
- Intensive Care Unit, Raymond Poincaré Hospital, Assistance Publique-Hôpitaux de Paris, Garches, France
| | - Michael Mustari
- Washington National Primate Research Center, Department of Ophthalmology, University of Washington Seattle, Seattle, WA, USA
| | - Hans Straka
- Department of Biology II, Ludwig-Maximilians-University Munich, Planegg-Martinsried, Germany
| | - Anja Kerstin Ellen Horn
- Institute of Anatomy and Cell Biology, Dept. I, Ludwig-Maximilians-University Munich, Pettenkoferstrasse 11, 80336, Munich, Germany.
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Hoh JFY. Myosin heavy chains in extraocular muscle fibres: Distribution, regulation and function. Acta Physiol (Oxf) 2021; 231:e13535. [PMID: 32640094 DOI: 10.1111/apha.13535] [Citation(s) in RCA: 11] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2020] [Accepted: 07/02/2020] [Indexed: 12/13/2022]
Abstract
This review examines kinetic properties and distribution of the 11 isoforms of myosin heavy chain (MyHC) expressed in extraocular muscle (EOM) fibre types and the regulation and function of these MyHCs. Although recruitment and discharge characteristics of ocular motoneurons during fixation and eye movements are well documented, work directly linking these properties with motor unit contractile speed and MyHC composition is lacking. Recruitment of motor units according to Henneman's size principle has some support in EOMs but needs consolidation. Both neurogenic and myogenic mechanisms regulate MyHC expression as in other muscle allotypes. Developmentally, multiply-innervated (MIFs) and singly-innervated fibres (SIFs) are derived presumably from distinct myoblast lineages, ending up expressing MyHCs in the slow and fast ends of the kinetic spectrum respectively. They modulate the synaptic inputs of their motoneurons through different retrogradely transported neurotrophins, thereby specifying their tonic and phasic impulse patterns. Immunohistochemical analyses of EOMs regenerating in situ and in limb muscle beds suggest that the very impulse patterns driving various ocular movements equip effectors with appropriate MyHC compositions and speeds to accomplish their tasks. These experiments also suggest that satellite cells of SIFs and MIFs are distinct lineages expressing different MyHCs during regeneration. MyHC compositions and functional characteristics of orbital fibres show longitudinal variations that facilitate linear ocular rotation during saccades. Palisade endings on global MIFs are postulated to respond to active and passive tensions by triggering axon reflexes that play important roles during fixation, saccades and vergence. How EOMs implement Listings law during ocular rotation is discussed.
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Affiliation(s)
- Joseph F. Y. Hoh
- Discipline of Physiology and the Bosch Institute School of Medical Sciences Faculty of Medicine and Health The University of Sydney Sydney NSW Australia
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Zampieri N, de Nooij JC. Regulating muscle spindle and Golgi tendon organ proprioceptor phenotypes. CURRENT OPINION IN PHYSIOLOGY 2021; 19:204-210. [PMID: 33381667 PMCID: PMC7769215 DOI: 10.1016/j.cophys.2020.11.001] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Proprioception is an essential part of motor control. The main sensory subclasses that underlie this feedback control system - muscle spindle and Golgi tendon organ afferents - have been extensively characterized at a morphological and physiological level. More recent studies are beginning to reveal the molecular foundation for distinct proprioceptor subtypes, offering new insights into their developmental ontogeny and phenotypic diversity. This review intends to highlight some of these new findings.
