Neuromuscular Junction | Neuromuscular Transmission

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In this blog, we will learn about Neuromuscular junction and Neuromuscular transmission.

First we learn about Neuromuscular junction.
Neuro = neuron
muscular = muscles
So, this word simply means it is connection between neuron and muscles.

Neuromuscular Junction Definition 

Neuromuscular junction is the junction between terminal branch of the nerve fiber and muscle fiber. 

Our muscles can not work without this junction. We can not raise hand or can not open eyes if this junction not work.

When brain or spinal cord send message to raise hand. It comes upto muscles through neuron to neuron. Finally last neuron give this message to particular muscle through this junction. So, this topic is very important and interesting.

To understand this junction, we need to understand structure of neuron and muscles.

The structure of neuron includes dendrites, soma (body) & axon. The last part of axon is called axon terminal where axon devides into so many branches. This branches of axon comes in contact with muscles fiber. 

 

If we zoom junction, it looks like as shown in image. The axon part contents synaptic vessicles which are full of chemical. It has mitochondria which gives power to make process. Space between nerve fiber and muscle fiber is known as synaptic cleft. This is the whole understanding of Neuromuscular junction. Now, we will learn about transmission. It means that how impulse transmit from nerve fiber to muscle.

 

 

Neuromuscular Transmission

Neuromuscular transmission is defined as the transfer of information from motor nerve ending to the muscle fiber through neuromuscular junction. It is the mechanism by which the motor nerve impulses initiate muscle contraction.

Events of Neuromuscular Transmission

  1. Action potential reach to axon terminal
  2. Entry of Ca+ in axon terminal
  3. Release of Acetylcholine 
  4. Acetylcholine receptor complex
  5. Development of endplate potential
  6. Development of action potential
  7. Destruction of acetylcholine

We will learn transmission in these 7 stages with images. So, it will be easier to understand.

  1. Action potential reach to axon terminal

    For this transmission, it is important that one message reaches to axon terminal. It is known as action potential.

  2. Entry of Ca+ in axon terminal

    When action potential reaches to axon terminal, it opens the voltage gated calcium channels in the membrane of axon terminal. Calcium ions from extracellular fluid (ECF) enter the axon terminal.

  3. Release of Acetylcholine

    action potential —> Ca+ entry —> Ach release

    There are many synaptic vessicles in the axon terminal, which are full of chemical named acetylcholine (Ach). When calcium ion enters in the axon terminal, these synaptic vessicles becomes weak. At the movement, chemical acetylcholine (Ach) releases from this vessicles.

  4. Acetylcholine receptor complex

    By exocytosis, acetylcholine diffuses through the presynaptic membrane and enters the synaptic cleft. It means that Ach comes in between the nerve fiber and muscle fiber. Now, Acetylcholine (Ach) makes joint with receptor in postsynaptic membrane and form acetylcholine-receptor complex.

  5. Development of endplate potential

    After forming this Ach-receptor complex, it increases the permeability of postsynaptic membrane for sodium by opening the ligand-gated sodium channels. Now, sodium ions from ECF enter the neuromuscular junction through these channels. And there, sodium ions alter the resting membrane potential and develops the electrical potential called the endplate potential.

  6. Development of endplate potential to action potential

    Endplate potential is the changes in resting membrane potential when an impulse reaches the neuromuscular junction. Resting membrane potential at neuromuscular junction is -90 mV. When sodium ions enter inside, slight depolarization occurs up to -60 mV, which is called endplate potential.

    Ach-receptor complex —> Na+ entry —> action potential

    Miniature endplate potential cannot produce action potential in the muscles. When more and more quantity of acetylcholine are release continuously, the miniature endplate potentials are sum up together and finally produce endplate potential resulting in action potential in the muscle.

  7. Destruction of acetylcholine

    After releasing of the acetylcholine, it destroy within one millisecond by acetylcholinesterase enzyme. It means that this chemical works in very short time & destroy very quickly.

    The acetylcholine is so potent, that even this short duration of 1 millisecond is sufficient to excite the muscle fiber. Rapid destruction of acetylcholine has significant importance. Also it prevents the repeated excitation of the muscle fiber and allows the muscle to relax.

This way muscle contraction occurs. If you think to blink your eyes or want to raise your hand. These all steps needed to be function. 

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