Above: Overview of Toxic mechanism, this diagram can be broken down into three stages and is related to the sequential diagrams below.
Source - Modified from http://chemdef.apgea.army.mil/textbook/Ch-33.pdf
The first stage of toxicity comprises the recognition and binding of the carbonyl group at the end of the heavy chain (marked on as a black spot on the figure above) to a receptor or acceptor of a neuron.
Above : Stage 1- Binding of Toxin to receptor or acceptor via the heavy chain part of the toxin.
The next stage involves the transfer of the toxin to within the neuron through a pathway generally known as Receptor Mediated Endocytosis (RME). This is a pathway shared by several other toxins, as well as many hormones and some viruses. This process is referred to as the internalization step. The net result is the formation of a toxin containing vesicle within the neuron.
Above : Stage 2- Internalization step. Botulinum Toxin enters the neuron via Receptor Mediated Endocytosis.
Cleavage of the disulphide bond then leads to the release of the light chain part of the molecule into the cytoplasm.
This movement of the light chain leads to the third and final stage of toxicity, as it crosses to the cytoplasm it demonstrates enzyme like properties and catalyses specific reactions that consequently leads to the prevention of neurotransmitter release.
This effectively causes a break in the neuro-muscular junction, resulting in the paralysis of the associated muscle. Hence the ill health effects of Botulinum Toxins on human health (i.e. Botulism).
Above : Stage 3 - Enzymatic activity of Toxin catalyses reactions within the neuron leading to ceasing of neuron operation.
This mechanism is irreversible for a given neuron, although it is not destroyed, the neuron is rendered inactive forever. If the organism survives such an attack, eventually neuromuscular control is regained as a result of nerve sprouting. This means it is possible to make a full recovery from Botulism.
See the Cosmetics page for more detail of recovery mechanism, and an example of where neuron recovery is undesirable!