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Affiliation(s)
- Niccolò Zampieri
- Max-Delbrück-Center for Molecular Medicine Berlin-Buch, Robert-Rössle-Str. 10, 13125 Berlin, Germany
| | - Joriene C. de Nooij
- Dept. of Neurology, Vagelos College of Physicians and Surgeons, Columbia University Medical Center, 630 West 168th Street, New York, NY 10032.,Columbia University Motor Neuron Center, Columbia University Medical Center, 630 West 168th Street, New York, NY 10032.,Corresponding author:
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Molinaro F, Fusi G, Aglianò M, Volpi N, Franci D, Lorenzoni P, Messina M, Galgano A, Grasso F, Plessi C, Messina M, Angotti R. Clinically Differentiated Abnormalities of the Architecture and Expression of Myosin Isoforms of the Human Cremaster Muscle in Cryptorchidism and Retractile Testis. Urol Int 2020; 104:891-901. [PMID: 32674099 DOI: 10.1159/000508432] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2020] [Accepted: 05/03/2020] [Indexed: 11/19/2022]
Abstract
AIM To describe architecture and expression of myosin isoforms of the human cremaster muscle (CM) and to individuate changes in clinically differentiated abnormalities of testicular descent: cryptorchidism or undescended testis (UDT) and retractile testis (RT). BACKGROUND The CM is a nonsomitic striated muscle differentiating from mesenchyme of the gubernaculum testis. Morphofunctional and molecular peculiarities linked to its unique embryological origin are not yet completely defined. Its role in abnormalities of testicular descent is being investigated. SUBJECTS AND METHODS Biopsy samples were obtained from corrective surgery in cases of cryptorchidism, retractile testis, inguinal hernia, or hydrocele. Muscle specimens were processed for morphology, histochemistry, and immunohistology. RESULTS AND CONCLUSIONS The CM differs from the skeletal muscles both for morphological and molecular characteristics. The presence of fascicles with different characterization and its myosinic pattern suggested that the CM could be included in the specialized muscle groups, such as the extrinsic ocular muscles (EOMs) and laryngeal and masticatory muscles. The embryological origin from the nonsomitic mesoderm is, also for the CM, the basis of distinct molecular pathways. In UDT, the histological alterations of CM are suggestive of denervation; the genitofemoral nerve and its molecular messengers directed to this muscle are likely defective. Compared with the other samples, RT has a distinct myosinic pattern; therefore, it has been considered a well-defined entity with respect to the other testicular descent abnormalities.
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Affiliation(s)
- Francesco Molinaro
- Division of Pediatric Surgery, Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Giulia Fusi
- Division of Pediatric Surgery, Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy,
| | - Margherita Aglianò
- Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Nila Volpi
- Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Daniela Franci
- Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Paola Lorenzoni
- Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Maddalena Messina
- Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Angela Galgano
- Division of Pediatric Surgery, Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Francesco Grasso
- Division of Pediatric Surgery, Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Carlotta Plessi
- Division of Pediatric Surgery, Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Mario Messina
- Division of Pediatric Surgery, Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
| | - Rossella Angotti
- Division of Pediatric Surgery, Department of Medicine, Surgery and Neuroscience, University of Siena, Siena, Italy
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Yokoyama T, Settai K, Nakamuta N, Yamamoto Y. Vesicular glutamate transporter 2-immunoreactive afferent nerve terminals in rat carotid sinus baroreceptors. Acta Histochem 2020; 122:151469. [PMID: 31784233 DOI: 10.1016/j.acthis.2019.151469] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2019] [Revised: 10/24/2019] [Accepted: 11/14/2019] [Indexed: 11/17/2022]
Abstract
Sensory nerve endings respond to various stimuli and subsequently transmit afferent informations to central nervous system, but their responsibility has been suggested to be modulated by glutamate. In the present study, we examined the immunohistochemical localization of vesicular glutamate transporter 1 (vGLUT1) and vGLUT2 in baroreceptor nerve endings immunoreactive for P2X2 and P2X3 purinoceptors in the rat carotid sinus by immunohistochemistry of whole-mount preparations with confocal scanning laser microscopy. P2X3-immunoreactive flat leaf-like axon terminals were immunoreactive to vGLUT2, but not to vGLUT1. Among members of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex, immunoreactivities for synaptosomal-associated protein, 25 kDa, Syntaxin1, and vesicle-associated membrane protein 2 (VAMP2) were localized in P2X2- and P2X3-immunoreactive axon terminals. Punctate immunoreactive products for VAMP2 and vGLUT2 were co-localized in axon terminals. These results suggest that vGLUT2 is localized in P2X3-immunoreactive baroreceptor terminals in the carotid sinus, and these terminals may release glutamate by exocytosis in order to modulate baroreceptor function in the carotid sinus.
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Affiliation(s)
- Takuya Yokoyama
- Department of Anatomy (Cell Biology), Iwate Medical University, Yahaba, Japan
| | - Kazuya Settai
- Laboratory of Veterinary Anatomy and Cell Biology, Faculty of Agriculture, Iwate University, Morioka, Japan
| | - Nobuaki Nakamuta
- Laboratory of Veterinary Anatomy and Cell Biology, Faculty of Agriculture, Iwate University, Morioka, Japan
| | - Yoshio Yamamoto
- Laboratory of Veterinary Anatomy and Cell Biology, Faculty of Agriculture, Iwate University, Morioka, Japan.
