Neurotransmitters are small diffusible molecules such as acetylcholine, serotonin etc, which communicate nerve impulses across most synapses i.e the junction between two neurons.
They are regarded as chemical messengers because they are chemical substances, and they do aid transmission of nerve impulses for the neuron
If the environment changes quickly and no individual members of a species are able to survive in the new environment, then the species will become extinct in that environment
