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Neuromuscular junction and transmission

The neuromuscular junction is the synaptic connection between a motor neuron terminal and a muscle fiber. Here, action potential is transmitted from the neuron terminal to the muscle, initiating muscle contraction as described in the following steps:

  1. A muscle contraction is initiated by the arrival of a nervous signal reaching the synapse of a neuromuscular junction.

  2. Acetylcholine is released from synaptic vesicles and binds to receptors on the muscle cell, initiating an electrical signal in the muscle cell membrane.

  3. The electrical signal spread across the muscle cell membrane and enters into T-tubules, which are the invaginations of the plasma membrane.

  4. The electrical signal spread from the T-tubules to the sarcoplasmic reticulum where calcium is released into the cytoplasm of the muscle cell.

  5. Calcium binds to the troponin molecule, initiating the muscle contraction (see sliding filament theory).

Visualisation of 5 steps from nerve signal to muscle contraction. Image number 1 presents the general view on the muscle cell presented cross section of pink tube, to which nerve cells are connected by synapse, shown here as yellow, branched stripes. Image number two shows the synapse, muscle cell close up, in which tiny red spheres are released from the end of the yellow stripe, and they travel through the cell membrane using transport channels. Image number three presents the invagination in that cell membrane, and arrows presenting the path of the signal, coming from the yellow stripe to the inside of the invagination, which end is adherent to dark yellow rectangular structure, filled with tiny white sphere. Image number 4 presents the release of those tiny white spheres from the rectangular shaped structure down towards the image number five, where they bind to green spheres aligned next to each other in the helix shape. To few of those green spheres, purple claw like structure is bound, and it extends from dark purple, horizontal, tube like structure. To this claw like structure, one molecule of ADP and phosphate group is attached.

Figure 1. Diagram showing the transition of action potential at a neuromuscular junction.

References: - "Contraction of a Muscle Fiber " © 1999-2020, Rice University. Licensed under a Creative Commons Attribution 4.0 International License