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The Motor End-Plate 1.

Purpose:

The motor end-plate is the connection between the motor nerve and the skeletal muscle cell.

 

A. Definitions and Structural components required:

  1. A presynaptic membrane in the distal part of the axon
  2. This presynaptic membrane contains Na+, K+ and Ca2+ ion channels.
  3. Vesicles in the presynaptic cell that contains the (neuro) transmitter.
  4. Neuro-transmitter. In the motor endplate, the neuro-transmitter is always acetylcholine (ACh)
  5. A postsynaptic membrane of the muscle cell. These are typically folded (guess why?)
  6. Receptor operated channels located in the postsynaptic membrane

 

 

 

B. Functional Steps :

1.

A nerve action potential propagates down the axon towards the pre-synaptic membrane (shown in red in the diagram).

2.

The action potential, in this last part of the axon, opens the Ca2+ channels.

3.

Because of the concentration gradient (less calcium inside and more outside), calcium ions will flow into the cell.

4.

This intracellular calcium will induce one of the vesicles to move towards the pre-synaptic membrane.

5.

Once at the pre-synaptic membrane, the vesicles will fuse with the membrane and release its content (acetylcholine) into the synaptic cleft. This process is called exocytosis.

6.

The transmitter molecules (Acetycholine = ACh) diffuses through the synaptic cleft and some will reach the post-synaptic membrane.

small size sketch of a motor end-plate

7.

These acetylcholine molecules will then couple to the specific receptors (ACh-receptors) located in the post-synaptic membrane.

8.

The ACh-receptors are linked to ion channels (=receptor operated channels = ROC).

9.

The coupling of Acetylcholine to the ACh-receptor will open that particular channel.

10.

As more and more transmitters attach to the receptors, more and more channels will open.

11.

Opening of the channels will cause a flow of Na+ ions into the post-synaptic cell (=influx). (why?)

12.

These positive ions will cause a depolarization around that membrane.

13.

This potential is called a generator potential.

14.

When the generator potential reaches threshold, an action potential is generated.

15.

The action potential, once initiated, will propagate along the muscle membrane all around the cell. and into the transversal tubuli.

16.

This will start the process of contractions in the sarcomeres (sarcomere).

 

 

 

C. Acetylcholinesterase (=AChE):

1.

In the synaptic cleft, between the pre- and post-synaptic cleft, there is an enzyme, called acetylcholinesterase (= AchE)

2.

This acetylcholinesterase breaks down the acetylcholine.

3.

This breaking-down is a necessary step to stop the ACh from coupling continuously to the ACh-receptors.

4.

Without this enzyme, the post-synaptic membrane would be constantly depolarized (= above threshold) which will no longer induce new action potentials.

 

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