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Lienbacher K, Sänger K, Strassburger S, Ehrt O, Rudolph G, Barnerssoi M, Horn AKE. Extraocular muscles involved in convergence are innervated by an additional set of palisade endings that may differ in their excitability: A human study. PROGRESS IN BRAIN RESEARCH 2019; 248:127-137. [PMID: 31239126 DOI: 10.1016/bs.pbr.2019.04.005] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
Palisade endings are located at the myotendinous junction of extraocular muscles in most mammals. Irrespective of their unclarified function as motor or sensory nerve endings, a specialized role in convergence is proposed, based on their high number in the medial rectus muscle (MR). Further support comes from a study in monkey demonstrating that only the MR and inferior rectus muscle (IR) contain an additional population of palisade endings that express the calcium-binding protein calretinin (CR) in addition to choline acetyltransferase (ChAT). Here we studied, whether CR-positive palisade endings are present in human as well and confined to extraocular muscles most active during convergence. The systematic analysis of all eye muscles of 17 human specimen revealed that only the MR and IR contain an additional population of CR-positive palisade endings and multiple en-grappe endings, which target non-twitch muscle fibers along their whole length. Approximately 80% of all palisade endings in the MR expressed CR. Furthermore, the intrafusal muscle fibers of some muscle spindles in the MR were innervated by CR-positive annulospiral nerve endings that transmit the signals of muscle length changes to the brain. All extraocular muscles contained few thin CR-positive, but ChAT-negative nerve fibers, possibly representing free sensory or autonomic endings arising from the trigeminal ganglion. As in monkey, in the medial periphery of the human oculomotor nucleus ChAT-positive neurons were found to co-express CR. Therefore these neurons most likely represent the cell bodies of CR-positive palisade endings in the MR. Unlike in monkey, these neurons do not lie within a compact cell group, but are more scattered. In conclusion, the MR and IR in human contain two histochemically different populations of palisade and multiple endings that may contribute to ocular alignment and convergence in a different way.
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Affiliation(s)
- Karoline Lienbacher
- Faculty of Medicine, Institute of Anatomy and Cell Biology I, Ludwig-Maximilians-University, Munich, Germany; Center for Vertigo and Balance Disorders DSGZ, Ludwig-Maximilians-University, Munich, Germany
| | - Kathrin Sänger
- Faculty of Medicine, Institute of Anatomy and Cell Biology I, Ludwig-Maximilians-University, Munich, Germany
| | - Sebastian Strassburger
- Faculty of Medicine, Institute of Anatomy and Cell Biology I, Ludwig-Maximilians-University, Munich, Germany
| | - Oliver Ehrt
- Department of Ophthalmology, University Hospital, Ludwig-Maximilians-University, Munich, Germany
| | - Günther Rudolph
- Department of Ophthalmology, University Hospital, Ludwig-Maximilians-University, Munich, Germany
| | - Miriam Barnerssoi
- Faculty of Medicine, Institute of Anatomy and Cell Biology I, Ludwig-Maximilians-University, Munich, Germany
| | - Anja K E Horn
- Faculty of Medicine, Institute of Anatomy and Cell Biology I, Ludwig-Maximilians-University, Munich, Germany; Center for Vertigo and Balance Disorders DSGZ, Ludwig-Maximilians-University, Munich, Germany.
