PRINCIPLES OF TOXICOLOGY

412 PROPERTIES AND EFFECTS OF NATURAL TOXINS AND VENOMS
Figure 17.1a Common sites of neurotoxin action at the mammalian neuromuscular junction. Toxins can affect
the (A) nerve action potential, (B) release of neurotransmitter from the nerve terminal, (C) the membrane
depolarizing response of neurotransmitter receptors to neurotransmitter, (D) muscle action potential, and (E)
coupling of muscle membrane depolarization with calcium release from the sarcoplasmic reticulum, mediated by
transverse invaginations of the cell membrane called t-tubules.
Figure 17.1b A molecular view of the nicotinic acetylcholine receptor found in the skeletal muscle membrane.
The receptor is a pentamer consisting of four different polypeptide subunits; the two α subunits must be occupied
by an agonist like ACh to open the ion channel in the center. Three potential sites of toxin action on the nicotinic
receptor are shown: circular molecule (A), toxin binding to the neurotransmitter (acetylcholine) recognition site;
triangular molecule (B), toxin binding to the edge of the receptor protein, interfering with its interaction with the
lipid bilayer; square molecule (C), toxin entering and directly blocking the ion channel.