what neurotransmitter is released at the neuromuscular junction
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Neuromuscular junction
Neuromuscular junction
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Neuromuscular junction
Electron micrograph showing a cross section through the neuromuscular junction. T is the axon terminal, M is the muscle fiber. The arrow shows junctional folds with basal lamina. Active zones are visible on the tips between the folds. Scale is 0.3 μm. Source: NIMH
Detailed view of a neuromuscular junction:
Presynaptic terminal
Sarcolemma Synaptic vesicle
Nicotinic acetylcholine receptor
Mitochondrion Details Identifiers Latin MeSH D009469 TH H2.00.06.1.02001 FMA 61803
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At the neuromuscular junction, the nerve fiber is able to transmit a signal to the muscle fiber by releasing ACh (and other substances), causing muscle contraction.
Muscles will contract or relax when they receive signals from the nervous system. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows:(1) The action potential reaches the axon terminal. (2) Voltage-dependent calcium gates open, allowing calcium to enter the axon terminal. (3) Neurotransmitter vesicles fuse with the presynaptic membrane and ACh is released into the synaptic cleft via exocytosis. (4) ACh binds to postsynaptic receptors on the sarcolemma. (5) This binding causes ion channels to open and allows sodium and other cations to flow across the membrane into the muscle cell. (6) The flow of sodium ions across the membrane into and potassium ions out of the muscle cell generates an action potential which travels to the myofibril and results in muscle contraction.Labels:A: Motor Neuron AxonB: Axon TerminalC. Synaptic CleftD. Muscle CellE. Part of a Myofibril
A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber.[1]
It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction.
Muscles require innervation to function—and even just to maintain muscle tone, avoiding atrophy. In the neuromuscular system nerves from the central nervous system and the peripheral nervous system are linked and work together with muscles.[2] Synaptic transmission at the neuromuscular junction begins when an action potential reaches the presynaptic terminal of a motor neuron, which activates voltage-gated calcium channels to allow calcium ions to enter the neuron. Calcium ions bind to sensor proteins (synaptotagmin) on synaptic vesicles, triggering vesicle fusion with the cell membrane and subsequent neurotransmitter release from the motor neuron into the synaptic cleft. In vertebrates, motor neurons release acetylcholine (ACh), a small molecule neurotransmitter, which diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the cell membrane of the muscle fiber, also known as the sarcolemma. nAChRs are ionotropic receptors, meaning they serve as ligand-gated ion channels. The binding of ACh to the receptor can depolarize the muscle fiber, causing a cascade that eventually results in muscle contraction.
Neuromuscular junction diseases can be of genetic and autoimmune origin. Genetic disorders, such as Congenital myasthenic syndrome, can arise from mutated structural proteins that comprise the neuromuscular junction, whereas autoimmune diseases, such as myasthenia gravis, occur when antibodies are produced against nicotinic acetylcholine receptors on the sarcolemma.
Contents
1 Structure and function
1.1 Quantal transmission
1.2 Acetylcholine receptors
2 Development 3 Research methods
4 Toxins that affect the neuromuscular junction
4.1 Nerve gases 4.2 Botulinum toxin 4.3 Tetanus toxin 4.4 Latrotoxin 4.5 Snake venom 5 Diseases 5.1 Autoimmune
5.1.1 Myasthenia gravis
5.1.1.1 Neonatal MG
5.1.2 Lambert-Eaton myasthenic syndrome
5.1.3 Neuromyotonia 5.2 Genetic
5.2.1 Congenital myasthenic syndromes
6 See also 7 External links 8 Further reading 9 References
Structure and function[edit]
Motor Endplate
Quantal transmission[edit]
At the neuromuscular junction presynaptic motor axons terminate 30 nanometers from the cell membrane or sarcolemma of a muscle fiber. The sarcolemma at the junction has invaginations called postjunctional folds, which increase its surface area facing the synaptic cleft.[3] These postjunctional folds form the motor endplate, which is studded with nicotinic acetylcholine receptors (nAChRs) at a density of 10,000 receptors/micrometer2.[4] The presynaptic axons terminate in bulges called terminal boutons (or presynaptic terminals) that project toward the postjunctional folds of the sarcolemma. In the frog each motor nerve terminal contains about 300,000 vesicles, with an average diameter of 0.05 micrometers. The vesicles contain acetylcholine. Some of these vesicles are gathered into groups of fifty, positioned at active zones close to the nerve membrane. Active zones are about 1 micrometer apart. The 30 nanometer cleft between nerve ending and endplate contains a meshwork of acetylcholinesterase (AChE) at a density of 2,600 enzyme molecules/micrometer2, held in place by the structural proteins dystrophin and rapsyn. Also present is the receptor tyrosine kinase protein MuSK, a signaling protein involved in the development of the neuromuscular junction, which is also held in place by rapsyn.[3]
Glutamate at the Vertebrate Neuromuscular Junction: From Modulation to Neurotransmission
Although acetylcholine is the major neurotransmitter operating at the skeletal neuromuscular junction of many invertebrates and of vertebrates, glutamate participates in modulating cholinergic transmission and plastic changes in the last. Presynaptic ...
