Chemicals called are stored in neurons and released when the cell is stimulated by a signal.
toxins
cytokines
chemokines
neurotransmitters

Neurotransmitters

Explanation:

Neurotransmitters are the chemicals present in the body used for the propagation of action potential. These neurotransmitters are also known as the body chemical messengers.

Neurotransmitters are stored in the neural cells. These chemicals are generally released when the cell is stimulated by the specific signal. The effect of neurotransmitter may be excitatory or inhibitory.

Thus, the answer is option (4).

answer; ///i believe its: ///(embolus);

climate change and pollution