Neuromuscular Junction (NMJ) Structure and Action

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Neuromuscular Junction (NMJ) Structure and Action

Neuromuscular Junction (NMJ) Structure and Action



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A neuromuscular junction (or myoneural junction) is a chemical synapse formed by the contact between a motor neuron and a muscle fiber.[1] It is at the neuromuscular junction that a motor neuron is able 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. Synaptic transmission at the neuromuscular junction begins when an action potential reaches the presynaptic terminal of a motor neuron, which activates voltage-dependent 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 Duchenne muscular dystrophy, 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.

The neuromuscular junction differs from chemical synapses between neurons. Presynaptic motor axons stop 30 nanometers from the sarcolemma, the cell membrane of a muscle cell. This 30-nanometer space forms the synaptic cleft through which signalling molecules are released. The sarcolemma has invaginations called postjunctional folds, which increase the surface area of the membrane exposed to the synaptic cleft.[2] These postjunctional folds form what is referred to as the motor endplate, which possess nicotinic acetylcholine receptors (nAChRs) at a density of 10,000 receptors/micrometer2 in skeletal muscle.[3] The presynaptic axons form bulges called terminal boutons (or presynaptic terminals) that project into the postjunctional folds of the sarcolemma. The presynaptic terminals have active zones that contain vesicles, quanta, full of acetylcholine molecules. These vesicles can fuse with the presynaptic membrane and release ACh molecules into the synaptic cleft via exocytosis after depolarization.[2] AChRs are localized opposite the presynaptic terminals by protein scaffolds at the postjunctional folds of the sarcolemma. Dystrophin, a structural protein, connects the sarcomere, sarcolemma, and extracellular matrix components. Rapsyn is another protein that docks AChRs and structural proteins to the cytoskeleton. 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.[2]

Mechanism of action
The neuromuscular junction is where a neuron activates a muscle to contract. Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca2+ ions flow from the extracellular fluid into the presynaptic neuron’s cytosol. This influx of Ca2+ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron’s cell membrane through SNARE proteins. Fusion of the vesicular membrane with the presynaptic cell membrane results in the emptying of the vesicle’s contents (acetylcholine) into the synaptic cleft, a process known as exocytosis. Acetylcholine diffuses into the synaptic cleft and can bind to the nicotinic acetylcholine receptors on the motor endplate.

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50 Comments

  • Posted on Sep 06, 2018 at 16:12 pm

    You are awesome awesome no words to say please keep making videos it gives me relief from pressure of study

  • Posted on Sep 29, 2018 at 18:05 pm

    Thank u 🤗🤗

  • Posted on Sep 29, 2018 at 19:27 pm

    Your voice is so kind and calm. It's lovely to listen to you.

  • Posted on Oct 10, 2018 at 02:35 am

    Pretty sure the acetylcholine receptors are ligand gated not voltage gated and they also let potassium pass through as well.

  • Posted on Oct 28, 2018 at 16:17 pm

    Loved the video, it's short and to the point 👍

  • Posted on Nov 02, 2018 at 10:25 am

    Ligand gated sodium channels daddy

  • Posted on Dec 05, 2018 at 01:19 am

    Gr8 video
    I appreciate the simplicity in your video
    Others videos just make it too complex to understand for a layman

  • Posted on Jan 06, 2019 at 08:25 am

    mn

  • Posted on Jan 30, 2019 at 18:58 pm

    Ach receptors are ligand gated sodium channels not voltage gated…it allows only ions to influx in post snaptic membrane.

  • Posted on Apr 01, 2019 at 08:11 am

    This ia awesome!!😎😎

  • Posted on Apr 11, 2019 at 05:42 am

    Superbbbbbb

  • Posted on Apr 21, 2019 at 07:14 am

    Thanks sir it helped me a lot 🙏

  • Posted on Jul 18, 2019 at 11:08 am

    Great and clear explanation

  • Posted on Sep 17, 2019 at 06:21 am

    Thanks a lot..it was quite helpful…

  • Posted on Nov 22, 2019 at 17:51 pm

    Nice video sir

  • Posted on Dec 05, 2019 at 05:47 am

    Wow,pretty good at the understanding.