Cells. 2019 Sep; 8(9): 996.
Published online 2019 Aug 28. doi: 10.3390/cells8090996
PMCID: PMC6770210 PMID: 31466388
Glutamate at the Vertebrate Neuromuscular Junction: From Modulation to Neurotransmission
Maria Nicol Colombo and Maura Francolini*
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This article has been cited by other articles in PMC.
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Abstract
Although acetylcholine is the major neurotransmitter operating at the skeletal neuromuscular junction of many invertebrates and of vertebrates, glutamate participates in modulating cholinergic transmission and plastic changes in the last. Presynaptic terminals of neuromuscular junctions contain and release glutamate that contribute to the regulation of synaptic neurotransmission through its interaction with pre- and post-synaptic receptors activating downstream signaling pathways that tune synaptic efficacy and plasticity. During vertebrate development, the chemical nature of the neurotransmitter at the vertebrate neuromuscular junction can be experimentally shifted from acetylcholine to other mediators (including glutamate) through the modulation of calcium dynamics in motoneurons and, when the neurotransmitter changes, the muscle fiber expresses and assembles new receptors to match the nature of the new mediator. Finally, in adult rodents, by diverting descending spinal glutamatergic axons to a denervated muscle, a functional reinnervation can be achieved with the formation of new neuromuscular junctions that use glutamate as neurotransmitter and express ionotropic glutamate receptors and other markers of central glutamatergic synapses. Here, we summarize the past and recent experimental evidences in support of a role of glutamate as a mediator at the synapse between the motor nerve ending and the skeletal muscle fiber, focusing on the molecules and signaling pathways that are present and activated by glutamate at the vertebrate neuromuscular junction.
Keywords: neuromuscular junction, glutamate, acetylcholine, neurotransmitter, receptor, transporterGo to:
1. Role of Glutamate as Modulator of Cholinergic Transmission and Plasticity at the Skeletal Neuromuscular Junction of Vertebrates
Acetylcholine (ACh) is the principal neurotransmitter at the vertebrate neuromuscular junction (NMJ), however since the discovery that motoneurons and presynaptic terminals of rodent endplates from the hindlimb muscles extensor digitorum longus (EDL) and soleus are positive for glutamate labelling [1,2], it has been proposed that glutamate may participate in modulating cholinergic transmission at the NMJ of vertebrates.
A role of glutamate as modulator of cholinergic transmission and plasticity was initially suggested by the observation that its concentration was higher in the fast twitch EDL than in the slow twitch soleus muscle NMJ [2], and by more recent observations showing that glutamate immunoreactivity was higher among slow muscle fibers with respect to intermediate and fast axial muscles in adult zebrafish in control conditions, and that only in fast muscles its level was increased upon exercise training and/or during nerve/muscle regeneration following injury [3].
1.1. Glutamate Transporters at the Neuromuscular Junction in Vertebrates
The precise localization of glutamate in motoneurons and specifically at presynaptic terminals of NMJs remains elusive, and its presence within synaptic vesicles has never been unambiguously demonstrated [2]. Data supporting the presence of vesicular glutamate transporters at the NMJ and cholinergic synapses are rather contradictory [1,4,5,6,7,8]. Finally, besides the vesicular glutamate transporters, other molecules involved in handling glutamate at central synapses, namely glial glutamate transporters, are present at the NMJ in rodents, supporting the notion that glutamate is released at the motor endplate.
1.2. Vesicular Glutamate Transporters
Three main types of vesicular glutamate transporters are expressed in the excitatory glutamatergic synapses in the central nervous system (CNS) of vertebrates (VGLUTs 1–3); these transporters fill synaptic vesicle with the neurotransmitter thanks to the proton gradient across the vesicle membrane. Evidences showing the presence of vesicular glutamate transporters at NMJs are rather scanty and somehow contradictory: Herzog and colleagues demonstrated that rat spinal motoneurons express the vesicular glutamate transporters 1 and 2 (VGLUT1 and VGLUT2) but these are confined at the central nerve terminals that contact Renshaw inhibitory interneurons and they are not found at neuromuscular synapses [5]. Indeed, at central synapses, mammalian motoneurons corelease ACh and glutamate [7].
Conversely, NMJs immunoreactive for VGLUT1 were found in mouse striated esophageal muscles [6,9], while NMJs of other somitic and branchiogenic muscles (i.e., soleus, tibilias anterior, masseter) were negative for all the three isoforms of the transporter [6]. Recently, VGlut1 (but not VGlut2) positive puncta, in close proximity to vesicular ACh transporter (VAChT) positive areas, were observed in adult zebrafish motoneurons innervating the lateral axial muscle [3].
