Final answer:
Acetylcholine is found in all animal taxa and acts as a neurotransmitter, binding to ionotropic receptors to initiate a cellular response and is integral to neuromuscular junction function.
Step-by-step explanation:
The substance which is found in all animal taxa and acts as a neurotransmitter is acetylcholine. Acetylcholine acts like the endogenous neurotransmitter by binding to its receptor, specifically the ionotropic receptor. This type of receptor acts as an ion channel gate and opens by the binding of the neurotransmitter, allowing ions to flow through and initiate a response. Acetylcholine can also interact with metabotropic receptors influencing various physiological functions. Besides its role in the central nervous system, it is also notable for its function at the neuromuscular junction, where it triggers muscle contractions.
Neurotransmitters are compounds that are released at the synapse of neurons to stimulate or suppress the actions of other cells. A unique property of some neurotransmitters is their ability to interfere with neurotransmitter reuptake or to block voltage-gated calcium ion channels, affecting the strength and duration of the signal. Substances like GABA act within their specific system—GABAergic neurotransmission—to function as the main inhibitory neurotransmitter in the central nervous system.