  • Posted on Dec 10, 2019 at 08:06 am

    Sir aapne diagram bnaya y bna kr labelling kr du to itna sufficient h

  • Posted on Jan 09, 2020 at 17:04 pm

    Thank you !!!!! ❤❤❤❤

  • Posted on Feb 21, 2020 at 08:50 am

    Thank u sir very easy and clear explanation with drawing it helped me lot.

  • Posted on Feb 28, 2020 at 07:14 am

    Thanks a lot sir for kind information about neuromuscular junction . I,m from india

  • Posted on Mar 22, 2020 at 05:49 am

    Thank you for this video! Very clear and concise. Take care, be well

  • Posted on May 18, 2020 at 03:57 am

    Ach receptors are ligand gated Na + ion channel not the voltage gated Na + ion channel

  • Posted on Jun 18, 2020 at 11:19 am

    Thnku sir

  • Posted on Jun 24, 2020 at 15:54 pm

    Thanks sir this video very simple and helpful

  • Posted on Aug 02, 2020 at 10:05 am

    I have a question, does this mean that calcium is needed for acetylcholine realease and also for the contraction of muscles?, cause u said calcium causes release of snare proteins which cause exocytosis of synaptic vessicles releasing acetylcholine
    Please expantiate

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  • Posted on Oct 13, 2020 at 09:12 am

    thank youuu understandable enoughh

  • Posted on Nov 24, 2020 at 16:38 pm

    Thank u so much i was dying

  • Posted on Dec 06, 2020 at 06:23 am

    👍

  • Posted on Dec 21, 2020 at 07:31 am

    Thnx for videos

  • Posted on Jan 23, 2021 at 10:39 am

    Wave TQ sir it's very easy, clear,clean explanation with diagram nd intresting

  • Posted on Feb 03, 2021 at 17:51 pm

    Amazing💡

  • Posted on Feb 12, 2021 at 08:59 am

    Thanku

  • Posted on Feb 15, 2021 at 05:01 am

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  • Posted on Feb 28, 2021 at 13:04 pm

    THANKYOU IT HELPS A LOT DURING QUICKREVISION

  • Posted on Mar 31, 2021 at 14:34 pm

    You're beautiful Pooja

  • Posted on Apr 08, 2021 at 11:43 am

    Lovely video sir
    The voice was in 0.25x and video was in 1.5x😂
    Anyways lovely explanation sir 🙏

  • Posted on Apr 16, 2021 at 01:33 am

    Please upload more videos please

  • Posted on Apr 25, 2021 at 08:36 am

    Sir your voice is so angelic and calming
    Thanks sir😊💜🤩😇

  • Posted on May 02, 2021 at 04:50 am

    Thanq sir it is very clear and concise
    Keep on like this and send more vedios like these in easy way .
    And this is very useful to many students thanq sir

  • Posted on May 31, 2021 at 09:15 am

    That was really nice sir

  • Posted on Jul 23, 2021 at 07:49 am

    Thanks sir keep it up 👍

  • Posted on Aug 10, 2021 at 00:22 am

    Nice vdeo helpful sir

  • Posted on Aug 28, 2021 at 10:11 am

    Thank u sir

  • Posted on Sep 16, 2021 at 02:54 am

    Thanks 😊 bhai

  • Posted on Oct 30, 2021 at 01:35 am

    This is really excellent explanation ❣️❣️❣️. Thank's a lot ✨✨✨

  • Posted on Nov 15, 2021 at 13:30 pm

    Alhamdulillah… Many many thanks..

  • Posted on Nov 24, 2021 at 13:38 pm

    Sir 🔥🔥🔥🔥🔥

  • Posted on Jan 03, 2022 at 11:44 am

    Thank u soooo much

  • Posted on Jan 04, 2022 at 15:16 pm

    Is that Diagram of Synapse and NMJ Ar Same or ya different??? Can anyone pls tell???

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