The Neuromuscular Junction and synaptic excitation

The motor end plate is a good example to use for synaptic excitation because the resultant effect can be seen/measured etc as muscle contraction.

The motor end plate

  • The first thing to note is that post-synaptic zones (including the motor end plate) do NOT (for learning purposes) contain any voltage gated channels.  The post-synaptic membrane does contain transmitter-receptor channels
    • In the case of the motor end plate, the transmitter is acetylcholine and thus acetylcholine receptors (nicotinic receptors)
      • (Nicotinic receptors and pentameric and require binding of two ACh molecules for a conformational change)
      • In myaesthenia gravis, there is destruction of these receptors
      • Curare and botulinum blocks these receptors
    • Opening of these channels allow flow of both Na AND K into and out of the cell (respectively).  Because the resting potential is much closer to the equilibrium potential of K, the electrochemical gradient for the movement of Na is greater, and more Na moves into the cell, causing depolarisation.
      • This depolarisation is the end-plate potential
    • The depolarisation of the membrane spreads to nearby segments of the cell (i.e. NOT post-synaptic segments) that DO contain voltage gated channels.  When the end plate potential reaches threshold, these open and generate an action potential.
VOC: Voltage opening channels; ROC: Receptor opening channels
  • Acetycholine is broken down in the synaptic cleft by acetylcholinesterase, and the signal stops.

2 thoughts on “The Neuromuscular Junction and synaptic excitation”

  1. Because the resting potential is much closer to the equilibrium potential of K, the electrochemical gradient for the movement of Na is greater, and more Na moves into the cell, causing depolarisation.
    This depolarisation is the end-plate potential

    I need to understand this

    1. The resting membrane potential of most cells is always negative (varies between cells but in general around -70mV- this is mainly due to the Na/K/ATPase pump), and the equilibrium potentials of K is -80mV; Na +60mV. The concentrations of Na is also much higher outside of the cell. Therefore, both electrical and chemical gradients drive Na into the cell during depolarisation.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: