Neuromuscular junctions in slow- and fast-twitch muscle

  • Dr Guy Bewick, Aberdeen Univ.
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Neuromuscular junctions in slow- and fast-twitch muscle. Dr Guy Bewick, Aberdeen Univ.. Attribution-NonCommercial 4.0 International (CC BY-NC 4.0). Source: Wellcome Collection.

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A confocal micrograph of living nerve terminals on rat skeletal muscle fibre. It shows immature (top, 3 weeks after birth) and mature (bottom, 18 weeks after birth) neuromuscular junctions on soleus (slow-twitch, postural) and extensor digitorum longus (EDL, fast -twitch and non-postural) rat muscle fibres. Both types of NMJs increase in size with age but, while in EDL they remain ovoid, those in soleus elongate. Fluorescently labelled alpha-bungarotoxin, a snake toxin, has been used to visualise the regions of highly concentrated receptor proteins on the musce fibre surface. These align precisely with branches of the overlying motor nerve terminal. When activated, the terminal releases chemicals which bind onto the receptors, making the muscle fibre contract. The lower panel shows how the ratio of length to breadth in labelled NMJs diverges with age in the two muscle types.

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