The most infamous spiders in the United States, as far as being dangerous to humans, are the brown recluse and the black widow spider. The brown recluse is understandably dangerous, as their toxin causes necrosis, and a chunk of flesh as large as a softball in diameter eventually rots away unless there is early intervention. However, I am going to discuss the black widow. I shot some photographs recently, and recalled discussions in Neurobiology of Cognition about latrotoxin (named after the genus of the black widow, latrodectus).
Latrotoxins are the active ingredient in black widow venom which causes symptoms, known as latrodectism, in humans. The toxins are large molecules, and the most studied of the latrotoxins, alpha-latrotoxin, acts presynaptically to stimulate the release of neurotransmitters. The toxin forms a tetramer, so four alpha-latrotoxin molecules group together and form an ion pore. When the tetramer enters the cell membrane, the pore allows an influx of calcium ions, which results in neurotransmitter release. At nerve endings, it causes muscles to remain contracted, resulting in muscle cramps and pain - particularly in the abdomen. Rarely, people with heart problems can suffer complications. Luckily for us all, antivenin is readily available and there has not been a death since the 1940s.