Respiratory timescale

As noted above, ACh produces an electrical impulse at the post-junctional membrane of the chohnergic synapse which is either transmitted on to another neurone or to smooth or skeletal muscle causing contraction. This occurs by binding of the ACh molecules at special receptor sites which control entiy of ions into a channel. At the NMJ, there are two such sites at the entrance of an ion channel in the post-junctional membrane. When ACh combines with these, the channel is opened and there is an ion flow through it leading to depolarisation of the membrane. In the case of NMJ, this causes the associated muscle fibre to contract. The action of the ACh is terminated by AChE. If, however, the ACh receptor sites are blocked, the transmitter cannot attach to them and depolarisation cannot take place. Toxic substances such as curare, alpha-bungarotoxin from krait snakes and black widow spider venom can do this. The overall main chnical effect is the same as for BoTx, namely a paralysis which leads to respiratory failure and arrest. However, the timescale of a post-junctional block is shorter than a pre-junctional ACh release block and the... 

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