The Motor End-Plate 2.

A few additional points:


D. Receptor Operated Channels and Voltage Operated Channels:

Receptor-operated channels (ROC) are 'operated' by the transmitter coupling to the receptor.

Voltage-operated channels (VOC) are 'operated' by voltage (this is the voltage across the membrane; which, at rest, it positive inside and positive outside).

Step 1: Acetylcholine couples to the receptor and thereby opens the receptor operated channels.

Step 2: This will cause an influx of Na+ ions into the cell and depolarize the resting potential. This is called the generator potential

Step 3: Once the generator potential crosses the threshold, the voltage operated channels, which are located in the neighbouring membrane, will open.

Step 4: The opening of these channels will induce a rapid Na+ influx and the initiation of an action potential.




E. Comparing a generator potential to an action potential:

There are several important differences between a generator potential and an action potential:

  1. the generator potential does not propagate and is therefore a local phenomenon whereas the action potentials can and do propagate.
  2. A generator potential is graded; it is small when few transmitters are coupled to their receptors and becomes larger when a lot of transmitters are attached (so, not the famous all-or-none law!).
  3. Therefore, if the generator potential does not reach the threshold, then there will be no action potential.
  4. The generator potential also does not have a refractory period whereas the action potential does.


F. Comparing a motor-end plate to a synaps:

The function of the motor endplate is very similar to that of a chemical synaps but there are some important differences:

  1. the post-synaptic membrane is folded. This is to increase the surface area and therefore the number of receptors (in the synaps the post-synaptic membrane is not folded)
  2. the post-synaptic membrane always depolarizes and never hyperpolarizes (in the synaps it can also hyperpolarize)
  3. there is only one type of transmitter: Acetycholine (in synapses in the brain, there are many types of transmitters)
  4. transmission from nerve to muscle is always successful (in the chemical synaps, successful transmission depends on the number of EPSP’s and IPSP’s generated).


G. Curare and Curare-mimetica :


It is important to realize that the breaking-down of the transmitter takes place in the synaptic cleft. This is actually extracellular space (= outside the cell).


This makes it easy to influence this mechanism by poisons or drugs.


For example, curare is a well-known poison that competes with acetylcholine (ACh) to occupy the receptor-operated channels.


But, in contrast to ACh, it does not open the channels and therefore does not induce a generator potential.


As curare does not induce a generator potential, it does not induce action potentials and therefore no contractions. Effectively, curare induces paralysis.


Curare was used by the South American Indians to kill their preys (and humans too!). The question now is, why does curare kill a mammalian organism (like humans)?


At this point, students will often say that curare also blocks the heart.


But the heart does not have motor-end plates (it is an autonomic organ). So curare can not block these non-existing ACh-receptors.


But the respiration in the body is done by the muscles of the chest. And these are skeletal muscles.


So, curare paralyses the muscles of the respiration and therefore the unlucky prey is killed by suffocation.


Nowadays, the pharmaceutical industry has developed curera-mimetica (= works like curare).


With these drugs, one can temporarily block all skeletal muscle activity. This is useful for example during surgery.


But then of course, one must also make sure that the respiration is not stopped. This respiration is taken over by a mechanical ventilator.


Probably, even more important, is to make the patient unconscious.


Because, if the patient during surgery is not unconscious, he is effectively paralysed, and cannot scream, move or talk!


This is the stuff of nightmares!


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