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Abstract
Extraocular muscles contain two types of muscle fibers according to their innervation pattern: singly innervated muscle fibers (SIFs), similar to most skeletal muscle fibers, and multiply innervated muscle fibers (MIFs). Morphological studies have revealed that SIF and MIF motoneurons are segregated anatomically and receive different proportions of certain afferents, suggesting that while SIF motoneurons would participate in the whole repertoire of eye movements, MIF motoneurons would contribute only to slow eye movements and fixations. We have tested that proposal by performing single-unit recordings, in alert behaving cats, of electrophysiologically identified MIF and SIF motoneurons in the abducens nucleus. Our results show that both types of motoneuron discharge in relation to eye position and velocity, displaying a tonic-phasic firing pattern for different types of eye movement (saccades, vestibulo-ocular reflex, vergence) and gaze-holding. However, MIF motoneurons presented an overall reduced firing rate compared with SIF motoneurons, and had significantly lower recruitment threshold and also lower eye position and velocity sensitivities. Accordingly, MIF motoneurons could control mainly gaze in the off-direction, when less force is needed, whereas SIF motoneurons would contribute to increase muscle tension progressively toward the on-direction as more force is required. Anatomically, MIF and SIF motoneurons distributed intermingled within the abducens nucleus, with MIF motoneurons being smaller and having a lesser somatic synaptic coverage. Our data demonstrate the functional participation of both MIF and SIF motoneurons in fixations and slow and phasic eye movements, although their discharge properties indicate a functional segregation.
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Lienbacher K, Ono S, Fleuriet J, Mustari M, Horn AKE. A Subset of Palisade Endings Only in the Medial and Inferior Rectus Muscle in Monkey Contain Calretinin. Invest Ophthalmol Vis Sci 2018; 59:2944-2954. [PMID: 30025142 PMCID: PMC5989861 DOI: 10.1167/iovs.18-24322] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2018] [Accepted: 05/06/2018] [Indexed: 12/11/2022] Open
Abstract
Purpose To further chemically characterize palisade endings in extraocular muscles in rhesus monkeys. Methods Extraocular muscles of three rhesus monkeys were studied for expression of the calcium-binding protein calretinin (CR) in palisade endings and multiple endings. The complete innervation was visualized with antibodies against the synaptosomal-associated protein of 25 kDa and combined with immunofluorescence for CR. Six rhesus monkeys received tracer injections of choleratoxin subunit B or wheat germ agglutinin into either the belly or distal myotendinous junction of the medial or inferior rectus muscle to allow retrograde tracing in the C-group of the oculomotor nucleus. Double-immunofluorescence methods were used to study the CR content in retrogradely labeled neurons in the C-group. Results A subgroup of palisade and multiple endings was found to express CR, only in the medial and inferior rectus muscle. In contrast, the en plaque endings lacked CR. Accordingly, within the tracer-labeled neurons of the C-group, a subgroup expressed CR. Conclusions The study indicates that two different neuron populations targeting nontwitch muscle fibers are present within the C-group for inferior rectus and medial rectus, respectively, one expressing CR, one lacking CR. It is possible that the CR-negative neurons represent the basic population for all extraocular muscles, whereas the CR-positive neurons giving rise to CR-positive palisade endings represent a specialized, perhaps more excitable type of nerve ending in the medial and inferior rectus muscles, being more active in vergence. The malfunction of this CR-positive population of neurons that target nontwitch muscle fibers could play a significant role in strabismus.
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Affiliation(s)
- Karoline Lienbacher
- Institute of Anatomy and Cell Biology, Ludwig-Maximilians-Universität, Munich, Germany
- German Center for Vertigo and Balance Disorders, Klinikum Grosshadern, Ludwig-Maximilians Universität, Munich, Germany
| | - Seiji Ono
- Faculty of Health and Sport Science, University of Tsukuba, Tsukuba, Ibaraki, Japan
| | - Jérome Fleuriet
- Washington National Primate Research Center, Seattle, Washington, United States
- Department of Ophthalmology, University of Washington, Seattle, Washington, United States
| | - Michael Mustari
- Washington National Primate Research Center, Seattle, Washington, United States
- Department of Ophthalmology, University of Washington, Seattle, Washington, United States
| | - Anja K. E. Horn
- Institute of Anatomy and Cell Biology, Ludwig-Maximilians-Universität, Munich, Germany
- German Center for Vertigo and Balance Disorders, Klinikum Grosshadern, Ludwig-Maximilians Universität, Munich, Germany
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Brownstone RM, Lancelin C. Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis. J Neurophysiol 2018; 119:1782-1794. [PMID: 29384454 PMCID: PMC6008087 DOI: 10.1152/jn.00331.2017] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.
