The nervous system sends signals by an electrical impulse that travels along the length of the nerve until it reaches a junction with another nerve cell (synaptic cleft), or a muscle cell (neuromuscular junction). When the impulse reaches a junction, the influx of ions stimulates the release of vesicles containing neurotransmitters, such as acetylcholine, which diffuse across the gap and bind to receptors on the adjoining nerve or muscle cell, thereby continuing the response.
There are two types of acetylcholine receptors (AchR): muscarinic-type, which are primarily neuronal, and nicotinic-type, which are either neuronal or muscle-type. The venom from the many-banded krait (Bungarus multicinctus) contains toxins that can bind to each type of receptor: the a-bungarotoxins act primarily on nicotinic AchRs at the neuromuscular junction, the k-bungarotoxins act primarily on nicotinic AchRs in neuronal tissue, and there are also muscarinic AchR-binding toxins. These toxins show almost irreversible binding to the receptors, competitively inhibiting acetylcholine binding and, consequently, inhibiting the acetylcholine-induced electrical response. Both a- and k-bungarotoxins are three-finger toxins, their characteristic ‘3-finger’ structure being determined by disulphide bonds. This 3-finger fold allows for variation in structure, which can alter the function and selectivity of molecular targets. Evolutionary divergence has given rise to over 100 other post-synaptic a-neurotoxins found in Elapidae and Hydrophiidae, which may be related to the 3-finger proteins of vertebrates that play a significant role in cell-cell adhesion