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 venom of black widow spider is neurotoxic and contains α-latrotoxin. This toxin binds to the surface receptors of neurosecretory PC12 cells and vertebrate synapses and activates the release of the neurotransmitter acetylcholine.  

Acetylcholine is the neurotransmitter which plays important role in muscle contraction so the toxin allow the continuous release of the neurotransmitter which cause contraction of smooth muscle in airways and other muscle in the body and cause breathing difficulties and muscle spasms.