During a rest stop while hiking, Phil was bitten by a black widow spider. Shortly after being bitten, he started having breathing difficulties, then experienced muscle spasms. The symptoms he experienced occurred because the black widow spider's venom:.

caused acetylcholine to be continuously released by the motor neurons.

Explanation:

The black widow spider releases a venom on the bite. The name of the venom is the alpha-latrotoxin. The alpha-latrotoxin is a neurotoxin produced by the venom of the spiders. This venom is responsible for the release of latrodectism. The person experiences the release of acetylcholine in excess from the motor neurons due to the venom.  

The release of excess acetylcholine causes contraction of the muscles and the person may experience a muscle spasm.

pathogens is the answer