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Affiliation(s)
- Robert M Brownstone
- Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London , London , United Kingdom
| | - Camille Lancelin
- Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London , London , United Kingdom
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14
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Paduca A, Bruenech JR. Neuroanatomical Structures in Human Extraocular Muscles and Their Potential Implication in the Development of Oculomotor Disorders. J Pediatr Ophthalmol Strabismus 2018; 55:14-22. [PMID: 28991350 DOI: 10.3928/01913913-20170703-16] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/30/2016] [Accepted: 05/08/2017] [Indexed: 11/20/2022]
Abstract
The potential role of sensory feedback from human extraocular muscles has been subjected to considerable speculation in the ophthalmic literature. Extraocular muscles pull against a fairly even load and do not initiate a stretch reflex, even when the eyes are directed toward the boundaries of their respective field of action. These unique working conditions and physiological properties have led to the notion that the sensory signal arising from receptors in extraocular muscles differs from the conventional proprioceptive signal arising from their somatic counterparts. The interest in the receptors at the myotendinous junction of human extraocular muscles has been renewed due to their alleged role in the development of binocular vision and their potential implication in the etiology of binocular vision anomalies. The idea that extraocular muscles provide knowledge of eye position and whether this function can be affected by surgical intervention has initiated several clinical and neuroanatomical studies. Many of these studies support this concept and suggest that surgical procedures that impose only minimal interference with the proprioceptive signal will give a better postoperative result. However, other studies contradict this view because the afferent capacity of the receptors can be questioned and some uncertainties remain. The purpose of this study was to review the related literature and discuss the possible role of ocular proprioceptors in relation to binocular vision and the development of eye motility disorders. [J Pediatr Ophthalmol Strabismus. 2018;55(1):14-22.].
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15
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Bohlen MO, Warren S, Mustari MJ, May PJ. Examination of feline extraocular motoneuron pools as a function of muscle fiber innervation type and muscle layer. J Comp Neurol 2016; 525:919-935. [PMID: 27588695 DOI: 10.1002/cne.24111] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2016] [Revised: 08/08/2016] [Accepted: 08/18/2016] [Indexed: 01/13/2023]
Abstract
This study explores two points related to the pattern of innervation of the extraocular muscles. First, species differences exist in the location of the motoneurons supplying multiply innervated fibers (MIFs) and singly innervated fibers (SIFs) in eye muscles. MIF motoneurons are located outside the extraocular nuclei in primates, but are intermixed with SIF motoneurons within rat extraocular nuclei. To test whether this difference is related to visual capacity and frontal placement of eyes, we injected retrograde tracers into the medial rectus muscle of the cat, a highly visual nonprimate with frontally placed eyes. Distal injections labeled smaller MIF motoneurons located ventrolaterally and rostrally within the oculomotor nucleus (III). More central injections also labeled a separate population of larger cells located dorsally in III. Thus, the cat shares with the nocturnal rat the feature of having MIF motoneurons located within the bounds of III. On the other hand, just as with monkeys, cats show segregation of the MIF and SIF medial rectus motoneuron pools, albeit in a different pattern. Second, extraocular muscles are divided into two layers; the inner, global layer inserts into the sclera, and the outer, orbital layer inserts into the connective tissue pulley. To test whether these layers are supplied by anatomically discrete motoneuron pools, we injected tracer into the orbital layer of the cat lateral rectus muscle. No evidence of either morphological or distributional differences was found, suggesting that the functional differences in these layers may be due mainly to their orbital anatomy, not their innervation. J. Comp. Neurol. 525:919-935, 2017. © 2016 Wiley Periodicals, Inc.
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Affiliation(s)
- Martin O Bohlen
- Program in Neuroscience, University of Mississippi Medical Center, Jackson, Mississippi, 39216
| | - Susan Warren
- Department of Neurobiology and Anatomical Sciences, University of Mississippi Medical Center, Jackson, Mississippi, 39216
| | - Michael J Mustari
- National Primate Research Center, University of Washington, Seattle, Washington, 98195
| | - Paul J May
- Department of Neurobiology and Anatomical Sciences, University of Mississippi Medical Center, Jackson, Mississippi, 39216.,Department of Ophthalmology, University of Mississippi Medical Center, Jackson, Mississippi, 39216.,Department of Neurology, University of Mississippi Medical Center, Jackson, Mississippi, 39216
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16
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Sun LD, Goldberg ME. Corollary Discharge and Oculomotor Proprioception: Cortical Mechanisms for Spatially Accurate Vision. Annu Rev Vis Sci 2016; 2:61-84. [PMID: 28532350 PMCID: PMC5691365 DOI: 10.1146/annurev-vision-082114-035407] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
A classic problem in psychology is understanding how the brain creates a stable and accurate representation of space for perception and action despite a constantly moving eye. Two mechanisms have been proposed to solve this problem: Herman von Helmholtz's idea that the brain uses a corollary discharge of the motor command that moves the eye to adjust the visual representation, and Sir Charles Sherrington's idea that the brain measures eye position to calculate a spatial representation. Here, we discuss the cognitive, neuropsychological, and physiological mechanisms that support each of these ideas. We propose that both are correct: A rapid corollary discharge signal remaps the visual representation before an impending saccade, computing accurate movement vectors; and an oculomotor proprioceptive signal enables the brain to construct a more accurate craniotopic representation of space that develops slowly after the saccade.