On the other hand, it was shown that VGLUT3 was present in adult rat skeletal muscle homogenates, and VGLUT3 like-immunoreactivity was found at the NMJs of the soleus, close to both ACh vesicular transporter (VAChT) positive areas and the ACh receptor clusters [4]. Strikingly, the coexistence of VGLUT1 and 2 and of the VAChT (and of nucleotide transporters) was reported on a high proportion of synaptic vesicles from the torpedo electric organ, a structure that is considered a classical model of cholinergic neurotransmission [8].
neuromuscular junction
neuromuscular junction, also called myoneural junction, site of chemical communication between a nerve fibre and a muscle cell. The neuromuscular junction is analogous to the synapse between two neurons. A nerve fibre divides into many terminal branches; each terminal ends on a region of muscle fibre called the end plate. Embedded in the end plate are thousands of receptors, which are long protein molecules that form channels through the membrane. Upon stimulation by a nerve impulse, the terminal releases the chemical neurotransmitter acetylcholine from synaptic vesicles. Acetylcholine then binds to the receptors, the channels open, and sodium ions flow into
neuromuscular junction
neuromuscular junction
biochemistry
Alternate titles: myoneural junction
By The Editors of Encyclopaedia Britannica • Edit History
Related Topics: action potential end-plate potential summation miniature end-plate potential temporal summation
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neuromuscular junction, also called myoneural junction, site of chemical communication between a nerve fibre and a muscle cell. The neuromuscular junction is analogous to the synapse between two neurons. A nerve fibre divides into many terminal branches; each terminal ends on a region of muscle fibre called the end plate. Embedded in the end plate are thousands of receptors, which are long protein molecules that form channels through the membrane. Upon stimulation by a nerve impulse, the terminal releases the chemical neurotransmitter acetylcholine from synaptic vesicles. Acetylcholine then binds to the receptors, the channels open, and sodium ions flow into the end plate. This initiates the end-plate potential, the electrical event that leads to contraction of the muscle fibre.The Editors of Encyclopaedia Britannica
This article was most recently revised and updated by Adam Augustyn.
tissue
tissue
biology
Alternate titles: tissue system
By The Editors of Encyclopaedia Britannica • Edit History
xylem; Scots pine See all media
Key People: Joseph E. Murray Ross Granville Harrison Sir Peter B. Medawar Jacques Loeb Marie-François-Xavier Bichat
Related Topics: blood connective tissue fluid lymph epithelium
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tissue, in physiology, a level of organization in multicellular organisms; it consists of a group of structurally and functionally similar cells and their intercellular material.By definition, tissues are absent from unicellular organisms. Even among the simplest multicellular species, such as sponges, tissues are lacking or are poorly differentiated. But multicellular animals and plants that are more advanced have specialized tissues that can organize and regulate an organism’s response to its environment.
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Plants
Bryophytes (liverworts, hornworts, and mosses) are nonvascular plants; i.e., they lack vascular tissues (phloem and xylem) as well as true leaves, stems, and roots. Instead bryophytes absorb water and nutrients directly through leaflike and stemlike structures or through cells comprising the gametophyte body.
In vascular plants, such as angiosperms and gymnosperms, cell division takes place almost exclusively in specific tissues known as meristems. Apical meristems, which are located at the tips of shoots and roots in all vascular plants, give rise to three types of primary meristems, which in turn produce the mature primary tissues of the plant. The three kinds of mature tissues are dermal, vascular, and ground tissues. Primary dermal tissues, called epidermis, make up the outer layer of all plant organs (e.g., stems, roots, leaves, flowers). They help deter excess water loss and invasion by insects and microorganisms. The vascular tissues are of two kinds: water-transporting xylem and food-transporting phloem. Primary xylem and phloem are arranged in vascular bundles that run the length of the plant from roots to leaves. The ground tissues, which comprise the remaining plant matter, include various support, storage, and photosynthetic tissues.
Secondary, or lateral, meristems, which are found in all woody plants and in some herbaceous ones, consist of the vascular cambium and the cork cambium. They produce secondary tissues from a ring of vascular cambium in stems and roots. Secondary phloem forms along the outer edge of the cambium ring, and secondary xylem (i.e., wood) forms along the inner edge of the cambium ring. The cork cambium produces a secondary dermal tissue (periderm) that replaces the epidermis along older stems and roots.
Animals
Early in the evolutionary history of animals, tissues became aggregated into organs, which themselves became divided into specialized parts. An early scientific classification of tissues divided them on the basis of the organ system of which they formed a part (e.g., nervous tissues). Embryologists have often classified tissues on the basis of their origin in the developing embryo; i.e., ectodermal, endodermal, and mesodermal tissues. Another method classified tissues into four broad groups according to cell composition: epithelial tissues, composed of cells that make up the body’s outer covering and the membranous covering of internal organs, cavities, and canals; endothelial tissues, composed of cells that line the inside of organs; stroma tissues, composed of cells that serve as a matrix in which the other cells are embedded; and connective tissues, a rather amorphous category composed of cells and an extracellular matrix that serve as a connection from one tissue to another.
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