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Affiliation(s)
- Linus D Sun
- Mahoney-Keck Center for Brain and Behavior Research, Department of Neuroscience, Columbia University College of Physicians and Surgeons, New York, NY 10032;
- Department of Neuroscience, Columbia University College of Physicians and Surgeons, New York, NY 10032
- Department of Ophthalmology, Columbia University College of Physicians and Surgeons, New York, NY 10032
- Division of Neurobiology and Behavior, New York State Psychiatric Institute, New York, NY 10032
| | - Michael E Goldberg
- Mahoney-Keck Center for Brain and Behavior Research, Department of Neuroscience, Columbia University College of Physicians and Surgeons, New York, NY 10032;
- Department of Neuroscience, Columbia University College of Physicians and Surgeons, New York, NY 10032
- Department of Neurology, Columbia University College of Physicians and Surgeons, New York, NY 10032
- Department of Psychiatry, Columbia University College of Physicians and Surgeons, New York, NY 10032
- Department of Ophthalmology, Columbia University College of Physicians and Surgeons, New York, NY 10032
- Kavli Institute for Neuroscience, Columbia University, New York, NY 10032
- Division of Neurobiology and Behavior, New York State Psychiatric Institute, New York, NY 10032
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Schiavi C. Extraocular Muscles Tension, Tonus, and Proprioception in Infantile Strabismus: Role of the Oculomotor System in the Pathogenesis of Infantile Strabismus-Review of the Literature. SCIENTIFICA 2016; 2016:5790981. [PMID: 27006860 PMCID: PMC4781980 DOI: 10.1155/2016/5790981] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/06/2015] [Accepted: 01/27/2016] [Indexed: 05/04/2023]
Abstract
The role played by the extraocular muscles (EOMs) in the etiology of concomitant infantile strabismus is still debated and it has not yet definitively established if the sensory anomalies in concomitant strabismus are a consequence or a primary cause of the deviation. The commonest theory supposes that most strabismus results from abnormal innervation of the EOMs, but the cause of this dysfunction and its origin, whether central or peripheral, are still unknown. The interaction between sensory factors and innervational factors, that is, esotonus, accommodation, convergence, divergence, and vestibular reflexes in visually immature infants with family predisposition, is suspected to create conditions that prevent binocular alignment from stabilizing and strengthening. Some role in the onset of fixation instability and infantile strabismus could be played by the feedback control of eye movements and by dysfunction of eye muscle proprioception during the critical period of development of the visual sensory system. A possible role in the onset, maintenance, or worsening of the deviation of abnormalities of muscle force which have their clinical equivalent in eye muscle overaction and underaction has been investigated under either isometric or isotonic conditions, and in essence no significant anomalies of muscle force have been found in concomitant strabismus.
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Affiliation(s)
- Costantino Schiavi
- Department of Experimental, Diagnostic, and Specialty Medicine (DIMES), University of Bologna, St. Orsola-Malpighi Teaching Hospital, Via P. Palagi 9, 40138 Bologna, Italy
- *Costantino Schiavi:
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18
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Zeeh C, Mustari MJ, Hess BJM, Horn AKE. Transmitter inputs to different motoneuron subgroups in the oculomotor and trochlear nucleus in monkey. Front Neuroanat 2015; 9:95. [PMID: 26257611 PMCID: PMC4513436 DOI: 10.3389/fnana.2015.00095] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2015] [Accepted: 07/06/2015] [Indexed: 11/13/2022] Open
Abstract
In all vertebrates the eyes are moved by six pairs of extraocular muscles enabling horizontal, vertical and rotatory movements. Recent work showed that each extraocular muscle is controlled by two motoneuronal groups: (1) Motoneurons of singly-innervated muscle fibers (SIF) that lie within the boundaries of motonuclei mediating a fast muscle contraction; and (2) motoneurons of multiply-innervated muscle fibers (MIF) in the periphery of motonuclei mediating a tonic muscle contraction. Currently only limited data about the transmitter inputs to the SIF and MIF motoneurons are available. Here we performed a quantitative study on the transmitter inputs to SIF and MIF motoneurons of individual muscles in the oculomotor and trochlear nucleus in monkey. Pre-labeled motoneurons were immunostained for GABA, glutamate decarboxylase, GABA-A receptor, glycine transporter 2, glycine receptor 1, and vesicular glutamate transporters 1 and 2. The main findings were: (1) the inhibitory control of SIF motoneurons for horizontal and vertical eye movements differs. Unlike in previous primate studies a considerable GABAergic input was found to all SIF motoneuronal groups, whereas a glycinergic input was confined to motoneurons of the medial rectus (MR) muscle mediating horizontal eye movements and to those of the levator palpebrae (LP) muscle elevating the upper eyelid. Whereas SIF and MIF motoneurons of individual eye muscles do not differ numerically in their GABAergic, glycinergic and vGlut2 input, vGlut1 containing terminals densely covered the supraoculomotor area (SOA) targeting MR MIF motoneurons. It is reasonable to assume that the vGlut1 input affects the near response system in the SOA, which houses the preganglionic neurons mediating pupillary constriction and accommodation and the MR MIF motoneurones involved in vergence.
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Affiliation(s)
- Christina Zeeh
- Institute of Anatomy and Cell Biology, Department I, Ludwig-Maximilians UniversityMunich, Germany
| | - Michael J. Mustari
- Washington National Primate Research Center and Department of Ophthalmology, University of WashingtonSeattle, WA, USA
| | - Bernhard J. M. Hess
- Vestibulo-Oculomotor Laboratory Zürich, Department of Neurology, University HospitalZürich, Switzerland
| | - Anja K. E. Horn
- Institute of Anatomy and Cell Biology, Department I, Ludwig-Maximilians UniversityMunich, Germany
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19
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Tang X, Büttner-Ennever JA, Mustari MJ, Horn AKE. Internal organization of medial rectus and inferior rectus muscle neurons in the C group of the oculomotor nucleus in monkey. J Comp Neurol 2015; 523:1809-23. [PMID: 25684641 DOI: 10.1002/cne.23760] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2014] [Revised: 02/08/2015] [Accepted: 02/09/2015] [Indexed: 11/11/2022]
Abstract
Mammalian extraocular muscles contain singly innervated twitch muscle fibers (SIF) and multiply innervated nontwitch muscle fibers (MIF). In monkey, MIF motoneurons lie around the periphery of oculomotor nuclei and have premotor inputs different from those of the motoneurons inside the nuclei. The most prominent MIF motoneuron group is the C group, which innervates the medial rectus (MR) and inferior rectus (IR) muscle. To explore the organization of both cell groups within the C group, we performed small injections of choleratoxin subunit B into the myotendinous junction of MR or IR in monkeys. In three animals the IR and MR myotendinous junction of one eye was injected simultaneously with different tracers (choleratoxin subunit B and wheat germ agglutinin). This revealed that both muscles were supplied by two different, nonoverlapping populations in the C group. The IR neurons lie adjacent to the dorsomedial border of the oculomotor nucleus, whereas MR neurons are located farther medially. A striking feature was the differing pattern of dendrite distribution of both cell groups. Whereas the dendrites of IR neurons spread into the supraoculomotor area bilaterally, those of the MR neurons were restricted to the ipsilateral side and sent a focused bundle dorsally to the preganglionic neurons of the Edinger-Westphal nucleus, which are involved in the "near response." In conclusion, MR and IR are innervated by independent neuron populations from the C group. Their dendritic branching pattern within the supraoculomotor area indicates a participation in the near response providing vergence but also reflects their differing functional roles.
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Affiliation(s)
- Xiaofang Tang
- Institute of Anatomy and Cell Biology, Department I, Ludwig-Maximilians-University of Munich, D-80336, Munich, Germany
| | - Jean A Büttner-Ennever
- Institute of Anatomy and Cell Biology, Department I, Ludwig-Maximilians-University of Munich, D-80336, Munich, Germany
| | - Michael J Mustari
- Washington National Primate Research Center and Department of Ophthalmology, University of Washington, Seattle, Washington, 98195
| | - Anja K E Horn
- Institute of Anatomy and Cell Biology, Department I, Ludwig-Maximilians-University of Munich, D-80336, Munich, Germany
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20
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Bruenech JR, Kjellevold Haugen IB. How does the structure of extraocular muscles and their nerves affect their function? Eye (Lond) 2014; 29:177-83. [PMID: 25397785 DOI: 10.1038/eye.2014.269] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2014] [Accepted: 10/11/2014] [Indexed: 11/09/2022] Open
Abstract
The sensory and motor control of human extraocular muscles (EOMs) have been subjected to considerable speculation in ophthalmic literature, often related to infranuclear structures such as the unique complement of muscle fibres and their associated sensory organs. The intrafusal fibres do not resemble their somatic counterparts and their peculiar morphology has raised questions about their proprioceptive capacity. No Golgi tendon organs have so far been observed and the myotendinous nerve endings, previously assumed to convey sensory information, have recently been argued to merely represent constituents of the efferent innervation serving the multiply innervated muscles fibres. These observations raise questions about the overall capacity to monitor the activity created by the generous efferent nerve supply observed in these muscles. Furthermore, the argued independent activity of muscular layers and compartments suggest that the required feedback must be highly structured and more specific than previously assumed. Yet, uncertainty about the source of such information remains. The purpose of this paper is to provide a short review of neuromuscular properties of human extraocular muscles. Their functional implications and the most reputable sources of proprioception will also be discussed. The promoted views are based on pertinent literature and previous research undertaken by the authors.
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Affiliation(s)
- J R Bruenech
- Biomedical Research Unit, Faculty of Health Sciences, Buskerud and Vestfold University College, Campus Kongsberg, Kongsberg, Norway
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21
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Erichsen JT, Wright NF, May PJ. Morphology and ultrastructure of medial rectus subgroup motoneurons in the macaque monkey. J Comp Neurol 2014; 522:626-41. [PMID: 23897455 DOI: 10.1002/cne.23437] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2013] [Revised: 07/09/2013] [Accepted: 07/18/2013] [Indexed: 12/21/2022]
Abstract
There are two muscle fiber types in extraocular muscles: those receiving a single motor endplate, termed singly innervated fibers (SIFs), and those receiving multiple small terminals along their length, termed multiply innervated fibers (MIFs). In monkeys, these two fiber types receive input from different motoneuron pools: SIF motoneurons found within the extraocular motor nuclei, and MIF motoneurons found along their periphery. For the monkey medial rectus muscle, MIF motoneurons are found in the C-group, while SIF motoneurons lie in the A- and B-groups. We analyzed the somatodendritic morphology and ultrastructure of these three subgroups of macaque medial rectus motoneurons to better understand the structural determinants controlling the two muscle fiber types. The dendrites of A- and B-group motoneurons lay within the oculomotor nucleus, but those of the C-group motoneurons were located outside the nucleus, and extended into the preganglionic Edinger-Westphal nucleus. A- and B-group motoneurons were very similar ultrastructurally. In contrast, C-group motoneurons displayed significantly fewer synaptic contacts on their somata and proximal dendrites, and those contacts were smaller in size and lacked dense-cored vesicles. However, the synaptic structure of C-group distal dendrites was quite similar to that observed for A- and B-group motoneurons. Our anatomical findings suggest that C-group MIF motoneurons have different physiological properties than A- and B-group SIF motoneurons, paralleling their different muscle fiber targets. Moreover, primate C-group motoneurons have evolved a special relationship with the preganglionic Edinger-Westphal nucleus, suggesting these motoneurons play an important role in near triad convergence to support increased near work requirements.
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Affiliation(s)
- Jonathan T Erichsen
- School of Optometry and Vision Sciences, Cardiff University, Cardiff, CF24 4HQ, Wales, UK
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